Internet DRAFT - draft-harkins-salted-eap-pwd
draft-harkins-salted-eap-pwd
Internet Engineering Task Force D. Harkins
Internet-Draft HP Enterprise
Updates: 5931 (if approved) November 23, 2016
Intended status: Informational
Expires: May 27, 2017
Adding Support for Salted Password Databases to EAP-pwd
draft-harkins-salted-eap-pwd-08
Abstract
EAP-pwd is an EAP method that uses a shared password for
authentication using a technique that is resistant to dictionary
attack. It included support for raw keys and [RFC2759]-style double
hashing of a password but did not include support for salted
passwords. There are many existing databases of salted passwords and
it is desirable to allow their use with EAP-pwd.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 27, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Harkins Expires May 27, 2017 [Page 1]
Internet-Draft NaCl'd EAP-pwd November 2016
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Keyword Definition . . . . . . . . . . . . . . . . . . . 3
2. Salted Passwords in EAP-pwd . . . . . . . . . . . . . . . . . 3
2.1. Password Pre-Processing . . . . . . . . . . . . . . . . . 3
2.2. The Salting of a Password . . . . . . . . . . . . . . . . 5
2.3. Using UNIX crypt . . . . . . . . . . . . . . . . . . . . 5
2.4. Using scrypt . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Using PBKDF2 . . . . . . . . . . . . . . . . . . . . . . 6
2.6. Protocol Modifications . . . . . . . . . . . . . . . . . 6
2.7. Payload Modifications . . . . . . . . . . . . . . . . . . 7
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
1.1. Background
Databases of stored passwords present an attractive target for
attack--get access to the database, learn the passwords. To confound
such attacks a random "salt" was hashed with the password and the
resulting digest stored, along with the salt, instead of the raw
password. This has the effect of randomizing the password so if two
distinct users have chosen the same password the stored, and salted,
password will be different. It also requires an adversary who has
compromised the security of the stored database to launch a
dictionary attack per entry to recover passwords.
Dictionary attacks, especially using custom hardware, represent real-
world attacks and merely salting a password is insufficient to
protect a password database. To address these attacks an sequential
memory hard function such as described in [RFC7914] is used.
While salting a password database is not sufficient to deal with many
real-world attacks the historic popularity of password salting means
there are a large number of such databases deployed and EAP-pwd needs
to be able to support them. In addition, EAP-pwd needs to be able to
Harkins Expires May 27, 2017 [Page 2]
Internet-Draft NaCl'd EAP-pwd November 2016
support databases using more modern sequential memory hard functions
for protection.
EAP-pwd imposes an additional security requirement on a database of
salted passwords that otherwise would not exist, see Section 5.
1.2. Keyword Definition
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Salted Passwords in EAP-pwd
2.1. Password Pre-Processing
EAP-pwd is based on the "dragonfly" password-authenticated key
exchange (PAKE)--see [RFC7664]. This is a balanced PAKE and requires
that each party to the protocol obtain an identical representation of
a processed password (see Section 5). Salting of a password is
therefore treated as an additional password pre-processing technique
of EAP-pwd. The salt and digest to use is conveyed to the peer by
the server and the password is processed prior to fixing the password
element (see Section 2.8.3 of [RFC5931]).
This memo defines eight (8) new password pre-processing techniques
for EAP-pwd:
o TBD1: a random salt with SHA-1 ([SHS])
o TBD2: a random salt with SHA-256 ([SHS])
o TBD3: a random salt with SHA-512 ([SHS])
o TBD4: UNIX crypt() ([CRY])
o TBD5: scrypt ([RFC7914])
o TBD6: PBKDF2 with SHA-256 ([RFC2898])
o TBD7: PBKDF2 with SHA-512 ([RFC2898])
o TBD8: SASLprep then a random salt with SHA-1 ([SHS])
o TBD9: SASLprep then a random salt with SHA-256 ([SHS])
o TBD10: SASLprep then a random salt with SHA-512 ([SHS])
Harkins Expires May 27, 2017 [Page 3]
Internet-Draft NaCl'd EAP-pwd November 2016
o TBD11: SASLprep then UNIX crypt() ([CRY])
o TBD12: OpaqueString then scrypt ([RFC7914])
o TBD13: OpaqueString then PBKDF2 with SHA-256 ([RFC2898])
o TBD14: OpaqueString then PBKDF2 with SHA-512 ([RFC2898])
When passing salt, the size of the salt SHOULD be at least as long as
the message digest of the hash algorithm used. There is no guarantee
that deployed salted databases have followed this rule, and in the
interest of interoperability, an EAP peer SHOULD NOT abort an EAP-pwd
exchange if the length of the salt conveyed during the exchange is
less than the message digest of the indicated hash algorithm.
UNIX crypt(), scrypt, and PBKDF2 impose additional formatting
requirements on the passed salt. See below.
Plain salting techniques are included for support of existing
databases. scrypt and PBKDF2 techniques are RECOMMENDED for new
password database deployments.
SASLprep has been deprecated but databases treated with SASLprep
exist and it is necessary to provide code points for them. When
using SASLprep a password SHALL be considered a "stored string" per
[RFC3454] and unassigned code points are therefore prohibited. The
output of SASLprep SHALL be the binary representation of the
processed UTF-8 character string. Prohibted output and unassigned
codepoints encountered in SASLprep pre-processing SHALL cause a
failure of pre-processing, and the output SHALL NOT be used with EAP-
pwd.
When performing one of TBD12-TBD14 the password SHALL be a UTF-8
string and SHALL be pre-processed by applying the Preparation and
Enforcement steps of the OpaqueString profile in [RFC7613] to the
password. The output of OpaqueString, also a UTF-8 string, becomes
the EAP-pwd password and SHALL be hashed with the indicated
algorithm.
There is a large number of deployed password databases that use
[RFC7616]-style salting and hashing but these deployments require a
nonce contribution by the client (as well as the server) and EAP-pwd
does not have the capability to provide that information.
Harkins Expires May 27, 2017 [Page 4]
Internet-Draft NaCl'd EAP-pwd November 2016
2.2. The Salting of a Password
For both parties to derive the same salted password there needs to be
a canonical method of salting a password. When using EAP-pwd, a
password SHALL be salted by hashing the password followed by the salt
using the designated hash function:
salted-password = Hash(password | salt)
The server stores the salted-password, and the salt, in its database
and the client derives the salted-password on-the-fly.
2.3. Using UNIX crypt
Different algorithms are supported with the UNIX crypt() function.
The particular algorithm used is indicated by prepending an encoding
of "setting" to the passed salt. The specific algorithm used is
opaque to EAP-pwd as the entire salt, including the encoded
"setting", is passed as an opaque string for interpretation by
crypt(). The salted password used for EAP-pwd SHALL be the output of
crypt():
salted-password = crypt(password, salt)
The server stores the salted-password, and the encoded algorithm plus
salt, in its database and the client derives the salted-password on-
the-fly.
If the server indicates a crypt() algorithm that is unsupported by
the client, the exchange fails and the client MUST terminate the
connection.
2.4. Using scrypt
The scrypt function takes several parameters:
o N, the cost parameter
o r, the block size
o p, the parallelization parameter
o dkLen, the length of the output
These parameters are encoded into the "salt" field of the modified
EAP-pwd message. Parameters r and dkLen SHALL be 16-bit integers in
network order. The other parameters SHALL each be 32-bit integers in
Harkins Expires May 27, 2017 [Page 5]
Internet-Draft NaCl'd EAP-pwd November 2016
network order. The "salt" field that gets transmitted in EAP-pwd
SHALL therefore be:
N || r || p || dkLen || salt
where || represents concatenation.
The value of N represents the exponent taken to the power of two in
order to determine the CPU/Memory cost of scrypt-- i.e. the value is
2^N. Per [RFC7914] the resulting CPU/Memory cost value SHALL be less
than 2^(128 * r / 8) and the value p SHALL be less than or equal to
((2^32 - 1) * 32) / (128 * r).
Note: EAP-pwd uses the salted password directly as the authentication
credential and will hash it with a counter in order to obtain a
secret element in a finite field. Therefore it makes little sense to
use dkLen greater than the digest of the underlying hash function but
the capability is provided to do so anyway.
2.5. Using PBKDF2
The PBKDF2 function requires two parameters:
o c, the iteration count
o dkLen, the length of the output
These parameters are encoded into the "salt" field of the modified
EAP-pwd message. The parameters SHALL be 16-bit integers in network
order. The "salt" field that gets transmitted in EAP-pwd SHALL
therefore be:
c || dkLen || salt
where || represents concatenation.
Note: EAP-pwd uses the salted password directly as the authentication
credential and will hash it with a counter in order to obtain a
secret element in a finite field. Therefore it makes little sense to
use dkLen greater than the digest of the underlying hash function but
the capability is provided to do so anyway.
2.6. Protocol Modifications
Like all EAP methods, EAP-pwd is server initiated and the initial
identity supplied by the client is not useful for authentication
purposes. Because of this, the server is required to indicate its
intentions, including the password pre-processing it wishes to use,
Harkins Expires May 27, 2017 [Page 6]
Internet-Draft NaCl'd EAP-pwd November 2016
before it knows the true identity of the client. This prevents the
server from supporting multiple salt digests simultaneously in a
single password database. To support multiple salt digests
simultaneously, it is necessary to maintain multiple password
databases and use the routable portion of the client identity to
select one when initiating EAP-pwd.
The server uses the EAP-pwd-ID/Request to indicate the password pre-
processing technique. The client indicates its acceptance of the
password pre-processing technique and identifies itself in the EAP-
pwd-ID/Response. If the cient does not accept any of the offered
pre-processing techniques it SHALL terminate the exchange. Upon
receipt of the EAP-pwd-ID/Response, the server knows the identity of
the client and can look up the client's salted password and the salt
from the database. The server adds the length of the salt and the
salt itself to the EAP-pwd-Commit/Request message (see Section 2.7).
The server can fix the password element (Section 2.8.3 of [RFC5931])
as soon as the salted password has been looked up in the database.
The client, though, is required to wait until receipt of the server's
EAP-pwd-Commit/Request before it begins fixing the password element.
2.7. Payload Modifications
When a salted password pre-processing technique is agreed upon during
the EAP-pwd-ID exchange the EAP-pwd-Commit payload is modified to
include the salt and salt length (see Figure 1). The server passes
the salt and salt length in the EAP-pwd-Commit/Request; the client's
EAP-pwd-Commit/Response is unchanged and it MUST NOT echo the salt
length and salt in its EAP-pwd-Commit/Response.
Harkins Expires May 27, 2017 [Page 7]
Internet-Draft NaCl'd EAP-pwd November 2016
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| salt-len | |
+-+-+-+-+-+-+-+-+ ~
~ Salt +-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~
| |
~ Element ~
| |
~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~
| |
~ Scalar +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Salted EAP-pwd-Commit/Request
The "salt-len" SHALL be non-zero and indicates the length, in octets,
of the salt that follows. The salt SHALL be a binary string. The
Element and Scalar are encoded according to Section 3.3 of [RFC5931].
Note: when a non-salted password pre-processing method is used, for
example, any of the methods from [RFC5931], the EAP-pwd-Commit
payload MUST NOT be modified to include the salt and salt length.
3. Acknowledgements
Thanks to Stefan Winter and the eduroam project for its continued
interest in using EAP-pwd. Thanks to Simon Josefsson for his advice
on support for scrypt and PBKDF2.
4. IANA Considerations
IANA is instructed to allocate fourteen (14) values from the
"password preprocessing method registry" established by [RFC5931] and
replace TBD1, TBD2, TBD3, TBD4, TBD5, TBD6, TBD7, TBD8, TBD9, TBD10,
TBD11, TBD12, TBD13, and TBD14 above with the values assigned.
5. Security Considerations
EAP-pwd requires each side to produce an identical representation of
the (processed) password before the password element can be fixed.
This symmetry undercuts one of the benefits to salting a password
database because the salted password from a compromised database can
Harkins Expires May 27, 2017 [Page 8]
Internet-Draft NaCl'd EAP-pwd November 2016
be used directly to impersonate the EAP-pwd client--since the
plaintext password need not be recovered, no dictionary attack is
needed. While the immediate effect of such a compromise would be
compromise of the server, the per-user salt would still prevent the
adversary from recovering the password, barring a successful
dictionary attack, to use for other purposes.
Salted password databases used with EAP-pwd MUST be afforded the same
level of protection as databases of plaintext passwords.
Hashing a password with a salt increases the work factor for an
attacker to obtain the cleartext password but dedicated hardware
makes this increased work factor increasingly negligible in real-
world scenarios. Cleartext password databases SHOULD be protected
with a scheme that uses a sequential memory hard function such as
[RFC7914].
EAP-pwd sends the salt in the clear. If EAP-pwd is not tunneled in
another, encrypting, EAP method, an adversary that can observe
traffic from server to authenticator or from authenticator to client
would learn the salt used for a particular user. While knowledge of
a salt by an adversary may be of a somewhat dubious nature (pre-image
resistance of the hash function used will protect the client's
password and, as noted above, the database of salted passwords must
still be protected from disclosure), it does represent potential
additional meta-data in the hands of a untrusted third party.
6. References
6.1. Normative References
[CRY] "crypt(3) man page",
<http://man7.org/linux/man-pages/man3/crypt.3.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, DOI 10.17487/
RFC2898, September 2000,
<http://www.rfc-editor.org/info/rfc2898>.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, DOI
10.17487/RFC3454, December 2002,
<http://www.rfc-editor.org/info/rfc3454>.
Harkins Expires May 27, 2017 [Page 9]
Internet-Draft NaCl'd EAP-pwd November 2016
[RFC5931] Harkins, D. and G. Zorn, "Extensible Authentication
Protocol (EAP) Authentication Using Only a Password", RFC
5931, August 2010.
[RFC7613] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 7613, DOI
10.17487/RFC7613, August 2015,
<http://www.rfc-editor.org/info/rfc7613>.
[RFC7914] Percival, C. and S. Josefsson, "The scrypt Password-Based
Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914,
August 2016, <http://www.rfc-editor.org/info/rfc7914>.
[SHS] National Institute of Standards and Technology, , "Federal
Information Processing Standard Publication 180-4: Secure
Hash Standard (SHS)", March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
6.2. Informative References
[RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC
2759, DOI 10.17487/RFC2759, January 2000,
<http://www.rfc-editor.org/info/rfc2759>.
[RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616, DOI 10.17487/
RFC7616, September 2015,
<http://www.rfc-editor.org/info/rfc7616>.
[RFC7664] Harkins, D., Ed., "Dragonfly Key Exchange", RFC 7664, DOI
10.17487/RFC7664, November 2015,
<http://www.rfc-editor.org/info/rfc7664>.
Author's Address
Dan Harkins
HP Enterprise
1322 Crossman Avenue
Sunnyvale, CA 94089-1113
United States of America
Email: dharkins@arubanetworks.com
Harkins Expires May 27, 2017 [Page 10]
