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### draft-jonsson-pkcs1-v2dot1

 Last Version: draft-jonsson-pkcs1-v2dot1-00.txt Tracker Entry Date: 11-Feb-2003 Disposition: RFC3447 (diff)



Network Working Group                                         J. Jonsson
Internet Draft                                                B. Kaliski
expires in six months                                   RSA Laboratories
<draft-jonsson-pkcs1-v2dot1-00.txt>                          August 2002

PKCS #1: RSA Cryptography Specifications
Version 2.1

Status of this Memo

This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026 except that the right to produce derivative
works is not granted.

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Abstract

This memo represents a republication of PKCS #1 v2.1 from RSA
Laboratories' Public-Key Cryptography Standards (PKCS) series, and
change control is retained within the PKCS process.  The body of
this document is taken directly from the PKCS #1 v2.1 document.

This document provides recommendations for the implementation of
public-key cryptography based on the RSA algorithm.

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1.       Introduction.....................................3
2.       Notation.........................................4
3.       Key types........................................6
3.1      RSA public key...................................7
3.2      RSA private key..................................7
4.       Data conversion primitives.......................9
4.1      I2OSP............................................9
4.2      OS2IP...........................................10
5.       Cryptographic primitives........................11
5.1      Encryption and decryption primitives............11
5.2      Signature and verification primitives...........13
6.       Overview of schemes.............................15
7.       Encryption schemes..............................16
7.1      RSAES-OAEP......................................17
7.2      RSAES-PKCS1-v1_5................................24
8.       Signature schemes with appendix.................28
8.1      RSASSA-PSS......................................29
8.2      RSASSA-PKCS1-v1_5...............................32
9        Encoding methods for signatures with appendix...36
9.1      EMSA-PSS........................................37
9.2      EMSA-PKCS1-v1_5.................................42
A.       ASN.1 syntax....................................44
A.1      RSA key representation..........................44
A.2      Scheme identification...........................46
B.       Supporting techniques...........................52
B.1      Hash functions..................................52
C.       ASN.1 module....................................55
D.       Intellectual property considerations............63
E.       Revision history................................63
F.       References......................................64
Security Considerations.........................69
Acknowledgements................................69

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1. Introduction

This document provides recommendations for the implementation of
public-key cryptography based on the RSA algorithm [42], covering
the following aspects:

* Cryptographic primitives

* Encryption schemes

* Signature schemes with appendix

* ASN.1 syntax for representing keys and for identifying the
schemes

The recommendations are intended for general application within
computer and communications systems, and as such include a fair
amount of flexibility. It is expected that application standards
based on these specifications may include additional constraints.
The recommendations are intended to be compatible with the standard
IEEE-1363-2000 [26] and draft standards currently being developed
by the ANSI X9F1 [1] and IEEE P1363 [27] working groups.

This document supersedes PKCS #1 version 2.0 [35][44] but includes
compatible techniques.

The organization of this document is as follows:

* Section 1 is an introduction.

* Section 2 defines some notation used in this document.

* Section 3 defines the RSA public and private key types.

* Sections 4 and 5 define several primitives, or basic
mathematical operations. Data conversion primitives are in
Section 4, and cryptographic primitives (encryption-decryption,
signature-verification) are in Section 5.

* Sections 6, 7, and 8 deal with the encryption and signature
schemes in this document. Section 6 gives an overview. Along
with the methods found in PKCS #1 v1.5, Section 7 defines an
OAEP-based [3] encryption scheme and Section 8 defines a
PSS-based [4][5] signature scheme with appendix.

* Section 9 defines the encoding methods for the signature
schemes in Section 8.

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* Appendix A defines the ASN.1 syntax for the keys defined in
Section 3 and the schemes in Sections 7 and 8.

* Appendix B defines the hash functions and the mask generation
function used in this document, including ASN.1 syntax for
the techniques.

* Appendix C gives an ASN.1 module.

* Appendices D, E, F and G cover intellectual property issues,
outline the revision history of PKCS #1, give references to
other publications and standards, and provide general
information about the Public-Key Cryptography Standards.

2. Notation

c              ciphertext representative, an integer between 0
and n-1

C              ciphertext, an octet string

d              RSA private exponent

d_i            additional factor r_i's CRT exponent, a positive
integer such that

e * d_i == 1 (mod (r_i-1)), i = 3, ..., u

dP             p's CRT exponent, a positive integer such that

e * dP == 1 (mod (p-1))

dQ             q's CRT exponent, a positive integer such that

e * dQ == 1 (mod (q-1))

e              RSA public exponent

EM             encoded message, an octet string

emBits         (intended) length in bits of an encoded message EM

emLen          (intended) length in octets of an encoded message EM

GCD(. , .)     greatest common divisor of two nonnegative integers

Hash           hash function

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hLen           output length in octets of hash function Hash

k              length in octets of the RSA modulus n

K              RSA private key

L              optional RSAES-OAEP label, an octet string

LCM(., ..., .) least common multiple of a list of nonnegative
integers

m              message representative, an integer between 0 and n-1

M              message, an octet string

mask           MGF output, an octet string

mgfSeed       seed from which mask is generated, an octet string

mLen          length in octets of a message M

n             RSA modulus, n = r_1 * r_2 * ... * r_u , u >= 2

(n, e)        RSA public key

p, q          first two prime factors of the RSA modulus n

qInv          CRT coefficient, a positive integer less than p such
that

q * qInv == 1 (mod p)

r_i           prime factors of the RSA modulus n, including
r_1 = p, r_2 = q, and additional factors if any

s             signature representative, an integer between 0 and
n-1

S             signature, an octet string

sLen          length in octets of the EMSA-PSS salt

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t_i           additional prime factor r_i's CRT coefficient, a
positive integer less than r_i such that

r_1 * r_2 * ... * r_(i-1) * t_i == 1 (mod r_i) ,

i = 3, ... , u

u             number of prime factors of the RSA modulus, u >= 2

x             a nonnegative integer

X             an octet string corresponding to x

xLen          (intended) length of the octet string X

0x            indicator of hexadecimal representation of an octet
or an octet string; "0x48" denotes the octet with
hexadecimal value 48; "(0x)48 09 0e" denotes the
string of three consecutive octets with hexadecimal
value 48, 09, and 0e, respectively

\lambda(n)    LCM(r_1-1, r_2-1, ... , r_u-1)

\xor          bit-wise exclusive-or of two octet strings

\ceil(.)      ceiling function; \ceil(x) is the smallest integer
larger than or equal to the real number x

||            concatenation operator

==            congruence symbol; a == b (mod n) means that the
integer n divides the integer a - b

Note. The CRT can be applied in a non-recursive as well as a
recursive way. In this document a recursive approach following
Garner's algorithm [22] is used. See also Note 1 in Section 3.2.

3. Key types

Two key types are employed in the primitives and schemes defined in
this document: RSA public key and RSA private key. Together, an RSA
public key and an RSA private key form an RSA key pair.

This specification supports so-called "multi-prime" RSA where the
modulus may have more than two prime factors. The benefit of
multi-prime RSA is lower computational cost for the decryption and
signature primitives, provided that the CRT (Chinese Remainder
Theorem) is used. Better performance can be achieved on single

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processor platforms, but to a greater extent on multiprocessor
platforms, where the modular exponentiations involved can be done
in parallel.

For a discussion on how multi-prime affects the security of the RSA
cryptosystem, the reader is referred to [49].

3.1 RSA public key

For the purposes of this document, an RSA public key consists of
two components:

n        the RSA modulus, a positive integer
e        the RSA public exponent, a positive integer

In a valid RSA public key, the RSA modulus n is a product of u
distinct odd primes r_i, i = 1, 2, ..., u, where u >= 2, and the
RSA public exponent e is an integer between 3 and n - 1 satisfying
GCD(e, \lambda(n)) = 1, where \lambda(n) = LCM(r_1 - 1, ...,
r_u - 1). By convention, the first two primes r_1 and r_2 may also
be denoted p and q respectively.

A recommended syntax for interchanging RSA public keys between
implementations is given in Appendix A.1.1; an implementation's
internal representation may differ.

3.2 RSA private key

For the purposes of this document, an RSA private key may have
either of two representations.

1. The first representation consists of the pair (n, d), where the
components have the following meanings:

n        the RSA modulus, a positive integer
d        the RSA private exponent, a positive integer

2. The second representation consists of a quintuple (p, q, dP, dQ,
qInv) and a (possibly empty) sequence of triplets (r_i, d_i, t_i),
i = 3, ..., u, one for each prime not in the quintuple, where the
components have the following meanings:

p        the first factor, a positive integer
q        the second factor, a positive integer
dP       the first factor's CRT exponent, a positive integer
dQ       the second factor's CRT exponent, a positive integer
qInv     the (first) CRT coefficient, a positive integer

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r_i   the i-th factor, a positive integer
d_i   the i-th factor's CRT exponent, a positive integer
t_i   the i-th factor's CRT coefficient, a positive integer

In a valid RSA private key with the first representation, the RSA
modulus n is the same as in the corresponding RSA public key and is
the product of u distinct odd primes r_i, i = 1, 2, ..., u, where u
>= 2. The RSA private exponent d is a positive integer less than n
satisfying

e * d == 1 (mod \lambda(n)),

where e is the corresponding RSA public exponent and \lambda(n) is
defined as in Section 3.1.

In a valid RSA private key with the second representation, the two
factors p and q are the first two prime factors of the RSA modulus
n (i.e., r_1 and r_2), the CRT exponents dP and dQ are positive
integers less than p and q respectively satisfying

e * dP == 1 (mod (p-1))
e * dQ == 1 (mod (q-1)) ,

and the CRT coefficient qInv is a positive integer less than p
satisfying

q * qInv == 1 (mod p).

If u > 2, the representation will include one or more triplets
(r_i, d_i, t_i), i = 3, ..., u. The factors r_i are the additional
prime factors of the RSA modulus n. Each CRT exponent d_i (i = 3,
..., u) satisfies

e * d_i == 1 (mod (r_i - 1)).

Each CRT coefficient t_i (i = 3, ..., u) is a positive integer less
than r_i satisfying

R_i * t_i == 1 (mod r_i) ,

where R_i = r_1 * r_2 * ... * r_(i-1).

A recommended syntax for interchanging RSA private keys between
implementations, which includes components from both
representations, is given in Appendix A.1.2; an implementation's
internal representation may differ.

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Notes.

1. The definition of the CRT coefficients here and the formulas
that use them in the primitives in Section 5 generally follow
However, for compatibility with the representations of RSA
private keys in PKCS #1 v2.0 and previous versions, the roles of
p and q are reversed compared to the rest of the primes. Thus,
the first CRT coefficient, qInv, is defined as the inverse of
q mod p, rather than as the inverse of R_1 mod r_2, i.e., of
p mod q.

2. Quisquater and Couvreur [40] observed the benefit of applying
the Chinese Remainder Theorem to RSA operations.

4. Data conversion primitives

Two data conversion primitives are employed in the schemes defined
in this document:

* I2OSP - Integer-to-Octet-String primitive

* OS2IP - Octet-String-to-Integer primitive

For the purposes of this document, and consistent with ASN.1
syntax, an octet string is an ordered sequence of octets (eight-bit
bytes). The sequence is indexed from first (conventionally,
leftmost) to last (rightmost). For purposes of conversion to and
from integers, the first octet is considered the most significant
in the following conversion primitives.

4.1 I2OSP

I2OSP converts a nonnegative integer to an octet string of a
specified length.

I2OSP (x, xLen)

Input:
x        nonnegative integer to be converted
xLen     intended length of the resulting octet string

Output:
X        corresponding octet string of length xLen

Error: "integer too large"

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Steps:

1. If x >= 256^xLen, output "integer too large" and stop.

2. Write the integer x in its unique xLen-digit representation in
base 256:

x = x_(xLen-1) 256^(xLen-1) + x_(xLen-2) 256^(xLen-2) + ...
+ x_1 256 + x_0,

where 0 <= x_i < 256 (note that one or more leading digits will
be zero if x is less than 256^(xLen-1)).

3. Let the octet X_i have the integer value x_(xLen-i) for 1 <= i
<= xLen. Output the octet string

X = X_1 X_2 ... X_xLen.

4.2 OS2IP

OS2IP converts an octet string to a nonnegative integer.

OS2IP (X)

Input:
X        octet string to be converted

Output:
x        corresponding nonnegative integer

Steps:

1. Let X_1 X_2 ... X_xLen be the octets of X from first to last,
and let x_(xLen-i) be the integer value of the octet X_i for
1 <= i <= xLen.

2. Let x = x_(xLen-1) 256^(xLen-1) + x_(xLen-2) 256^(xLen-2) + ...
+ x_1 256 + x_0.

3. Output x.

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5. Cryptographic primitives

Cryptographic primitives are basic mathematical operations on which
cryptographic schemes can be built. They are intended for
implementation in hardware or as software modules, and are not
intended to provide security apart from a scheme.

Four types of primitive are specified in this document, organized
in pairs: encryption and decryption; and signature and
verification.

The specifications of the primitives assume that certain conditions
are met by the inputs, in particular that RSA public and private
keys are valid.

5.1 Encryption and decryption primitives

An encryption primitive produces a ciphertext representative from a
message representative under the control of a public key, and a
decryption primitive recovers the message representative from the
ciphertext representative under the control of the corresponding
private key.

One pair of encryption and decryption primitives is employed in the
encryption schemes defined in this document and is specified here:
operation, with different keys as input.

The primitives defined here are the same as IFEP-RSA/IFDP-RSA in
IEEE Std 1363-2000 [26] (except that support for multi-prime RSA
has been added) and are compatible with PKCS #1 v1.5.

The main mathematical operation in each primitive is
exponentiation.

5.1.1 RSAEP

RSAEP ((n, e), m)

Input:
(n, e)   RSA public key
m        message representative, an integer between 0 and n - 1

Output:
c        ciphertext representative, an integer between 0 and n - 1

Error: "message representative out of range"

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Assumption: RSA public key (n, e) is valid

Steps:

1. If the message representative m is not between 0 and n - 1,
output "message representative out of range" and stop.

2. Let c = m^e mod n.

3. Output c.

Input:
K        RSA private key, where K has one of the following forms:
- a pair (n, d)
- a quintuple (p, q, dP, dQ, qInv) and a possibly empty
sequence of triplets (r_i, d_i, t_i), i = 3, ..., u
c        ciphertext representative, an integer between 0 and n - 1

Output:
m        message representative, an integer between 0 and n - 1

Error: "ciphertext representative out of range"

Assumption: RSA private key K is valid

Steps:

1. If the ciphertext representative c is not between 0 and n - 1,
output "ciphertext representative out of range" and stop.

2. The message representative m is computed as follows.

a. If the first form (n, d) of K is used, let m = c^d mod n.

b. If the second form (p, q, dP, dQ, qInv) and (r_i, d_i, t_i)
of K is used, proceed as follows:

i.    Let m_1 = c^dP mod p and m_2 = c^dQ mod q.

ii.   If u > 2, let m_i = c^(d_i) mod r_i, i = 3, ..., u.

iii.  Let h = (m_1 - m_2) * qInv mod p.

iv.   Let m = m_2 + q * h.

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v.    If u > 2, let R = r_1 and for i = 3 to u do

1. Let R = R * r_(i-1).

2. Let h = (m_i - m) * t_i mod r_i.

3. Let m = m + R * h.

3.   Output m.

Note. Step 2.b can be rewritten as a single loop, provided that one
reverses the order of p and q. For consistency with PKCS #1 v2.0,
however, the first two primes p and q are treated separately from

5.2 Signature and verification primitives

A signature primitive produces a signature representative from a
message representative under the control of a private key, and a
verification primitive recovers the message representative from the
signature representative under the control of the corresponding
public key. One pair of signature and verification primitives is
employed in the signature schemes defined in this document and is
specified here: RSASP1/RSAVP1.

The primitives defined here are the same as IFSP-RSA1/IFVP-RSA1 in
IEEE 1363-2000 [26] (except that support for multi-prime RSA has
been added) and are compatible with PKCS #1 v1.5.

The main mathematical operation in each primitive is
exponentiation, as in the encryption and decryption primitives of
Section 5.1. RSASP1 and RSAVP1 are the same as RSADP and RSAEP
except for the names of their input and output arguments; they are
distinguished as they are intended for different purposes.

5.2.1 RSASP1

RSASP1 (K, m)

Input:
K        RSA private key, where K has one of the following forms:
- a pair (n, d)
- a quintuple (p, q, dP, dQ, qInv) and a (possibly empty)
sequence of triplets (r_i, d_i, t_i), i = 3, ..., u
m        message representative, an integer between 0 and n - 1

Output:
s        signature representative, an integer between 0 and n - 1

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Error: "message representative out of range"

Assumption: RSA private key K is valid

Steps:

1. If the message representative m is not between 0 and n - 1,
output "message representative out of range" and stop.

2. The signature representative s is computed as follows.

a. If the first form (n, d) of K is used, let s = m^d mod n.

b. If the second form (p, q, dP, dQ, qInv) and (r_i, d_i, t_i)
of K is used, proceed as follows:

i.    Let s_1 = m^dP mod p and s_2 = m^dQ mod q.

ii.   If u > 2, let s_i = m^(d_i) mod r_i, i = 3, ..., u.

iii.  Let h = (s_1 - s_2) * qInv mod p.

iv.   Let s = s_2 + q * h.

v.    If u > 2, let R = r_1 and for i = 3 to u do

1. Let R = R * r_(i-1).

2. Let h = (s_i - s) * t_i mod r_i.

3. Let s = s + R * h.

3. Output s.

Note. Step 2.b can be rewritten as a single loop, provided that one
reverses the order of p and q. For consistency with PKCS #1 v2.0,
however, the first two primes p and q are treated separately from

5.2.2 RSAVP1

RSAVP1 ((n, e), s)

Input:
(n, e)   RSA public key
s        signature representative, an integer between 0 and n - 1

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Output:
m        message representative, an integer between 0 and n - 1

Error: "signature representative out of range"

Assumption: RSA public key (n, e) is valid

Steps:

1. If the signature representative s is not between 0 and n - 1,
output "signature representative out of range" and stop.

2. Let m = s^e mod n.

3. Output m.

6. Overview of schemes

A scheme combines cryptographic primitives and other techniques to
achieve a particular security goal. Two types of scheme are
specified in this document: encryption schemes and signature
schemes with appendix.

The schemes specified in this document are limited in scope in that
their operations consist only of steps to process data with an RSA
public or private key, and do not include steps for obtaining or
validating the key. Thus, in addition to the scheme operations, an
application will typically include key management operations by
which parties may select RSA public and private keys for a scheme
operation. The specific additional operations and other details are
outside the scope of this document.

As was the case for the cryptographic primitives (Section 5), the
specifications of scheme operations assume that certain conditions
are met by the inputs, in particular that RSA public and private
keys are valid. The behavior of an implementation is thus
unspecified when a key is invalid. The impact of such unspecified
behavior depends on the application. Possible means of addressing
key validation include explicit key validation by the application;
key validation within the public-key infrastructure; and assignment
of liability for operations performed with an invalid key to the
party who generated the key.

A generally good cryptographic practice is to employ a given RSA
key pair in only one scheme. This avoids the risk that
vulnerability in one scheme may compromise the security of the
other, and may be essential to maintain provable security. While
RSAES-PKCS1-v1_5 (Section 7.2) and RSASSA-PKCS1-v1_5 (Section 8.2)

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interactions (indeed, this is the model introduced by PKCS #1
v1.5), such a combined use of an RSA key pair is not recommended
for new applications.

To illustrate the risks related to the employment of an RSA key
pair in more than one scheme, suppose an RSA key pair is employed
in both RSAES-OAEP (Section 7.1) and RSAES-PKCS1-v1_5. Although
RSAES-OAEP by itself would resist attack, an opponent might be able
to exploit a weakness in the implementation of RSAES-PKCS1-v1_5 to
recover messages encrypted with either scheme. As another example,
suppose an RSA key pair is employed in both RSASSA-PSS (Section
8.1) and RSASSA-PKCS1-v1_5. Then the security proof for RSASSA-PSS
would no longer be sufficient since the proof does not account for
the possibility that signatures might be generated with a second
scheme. Similar considerations may apply if an RSA key pair is
employed in one of the schemes defined here and in a variant
defined elsewhere.

7. Encryption schemes

For the purposes of this document, an encryption scheme consists of
an encryption operation and a decryption operation, where the
encryption operation produces a ciphertext from a message with a
recipient's RSA public key, and the decryption operation recovers
the message from the ciphertext with the recipient's corresponding
RSA private key.

An encryption scheme can be employed in a variety of applications.
A typical application is a key establishment protocol, where the
message contains key material to be delivered confidentially from
one party to another. For instance, PKCS #7 [45] employs such a
protocol to deliver a content-encryption key from a sender to a
recipient; the encryption schemes defined here would be suitable
key-encryption algorithms in that context.

Two encryption schemes are specified in this document: RSAES-OAEP
and RSAES-PKCS1-v1_5. RSAES-OAEP is recommended for new
applications; RSAES-PKCS1-v1_5 is included only for compatibility
with existing applications, and is not recommended for new
applications.

The encryption schemes given here follow a general model similar to
that employed in IEEE Std 1363-2000 [26], combining encryption and
decryption primitives with an encoding method for encryption. The
encryption operations apply a message encoding operation to a
message to produce an encoded message, which is then converted to
an integer message representative. An encryption primitive is

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applied to the message representative to produce the ciphertext.
Reversing this, the decryption operations apply a decryption
primitive to the ciphertext to recover a message representative,
which is then converted to an octet string encoded message. A
message decoding operation is applied to the encoded message to
recover the message and verify the correctness of the decryption.

To avoid implementation weaknesses related to the way errors are
handled within the decoding operation (see [6] and [36]), the
encoding and decoding operations for RSAES-OAEP and
RSAES-PKCS1-v1_5 are embedded in the specifications of the
respective encryption schemes rather than defined in separate
specifications. Both encryption schemes are compatible with the
corresponding schemes in PKCS #1 v2.0.

7.1 RSAES-OAEP

RSAES-OAEP combines the RSAEP and RSADP primitives (Sections 5.1.1
and 5.1.2) with the EME-OAEP encoding method (step 1.b in Section
7.1.1 and step 3 in Section 7.1.2). EME-OAEP is based on Bellare
and Rogaway's Optimal Asymmetric Encryption scheme [3]. (OAEP
stands for "Optimal Asymmetric Encryption Padding."). It is
compatible with the IFES scheme defined in IEEE Std 1363-2000 [26],
where the encryption and decryption primitives are IFEP-RSA and
IFDP-RSA and the message encoding method is EME-OAEP. RSAES-OAEP
can operate on messages of length up to k - 2hLen - 2 octets, where
hLen is the length of the output from the underlying hash function
and k is the length in octets of the recipient's RSA modulus.

Assuming that computing e-th roots modulo n is infeasible and the
mask generation function in RSAES-OAEP has appropriate properties,
RSAES-OAEP is semantically secure against adaptive chosen-
ciphertext attacks. This assurance is provable in the sense that
the difficulty of breaking RSAES-OAEP can be directly related to
the difficulty of inverting the RSA function, provided that the
mask generation function is viewed as a black box or random oracle;
see [21] and the note below for further discussion.

Both the encryption and the decryption operations of RSAES-OAEP
take the value of a label L as input. In this version of PKCS #1, L
is the empty string; other uses of the label are outside the scope
of this document. See Appendix A.2.1 for the relevant ASN.1
syntax.

RSAES-OAEP is parameterized by the choice of hash function and mask
generation function. This choice should be fixed for a given RSA
key. Suggested hash and mask generation functions are given in
Appendix B.

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Note. Recent results have helpfully clarified the security
properties of the OAEP encoding method [3] (roughly the procedure
described in step 1.b in Section 7.1.1). The background is as
follows. In 1994, Bellare and Rogaway [3] introduced a security
concept that they denoted plaintext awareness (PA94). They proved
that if a deterministic public-key encryption primitive (e.g.,
RSAEP) is hard to invert without the private key, then the
corresponding OAEP-based encryption scheme is plaintext-aware (in
the random oracle model), meaning roughly that an adversary cannot
produce a valid ciphertext without actually "knowing" the
underlying plaintext. Plaintext awareness of an encryption scheme
is closely related to the resistance of the scheme against
chosen-ciphertext attacks. In such attacks, an adversary is given
the opportunity to send queries to an oracle simulating the
decryption primitive. Using the results of these queries, the
adversary attempts to decrypt a challenge ciphertext.

However, there are two flavors of chosen-ciphertext attacks, and
PA94 implies security against only one of them. The difference
relies on what the adversary is allowed to do after she is given
the challenge ciphertext. The indifferent attack scenario (denoted
CCA1) does not admit any queries to the decryption oracle after the
scenario (denoted CCA2) does (except that the decryption oracle
refuses to decrypt the challenge ciphertext once it is published).
In 1998, Bellare and Rogaway, together with Desai and Pointcheval
[2], came up with a new, stronger notion of plaintext awareness
(PA98) that does imply security against CCA2.

To summarize, there have been two potential sources for
misconception: that PA94 and PA98 are equivalent concepts; or that
CCA1 and CCA2 are equivalent concepts. Either assumption leads to
the conclusion that the Bellare-Rogaway paper implies security of
OAEP against CCA2, which it does not.

(Footnote: It might be fair to mention that PKCS #1 v2.0 cites [3]
and claims that "a chosen ciphertext attack is ineffective against
a plaintext-aware encryption scheme such as RSAES-OAEP" without
specifying the kind of plaintext awareness or chosen ciphertext
attack considered.)

OAEP has never been proven secure against CCA2; in fact, Victor
Shoup [48] has demonstrated that such a proof does not exist in the
general case. Put briefly, Shoup showed that an adversary in the
CCA2 scenario who knows how to partially invert the encryption
primitive but does not know how to invert it completely may well be
able to break the scheme. For example, one may imagine an attacker
who is able to break RSAES-OAEP if she knows how to recover all but

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the first 20 bytes of a random integer encrypted with RSAEP. Such
an attacker does not need to be able to fully invert RSAEP, because
she does not use the first 20 octets in her attack.

Still, RSAES-OAEP is secure against CCA2, which was proved by
Fujisaki, Okamoto, Pointcheval, and Stern [21] shortly after the
announcement of Shoup's result. Using clever lattice reduction
techniques, they managed to show how to invert RSAEP completely
given a sufficiently large part of the pre-image. This observation,
combined with a proof that OAEP is secure against CCA2 if the
underlying encryption primitive is hard to partially invert, fills
the gap between what Bellare and Rogaway proved about RSAES-OAEP
and what some may have believed that they proved. Somewhat
paradoxically, we are hence saved by an ostensible weakness in
RSAEP (i.e., the whole inverse can be deduced from parts of it).

Unfortunately however, the security reduction is not efficient for
concrete parameters. While the proof successfully relates an
algorithm Inv inverting RSA, the probability of success for Inv is
only approximately \epsilon^2 / 2^18, where \epsilon is the

(Footnote: In [21] the probability of success for the inverter was
\epsilon^2 / 4. The additional factor 1 / 2^16 is due to the eight
fixed zero bits at the beginning of the encoded message EM, which
are not present in the variant of OAEP considered in [21] (Inv must
apply Adv twice to invert RSA, and each application corresponds to
a factor 1 / 2^8).)

In addition, the running time for Inv is approximately t^2, where t
is the running time of the adversary. The consequence is that we
cannot exclude the possibility that attacking RSAES-OAEP is
considerably easier than inverting RSA for concrete parameters.
Still, the existence of a security proof provides some assurance
that the RSAES-OAEP construction is sounder than ad hoc
constructions such as RSAES-PKCS1-v1_5.

Hybrid encryption schemes based on the RSA-KEM key encapsulation
paradigm offer tight proofs of security directly applicable to
concrete parameters; see [30] for discussion. Future versions of
PKCS #1 may specify schemes based on this paradigm.

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7.1.1 Encryption operation

RSAES-OAEP-ENCRYPT ((n, e), M, L)

Options:
Hash    hash function (hLen denotes the length in octets of the
hash function output)

Input:
(n, e)  recipient's RSA public key (k denotes the length in octets
of the RSA modulus n)
M       message to be encrypted, an octet string of length mLen,
where mLen <= k - 2hLen - 2
L       optional label to be associated with the message; the default
value for L, if L is not provided, is the empty string

Output:
C       ciphertext, an octet string of length k

Errors: "message too long"; "label too long"

Assumption: RSA public key (n, e) is valid

Steps:

1. Length checking:

a. If the length of L is greater than the input limitation for
the hash function (2^61 - 1 octets for SHA-1), output "label
too long" and stop.

b. If mLen > k - 2hLen - 2, output "message too long" and stop.

2. EME-OAEP encoding (see Figure 1 below):

a. If the label L is not provided, let L be the empty string.
Let lHash = Hash(L), an octet string of length hLen (see the
note below).

b. Generate an octet string PS consisting of k - mLen - 2hLen -
2 zero octets. The length of PS may be zero.

c. Concatenate lHash, PS, a single octet with hexadecimal value
0x01, and the message M to form a data block DB of length k -
hLen - 1 octets as

DB = lHash || PS || 0x01 || M.

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d. Generate a random octet string seed of length hLen.

e. Let dbMask = MGF(seed, k - hLen - 1).

i. Concatenate a single octet with hexadecimal value 0x00,
length k octets as

3. RSA encryption:

a. Convert the encoded message EM to an integer message
representative m (see Section 4.2):

m = OS2IP (EM).

b. Apply the RSAEP encryption primitive (Section 5.1.1) to the
RSA public key (n, e) and the message representative m to
produce an integer ciphertext representative c:

c = RSAEP ((n, e), m).

c. Convert the ciphertext representative c to a ciphertext C of
length k octets (see Section 4.1):

C = I2OSP (c, k).

4. Output the ciphertext C.

Note. If L is the empty string, the corresponding hash value lHash
has the following hexadecimal representation for different choices
of Hash:

SHA-1:   (0x)da39a3ee 5e6b4b0d 3255bfef 95601890 afd80709
SHA-256: (0x)e3b0c442 98fc1c14 9afbf4c8 996fb924 27ae41e4 649b934c
a495991b 7852b855
SHA-384: (0x)38b060a7 51ac9638 4cd9327e b1b1e36a 21fdb711 14be0743
4c0cc7bf 63f6e1da 274edebf e76f65fb d51ad2f1 4898b95b
SHA-512: (0x)cf83e135 7eefb8bd f1542850 d66d8007 d620e405 0b5715dc
83f4a921 d36ce9ce 47d0d13c 5d85f2b0 ff8318d2 877eec2f
63b931bd 47417a81 a538327a f927da3e

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__________________________________________________________________

+----------+---------+-------+
DB = |  lHash   |    PS   |   M   |
+----------+---------+-------+
|
+----------+              V
|   seed   |--> MGF ---> xor
+----------+              |
|                   |
+--+     V                   |
|00|    xor <----- MGF <-----|
+--+     |                   |
|      |                   |
V      V                   V
+--+----------+----------------------------+
+--+----------+----------------------------+
__________________________________________________________________

Figure 1: EME-OAEP encoding operation. lHash is the hash of the
optional label L. Decoding operation follows reverse steps to
recover M and verify lHash and PS.

7.1.2 Decryption operation

RSAES-OAEP-DECRYPT (K, C, L)

Options:
Hash     hash function (hLen denotes the length in octets of the
hash function output)

Input:
K        recipient's RSA private key (k denotes the length in
octets of the RSA modulus n)
C        ciphertext to be decrypted, an octet string of length k,
where k = 2hLen + 2
L        optional label whose association with the message is to be
verified; the default value for L, if L is not provided,
is the empty string

Output:
M        message, an octet string of length mLen, where mLen ú k -
2hLen - 2

Error: "decryption error"

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Steps:

1. Length checking:

a. If the length of L is greater than the input limitation for
the hash function (2^61 - 1 octets for SHA-1), output
"decryption error" and stop.

b. If the length of the ciphertext C is not k octets, output
"decryption error" and stop.

c. If k < 2hLen + 2, output "decryption error" and stop.

2.    RSA decryption:

a. Convert the ciphertext C to an integer ciphertext
representative c (see Section 4.2):

c = OS2IP (C).

b. Apply the RSADP decryption primitive (Section 5.1.2) to the
RSA private key K and the ciphertext representative c to
produce an integer message representative m:

If RSADP outputs "ciphertext representative out of range"
(meaning that c >= n), output "decryption error" and stop.

c. Convert the message representative m to an encoded message EM
of length k octets (see Section 4.1):

EM = I2OSP (m, k).

3. EME-OAEP decoding:

a. If the label L is not provided, let L be the empty string.
Let lHash = Hash(L), an octet string of length hLen (see the
note in Section 7.1.1).

b. Separate the encoded message EM into a single octet Y, an
octet string maskedSeed of length hLen, and an octet string
maskedDB of length k - hLen - 1 as

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e. Let dbMask = MGF(seed, k - hLen - 1).

g. Separate DB into an octet string lHash' of length hLen, a
(possibly empty) padding string PS consisting of octets with
hexadecimal value 0x00, and a message M as

DB = lHash' || PS || 0x01 || M.

If there is no octet with hexadecimal value 0x01 to separate
PS from M, if lHash does not equal lHash', or if Y is
nonzero, output "decryption error" and stop. (See the note
below.)

4. Output the message M.

Note. Care must be taken to ensure that an opponent cannot
distinguish the different error conditions in Step 3.f, whether by
error message or timing, or, more generally, learn partial
information about the encoded message EM. Otherwise an opponent may
be able to obtain useful information about the decryption of the
ciphertext C, leading to a chosen-ciphertext attack such as the one
observed by Manger [36].

7.2 RSAES-PKCS1-v1_5

RSAES-PKCS1-v1_5 combines the RSAEP and RSADP primitives (Sections
5.1.1 and 5.1.2) with the EME-PKCS1-v1_5 encoding method (step 1 in
Section 7.2.1 and step 3 in Section 7.2.2). It is mathematically
equivalent to the encryption scheme in PKCS #1 v1.5.
RSAES-PKCS1-v1_5 can operate on messages of length up to k - 11
octets (k is the octet length of the RSA modulus), although care
should be taken to avoid certain attacks on low-exponent RSA due to
Coppersmith, Franklin, Patarin, and Reiter when long messages are
encrypted (see the third bullet in the notes below and [10]; [14]
contains an improved attack). As a general rule, the use of this
scheme for encrypting an arbitrary message, as opposed to a
randomly generated key, is not recommended.

It is possible to generate valid RSAES-PKCS1-v1_5 ciphertexts
without knowing the corresponding plaintexts, with a reasonable
probability of success. This ability can be exploited in a chosen-
ciphertext attack as shown in [6]. Therefore, if RSAES-PKCS1-v1_5
is to be used, certain easily implemented countermeasures should be
taken to thwart the attack found in [6]. Typical examples include

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the addition of structure to the data to be encoded, rigorous
checking of PKCS #1 v1.5 conformance (and other redundancy) in
decrypted messages, and the consolidation of error messages in a
client-server protocol based on PKCS #1 v1.5. These can all be
effective countermeasures and do not involve changes to a PKCS #1
v1.5-based protocol. See [7] for a further discussion of these and
other countermeasures. It has recently been shown that the security
of the SSL/TLS handshake protocol [17], which uses RSAES-PKCS1-v1_5
and certain countermeasures, can be related to a variant of the RSA
problem; see [32] for discussion.

Note. The following passages describe some security recommendations
pertaining to the use of RSAES-PKCS1-v1_5. Recommendations from
version 1.5 of this document are included as well as new
intervening years.

* It is recommended that the pseudorandom octets in step 2 in
Section 7.2.1 be generated independently for each encryption
process, especially if the same data is input to more than one
encryption process. H…stad's results [24] are one motivation for
this recommendation.

* The padding string PS in step 2 in Section 7.2.1 is at least
eight octets long, which is a security condition for public-key
operations that makes it difficult for an attacker to recover
data by trying all possible encryption blocks.

* The pseudorandom octets can also help thwart an attack due to
Coppersmith et al. [10] (see [14] for an improvement of the
attack) when the size of the message to be encrypted is kept
small. The attack works on low-exponent RSA when similar
messages are encrypted with the same RSA public key. More
specifically, in one flavor of the attack, when two inputs to
RSAEP agree on a large fraction of bits (8/9) and low-exponent
RSA (e = 3) is used to encrypt both of them, it may be possible
to recover both inputs with the attack. Another flavor of the
attack is successful in decrypting a single ciphertext when a
large fraction (2/3) of the input to RSAEP is already known. For
typical applications, the message to be encrypted is short
(e.g., a 128-bit symmetric key) so not enough information will
be known or common between two messages to enable the attack.
However, if a long message is encrypted, or if part of a message
is known, then the attack may be a concern. In any case, the
RSAES-OAEP scheme overcomes the attack.

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7.2.1 Encryption operation

RSAES-PKCS1-V1_5-ENCRYPT ((n, e), M)

Input:
(n, e)   recipient's RSA public key (k denotes the length in octets
of the modulus n)
M        message to be encrypted, an octet string of length mLen,
where mLen <= k - 11

Output:
C        ciphertext, an octet string of length k

Error: "message too long"

Steps:

1. Length checking: If mLen > k - 11, output "message too long" and
stop.

2. EME-PKCS1-v1_5 encoding:

a. Generate an octet string PS of length k - mLen - 3 consisting
of pseudo-randomly generated nonzero octets. The length of PS
will be at least eight octets.

b. Concatenate PS, the message M, and other padding to form an
encoded message EM of length k octets as

EM = 0x00 || 0x02 || PS || 0x00 || M.

3. RSA encryption:

a. Convert the encoded message EM to an integer message
representative m (see Section 4.2):

m = OS2IP (EM).

b. Apply the RSAEP encryption primitive (Section 5.1.1) to the
RSA public key (n, e) and the message representative m to
produce an integer ciphertext representative c:

c = RSAEP ((n, e), m).

c. Convert the ciphertext representative c to a ciphertext C of
length k octets (see Section 4.1):

C = I2OSP (c, k).

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4. Output the ciphertext C.

7.2.2 Decryption operation

RSAES-PKCS1-V1_5-DECRYPT (K, C)

Input:
K        recipient's RSA private key
C        ciphertext to be decrypted, an octet string of length k,
where k is the length in octets of the RSA modulus n

Output:
M        message, an octet string of length at most k - 11

Error: "decryption error"

Steps:

1. Length checking: If the length of the ciphertext C is not k
octets (or if k < 11), output "decryption error" and stop.

2. RSA decryption:

a. Convert the ciphertext C to an integer ciphertext
representative c (see Section 4.2):

c = OS2IP (C).

b. Apply the RSADP decryption primitive (Section 5.1.2) to the
RSA private key (n, d) and the ciphertext representative c to
produce an integer message representative m:

m = RSADP ((n, d), c).

If RSADP outputs "ciphertext representative out of range"
(meaning that c >= n), output "decryption error" and stop.

c. Convert the message representative m to an encoded message EM
of length k octets (see Section 4.1):

EM = I2OSP (m, k).

3. EME-PKCS1-v1_5 decoding: Separate the encoded message EM into an
octet string PS consisting of nonzero octets and a message M as

EM = 0x00 || 0x02 || PS || 0x00 || M.

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If the first octet of EM does not have hexadecimal value 0x00,
if the second octet of EM does not have hexadecimal value 0x02,
if there is no octet with hexadecimal value 0x00 to separate PS
from M, or if the length of PS is less than 8 octets, output
"decryption error" and stop. (See the note below.)

4. Output M.

Note. Care shall be taken to ensure that an opponent cannot
distinguish the different error conditions in Step 3, whether by
error message or timing. Otherwise an opponent may be able to
obtain useful information about the decryption of the ciphertext C,
leading to a strengthened version of Bleichenbacher's attack [6];
compare to Manger's attack [36].

8. Signature schemes with appendix

For the purposes of this document, a signature scheme with appendix
consists of a signature generation operation and a signature
verification operation, where the signature generation operation
produces a signature from a message with a signer's RSA private
key, and the signature verification operation verifies the
signature on the message with the signer's corresponding RSA public
key. To verify a signature constructed with this type of scheme it
is necessary to have the message itself. In this way, signature
schemes with appendix are distinguished from signature schemes with
message recovery, which are not supported in this document.

A signature scheme with appendix can be employed in a variety of
applications. For instance, the signature schemes with appendix
defined here would be suitable signature algorithms for X.509
certificates [28]. Related signature schemes could be employed in
PKCS #7 [45], although for technical reasons the current version of
PKCS #7 separates a hash function from a signature scheme, which is
different than what is done here; see the note in Appendix A.2.3
for more discussion.

Two signature schemes with appendix are specified in this document:
RSASSA-PSS and RSASSA-PKCS1-v1_5. Although no attacks are known
against RSASSA-PKCS1-v1_5, in the interest of increased robustness,
RSASSA-PSS is recommended for eventual adoption in new
applications. RSASSA-PKCS1-v1_5 is included for compatibility with
existing applications, and while still appropriate for new
applications, a gradual transition to RSASSA-PSS is encouraged.

The signature schemes with appendix given here follow a general
model similar to that employed in IEEE Std 1363-2000 [26],
combining signature and verification primitives with an encoding

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method for signatures. The signature generation operations apply a
message encoding operation to a message to produce an encoded
message, which is then converted to an integer message
representative. A signature primitive is applied to the message
representative to produce the signature. Reversing this, the
signature verification operations apply a signature verification
primitive to the signature to recover a message representative,
which is then converted to an octet string encoded message. A
verification operation is applied to the message and the encoded
message to determine whether they are consistent.

If the encoding method is deterministic (e.g., EMSA-PKCS1-v1_5),
the verification operation may apply the message encoding operation
to the message and compare the resulting encoded message to the
previously derived encoded message. If there is a match, the
signature is considered valid. If the method is randomized (e.g.,
EMSA-PSS), the verification operation is typically more
complicated. For example, the verification operation in EMSA-PSS
extracts the random salt and a hash output from the encoded message
and checks whether the hash output, the salt, and the message are
consistent; the hash output is a deterministic function in terms of
the message and the salt.

For both signature schemes with appendix defined in this document,
the signature generation and signature verification operations are
readily implemented as "single-pass" operations if the signature is
placed after the message. See PKCS #7 [45] for an example format in
the case of RSASSA-PKCS1-v1_5.

8.1 RSASSA-PSS

RSASSA-PSS combines the RSASP1 and RSAVP1 primitives with the
EMSA-PSS encoding method. It is compatible with the IFSSA scheme as
amended in the IEEE P1363a draft [27], where the signature and
verification primitives are IFSP-RSA1 and IFVP-RSA1 as defined in
IEEE Std 1363-2000 [26] and the message encoding method is EMSA4.
EMSA4 is slightly more general than EMSA-PSS as it acts on bit
strings rather than on octet strings. EMSA-PSS is equivalent to
EMSA4 restricted to the case that the operands as well as the hash
and salt values are octet strings.

The length of messages on which RSASSA-PSS can operate is either
unrestricted or constrained by a very large number, depending on
the hash function underlying the EMSA-PSS encoding method.

Assuming that computing e-th roots modulo n is infeasible and the
hash and mask generation functions in EMSA-PSS have appropriate
properties, RSASSA-PSS provides secure signatures. This assurance

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is provable in the sense that the difficulty of forging signatures
can be directly related to the difficulty of inverting the RSA
function, provided that the hash and mask generation functions are
viewed as black boxes or random oracles. The bounds in the security
proof are essentially "tight", meaning that the success probability
and running time for the best forger against RSASSA-PSS are very
close to the corresponding parameters for the best RSA inversion
algorithm; see [4][13][31] for further discussion.

In contrast to the RSASSA-PKCS1-v1_5 signature scheme, a hash
function identifier is not embedded in the EMSA-PSS encoded
message, so in theory it is possible for an adversary to substitute
a different (and potentially weaker) hash function than the one
selected by the signer. Therefore, it is recommended that the
EMSA-PSS mask generation function be based on the same hash
function. In this manner the entire encoded message will be
dependent on the hash function and it will be difficult for an
opponent to substitute a different hash function than the one
intended by the signer. This matching of hash functions is only for
the purpose of preventing hash function substitution, and is not
necessary if hash function substitution is addressed by other means
(e.g., the verifier accepts only a designated hash function). See
[34] for further discussion of these points. The provable security
of RSASSA-PSS does not rely on the hash function in the mask
generation function being the same as the hash function applied to
the message.

RSASSA-PSS is different from other RSA-based signature schemes in
that it is probabilistic rather than deterministic, incorporating a
randomly generated salt value. The salt value enhances the security
of the scheme by affording a "tighter" security proof than
deterministic alternatives such as Full Domain Hashing (FDH); see
[4] for discussion. However, the randomness is not critical to
security. In situations where random generation is not possible, a
fixed value or a sequence number could be employed instead, with
the resulting provable security similar to that of FDH [12].

8.1.1 Signature generation operation

RSASSA-PSS-SIGN (K, M)

Input:
K        signer's RSA private key
M        message to be signed, an octet string

Output:
S        signature, an octet string of length k, where k is the
length in octets of the RSA modulus n

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Errors: "message too long;" "encoding error"

Steps:
1. EMSA-PSS encoding: Apply the EMSA-PSS encoding operation
(Section 9.1.1) to the message M to produce an encoded message
EM of length \ceil ((modBits - 1)/8) octets such that the bit
length of the integer OS2IP (EM) (see Section 4.2) is at most
modBits - 1, where modBits is the length in bits of the RSA
modulus n:

EM = EMSA-PSS-ENCODE (M, modBits - 1).

Note that the octet length of EM will be one less than k if
modBits - 1 is divisible by 8 and equal to k otherwise. If the
encoding operation outputs "message too long," output "message
too long" and stop. If the encoding operation outputs "encoding
error," output "encoding error" and stop.

2. RSA signature:

a. Convert the encoded message EM to an integer message
representative m (see Section 4.2):

m = OS2IP (EM).

b. Apply the RSASP1 signature primitive (Section 5.2.1) to the
RSA private key K and the message representative m to produce
an integer signature representative s:

s = RSASP1 (K, m).

c. Convert the signature representative s to a signature S of
length k octets (see Section 4.1):

S = I2OSP (s, k).

3. Output the signature S.

8.1.2 Signature verification operation

RSASSA-PSS-VERIFY ((n, e), M, S)

Input:
(n, e)   signer's RSA public key
M        message whose signature is to be verified, an octet string
S        signature to be verified, an octet string of length k,
where k is the length in octets of the RSA modulus n

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Output:
"valid signature" or "invalid signature"

Steps:

1. Length checking: If the length of the signature S is not k
octets, output "invalid signature" and stop.

2. RSA verification:

a. Convert the signature S to an integer signature
representative s (see Section 4.2):

s = OS2IP (S).

b. Apply the RSAVP1 verification primitive (Section 5.2.2) to
the RSA public key (n, e) and the signature representative s
to produce an integer message representative m:

m = RSAVP1 ((n, e), s).

If RSAVP1 output "signature representative out of range,"
output "invalid signature" and stop.

c. Convert the message representative m to an encoded message EM
of length emLen = \ceil ((modBits - 1)/8) octets, where
modBits is the length in bits of the RSA modulus n (see
Section 4.1):

EM = I2OSP (m, emLen).

Note that emLen will be one less than k if modBits - 1 is
divisible by 8 and equal to k otherwise. If I2OSP outputs
"integer too large," output "invalid signature" and stop.

3. EMSA-PSS verification: Apply the EMSA-PSS verification operation
(Section 9.1.2) to the message M and the encoded message EM to
determine whether they are consistent:

Result = EMSA-PSS-VERIFY (M, EM, modBits - 1).

4. If Result = "consistent," output "valid signature." Otherwise,
output "invalid signature."

8.2 RSASSA-PKCS1-v1_5

RSASSA-PKCS1-v1_5 combines the RSASP1 and RSAVP1 primitives with
the EMSA-PKCS1-v1_5 encoding method. It is compatible with the

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IFSSA scheme defined in IEEE Std 1363-2000 [26], where the
signature and verification primitives are IFSP-RSA1 and IFVP-RSA1
and the message encoding method is EMSA-PKCS1-v1_5 (which is not
defined in IEEE Std 1363-2000, but is in the IEEE P1363a draft
[27]).

The length of messages on which RSASSA-PKCS1-v1_5 can operate is
either unrestricted or constrained by a very large number,
depending on the hash function underlying the EMSA-PKCS1-v1_5
method.

Assuming that computing e-th roots modulo n is infeasible and the
hash function in EMSA-PKCS1-v1_5 has appropriate properties,
RSASSA-PKCS1-v1_5 is conjectured to provide secure signatures. More
precisely, forging signatures without knowing the RSA private key
is conjectured to be computationally infeasible. Also, in the
encoding method EMSA-PKCS1-v1_5, a hash function identifier is
embedded in the encoding. Because of this feature, an adversary
trying to find a message with the same signature as a previously
signed message must find collisions of the particular hash function
being used; attacking a different hash function than the one
selected by the signer is not useful to the adversary. See [34] for
further discussion.

Note. As noted in PKCS #1 v1.5, the EMSA-PKCS1-v1_5 encoding method
has the property that the encoded message, converted to an integer
message representative, is guaranteed to be large and at least
somewhat "random". This prevents attacks of the kind proposed by
Desmedt and Odlyzko [16] where multiplicative relationships between
message representatives are developed by factoring the message
representatives into a set of small values (e.g., a set of small
primes). Coron, Naccache, and Stern [15] showed that a stronger
form of this type of attack could be quite effective against some
instances of the ISO/IEC 9796-2 signature scheme. They also
analyzed the complexity of this type of attack against the
EMSA-PKCS1-v1_5 encoding method and concluded that an attack would
be impractical, requiring more operations than a collision search
on the underlying hash function (i.e., more than 2^80 operations).
Coppersmith, Halevi, and Jutla [11] subsequently extended Coron et
al.'s attack to break the ISO/IEC 9796-1 signature scheme with
message recovery. The various attacks illustrate the importance of
carefully constructing the input to the RSA signature primitive,
particularly in a signature scheme with message recovery.
Accordingly, the EMSA-PKCS-v1_5 encoding method explicitly
includes a hash operation and is not intended for signature schemes
with message recovery. Moreover, while no attack is known against
the EMSA-PKCS-v1_5 encoding method, a gradual transition to

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EMSA-PSS is recommended as a precaution against future
developments.

8.2.1 Signature generation operation

RSASSA-PKCS1-V1_5-SIGN (K, M)

Input:
K        signer's RSA private key
M        message to be signed, an octet string

Output:
S        signature, an octet string of length k, where k is the
length in octets of the RSA modulus n

Errors: "message too long"; "RSA modulus too short"

Steps:

1. EMSA-PKCS1-v1_5 encoding: Apply the EMSA-PKCS1-v1_5 encoding
operation (Section 9.2) to the message M to produce an encoded
message EM of length k octets:

EM = EMSA-PKCS1-V1_5-ENCODE (M, k).

If the encoding operation outputs "message too long," output
"message too long" and stop. If the encoding operation outputs
"intended encoded message length too short," output "RSA modulus
too short" and stop.

2. RSA signature:

a. Convert the encoded message EM to an integer message
representative m (see Section 4.2):

m = OS2IP (EM).

b. Apply the RSASP1 signature primitive (Section 5.2.1) to the
RSA private key K and the message representative m to produce
an integer signature representative s:

s = RSASP1 (K, m).

c. Convert the signature representative s to a signature S of
length k octets (see Section 4.1):

S = I2OSP (s, k).

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3. Output the signature S.

8.2.2 Signature verification operation

RSASSA-PKCS1-V1_5-VERIFY ((n, e), M, S)

Input:
(n, e)   signer's RSA public key
M        message whose signature is to be verified, an octet string
S        signature to be verified, an octet string of length k,
where k is the length in octets of the RSA modulus n

Output:
"valid signature" or "invalid signature"

Errors: "message too long"; "RSA modulus too short"

Steps:

1. Length checking: If the length of the signature S is not k
octets, output "invalid signature" and stop.

2. RSA verification:

a. Convert the signature S to an integer signature
representative s (see Section 4.2):

s = OS2IP (S).

b. Apply the RSAVP1 verification primitive (Section 5.2.2) to
the RSA public key (n, e) and the signature representative s
to produce an integer message representative m:

m = RSAVP1 ((n, e), s).

If RSAVP1 outputs "signature representative out of range,"
output "invalid signature" and stop.

c. Convert the message representative m to an encoded message EM
of length k octets (see Section 4.1):

EM' = I2OSP (m, k).

If I2OSP outputs "integer too large," output "invalid
signature" and stop.

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3. EMSA-PKCS1-v1_5 encoding: Apply the EMSA-PKCS1-v1_5 encoding
operation (Section 9.2) to the message M to produce a second
encoded message EM' of length k octets:

EM' = EMSA-PKCS1-V1_5-ENCODE (M, k).

If the encoding operation outputs "message too long," output
"message too long" and stop. If the encoding operation outputs
"intended encoded message length too short," output "RSA modulus
too short" and stop.

4. Compare the encoded message EM and the second encoded message
EM'. If they are the same, output "valid signature"; otherwise,
output "invalid signature."

Note. Another way to implement the signature verification operation
is to apply a "decoding" operation (not specified in this document)
to the encoded message to recover the underlying hash value, and
then to compare it to a newly computed hash value. This has the
advantage that it requires less intermediate storage (two hash
values rather than two encoded messages), but the disadvantage that

9. Encoding methods for signatures with appendix

Encoding methods consist of operations that map between octet
string messages and octet string encoded messages, which are
converted to and from integer message representatives in the
schemes. The integer message representatives are processed via the
primitives. The encoding methods thus provide the connection
between the schemes, which process messages, and the primitives.

An encoding method for signatures with appendix, for the purposes
of this document, consists of an encoding operation and optionally
a verification operation. An encoding operation maps a message M to
an encoded message EM of a specified length. A verification
operation determines whether a message M and an encoded message EM
are consistent, i.e., whether the encoded message EM is a valid
encoding of the message M.

The encoding operation may introduce some randomness, so that
different applications of the encoding operation to the same
message will produce different encoded messages, which has benefits
for provable security. For such an encoding method, both an
encoding and a verification operation are needed unless the
verifier can reproduce the randomness (e.g., by obtaining the salt
value from the signer). For a deterministic encoding method only an
encoding operation is needed.

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Two encoding methods for signatures with appendix are employed in
the signature schemes and are specified here: EMSA-PSS and
EMSA-PKCS1-v1_5.

9.1 EMSA-PSS

This encoding method is parameterized by the choice of hash
function, mask generation function, and salt length. These options
should be fixed for a given RSA key, except that the salt length
can be variable (see [31] for discussion). Suggested hash and mask
generation functions are given in Appendix B. The encoding method
is based on Bellare and Rogaway's Probabilistic Signature Scheme
(PSS) [4][5]. It is randomized and has an encoding operation and a
verification operation.

Figure 2 illustrates the encoding operation.

__________________________________________________________________

+-----------+
|     M     |
+-----------+
|
V
Hash
|
V
+--------+----------+----------+
M' = |Padding1|  mHash   |   salt   |
+--------+----------+----------+
|
+--------+----------+     V
+--------+----------+     |
|               |
V               |    +--+
xor <--- MGF <---|    |bc|
|               |    +--+
|               |      |
V               V      V
+-------------------+----------+--+
+-------------------+----------+--+
__________________________________________________________________

Figure 2: EMSA-PSS encoding operation. Verification operation
follows reverse steps to recover salt, then forward steps to
recompute and compare H.

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Notes.

1. The encoding method defined here differs from the one in Bellare
and Rogaway's submission to IEEE P1363a [5] in three respects:

* It applies a hash function rather than a mask generation
function to the message. Even though the mask generation
function is based on a hash function, it seems more natural
to apply a hash function directly.

* The value that is hashed together with the salt value is the
string (0x)00 00 00 00 00 00 00 00 || mHash rather than the
message M itself. Here, mHash is the hash of M. Note that the
hash function is the same in both steps. See Note 3 below for
further discussion. (Also, the name "salt" is used instead of
"seed", as it is more reflective of the value's role.)

* The encoded message in EMSA-PSS has nine fixed bits; the
first bit is 0 and the last eight bits form a "trailer
field", the octet 0xbc. In the original scheme, only the
first bit is fixed. The rationale for the trailer field is
for compatibility with the Rabin-Williams IFSP-RW signature
primitive in IEEE Std 1363-2000 [26] and the corresponding
primitive in the draft ISO/IEC 9796-2 [29].

2. Assuming that the mask generation function is based on a hash
function, it is recommended that the hash function be the same
as the one that is applied to the message; see Section 8.1 for
further discussion.

3. Without compromising the security proof for RSASSA-PSS, one may
perform steps 1 and 2 of EMSA-PSS-ENCODE and EMSA-PSS-VERIFY
(the application of the hash function to the message) outside
the module that computes the rest of the signature operation, so
that mHash rather than the message M itself is input to the
module. In other words, the security proof for RSASSA-PSS still
holds even if an opponent can control the value of mHash. This
is convenient if the module has limited I/O bandwidth, e.g., a
smart card. Note that previous versions of PSS [4][5] did not
have this property. Of course, it may be desirable for other
security reasons to have the module process the full
message. For instance, the module may need to "see" what it is
signing if it does not trust the component that computes the
hash value.

4. Typical salt lengths in octets are hLen (the length of the
output of the hash function Hash) and 0. In both cases the
security of RSASSA-PSS can be closely related to the hardness of

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inverting RSAVP1. Bellare and Rogaway [4] give a tight lower
bound for the security of the original RSA-PSS scheme, which
corresponds roughly to the former case, while Coron [12] gives a
lower bound for the related Full Domain Hashing scheme, which
corresponds roughly to the latter case. In [13] Coron provides a
general treatment with various salt lengths ranging from 0 to
security proofs in [4][13] to address the differences between
the original and the present version of RSA-PSS as listed in
Note 1 above.

5. As noted in IEEE P1363a [27], the use of randomization in
signature schemes - such as the salt value in EMSA-PSS - may
provide a "covert channel" for transmitting information other
than the message being signed. For more on covert channels, see
[50].

9.1.1 Encoding operation

EMSA-PSS-ENCODE (M, emBits)

Options:

Hash     hash function (hLen denotes the length in octets of the
hash function output)
sLen     intended length in octets of the salt

Input:
M        message to be encoded, an octet string
emBits   maximal bit length of the integer OS2IP (EM) (see Section
4.2), at least 8hLen + 8sLen + 9

Output:
EM       encoded message, an octet string of length emLen =
\ceil (emBits/8)

Errors:  "encoding error"; "message too long"

Steps:

1. If the length of M is greater than the input limitation for the
hash function (2^61 - 1 octets for SHA-1), output "message too
long" and stop.

2. Let mHash = Hash(M), an octet string of length hLen.

3. If emLen < hLen + sLen + 2, output "encoding error" and stop.

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4. Generate a random octet string salt of length sLen; if sLen = 0,
then salt is the empty string.

5. Let

M' = (0x)00 00 00 00 00 00 00 00 || mHash || salt;

M' is an octet string of length 8 + hLen + sLen with eight
initial zero octets.

6. Let H = Hash(M'), an octet string of length hLen.

7. Generate an octet string PS consisting of emLen - sLen - hLen -
2 zero octets. The length of PS may be 0.

8. Let DB = PS || 0x01 || salt; DB is an octet string of length
emLen - hLen - 1.

9. Let dbMask = MGF(H, emLen - hLen - 1).

11. Set the leftmost 8emLen - emBits bits of the leftmost octet in

12. Let EM = maskedDB || H || 0xbc.

13. Output EM.

9.1.2 Verification operation

EMSA-PSS-VERIFY (M, EM, emBits)

Options:
Hash     hash function (hLen denotes the length in octets of the
hash function output)
sLen     intended length in octets of the salt

Input:
M        message to be verified, an octet string
EM       encoded message, an octet string of length emLen =
\ceil (emBits/8)
emBits   maximal bit length of the integer OS2IP (EM) (see Section
4.2), at least 8hLen + 8sLen + 9

Output:
"consistent" or "inconsistent"

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Steps:

1. If the length of M is greater than the input limitation for the
hash function (2^61 - 1 octets for SHA-1), output "inconsistent"
and stop.

2. Let mHash = Hash(M), an octet string of length hLen.

3. If emLen < hLen + sLen + 2, output "inconsistent" and stop.

4. If the rightmost octet of EM does not have hexadecimal value
0xbc, output "inconsistent" and stop.

5. Let maskedDB be the leftmost emLen - hLen - 1 octets of EM, and
let H be the next hLen octets.

6. If the leftmost 8emLen - emBits bits of the leftmost octet in
maskedDB are not all equal to zero, output "inconsistent" and
stop.

7. Let dbMask = MGF(H, emLen - hLen - 1).

9. Set the leftmost 8emLen - emBits bits of the leftmost octet in
DB to zero.

10. If the emLen - hLen - sLen - 2 leftmost octets of DB are not
zero or if the octet at position emLen - hLen - sLen - 1 (the
leftmost position is "position 1") does not have hexadecimal
value 0x01, output "inconsistent" and stop.

11. Let salt be the last sLen octets of DB.

12. Let

M' = (0x)00 00 00 00 00 00 00 00 || mHash || salt ;

M' is an octet string of length 8 + hLen + sLen with eight
initial zero octets.

13. Let H' = Hash(M'), an octet string of length hLen.

14. If H = H', output "consistent." Otherwise, output
"inconsistent."

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9.2 EMSA-PKCS1-v1_5

This encoding method is deterministic and only has an encoding
operation.

EMSA-PKCS1-v1_5-ENCODE (M, emLen)

Option:
Hash     hash function (hLen denotes the length in octets of the
hash function output)

Input:
M        message to be encoded
emLen    intended length in octets of the encoded message, at least
tLen + 11, where tLen is the octet length of the DER
encoding T of a certain value computed during the encoding
operation

Output:
EM       encoded message, an octet string of length emLen

Errors:
"message too long"; "intended encoded message length too short"

Steps:

1. Apply the hash function to the message M to produce a hash value
H:

H = Hash(M).

If the hash function outputs "message too long," output "message
too long" and stop.

2. Encode the algorithm ID for the hash function and the hash value
into an ASN.1 value of type DigestInfo (see Appendix A.2.4) with
the Distinguished Encoding Rules (DER), where the type
DigestInfo has the syntax

DigestInfo ::= SEQUENCE {
digestAlgorithm AlgorithmIdentifier,
digest OCTET STRING
}

The first field identifies the hash function and the second
contains the hash value. Let T be the DER encoding of the
DigestInfo value (see the notes below) and let tLen be the
length in octets of T.

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3. If emLen < tLen + 11, output "intended encoded message length
too short" and stop.

4. Generate an octet string PS consisting of emLen - tLen - 3
octets with hexadecimal value 0xff. The length of PS will be at
least 8 octets.

5. Concatenate PS, the DER encoding T, and other padding to form
the encoded message EM as

EM = 0x00 || 0x01 || PS || 0x00 || T.

6. Output EM.

Notes.

1. For the six hash functions mentioned in Appendix B.1, the DER
encoding T of the DigestInfo value is equal to the following:

MD2:     (0x)30 20 30 0c 06 08 2a 86 48 86 f7 0d 02 02 05 00 04
10 || H.
MD5:     (0x)30 20 30 0c 06 08 2a 86 48 86 f7 0d 02 05 05 00 04
10 || H.
SHA-1:   (0x)30 21 30 09 06 05 2b 0e 03 02 1a 05 00 04 14 || H.
SHA-256: (0x)30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00
04 20 || H.
SHA-384: (0x)30 41 30 0d 06 09 60 86 48 01 65 03 04 02 02 05 00
04 30 || H.
SHA-512: (0x)30 51 30 0d 06 09 60 86 48 01 65 03 04 02 03 05 00
04 40 || H.

2. In version 1.5 of this document, T was defined as the BER
encoding, rather than the DER encoding, of the DigestInfo value.
In particular, it is possible - at least in theory - that the
verification operation defined in this document (as well as in
version 2.0) rejects a signature that is valid with respect to
the specification given in PKCS #1 v1.5. This occurs if other
rules than DER are applied to DigestInfo (e.g., an indefinite
length encoding of the underlying SEQUENCE type). While this is
unlikely to be a concern in practice, a cautious implementer may
choose to employ a verification operation based on a BER
decoding operation as specified in PKCS #1 v1.5. In this manner,
compatibility with any valid implementation based on PKCS #1
v1.5 is obtained. Such a verification operation should indicate
whether the underlying BER encoding is a DER encoding and hence
whether the signature is valid with respect to the specification
given in this document.

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A. ASN.1 syntax

A.1 RSA key representation

This section defines ASN.1 object identifiers for RSA public and
private keys, and defines the types RSAPublicKey and
RSAPrivateKey. The intended application of these definitions
includes X.509 certificates, PKCS #8 [46], and PKCS #12 [47].

The object identifier rsaEncryption identifies RSA public and
private keys as defined in Appendices A.1.1 and A.1.2. The
parameters field associated with this OID in a value of type
AlgorithmIdentifier shall have a value of type NULL.

rsaEncryption    OBJECT IDENTIFIER ::= { pkcs-1 1 }

The definitions in this section have been extended to support
multi-prime RSA, but are backward compatible with previous
versions.

A.1.1 RSA public key syntax

An RSA public key should be represented with the ASN.1 type
RSAPublicKey:

RSAPublicKey ::= SEQUENCE {
modulus           INTEGER,  -- n
publicExponent    INTEGER   -- e
}

The fields of type RSAPublicKey have the following meanings:

* modulus is the RSA modulus n.

* publicExponent is the RSA public exponent e.

A.1.2 RSA private key syntax

An RSA private key should be represented with the ASN.1 type
RSAPrivateKey:

RSAPrivateKey ::= SEQUENCE {
version           Version,
modulus           INTEGER,  -- n
publicExponent    INTEGER,  -- e
privateExponent   INTEGER,  -- d
prime1            INTEGER,  -- p
prime2            INTEGER,  -- q

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exponent1         INTEGER,  -- d mod (p-1)
exponent2         INTEGER,  -- d mod (q-1)
coefficient       INTEGER,  -- (inverse of q) mod p
otherPrimeInfos   OtherPrimeInfos OPTIONAL
}

The fields of type RSAPrivateKey have the following meanings:

* version is the version number, for compatibility with future
revisions of this document. It shall be 0 for this version of
the document, unless multi-prime is used, in which case it shall
be 1.

Version ::= INTEGER { two-prime(0), multi(1) }
(CONSTRAINED BY
{-- version must be multi if otherPrimeInfos present --})

* modulus is the RSA modulus n.

* publicExponent is the RSA public exponent e.

* privateExponent is the RSA private exponent d.

* prime1 is the prime factor p of n.

* prime2 is the prime factor q of n.

* exponent1 is d mod (p - 1).

* exponent2 is d mod (q - 1).

* coefficient is the CRT coefficient q^(-1) mod p.

* otherPrimeInfos contains the information for the additional
primes r_3, ..., r_u, in order. It shall be omitted if version
is 0 and shall contain at least one instance of OtherPrimeInfo
if version is 1.

OtherPrimeInfos ::= SEQUENCE SIZE(1..MAX) OF OtherPrimeInfo

OtherPrimeInfo ::= SEQUENCE {
prime             INTEGER,  -- ri
exponent          INTEGER,  -- di
coefficient       INTEGER   -- ti
}

The fields of type OtherPrimeInfo have the following meanings:

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* prime is a prime factor r_i of n, where i þ 3.

* exponent is d_i = d mod (r_i - 1).

* coefficient is the CRT coefficient t_i = (r_1 * r_2 * ... *
r_(i-1))^(-1) mod r_i.

Note. It is important to protect the RSA private key against both
disclosure and modification. Techniques for such protection are
outside the scope of this document. Methods for storing and
distributing private keys and other cryptographic data are
described in PKCS #12 and #15.

A.2 Scheme identification

This section defines object identifiers for the encryption and
signature schemes. The schemes compatible with PKCS #1 v1.5 have
the same definitions as in PKCS #1 v1.5. The intended application
of these definitions includes X.509 certificates and PKCS #7.

Here are type identifier definitions for the PKCS #1 OIDs:

PKCS1Algorithms    ALGORITHM-IDENTIFIER ::= {
{ OID rsaEncryption              PARAMETERS NULL } |
{ OID md2WithRSAEncryption       PARAMETERS NULL } |
{ OID md5WithRSAEncryption       PARAMETERS NULL } |
{ OID sha1WithRSAEncryption      PARAMETERS NULL } |
{ OID sha256WithRSAEncryption    PARAMETERS NULL } |
{ OID sha384WithRSAEncryption    PARAMETERS NULL } |
{ OID sha512WithRSAEncryption    PARAMETERS NULL } |
{ OID id-RSAES-OAEP PARAMETERS RSAES-OAEP-params } |
PKCS1PSourceAlgorithms                             |
{ OID id-RSASSA-PSS PARAMETERS RSASSA-PSS-params } ,
...  -- Allows for future expansion --
}

A.2.1 RSAES-OAEP

The object identifier id-RSAES-OAEP identifies the RSAES-OAEP
encryption scheme.

id-RSAES-OAEP    OBJECT IDENTIFIER ::= { pkcs-1 7 }

The parameters field associated with this OID in a value of type
AlgorithmIdentifier shall have a value of type RSAES-OAEP-params:

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RSAES-OAEP-params ::= SEQUENCE {
hashAlgorithm     [0] HashAlgorithm    DEFAULT sha1,
pSourceAlgorithm  [2] PSourceAlgorithm DEFAULT pSpecifiedEmpty
}

The fields of type RSAES-OAEP-params have the following meanings:

* hashAlgorithm identifies the hash function. It shall be an
algorithm ID with an OID in the set OAEP-PSSDigestAlgorithms.
For a discussion of supported hash functions, see Appendix B.1.

HashAlgorithm ::= AlgorithmIdentifier {
{OAEP-PSSDigestAlgorithms}
}

OAEP-PSSDigestAlgorithms    ALGORITHM-IDENTIFIER ::= {
{ OID id-sha1 PARAMETERS NULL   }|
{ OID id-sha256 PARAMETERS NULL }|
{ OID id-sha384 PARAMETERS NULL }|
{ OID id-sha512 PARAMETERS NULL },
...  -- Allows for future expansion --
}

The default hash function is SHA-1:

sha1    HashAlgorithm ::= {
algorithm   id-sha1,
parameters  SHA1Parameters : NULL
}

SHA1Parameters ::= NULL

shall be an algorithm ID with an OID in the set
PKCS1MGFAlgorithms, which for this version shall consist of
id-mgf1, identifying the MGF1 mask generation function (see
Appendix B.2.1). The parameters field associated with id-mgf1
shall be an algorithm ID with an OID in the set
OAEP-PSSDigestAlgorithms, identifying the hash function on which
MGF1 is based.

{PKCS1MGFAlgorithms}
}

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PKCS1MGFAlgorithms    ALGORITHM-IDENTIFIER ::= {
{ OID id-mgf1 PARAMETERS HashAlgorithm },
...  -- Allows for future expansion --
}

The default mask generation function is MGF1 with SHA-1:

algorithm   id-mgf1,
parameters  HashAlgorithm : sha1
}

* pSourceAlgorithm identifies the source (and possibly the value)
of the label L. It shall be an algorithm ID with an OID in the
set PKCS1PSourceAlgorithms, which for this version shall consist
of id-pSpecified, indicating that the label is specified
explicitly. The parameters field associated with id-pSpecified
shall have a value of type OCTET STRING, containing the
label. In previous versions of this specification, the term
"encoding parameters" was used rather than "label", hence the
name of the type below.

PSourceAlgorithm ::= AlgorithmIdentifier {
{PKCS1PSourceAlgorithms}
}

PKCS1PSourceAlgorithms    ALGORITHM-IDENTIFIER ::= {
{ OID id-pSpecified PARAMETERS EncodingParameters },
...  -- Allows for future expansion --
}

id-pSpecified    OBJECT IDENTIFIER ::= { pkcs-1 9 }

EncodingParameters ::= OCTET STRING(SIZE(0..MAX))

The default label is an empty string (so that lHash will contain
the hash of the empty string):

pSpecifiedEmpty    PSourceAlgorithm ::= {
algorithm   id-pSpecified,
parameters  EncodingParameters : emptyString
}

emptyString    EncodingParameters ::= ''H

If all of the default values of the fields in RSAES-OAEP-params
are used, then the algorithm identifier will have the following
value:

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rSAES-OAEP-Default-Identifier  RSAES-AlgorithmIdentifier ::= {
algorithm   id-RSAES-OAEP,
parameters  RSAES-OAEP-params : {
hashAlgorithm       sha1,
pSourceAlgorithm    pSpecifiedEmpty
}
}

RSAES-AlgorithmIdentifier ::= AlgorithmIdentifier {
{PKCS1Algorithms}
}

A.2.2 RSAES-PKCS1-v1_5

The object identifier rsaEncryption (see Appendix A.1) identifies
the RSAES-PKCS1-v1_5 encryption scheme. The parameters field
associated with this OID in a value of type AlgorithmIdentifier
shall have a value of type NULL. This is the same as in PKCS #1
v1.5.

rsaEncryption    OBJECT IDENTIFIER ::= { pkcs-1 1 }

A.2.3 RSASSA-PSS

The object identifier id-RSASSA-PSS identifies the RSASSA-PSS
encryption scheme.

id-RSASSA-PSS    OBJECT IDENTIFIER ::= { pkcs-1 10 }

The parameters field associated with this OID in a value of type
AlgorithmIdentifier shall have a value of type RSASSA-PSS-params:

RSASSA-PSS-params ::= SEQUENCE {
hashAlgorithm      [0] HashAlgorithm    DEFAULT sha1,
saltLength         [2] INTEGER          DEFAULT 20,
trailerField       [3] TrailerField     DEFAULT trailerFieldBC
}

The fields of type RSASSA-PSS-params have the following meanings:

* hashAlgorithm identifies the hash function. It shall be an
algorithm ID with an OID in the set OAEP-PSSDigestAlgorithms
(see Appendix A.2.1). The default hash function is SHA-1.

shall be an algorithm ID with an OID in the set

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PKCS1MGFAlgorithms (see Appendix A.2.1). The default mask
generation function is MGF1 with SHA-1. For MGF1 (and more
generally, for other mask generation functions based on a hash
function), it is recommended that the underlying hash function
be the same as the one identified by hashAlgorithm; see Note 2
in Section 9.1 for further comments.

* saltLength is the octet length of the salt. It shall be an
integer. For a given hashAlgorithm, the default value of
saltLength is the octet length of the hash value. Unlike the
other fields of type RSASSA-PSS-params, saltLength does not need
to be fixed for a given RSA key pair.

* trailerField is the trailer field number, for compatibility with
the draft IEEE P1363a [27]. It shall be 1 for this version of
the document, which represents the trailer field with
hexadecimal value 0xbc. Other trailer fields (including the
trailer field HashID || 0xcc in IEEE P1363a) are not supported
in this document.

TrailerField ::= INTEGER { trailerFieldBC(1) }

If the default values of the hashAlgorithm, maskGenAlgorithm,
and trailerField fields of RSASSA-PSS-params are used, then the
algorithm identifier will have the following value:

rSASSA-PSS-Default-Identifier  RSASSA-AlgorithmIdentifier ::= {
algorithm   id-RSASSA-PSS,
parameters  RSASSA-PSS-params : {
hashAlgorithm       sha1,
saltLength          20,
trailerField        trailerFieldBC
}
}

RSASSA-AlgorithmIdentifier ::=
AlgorithmIdentifier { {PKCS1Algorithms} }

Note. In some applications, the hash function underlying a
signature scheme is identified separately from the rest of the
operations in the signature scheme. For instance, in PKCS #7 [45],
a hash function identifier is placed before the message and a
"digest encryption" algorithm identifier (indicating the rest of
the operations) is carried with the signature. In order for PKCS #7
to support the RSASSA-PSS signature scheme, an object identifier
would need to be defined for the operations in RSASSA-PSS after the
hash function (analogous to the RSAEncryption OID for the

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RSASSA-PKCS1-v1_5 scheme). S/MIME CMS [25] takes a different
approach. Although a hash function identifier is placed before the
message, an algorithm identifier for the full signature scheme may
be carried with a CMS signature (this is done for DSA signatures).
Following this convention, the id-RSASSA-PSS OID can be used to
identify RSASSA-PSS signatures in CMS. Since CMS is considered the
successor to PKCS #7 and new developments such as the addition of
support for RSASSA-PSS will be pursued with respect to CMS rather
than PKCS #7, an OID for the "rest of" RSASSA-PSS is not defined in
this version of PKCS #1.

A.2.4 RSASSA-PKCS1-v1_5

The object identifier for RSASSA-PKCS1-v1_5 shall be one of the
following. The choice of OID depends on the choice of hash
algorithm: MD2, MD5, SHA-1, SHA-256, SHA-384, or SHA-512. Note that
if either MD2 or MD5 is used, then the OID is just as in PKCS #1
v1.5. For each OID, the parameters field associated with this OID
in a value of type AlgorithmIdentifier shall have a value of type
NULL. The OID should be chosen in accordance with the following
table:

Hash algorithm   OID
--------------------------------------------------------
MD2              md2WithRSAEncryption    ::= {pkcs-1 2}
MD5              md5WithRSAEncryption    ::= {pkcs-1 4}
SHA-1            sha1WithRSAEncryption   ::= {pkcs-1 5}
SHA-256          sha256WithRSAEncryption ::= {pkcs-1 11}
SHA-384          sha384WithRSAEncryption ::= {pkcs-1 12}
SHA-512          sha512WithRSAEncryption ::= {pkcs-1 13}

The EMSA-PKCS1-v1_5 encoding method includes an ASN.1 value of type
DigestInfo, where the type DigestInfo has the syntax

DigestInfo ::= SEQUENCE {
digestAlgorithm DigestAlgorithm,
digest OCTET STRING
}

digestAlgorithm identifies the hash function and shall be an
algorithm ID with an OID in the set PKCS1-v1-5DigestAlgorithms. For
a discussion of supported hash functions, see Appendix B.1.

DigestAlgorithm ::=
AlgorithmIdentifier { {PKCS1-v1-5DigestAlgorithms} }

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PKCS1-v1-5DigestAlgorithms    ALGORITHM-IDENTIFIER ::= {
{ OID id-md2 PARAMETERS NULL    }|
{ OID id-md5 PARAMETERS NULL    }|
{ OID id-sha1 PARAMETERS NULL   }|
{ OID id-sha256 PARAMETERS NULL }|
{ OID id-sha384 PARAMETERS NULL }|
{ OID id-sha512 PARAMETERS NULL }
}

B. Supporting techniques

This section gives several examples of underlying functions
supporting the encryption schemes in Section 7 and the encoding
methods in Section 9. A range of techniques is given here to allow
compatibility with existing applications as well as migration to
new techniques. While these supporting techniques are appropriate
for applications to implement, none of them is required to be
implemented. It is expected that profiles for PKCS #1 v2.1 will be
developed that specify particular supporting techniques.

This section also gives object identifiers for the supporting
techniques.

B.1 Hash functions

Hash functions are used in the operations contained in Sections 7
and 9. Hash functions are deterministic, meaning that the output is
completely determined by the input. Hash functions take octet
strings of variable length, and generate fixed length octet
strings. The hash functions used in the operations contained in
Sections 7 and 9 should generally be collision-resistant. This
means that it is infeasible to find two distinct inputs to the hash
function that produce the same output. A collision-resistant hash
function also has the desirable property of being one-way; this
means that given an output, it is infeasible to find an input whose
hash is the specified output. In addition to the requirements, the
hash function should yield a mask generation function (Appendix
B.2) with pseudorandom output.

Six hash functions are given as examples for the encoding methods
in this document: MD2 [33], MD5 [41], SHA-1 [38], and the proposed
algorithms SHA-256, SHA-384, and SHA-512 [39]. For the RSAES-OAEP
encryption scheme and EMSA-PSS encoding method, only SHA-1 and
SHA-256/384/512 are recommended. For the EMSA-PKCS1-v1_5 encoding
method, SHA-1 or SHA-256/384/512 are recommended for new
applications. MD2 and MD5 are recommended only for compatibility
with existing applications based on PKCS #1 v1.5.

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The object identifiers id-md2, id-md5, id-sha1, id-sha256,
id-sha384, and id-sha512, identify the respective hash functions:

id-md2      OBJECT IDENTIFIER ::= {
digestAlgorithm(2) 2
}

id-md5      OBJECT IDENTIFIER ::= {
digestAlgorithm(2) 5
}

id-sha1    OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithms(2) 26
}

id-sha256    OBJECT IDENTIFIER ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1)
gov(101) csor(3) nistalgorithm(4) hashalgs(2) 1
}

id-sha384    OBJECT IDENTIFIER ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1)
gov(101) csor(3) nistalgorithm(4) hashalgs(2) 2
}

id-sha512    OBJECT IDENTIFIER ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1)
gov(101) csor(3) nistalgorithm(4) hashalgs(2) 3
}

The parameters field associated with these OIDs in a value of type
AlgorithmIdentifier shall have a value of type NULL.

Note. Version 1.5 of PKCS #1 also allowed for the use of MD4 in
signature schemes. The cryptanalysis of MD4 has progressed
significantly in the intervening years. For example, Dobbertin [18]
demonstrated how to find collisions for MD4 and that the first two
rounds of MD4 are not one-way [20]. Because of these results and
others (e.g. [8]), MD4 is no longer recommended. There have also
been advances in the cryptanalysis of MD2 and MD5, although not
enough to warrant removal from existing applications. Rogier and
Chauvaud [43] demonstrated how to find collisions in a modified
version of MD2. No one has demonstrated how to find collisions for
the full MD5 algorithm, although partial results have been found
(e.g. [9][19]).

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To address these concerns, SHA-1, SHA-256, SHA-384, or SHA-512 are
recommended for new applications. As of today, the best (known)
collision attacks against these hash functions are generic attacks
with complexity 2^(L/2), where L is the bit length of the hash
output. For the signature schemes in this document, a collision
attack is easily translated into a signature forgery. Therefore,
the value L / 2 should be at least equal to the desired security
level in bits of the signature scheme (a security level of B bits
means that the best attack has complexity 2^B). The same rule of
thumb can be applied to RSAES-OAEP; it is recommended that the bit
length of the seed (which is equal to the bit length of the hash
output) be twice the desired security level in bits.

A mask generation function takes an octet string of variable length
and a desired output length as input, and outputs an octet string
of the desired length. There may be restrictions on the length of
the input and output octet strings, but such bounds are generally
very large. Mask generation functions are deterministic; the octet
string output is completely determined by the input octet string.
The output of a mask generation function should be pseudorandom:
Given one part of the output but not the input, it should be
infeasible to predict another part of the output. The provable
security of RSAES-OAEP and RSASSA-PSS relies on the random nature
of the output of the mask generation function, which in turn relies
on the random nature of the underlying hash.

One mask generation function is given here: MGF1, which is based on
a hash function. MGF1 coincides with the mask generation functions
defined in IEEE Std 1363-2000 [26] and the draft ANSI X9.44
[1]. Future versions of this document may define other mask
generation functions.

B.2.1 MGF1

MGF1 is a Mask Generation Function based on a hash function.

Options:
Hash     hash function (hLen denotes the length in octets of the
hash function output)

Input:
mgfSeed  seed from which mask is generated, an octet string
maskLen  intended length in octets of the mask, at most 2^32 hLen

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Output:

Steps:

1. If maskLen > 2^32 hLen, output "mask too long" and stop.

2. Let T be the empty octet string.

3. For counter from 0 to \ceil (maskLen / hLen) - 1, do the
following:

a. Convert counter to an octet string C of length 4 octets (see
Section 4.1):

C = I2OSP (counter, 4) .

b. Concatenate the hash of the seed mgfSeed and C to the octet
string T:

T = T || Hash(mgfSeed || C) .

The object identifier id-mgf1 identifies the MGF1 mask generation
function:

id-mgf1    OBJECT IDENTIFIER ::= { pkcs-1 8 }

The parameters field associated with this OID in a value of type
AlgorithmIdentifier shall have a value of type hashAlgorithm,
identifying the hash function on which MGF1 is based.

C. ASN.1 module

PKCS-1 {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
modules(0) pkcs-1(1)
}

-- $Revision: 2.1$

-- This module has been checked for conformance with the ASN.1
-- standard by the OSS ASN.1 Tools

DEFINITIONS EXPLICIT TAGS ::=

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BEGIN

-- EXPORTS ALL
-- All types and values defined in this module are exported for use
-- in other ASN.1 modules.

IMPORTS

id-sha256, id-sha384, id-sha512
FROM NIST-SHA2 {
joint-iso-itu-t(2) country(16) us(840) organization(1)
gov(101) csor(3) nistalgorithm(4) modules(0) sha2(1)
};

-- ============================
--   Basic object identifiers
-- ============================

-- The DER encoding of this in hexadecimal is:
-- (0x)06 08
--        2A 86 48 86 F7 0D 01 01
--
pkcs-1    OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) 1
}

--
-- When rsaEncryption is used in an AlgorithmIdentifier the
-- parameters MUST be present and MUST be NULL.
--
rsaEncryption    OBJECT IDENTIFIER ::= { pkcs-1 1 }

--
-- When id-RSAES-OAEP is used in an AlgorithmIdentifier the
-- parameters MUST be present and MUST be RSAES-OAEP-params.
--
id-RSAES-OAEP    OBJECT IDENTIFIER ::= { pkcs-1 7 }

--
-- When id-pSpecified is used in an AlgorithmIdentifier the
-- parameters MUST be an OCTET STRING.
--
id-pSpecified    OBJECT IDENTIFIER ::= { pkcs-1 9 }

-- When id-RSASSA-PSS is used in an AlgorithmIdentifier the
-- parameters MUST be present and MUST be RSASSA-PSS-params.
--
id-RSASSA-PSS    OBJECT IDENTIFIER ::= { pkcs-1 10 }

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--
-- When the following OIDs are used in an AlgorithmIdentifier the
-- parameters MUST be present and MUST be NULL.
--
md2WithRSAEncryption       OBJECT IDENTIFIER ::= { pkcs-1 2 }
md5WithRSAEncryption       OBJECT IDENTIFIER ::= { pkcs-1 4 }
sha1WithRSAEncryption      OBJECT IDENTIFIER ::= { pkcs-1 5 }
sha256WithRSAEncryption    OBJECT IDENTIFIER ::= { pkcs-1 11 }
sha384WithRSAEncryption    OBJECT IDENTIFIER ::= { pkcs-1 12 }
sha512WithRSAEncryption    OBJECT IDENTIFIER ::= { pkcs-1 13 }

--
-- This OID really belongs in a module with the secsig OIDs.
--
id-sha1    OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithms(2) 26
}

--
-- OIDs for MD2 and MD5, allowed only in EMSA-PKCS1-v1_5.
--
id-md2 OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) digestAlgorithm(2) 2
}

id-md5 OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) digestAlgorithm(2) 5
}

--
-- When id-mgf1 is used in an AlgorithmIdentifier the parameters MUST
-- be present and MUST be a HashAlgorithm, for example sha1.
--
id-mgf1    OBJECT IDENTIFIER ::= { pkcs-1 8 }

-- ================
--   Useful types
-- ================

ALGORITHM-IDENTIFIER ::= CLASS {
&id    OBJECT IDENTIFIER  UNIQUE,
&Type  OPTIONAL
}
WITH SYNTAX { OID &id [PARAMETERS &Type] }

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--
-- Note: the parameter InfoObjectSet in the following definitions
-- allows a distinct information object set to be specified for sets
-- of algorithms such as:
-- DigestAlgorithms    ALGORITHM-IDENTIFIER ::= {
--     { OID id-md2  PARAMETERS NULL }|
--     { OID id-md5  PARAMETERS NULL }|
--     { OID id-sha1 PARAMETERS NULL }
-- }
--

AlgorithmIdentifier { ALGORITHM-IDENTIFIER:InfoObjectSet } ::=
SEQUENCE {
algorithm  ALGORITHM-IDENTIFIER.&id({InfoObjectSet}),
parameters
ALGORITHM-IDENTIFIER.&Type({InfoObjectSet}{@.algorithm})
OPTIONAL
}

-- ==============
--   Algorithms
-- ==============

--
-- Allowed EME-OAEP and EMSA-PSS digest algorithms.
--
OAEP-PSSDigestAlgorithms    ALGORITHM-IDENTIFIER ::= {
{ OID id-sha1 PARAMETERS NULL   }|
{ OID id-sha256 PARAMETERS NULL }|
{ OID id-sha384 PARAMETERS NULL }|
{ OID id-sha512 PARAMETERS NULL },
...  -- Allows for future expansion --
}

--
-- Allowed EMSA-PKCS1-v1_5 digest algorithms.
--
PKCS1-v1-5DigestAlgorithms    ALGORITHM-IDENTIFIER ::= {
{ OID id-md2 PARAMETERS NULL    }|
{ OID id-md5 PARAMETERS NULL    }|
{ OID id-sha1 PARAMETERS NULL   }|
{ OID id-sha256 PARAMETERS NULL }|
{ OID id-sha384 PARAMETERS NULL }|
{ OID id-sha512 PARAMETERS NULL }
}

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sha1    HashAlgorithm ::= {
algorithm   id-sha1,
parameters  SHA1Parameters : NULL
}

HashAlgorithm ::= AlgorithmIdentifier { {OAEP-PSSDigestAlgorithms} }

SHA1Parameters ::= NULL

--
-- Allowed mask generation function algorithms.
-- If the identifier is id-mgf1, the parameters are a HashAlgorithm.
--
PKCS1MGFAlgorithms    ALGORITHM-IDENTIFIER ::= {
{ OID id-mgf1 PARAMETERS HashAlgorithm },
...  -- Allows for future expansion --
}

--
-- Default AlgorithmIdentifier for id-RSAES-OAEP.maskGenAlgorithm and
--
algorithm   id-mgf1,
parameters  HashAlgorithm : sha1
}

MaskGenAlgorithm ::= AlgorithmIdentifier { {PKCS1MGFAlgorithms} }

--
-- Allowed algorithms for pSourceAlgorithm.
--
PKCS1PSourceAlgorithms    ALGORITHM-IDENTIFIER ::= {
{ OID id-pSpecified PARAMETERS EncodingParameters },
...  -- Allows for future expansion --
}

EncodingParameters ::= OCTET STRING(SIZE(0..MAX))

--
-- This identifier means that the label L is an empty string, so the
-- digest of the empty string appears in the RSA block before
--
pSpecifiedEmpty    PSourceAlgorithm ::= {
algorithm   id-pSpecified,
parameters  EncodingParameters : emptyString
}

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PSourceAlgorithm ::= AlgorithmIdentifier { {PKCS1PSourceAlgorithms} }

emptyString    EncodingParameters ::= ''H

--
-- Type identifier definitions for the PKCS #1 OIDs.
--
PKCS1Algorithms    ALGORITHM-IDENTIFIER ::= {
{ OID rsaEncryption              PARAMETERS NULL } |
{ OID md2WithRSAEncryption       PARAMETERS NULL } |
{ OID md5WithRSAEncryption       PARAMETERS NULL } |
{ OID sha1WithRSAEncryption      PARAMETERS NULL } |
{ OID sha256WithRSAEncryption    PARAMETERS NULL } |
{ OID sha384WithRSAEncryption    PARAMETERS NULL } |
{ OID sha512WithRSAEncryption    PARAMETERS NULL } |
{ OID id-RSAES-OAEP PARAMETERS RSAES-OAEP-params } |
PKCS1PSourceAlgorithms                             |
{ OID id-RSASSA-PSS PARAMETERS RSASSA-PSS-params } ,
...  -- Allows for future expansion --
}

-- ===================
--   Main structures
-- ===================

RSAPublicKey ::= SEQUENCE {
modulus           INTEGER,  -- n
publicExponent    INTEGER   -- e
}

--
-- Representation of RSA private key with information for the CRT
-- algorithm.
--
RSAPrivateKey ::= SEQUENCE {
version           Version,
modulus           INTEGER,  -- n
publicExponent    INTEGER,  -- e
privateExponent   INTEGER,  -- d
prime1            INTEGER,  -- p
prime2            INTEGER,  -- q
exponent1         INTEGER,  -- d mod (p-1)
exponent2         INTEGER,  -- d mod (q-1)
coefficient       INTEGER,  -- (inverse of q) mod p
otherPrimeInfos   OtherPrimeInfos OPTIONAL
}

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Version ::= INTEGER { two-prime(0), multi(1) }
(CONSTRAINED BY {
-- version must be multi if otherPrimeInfos present --
})

OtherPrimeInfos ::= SEQUENCE SIZE(1..MAX) OF OtherPrimeInfo

OtherPrimeInfo ::= SEQUENCE {
prime             INTEGER,  -- ri
exponent          INTEGER,  -- di
coefficient       INTEGER   -- ti
}

--
-- AlgorithmIdentifier.parameters for id-RSAES-OAEP.
-- Note that the tags in this Sequence are explicit.
--
RSAES-OAEP-params ::= SEQUENCE {
hashAlgorithm      [0] HashAlgorithm     DEFAULT sha1,
pSourceAlgorithm   [2] PSourceAlgorithm  DEFAULT pSpecifiedEmpty
}

--
-- Identifier for default RSAES-OAEP algorithm identifier.
-- The DER Encoding of this is in hexadecimal:
-- (0x)30 0D
--        06 09
--           2A 86 48 86 F7 0D 01 01 07
--        30 00
-- Notice that the DER encoding of default values is "empty".
--

rSAES-OAEP-Default-Identifier    RSAES-AlgorithmIdentifier ::= {
algorithm   id-RSAES-OAEP,
parameters  RSAES-OAEP-params : {
hashAlgorithm       sha1,
pSourceAlgorithm    pSpecifiedEmpty
}
}

RSAES-AlgorithmIdentifier ::=
AlgorithmIdentifier { {PKCS1Algorithms} }

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--
-- AlgorithmIdentifier.parameters for id-RSASSA-PSS.
-- Note that the tags in this Sequence are explicit.
--
RSASSA-PSS-params ::= SEQUENCE {
hashAlgorithm      [0] HashAlgorithm      DEFAULT sha1,
saltLength         [2] INTEGER            DEFAULT 20,
trailerField       [3] TrailerField       DEFAULT trailerFieldBC
}

TrailerField ::= INTEGER { trailerFieldBC(1) }

--
-- Identifier for default RSASSA-PSS algorithm identifier
-- The DER Encoding of this is in hexadecimal:
-- (0x)30 0D
--        06 09
--           2A 86 48 86 F7 0D 01 01 0A
--        30 00
-- Notice that the DER encoding of default values is "empty".
--
rSASSA-PSS-Default-Identifier    RSASSA-AlgorithmIdentifier ::= {
algorithm   id-RSASSA-PSS,
parameters  RSASSA-PSS-params : {
hashAlgorithm       sha1,
saltLength          20,
trailerField        trailerFieldBC
}
}

RSASSA-AlgorithmIdentifier ::=
AlgorithmIdentifier { {PKCS1Algorithms} }

--
-- Syntax for the EMSA-PKCS1-v1_5 hash identifier.
--
DigestInfo ::= SEQUENCE {
digestAlgorithm DigestAlgorithm,
digest OCTET STRING
}

DigestAlgorithm ::=
AlgorithmIdentifier { {PKCS1-v1-5DigestAlgorithms} }

END  -- PKCS1Definitions

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D. Intellectual property considerations

The RSA public-key cryptosystem is described in U.S. Patent
4,405,829, which expired on September 20, 2000. RSA Security
Inc. makes no other patent claims on the constructions described
in this document, although specific underlying techniques may be
covered.

Multi-prime RSA is described in U.S. Patent 5,848,159.

The University of California has indicated that it has a patent
pending on the PSS signature scheme [5]. It has also provided a
letter to the IEEE P1363 working group stating that if the PSS
signature scheme is included in an IEEE standard, "the University of
conforming implementation of PSS as a technique for achieving a
digital signature with appendix" [23]. The PSS signature scheme is
specified in the IEEE P1363a draft [27], which was in ballot
resolution when this document was published.

License to copy this document is granted provided that it is
identified as "RSA Security Inc. Public-Key Cryptography Standards
(PKCS)" in all material mentioning or referencing this document.

RSA Security Inc. makes no other representations regarding
intellectual property claims by other parties. Such determination
is the responsibility of the user.

E. Revision history

Versions 1.0 - 1.3

Versions 1.0 - 1.3 were distributed to participants in RSA Data
Security, Inc.'s Public-Key Cryptography Standards meetings in
February and March 1991.

Version 1.4

Version 1.4 was part of the June 3, 1991 initial public release
of PKCS. Version 1.4 was published as NIST/OSI Implementors'
Workshop document SEC-SIG-91-18.

Version 1.5

Version 1.5 incorporated several editorial changes, including
The following substantive changes were made:

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- Section 10: "MD4 with RSA" signature and verification processes
- Section 11: md4WithRSAEncryption object identifier was added.

Version 1.5 was republished as IETF RFC 2313.

Version 2.0

Version 2.0 incorporated major editorial changes in terms of
the document structure and introduced the RSAES-OAEP encryption
scheme. This version continued to support the encryption and
signature processes in version 1.5, although the hash algorithm
MD4 was no longer allowed due to cryptanalytic advances in the
intervening years. Version 2.0 was republished as IETF RFC 2437
[35].

Version 2.1

Version 2.1 introduces multi-prime RSA and the RSASSA-PSS
signature scheme with appendix along with several editorial
improvements. This version continues to support the schemes in
version 2.0.

F. References

[1]   ANSI X9F1 Working Group. ANSI X9.44 Draft D2: Key
Establishment Using Integer Factorization Cryptography.
Working Draft, March 2002.

[2]   M. Bellare, A. Desai, D. Pointcheval and P. Rogaway.
Relations Among Notions of Security for Public-Key Encryption
Schemes. In H. Krawczyk, editor, Advances in Cryptology -
Crypto '98, volume 1462 of Lecture Notes in Computer Science,
pp. 26 - 45. Springer Verlag, 1998.

[3]   M. Bellare and P. Rogaway. Optimal Asymmetric Encryption -
How to Encrypt with RSA. In A. De Santis, editor, Advances in
Cryptology - Eurocrypt '94, volume 950 of Lecture Notes in
Computer Science, pp. 92 - 111. Springer Verlag, 1995.

[4]   M. Bellare and P. Rogaway. The Exact Security of Digital
Signatures - How to Sign with RSA and Rabin. In U. Maurer,
editor, Advances in Cryptology - Eurocrypt '96, volume 1070
of Lecture Notes in Computer Science, pp. 399 - 416. Springer
Verlag, 1996.

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The Public-Key Cryptography Standards are specifications produced
by RSA Laboratories in cooperation with secure systems developers
worldwide for the purpose of accelerating the deployment of
public-key cryptography. First published in 1991 as a result of
meetings with a small group of early adopters of public-key
technology, the PKCS documents have become widely referenced and
implemented. Contributions from the PKCS series have become part of
many formal and de facto standards, including ANSI X9 and IEEE
P1363 documents, PKIX, SET, S/MIME, SSL/TLS, and WAP/WTLS.

Further development of PKCS occurs through mailing list discussions
and occasional workshops, and suggestions for improvement are

PKCS Editor
RSA Laboratories
174 Middlesex Turnpike
Bedford, MA  01730 USA
pkcs-editor@rsasecurity.com
http://www.rsasecurity.com/rsalabs/pkcs

Security Considerations

Security issues are discussed throughout this memo.

Acknowledgements

This document is based on a contribution of RSA Laboratories, a
the research center of RSA Security Inc.  Any substantial use of the
text from this document must acknowledge RSA Security Inc.  RSA
Security Inc. requests that all material mentioning or referencing
this document identify this as "RSA Security Inc. PKCS #1 v2.1".

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