S/MIME Working Group R. Housley Internet Draft SPYRUS expires in six months December 1997 Cryptographic Message Syntax Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet- Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document describes the Cryptographic Message Syntax. This syntax is used to digitally sign, digest, or encrypt arbitrary messages. The Cryptographic Message Syntax is derived from PKCS #7 version 1.5. Wherever possible, backward compatibility is preserved; however, changes were necessary to accommodate attribute certificate transfer and key agreement techniques for key management. This draft is being discussed on the ''ietf-smime'' mailing list. To subscribe, send a message to: ietf-smime-request@imc.org with the single ''subscribe'' word in the body of the message. Also, there is a Web site for the mailing list at: . Housley [Page 1] INTERNET DRAFT December 1997 1 Introduction This document describes the Cryptographic Message Syntax. This syntax is used to digitally sign or encrypt arbitrary messages. The Cryptographic Message Syntax describes an encapsulation syntax for data protection. It supports digital signatures and encryption. The syntax allows multiple encapsulation, so one encapsulation envelope can be nested inside another. Likewise, one party can digitally sign some previously encapsulated data. It also allows arbitrary attributes, such as signing time, to be authenticated along with the message content, and provides for other attributes such as countersignatures to be associated with a signature. The Cryptographic Message Syntax can support a variety of architectures for certificate-based key management, such as the one defined by the PKIX working group. The Cryptographic Message Syntax values are generated using ASN.1, using BER-encoding. Values are typically represented as octet strings. While many systems are capable of transmitting arbitrary octet strings reliably, it is well known that many electronic-mail systems are not. This document does not address mechanisms for encoding octet strings for reliable transmission in such environments. 2 General Overview The Cryptographic Message Syntax is general enough to support many different content types. This document defines three: data, signed data, and enveloped data. Other content types may be added in the future, and additional content types can be defined outside this document. The Cryptographic Message Syntax exports one content type, ContentInfo, as well as the various object identifiers. As a general design philosophy, content types permit single pass processing using indefinite-length Basic Encoding Rules (BER) encoding. Single-pass operation is especially helpful if content is large, stored on tapes, or is "piped" from another process. Single- pass operation has one significant drawback; it is difficult to perform encode operations using the Distinguished Encoding Rules (DER) encoding in a single pass since the lengths of the various components may not be known in advance. Since DER encoding is required by the signed-data content type, an extra pass may be necessary when a content type other than data is encapsulated. Housley [Page 2] INTERNET DRAFT December 1997 3 General Syntax The Cryptographic Message Syntax associates a content type with a content. The syntax shall have ASN.1 type ContentInfo: ContentInfo ::= SEQUENCE { contentType ContentType, content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL } ContentType ::= OBJECT IDENTIFIER The fields of ContentInfo have the following meanings: contentType indicates the type of content. It is an object identifier, which means it is a unique string of integers assigned by the authority that defines the content type. content is the content. The field is optional, although it is generally present. In the rare cases where it is absent, the intended value must be supplied by other means. The type of content can be determined uniquely by contentType, so the type defined along with the object identifier should not be a CHOICE type. The optional omission of the content field makes it possible to construct "external signatures." In the case of external signatures, the content being signed would be absent from the encapsulated ContentInfo value included in the signed-data content type. If the ContentInfo value is absent, the signatureValue is calculated as though the ContentInfo value were present. The presumed ContentInfo must have the content type set to id-data and the content omitted. 4 Data Content Type The data content type is identified by the following object identifier: id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 } The data content type is just an octet string. It shall have ASN.1 type Data: Data ::= OCTET STRING The data content type is intended to refer to arbitrary octet strings, such as ASCII text files; the interpretation is left to the Housley [Page 3] INTERNET DRAFT December 1997 application. Such strings need not have any internal structure (although they may; they could even be DER encoded). 5 Signed-data Content Type The signed-data content type consists of a content of any type and zero or more signature values. Any type of content can be signed by any number of signers in parallel. The typical application of the signed-data content type represents one signer's digital signature on content of the data content type. Another typical application disseminates certificates and certificate revocation lists (CRLs). The process by which signed data is constructed involves the following steps: 1. For each signer, a message digest, or hash value, is computed on the content with a signer-specific message-digest algorithm. If two signers employ the same message digest algorithm, then the message digest need be computed for only one of them. If the signer is authenticating any information other than the content (see Section 5.2), the message digest of the content and the other information are digested with the signer's message digest algorithm, and the result becomes the "message digest." 2. For each signer, the message digest is digitally signed using the signer's private key. 3. For each signer, the signature value and other signer-specific information are collected into a SignerInfo value, as defined in Section 5.2. Certificates and CRLs for each signer, and those not corresponding to any signer, are collected in this step. 4. The message digest algorithms for all the signers and the SignerInfo values for all the signers are collected together with the content into a SignedData value, as defined in Section 5.1. A recipient independently computes the message digest. This message digest and the signer's public key are used it to validate the signature value. The signer's public key is referenced by an issuer distinguished name and an issuer-specific serial number that uniquely identify the certificate containing the public key. The signer's certificate may be included in the SignedData certificates field. This section is divided into four parts. The first part describes the top-level type SignedData, the second part describes the per-signer information type SignerInfo, and the third and fourth parts describe Housley [Page 4] INTERNET DRAFT December 1997 the message digest calculation and signature generation processes. 5.1 SignedData Type The signed-data content type is identified by the following object identifier: id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 } The signed-data content type shall have ASN.1 type SignedData: SignedData ::= SEQUENCE { version Version, digestAlgorithms DigestAlgorithmIdentifiers, contentInfo ContentInfo, certificates [0] IMPLICIT CertificateSet OPTIONAL, crls [1] IMPLICIT CertificateRevocationLists OPTIONAL, signerInfos SignerInfos } DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier SignerInfos ::= SET OF SignerInfo The fields of type SignedData have the following meanings: version is the syntax version number. If no attribute certificates are present in the certificates field, then the value of version shall be 1; however, if attribute certificates are present, then the value of version shall be 3. digestAlgorithms is a collection of message digest algorithm identifiers. There may be any number of elements in the collection, including zero. Each element identifies the message digest algorithm, along with any associated parameters, used by one or more signer. The collection is intended to list the message digest algorithms employed by all of the signers, in any order, to facilitate one-pass signature verification. The message digesting process is described in Section 5.3. contentInfo is the content that is signed. It can have any type. certificates is a collection of certificates. It is intended that the set of certificates be sufficient to contain chains from a recognized "root" or "top-level certification authority" to all of the signers in the signerInfos field. There may be more certificates than necessary, and there may be certificates sufficient to contain chains from two or more independent top- Housley [Page 5] INTERNET DRAFT December 1997 level certification authorities. There may also be fewer certificates than necessary, if it is expected recipients have an alternate means of obtaining necessary certificates (e.g., from a previous set of certificates). If no attribute certificates are present in the collection, then the value of version shall be 1; however, if attribute certificates are present, then the value of version shall be 3. crls is a collection of certificate revocation lists (CRLs). It is intended that the set contain information sufficient to determine whether or not the certificates in the certificates field are valid, but such correspondence is not necessary. There may be more CRLs than necessary, and there may also be fewer CRLs than necessary. signerInfos is a collection of per-signer information. There may be any number of elements in the collection, including zero. In the degenerate case where there are no signers, the ContentInfo value being "signed" is irrelevant. In this case, the content type within the ContentInfo value being "signed" should be data, and the content field of the ContentInfo value should be omitted. 5.2 SignerInfo Type Per-signer information is represented in the type SignerInfo: SignerInfo ::= SEQUENCE { version Version, issuerAndSerialNumber IssuerAndSerialNumber, digestAlgorithm DigestAlgorithmIdentifier, authenticatedAttributes [0] IMPLICIT Attributes OPTIONAL, signatureAlgorithm SignatureAlgorithmIdentifier, signature SignatureValue, unauthenticatedAttributes [1] IMPLICIT Attributes OPTIONAL } Attributes ::= SET OF Attribute SignatureValue ::= OCTET STRING The fields of type SignerInfo have the following meanings: version is the syntax version number. It shall always be 1. issuerAndSerialNumber specifies the signer's certificate (and thereby the signer's public key) by issuer distinguished name and issuer-specific serial number. Housley [Page 6] INTERNET DRAFT December 1997 digestAlgorithm identifies the message digest algorithm, and any associated parameters, used by the signer. The message digest is computed over the content and authenticated attributes, if present. The message digest algorithm should be among those listed in the digestAlgorithms field of the superior SignerInfo value. The message digesting process is described in Section 5.3. authenticatedAttributes is a collection of attributes that are signed. The field is optional, but it must be present if the content type of the ContentInfo value being signed is not data. If the field is present, it must contain, at a minimum, two attributes: A content-type attribute having as its value the content type of the ContentInfo value being signed. Section 8.1 defines the content-type attribute. A message-digest attribute, having as its value the message digest of the content. Section 8.2 defines the message-digest attribute. Other attribute types that might be useful here, such as signing time, are defined in Section 8. signatureAlgorithm identifies the signature algorithm, and any associated parameters, used by the signer to generate the digital signature. signature is the result of digital signature generation, using the message digest and the signer's private key. unauthenticatedAttributes is a collection of attributes that are not signed. The field is optional. Attribute types that might be useful here, such as countersignatures, are defined in Section 10. 5.3 Message Digest Calculation Process The message digest calculation process computes a message digest on either the content being signed or the content together with the signer's authenticated attributes. In either case, the initial input to the message digest calculation process is the "value" of the content being signed. Specifically, the initial input is the content octets of the DER encoding of the content field of the ContentInfo value to which the signing process is applied. Only the contents octets of the DER encoding of that field are input to the message digest algorithm, not the identifier octets or the length octets. The result of the message digest calculation process depends on Housley [Page 7] INTERNET DRAFT December 1997 whether the authenticatedAttributes field is present. When the field is absent, the result is just the message digest of the content as described above. When the field is present, however, the result is the message digest of the complete DER encoding of the Attributes value contained in the authenticatedAttributes field. Since the Attributes value, when the field is present, must contain as attributes the content type and the message digest of the content, those values are indirectly included in the result. A separate encoding of the authenticatedAttributes field is performed for message digest calculation. The IMPLICIT [0] tag in the authenticatedAttributes field is not used for the DER encoding, rather an EXPLICIT SET OF tag is used. That is, the DER encoding of the SET OF tag, rather than of the IMPLICIT [0] tag, is to be included in the message digest calculation along with the length and contents octets of the Attributes value. When the content being signed has content type data and the authenticatedAttributes field is absent, then just the value of the data (e.g., the contents of a file) is input to the message digest calculation. This has the advantage that the length of the content being signed need not be known in advance of the encryption process. Although the identifier octets and the length octets are not included in the message digest calculation, they are still protected by other means. The length octets are protected by the nature of the message digest algorithm since it is computationally infeasible to find any two distinct messages of any length that have the same message digest. The fact that the message digest is computed on part of a DER encoding does not mean that DER is the required method of representing that part for data transfer. Indeed, it is expected that some implementations will store objects in forms other than their DER encodings, but such practices do not affect message digest computation. 5.4 Message Signature Generation Process The input to the signature generation process includes the result of the message digest calculation process and the signer's private key. The details of the signature generation depend on the signature algorithm employed. The object identifier, along with any parameters, that specifies the signature algorithm employed by the signer is carried in the signatureAlgorithm field. The signature value generated by the signer is encoded as an OCTET STRING and carried in the signature field. Housley [Page 8] INTERNET DRAFT December 1997 5.5 Message Signature Validation Process The input to the signature validation process includes the result of the message digest calculation process and the signer's public key. The details of the signature generation depend on the signature algorithm employed. The recipient may not rely on any message digest values computed by the originator. If the signedData signerInfo includes authenticatedAttributes, then the ASN.1 DER encoded content of the signedData object must be digested as described in section 5.3. For the signature to be valid, the message digest value calculated by the recipient must be the same as the value of the messageDigest attribute included in the authenticatedAttributes of the signedData signerInfo. 6 Enveloped-data Content Type The enveloped-data content type consists of an encrypted content of any type and encrypted content-encryption keys for one or more recipients. The combination of the encrypted content and one encrypted content-encryption key for a recipient is a "digital envelope" for that recipient. Any type of content can be enveloped for any number of recipients. The typical application of the enveloped-data content type will represent one or more recipients' digital envelopes on content of the data or signed-data content types. Enveloped-data is constructed by the following steps: 1. A content-encryption key for a particular content-encryption algorithm is generated at random. 2. The content-encryption key is encrypted for each recipient. The details of this encryption depend on the key management algorithm used, but three general techniques are supported: key transport: the content-encryption key is encrypted in the recipient's public key; key agreement: the recipient's public key and the sender's private key are used to generate a pairwise symmetric key, then the content-encryption key is encrypted in the pairwise symmetric key; and mail list keys: the content-encryption key is encrypted in a previously distributed symmetric key. Housley [Page 9] INTERNET DRAFT December 1997 3. For the originator and each recipient, the encrypted content- encryption key and other recipient-specific information are collected into a RecipientInfo value, defined in Section 6.2. 4. The content is encrypted with the content-encryption key. Content encryption may require that the content be padded to a multiple of some block size; see Section 6.3. 5. The RecipientInfo values for all the recipients are collected together with the encrypted content into a EnvelopedData value as defined in Section 6.1. A recipient opens the envelope by decrypting one of the encrypted content-encryption keys and decrypting the encrypted content with the recovered content-encryption key. This section is divided into four parts. The first part describes the top-level type EnvelopedData, the second part describes the per- recipient information type RecipientInfo, and the third and fourth parts describe the content-encryption and key-encryption processes. 6.1 EnvelopedData Type The enveloped-data content type is identified by the following object identifier: id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 } The enveloped-data content type shall have ASN.1 type EnvelopedData: EnvelopedData ::= SEQUENCE { version Version, originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL, recipientInfos RecipientInfos, encryptedContentInfo EncryptedContentInfo } OriginatorInfo ::= SEQUENCE { certs [0] IMPLICIT CertificateSet OPTIONAL, crls [1] IMPLICIT CertificateRevocationLists OPTIONAL, ukms [2] IMPLICIT UserKeyingMaterials OPTIONAL } RecipientInfos ::= SET OF RecipientInfo EncryptedContentInfo ::= SEQUENCE { contentType ContentType, contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier, encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL } Housley [Page 10] INTERNET DRAFT December 1997 EncryptedContent ::= OCTET STRING The fields of type EnvelopedData have the following meanings: version is the syntax version number. If originatorInfo is present, then version shall be 2. If any of the RecipientInfo structures included have a version of 2, then the version shall be 2. If originatorInfo is absent and all of the RecipientInfo structures are version 0, then version shall be 0. originatorInfo optionally provides information about the originator. It is present only if required by the key management algorithm. It may contain certificates, CRLs, and user keying material (UKMs): certs is a collection of certificates. certs may contain originator certificates associated with several different key management algorithms. The certificates contained in certs are intended to be sufficient to make chains from a recognized "root" or "top-level certification authority" to all recipients. However, certs may contain more certificates than necessary, and there may be certificates sufficient to make chains from two or more independent top-level certification authorities. Alternatively, certs may contain fewer certificates than necessary, if it is expected that recipients have an alternate means of obtaining necessary certificates (e.g., from a previous set of certificates). crls is a collection of CRLs. It is intended that the set contain information sufficient to determine whether or not the certificates in the certs field are valid, but such correspondence is not necessary. There may be more CRLs than necessary, and there may also be fewer CRLs than necessary. ukms is a collection of UKMs. The set includes a member for each key management algorithm employed by the originator that requires a UKM. In general, several recipients will use each UKM in the set. recipientInfos is a collection of per-recipient information. There must be at least one element in the collection. encryptedContentInfo is the encrypted content information. The fields of type EncryptedContentInfo have the following meanings: contentType indicates the type of content. Housley [Page 11] INTERNET DRAFT December 1997 contentEncryptionAlgorithm identifies the content-encryption algorithm, and any associated parameters, used to encrypt the content. The content-encryption process is described in Section 6.3. The same algorithm is used for all recipients. encryptedContent is the result of encrypting the content. The field is optional, and if the field is not present, its intended value must be supplied by other means. The recipientInfos field comes before the encryptedContentInfo field so that an EnvelopedData value may be processed in a single pass. 6.2 RecipientInfo Type Per-recipient information is represented in the type RecipientInfo: RecipientInfo ::= SEQUENCE { version Version, rid RecipientIdentifier, originatorCert [0] EXPLICIT EntityIdentifier OPTIONAL, keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier, encryptedKey EncryptedKey } RecipientIdentifier ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, rKeyId [0] IMPLICIT RecipientKeyIdentifier, mlKeyId [1] IMPLICIT MailListKeyIdentifier } RecipientKeyIdentifier ::= SEQUENCE { subjectKeyIdentifier SubjectKeyIdentifier, date GeneralizedTime OPTIONAL, other OtherKeyAttribute OPTIONAL } MailListKeyIdentifier ::= SEQUENCE { kekIdentifier OCTET STRING, date GeneralizedTime OPTIONAL, other OtherKeyAttribute OPTIONAL } EntityIdentifier ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, subjectKeyIdentifier SubjectKeyIdentifier } SubjectKeyIdentifier ::= OCTET STRING EncryptedKey ::= OCTET STRING The fields of type RecipientInfo have the following meanings: Housley [Page 12] INTERNET DRAFT December 1997 version is the syntax version number. If the OriginatorCert is absent and the RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the version shall be 0. If the OriginatorCert is present or the RecipientIdentifier is either the CHOICE rKeyId or mlKeyId, then the version shall be 2. rid specifies the recipient's certificate or key that was used by the sender to protect the content-encryption key. originatorCert optionally specifies the originator's certificate to be used by this recipient. This field should be included when the originator has more than one certificate containing a public key associated with the key management algorithm used for this recipient. keyEncryptionAlgorithm identifies the key-encryption algorithm, and any associated parameters, used to encrypt the content- encryption key for the recipient. The key-encryption process is described in Section 6.4. encryptedKey is the result of encrypting the content-encryption key for the recipient. The RecipientIdentifier is a CHOICE with three alternatives. The first two alternatives, issuerAndSerialNumber and rKeyId, specifies the recipient's certificate, and thereby the recipient's public key. The rKeyId alternative may optionally specify other parameters needed, such as the date. If the recipient's certificate contains a key transport public key, then the content-encryption key is encrypted with the recipient's public key. If the recipient's certificate contains a key agreement public key, then a pairwise symmetric key is established and used to encrypt the content- encryption key. The third alternative, mlKeyId, specifies a symmetric key encryption key that was previously distributed to the sender and recipient. The fields of type RecipientKeyIdentifier have the following meanings: subjectKeyIdentifier identifies the recipient's certificate by the X.509 subjectKeyIdentifier extension value. date is optional. When present, the date specifies which of the recipient's UKMs was used by the sender. other is optional. When present, this field contains additional information used by the recipient to locate the keying material used by the sender. Housley [Page 13] INTERNET DRAFT December 1997 The fields of type MailListKeyIdentifier have the following meanings: kekIdentifier identifies the key-encryption key that was previously distributed to the sender and the recipient. date is optional. When present, the date specifies a single key- encryption key from a set that was previously distributed to the sender and the recipient. other is optional. When present, this field contains additional information used by the recipient to locate the keying material used by the sender. 6.3 Content-encryption Process The input to the content-encryption process is the "value" of the content being enveloped. Specifically, the input is the content octets of a definite-length BER encoding of the content field of the ContentInfo value. Only the content octets of the BER encoding are encrypted, not the identifier octets or length octets; those other octets are not included. When the content being enveloped has content type data, then just the value of the data (e.g., the contents of a file) is encrypted. This has the advantage that the length of the content being encrypted need not be known in advance of the encryption process. The identifier octets and the length octets are not encrypted. The length octets may be protected implicitly by the encryption process, depending on the encryption algorithm. The identifier octets are not protected at all, although they can be recovered from the content type, assuming that the content type uniquely determines the identifier octets. Explicit protection of the identifier and length octets requires that the signed-data content type be employed prior to enveloping. A definite-length BER encoding is used to ensure that the bit indicating whether the length is definite or indefinite is not recorded in the enveloped-data content type. Definite-length encoding is more appropriate for simple types such as octet strings, so definite-length encoding is chosen. Some content-encryption algorithms assume the input length is a multiple of k octets, where k is greater than one. For such algorithms, the input shall be padded at the trailing end with k-(l mod k) octets all having value k-(l mod k), where l is the length of the input. In other words, the input is padded at the trailing end with one of the following strings: Housley [Page 14] INTERNET DRAFT December 1997 01 -- if l mod k = k-1 02 02 -- if l mod k = k-2 . . . k k ... k k -- if l mod k = 0 The padding can be removed unambiguously since all input is padded, including input values that are already a multiple of the block size, and no padding string is a suffix of another. This padding method is well-defined if and only if k is less than 256. 6.4 Key-encryption Process The input to the key-encryption process -- the value supplied to the recipient's key-encryption algorithm --is just the "value" of the content-encryption key. 7 Digested-data Content Type The digested-data content type consists of content of any type and a message digest of the content. The typical application of the digested-data content type will be to provide content integrity, and that the result generally becomes the input to the enveloped-data content type. Digested-data is constructed by the following steps: 1. A message digest is computed on the content with a message- digest algorithm. 2. The message-digest algorithm and the message digest are collected together with the content into a DigestedData value. A recipient verifies the message digest by comparing the message digest to an independently computed message digest. The digested-data content type is identified by the following object identifier: id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 } Housley [Page 15] INTERNET DRAFT December 1997 The digested-data content type shall have ASN.1 type DigestedData: DigestedData ::= SEQUENCE { version Version, digestAlgorithm DigestAlgorithmIdentifier, contentInfo ContentInfo, digest Digest } Digest ::= OCTET STRING The fields of type DigestedData have the following meanings: version is the syntax version number. It shall be 0. digestAlgorithm identifies the message digest algorithm, and any associated parameters, under which the content is digested. The message-digesting process is the same as in Section 5.3 in the case when there are no authenticated attributes. contentInfo is the content that is digested. It may have any of the defined content types. digest is the result of the message-digesting process. The ordering of the digestAlgorithm field, the contentInfo field, and the digest field makes it possible to process a DigestedData value in a single pass. 8 Encrypted-data Content Type The encrypted-data content type consists of encrypted content of any type. Unlike the enveloped-data content type, the encrypted-data content type has neither recipients nor encrypted content-encryption keys. Keys are assumed to be managed by other means. The typical application of the encrypted-data content type will be to encrypt the content of the data content type for local storage, perhaps where the encryption key is a password. The encrypted-data content type is identified by the following object identifier: id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 } Housley [Page 16] INTERNET DRAFT December 1997 The encrypted-data content type shall have ASN.1 type EncryptedData: EncryptedData ::= SEQUENCE { version Version, encryptedContentInfo EncryptedContentInfo } The fields of type EncryptedData have the following meanings: version is the syntax version number. It shall be 0. encryptedContentInfo is the encrypted content information, as defined in Section 6.1. 9 Useful Types This section defines types that are used other places in the document. The types are not listed in any particular order. 9.1 CertificateRevocationLists The CertificateRevocationLists type gives a set of certificate revocation lists (CRLs). It is intended that the set contain information sufficient to determine whether the certificates with which the set is associated are revoked or not. However, there may be more CRLs than necessary, or there may be fewer than necessary. The definition of CertificateList is imported from X.509. CertificateRevocationLists ::= SET OF CertificateList 9.2 ContentEncryptionAlgorithmIdentifier The ContentEncryptionAlgorithmIdentifier type identifies a content- encryption algorithm such as DES. A content-encryption algorithm supports encryption and decryption operations. The encryption operation maps an octet string (the message) to another octet string (the ciphertext) under control of a content-encryption key. The decryption operation is the inverse of the encryption operation. Context determines which operation is intended. The definition of AlgorithmIdentifier is imported from X.509. ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier 9.3 DigestAlgorithmIdentifier The DigestAlgorithmIdentifier type identifies a message-digest algorithm. Examples include SHA-1, MD2, and MD5. A message-digest Housley [Page 17] INTERNET DRAFT December 1997 algorithm maps an octet string (the message) to another octet string (the message digest). The definition of AlgorithmIdentifier is imported from X.509. DigestAlgorithmIdentifier ::= AlgorithmIdentifier 9.4 SignatureAlgorithmIdentifier The SignatureAlgorithmIdentifier type identifies a signature algorithm. Examples include DSS and RSA. A signature algorithm supports signature generation and verification operations. The signature generation operation uses the message digest and the signer's private key to generate a signature value. The signature verification operation uses the message digest and the signer's public key to determine whether or not a signature value is valid. Context determines which operation is intended. The definition of AlgorithmIdentifier is imported from X.509. SignatureAlgorithmIdentifier ::= AlgorithmIdentifier 9.5 CertificateChoices The CertificateChoices type gives either a PKCS #6 extended certificate, an X.509 certificate, or an X.509 attribute certificate. The PKCS #6 extended certificate is obsolete. It is included for backwards compatibility, and its use should be avoided. The definitions of Certificate and AttributeCertificate are imported from X.509. CertificateChoices ::= CHOICE { certificate Certificate, -- See X.509 extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete attrCert [1] IMPLICIT AttributeCertificate } -- See X.509 and X9.57 9.6 CertificateSet The CertificateSet type provides a set of certificates. It is intended that the set be sufficient to contain chains from a recognized "root" or "top-level certification authority" to all of the sender certificates with which the set is associated. However, there may be more certificates than necessary, or there may be fewer than necessary. The precise meaning of a "chain" is outside the scope of this document. Some applications may impose upper limits on the length of Housley [Page 18] INTERNET DRAFT December 1997 a chain; others may enforce certain relationships between the subjects and issuers of certificates within a chain. CertificateSet ::= SET OF CertificateChoices 9.7 IssuerAndSerialNumber The IssuerAndSerialNumber type identifies a certificate, and thereby an entity and a public key, by the distinguished name of the certificate issuer and an issuer-specific certificate serial number. The definition of Name is imported from X.501, and the definition of SerialNumber is imported from X.509. IssuerAndSerialNumber ::= SEQUENCE { issuer Name, serialNumber SerialNumber } SerialNumber ::= INTEGER 9.8 KeyEncryptionAlgorithmIdentifier The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption algorithm used to encrypt a content-encryption key. The encryption operation maps an octet string (the key) to another octet string (the encrypted key) under control of a key-encryption key. The decryption operation is the inverse of the encryption operation. Context determines which operation is intended. The details of encryption and decryption depend on the key management algorithm used. Key transport, key agreement, and previously distributed symmetric key-encrypting keys are supported. The definition of AlgorithmIdentifier is imported from X.509. KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier 9.9 Version The Version type gives a syntax version number, for compatibility with future revisions of this document. Version ::= INTEGER Housley [Page 19] INTERNET DRAFT December 1997 9.10 UserKeyingMaterial The UserKeyingMaterial type gives a syntax user keying material (UKM). Some key management algorithms require UKMs. The sender provides a UKM for the specific key management algorithm. The definition of AlgorithmIdentifier is imported from X.509. UserKeyingMaterial ::= SEQUENCE { algorithm AlgorithmIdentifier, ukm OCTET STRING } 9.11 UserKeyingMaterials The UserKeyingMaterial type provides a set of user keying materials (UKMs). This allows the sender to provide a UKM for each key management algorithm that requires one. UserKeyingMaterials ::= SET OF UserKeyingMaterial 9.12 OtherKeyAttribute The OtherKeyAttribute type gives a syntax for the inclusion of other key attributes that permit the recipient to select the key used by the sender. The attribute object identifier must be registered along with the syntax of the attribute itself. Use of this structure should be avoided since it may impede interoperability. OtherKeyAttribute ::= SEQUENCE { keyAttrId OBJECT IDENTIFIER, keyAttr ANY DEFINED BY keyAttrId OPTIONAL } 10 Useful Attributes This section defines attributes that may used with signed-data. All of these attributes ware originally defined in PKCS #9, and they are included here for easy reference. The attributes are not listed in any particular order. 10.1 Content Type The content-type attribute type specifies the content type of the ContentInfo value being signed in signed-data. The content-type attribute type is required if there are any authenticated attributes present. The content-type attribute is identified by the following object identifier: Housley [Page 20] INTERNET DRAFT December 1997 id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 } Content-type attribute values have ASN.1 type ContentType: ContentType ::= OBJECT IDENTIFIER A content-type attribute must have a single attribute value. 10.2 Message Digest The message-digest attribute type specifies the message digest of the contents octets of the DER encoding of the content field of the ContentInfo value being signed in signed-data, where the message digest is computed using the signer's message digest algorithm. The message-digest attribute type is required if there are any authenticated attributes present. The message-digest attribute is identified by the following object identifier: id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 } Message-digest attribute values have ASN.1 type MessageDigest: MessageDigest ::= OCTET STRING A message-digest attribute must have a single attribute value. 10.3 Signing Time The signing-time attribute type specifies the time at which the signer (purportedly) performed the signing process. The signing-time attribute type is intended for use in signed-data. The signing-time attribute is identified by the following object identifier: id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 } Signing-time attribute values have ASN.1 type SigningTime: Housley [Page 21] INTERNET DRAFT December 1997 SigningTime ::= Time Time ::= CHOICE { utcTime UTCTime, generalizedTime GeneralizedTime } Note: The definition of Time matches the one specified in the 1997 version of X.509. Dates through the year 2049 must be encoded as UTCTime, and dates in the year 2050 or later must be encoded as GeneralizedTime. A signing-time attribute must have a single attribute value. No requirement is imposed concerning the correctness of the signing time, and acceptance of a purported signing time is a matter of a recipient's discretion. It is expected, however, that some signers, such as time-stamp servers, will be trusted implicitly. 10.4 Countersignature The countersignature attribute type specifies one or more signatures on the contents octets of the DER encoding of the signatureValue field of a SignerInfo value in signed-data. Thus, the countersignature attribute type countersigns (signs in serial) another signature. The countersignature attribute must be an unauthenticated attribute; it cannot be an authenticated attribute. The signing-time attribute is identified by the following object identifier: id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 } Countersignature attribute values have ASN.1 type Countersignature: Countersignature ::= SignerInfo Countersignature values have the same meaning as SignerInfo values for ordinary signatures, except that: 1. The authenticatedAttributes field must contain a message- digest attribute if it contains any other attributes, but need not contain a content-type attribute, as there is no content type for countersignatures. 2. The input to the message-digesting process is the contents octets of the DER encoding of the signatureValue field of the SignerInfo value with which the attribute is associated. Housley [Page 22] INTERNET DRAFT December 1997 A countersignature attribute can have multiple attribute values. The fact that a countersignature is computed on a signature value means that the countersigning process need not know the original content input to the signing process. This has advantages both in efficiency and in confidentiality. A countersignature, since it has type SignerInfo, can itself contain a countersignature attribute. Thus it is possible to construct arbitrarily long series of countersignatures. 11 Supported Algorithms To be supplied. However, this section will list the must implement algorithms and other algorithms that may be implemented. Here are my current thoughts... MUST implement: DSS, SHA-1, Diffie-Hellman (X9.42), and Triple-DES CBC (with three keys). MAY implement: RSA (signature and key management), MD5, KEA, RC2, DES CBC, and SKIPJACK. Housley [Page 23] INTERNET DRAFT December 1997 Appendix A: ASN.1 Module CryptographicMessageSyntax { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) } DEFINITIONS IMPLICIT TAGS ::= BEGIN IMPORTS -- Directory Information Framework (X.501) Attribute, Name FROM InformationFramework { joint-iso-itu-t ds(5) modules(1) informationFramework(1) 3 } -- Directory Authentication Framework (X.509) AlgorithmIdentifier, AttributeCertificate, Certificate, CertificateList, CertificateSerialNumber FROM AuthenticationFramework { joint-iso-itu-t ds(5) module(1) authenticationFramework(7) 3 } ; -- Cryptographic Message Syntax ContentInfo ::= SEQUENCE { contentType ContentType, content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL } ContentType ::= OBJECT IDENTIFIER Data ::= OCTET STRING SignedData ::= SEQUENCE { version Version, digestAlgorithms DigestAlgorithmIdentifiers, contentInfo ContentInfo, certificates [0] IMPLICIT CertificateSet OPTIONAL, crls [1] IMPLICIT CertificateRevocationLists OPTIONAL, signerInfos SignerInfos } DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier Housley [Page 24] INTERNET DRAFT December 1997 SignerInfos ::= SET OF SignerInfo SignerInfo ::= SEQUENCE { version Version, issuerAndSerialNumber IssuerAndSerialNumber, digestAlgorithm DigestAlgorithmIdentifier, authenticatedAttributes [0] IMPLICIT Attributes OPTIONAL, signatureAlgorithm SignatureAlgorithmIdentifier, signature SignatureValue, unauthenticatedAttributes [1] IMPLICIT Attributes OPTIONAL } Attributes ::= SET OF Attribute SignatureValue ::= OCTET STRING EnvelopedData ::= SEQUENCE { version Version, originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL, recipientInfos RecipientInfos, encryptedContentInfo EncryptedContentInfo } OriginatorInfo ::= SEQUENCE { certs [0] IMPLICIT CertificateSet OPTIONAL, crls [1] IMPLICIT CertificateRevocationLists OPTIONAL, ukms [2] IMPLICIT UserKeyingMaterials OPTIONAL } RecipientInfos ::= SET OF RecipientInfo EncryptedContentInfo ::= SEQUENCE { contentType ContentType, contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier, encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL } EncryptedContent ::= OCTET STRING RecipientInfo ::= SEQUENCE { version Version, rid RecipientIdentifier, originatorCert [0] EXPLICIT EntityIdentifier OPTIONAL, keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier, encryptedKey EncryptedKey } RecipientIdentifier ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, rKeyId [0] IMPLICIT RecipientKeyIdentifier, mlKeyId [1] IMPLICIT MailListKeyIdentifier } Housley [Page 25] INTERNET DRAFT December 1997 RecipientKeyIdentifier ::= SEQUENCE { subjectKeyIdentifier SubjectKeyIdentifier, date GeneralizedTime OPTIONAL, other OtherKeyAttribute OPTIONAL } MailListKeyIdentifier ::= SEQUENCE { kekIdentifier OCTET STRING, date GeneralizedTime OPTIONAL, other OtherKeyAttribute OPTIONAL } EntityIdentifier ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, subjectKeyIdentifier SubjectKeyIdentifier } SubjectKeyIdentifier ::= OCTET STRING EncryptedKey ::= OCTET STRING DigestedData ::= SEQUENCE { version Version, digestAlgorithm DigestAlgorithmIdentifier, contentInfo ContentInfo, digest Digest } Digest ::= OCTET STRING EncryptedData ::= SEQUENCE { version Version, encryptedContentInfo EncryptedContentInfo } CertificateRevocationLists ::= SET OF CertificateList ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier DigestAlgorithmIdentifier ::= AlgorithmIdentifier SignatureAlgorithmIdentifier ::= AlgorithmIdentifier CertificateChoices ::= CHOICE { certificate Certificate, -- See X.509 extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete attrCert [1] IMPLICIT AttributeCertificate } -- See X.509 and X9.57 CertificateSet ::= SET OF CertificateChoices IssuerAndSerialNumber ::= SEQUENCE { issuer Name, serialNumber SerialNumber } Housley [Page 26] INTERNET DRAFT December 1997 SerialNumber ::= INTEGER KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier Version ::= INTEGER UserKeyingMaterial ::= SEQUENCE { algorithm AlgorithmIdentifier, ukm OCTET STRING } UserKeyingMaterials ::= SET OF UserKeyingMaterial OtherKeyAttribute ::= SEQUENCE { keyAttrId OBJECT IDENTIFIER, keyAttr ANY DEFINED BY keyAttrId OPTIONAL } -- Attributes MessageDigest ::= OCTET STRING SigningTime ::= Time Time ::= CHOICE { utcTime UTCTime, generalTime GeneralizedTime } Countersignature ::= SignerInfo -- Object Identifiers id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 } id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 } id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 } id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 } id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 } Housley [Page 27] INTERNET DRAFT December 1997 id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 } id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 } id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 } id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 } -- Obsolete Extended Certificate syntax from PKCS#6 ExtendedCertificateOrCertificate ::= CHOICE { certificate Certificate, extendedCertificate [0] IMPLICIT ExtendedCertificate } ExtendedCertificate ::= SEQUENCE { extendedCertificateInfo ExtendedCertificateInfo, signatureAlgorithm SignatureAlgorithmIdentifier, signature Signature } ExtendedCertificateInfo ::= SEQUENCE { version Version, certificate Certificate, attributes Attributes } Signature ::= BIT STRING END -- of CryptographicMessageSyntax Housley [Page 28] INTERNET DRAFT December 1997 References PKCS #6 RSA Laboratories. PKCS #6: Extended-Certificate Syntax Standard. Version 1.5, November 1993. PKCS #7 RSA Laboratories. PKCS #7: Cryptographic Message Syntax Standard. Version 1.5, November 1993. PKCS #9 RSA Laboratories. PKCS #9: Selected Attribute Types. Version 1.1, November 1993. X.208 CCITT. Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1). 1988. X.209 CCITT. Recommendation X.209: Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1). 1988. X.501 CCITT. Recommendation X.501: The Directory - Models. 1988. X.509 CCITT. Recommendation X.509: The Directory - Authentication Framework. 1988. Security Considerations The Cryptographic Message Syntax provides a method for digitally signing data, digesting data, and encrypting data. Implementations must protect the signer's private key. Compromise of the signer's private key permits masquerade. Implementations must protect the key management private key and the content-encryption key. Compromise of the key management private key may result in the disclosure of all messages protected with that key. Similarly, compromise of the content-encryption key may result in disclosure of the encrypted content. Author Address Russell Housley SPYRUS PO Box 1198 Herndon, VA 20172 USA housley@spyrus.com Housley [Page 29]