RFC 2630
Cryptographic Message Syntax
Network Working Group R. Housley Request for Comments: 2630 SPYRUS Category: Standards Track June 1999 Cryptographic Message Syntax Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved.Abstract
This document describes the Cryptographic Message Syntax. This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary messages. The Cryptographic Message Syntax is derived from PKCS #7 version 1.5 as specified in RFC 2315 [PKCS#7]. Wherever possible, backward compatibility is preserved; however, changes were necessary to accommodate attribute certificate transfer and key agreement techniques for key management.
Table of Contents
1 Introduction ................................................. 4 2 General Overview ............................................. 4 3 General Syntax ............................................... 5 4 Data Content Type ............................................ 5 5 Signed-data Content Type ..................................... 6 5.1 SignedData Type ......................................... 7 5.2 EncapsulatedContentInfo Type ............................ 8 5.3 SignerInfo Type ......................................... 9 5.4 Message Digest Calculation Process ...................... 11 5.5 Message Signature Generation Process .................... 12 5.6 Message Signature Verification Process .................. 12 6 Enveloped-data Content Type .................................. 12 6.1 EnvelopedData Type ...................................... 14 6.2 RecipientInfo Type ...................................... 15 6.2.1 KeyTransRecipientInfo Type ....................... 16 6.2.2 KeyAgreeRecipientInfo Type ....................... 17 6.2.3 KEKRecipientInfo Type ............................ 19 6.3 Content-encryption Process .............................. 20 6.4 Key-encryption Process .................................. 20 7 Digested-data Content Type ................................... 21 8 Encrypted-data Content Type .................................. 22 9 Authenticated-data Content Type .............................. 23 9.1 AuthenticatedData Type .................................. 23 9.2 MAC Generation .......................................... 25 9.3 MAC Verification ........................................ 26 10 Useful Types ................................................. 27 10.1 Algorithm Identifier Types ............................. 27 10.1.1 DigestAlgorithmIdentifier ...................... 27 10.1.2 SignatureAlgorithmIdentifier ................... 27 10.1.3 KeyEncryptionAlgorithmIdentifier ............... 28 10.1.4 ContentEncryptionAlgorithmIdentifier ........... 28 10.1.5 MessageAuthenticationCodeAlgorithm ............. 28 10.2 Other Useful Types ..................................... 28 10.2.1 CertificateRevocationLists ..................... 28 10.2.2 CertificateChoices ............................. 29 10.2.3 CertificateSet ................................. 29 10.2.4 IssuerAndSerialNumber .......................... 30 10.2.5 CMSVersion ..................................... 30 10.2.6 UserKeyingMaterial ............................. 30 10.2.7 OtherKeyAttribute .............................. 30
11 Useful Attributes ............................................ 31 11.1 Content Type ........................................... 31 11.2 Message Digest ......................................... 32 11.3 Signing Time ........................................... 32 11.4 Countersignature ....................................... 34 12 Supported Algorithms ......................................... 35 12.1 Digest Algorithms ...................................... 35 12.1.1 SHA-1 .......................................... 35 12.1.2 MD5 ............................................ 35 12.2 Signature Algorithms ................................... 36 12.2.1 DSA ............................................ 36 12.2.2 RSA ............................................ 36 12.3 Key Management Algorithms .............................. 36 12.3.1 Key Agreement Algorithms ....................... 36 12.3.1.1 X9.42 Ephemeral-Static Diffie-Hellman. 37 12.3.2 Key Transport Algorithms ....................... 38 12.3.2.1 RSA .................................. 39 12.3.3 Symmetric Key-Encryption Key Algorithms ........ 39 12.3.3.1 Triple-DES Key Wrap .................. 40 12.3.3.2 RC2 Key Wrap ......................... 41 12.4 Content Encryption Algorithms ........................... 41 12.4.1 Triple-DES CBC .................................. 42 12.4.2 RC2 CBC ......................................... 42 12.5 Message Authentication Code Algorithms .................. 42 12.5.1 HMAC with SHA-1 ................................. 43 12.6 Triple-DES and RC2 Key Wrap Algorithms .................. 43 12.6.1 Key Checksum .................................... 44 12.6.2 Triple-DES Key Wrap ............................. 44 12.6.3 Triple-DES Key Unwrap ........................... 44 12.6.4 RC2 Key Wrap .................................... 45 12.6.5 RC2 Key Unwrap .................................. 46 Appendix A: ASN.1 Module ........................................ 47 References ....................................................... 55 Security Considerations .......................................... 56 Acknowledgments .................................................. 58 Author's Address ................................................. 59 Full Copyright Statement ......................................... 60
1 Introduction
This document describes the Cryptographic Message Syntax. This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary messages. The Cryptographic Message Syntax describes an encapsulation syntax for data protection. It supports digital signatures, message authentication codes, 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 signed 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 [X.208-88], using BER-encoding [X.209-88]. 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 (CMS) is general enough to support many different content types. This document defines one protection content, ContentInfo. ContentInfo encapsulates a single identified content type, and the identified type may provide further encapsulation. This document defines six content types: data, signed-data, enveloped-data, digested-data, encrypted-data, and authenticated-data. Additional content types can be defined outside this document. An implementation that conforms to this specification must implement the protection content, ContentInfo, and must implement the data, signed-data, and enveloped-data content types. The other content types may be implemented if desired. As a general design philosophy, each content type permits 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) [X.509-88] encoding in a single pass since the lengths of the various components may not be known in advance. However, signed attributes within the signed-data content type and authenticated attributes within the authenticated-data content type require DER encoding. Signed attributes and authenticated attributes must be transmitted in DER form to ensure that recipients can verify a content that contains one or more unrecognized attributes. Signed attributes and authenticated attributes are the only CMS data types that require DER encoding.3 General Syntax
The Cryptographic Message Syntax (CMS) associates a content type identifier with a content. The syntax shall have ASN.1 type ContentInfo: ContentInfo ::= SEQUENCE { contentType ContentType, content [0] EXPLICIT ANY DEFINED BY contentType } ContentType ::= OBJECT IDENTIFIER The fields of ContentInfo have the following meanings: contentType indicates the type of the associated content. It is an object identifier; it is a unique string of integers assigned by an authority that defines the content type. content is the associated content. The type of content can be determined uniquely by contentType. Content types for data, signed-data, enveloped-data, digested-data, encrypted-data, and authenticated-data are defined in this document. If additional content types are defined in other documents, the ASN.1 type defined should not be a CHOICE type.4 Data Content Type
The following object identifier identifies the data content type: id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 } The data content type is intended to refer to arbitrary octet strings, such as ASCII text files; the interpretation is left to the application. Such strings need not have any internal structure
(although they could have their own ASN.1 definition or other structure). The data content type is generally encapsulated in the signed-data, enveloped-data, digested-data, encrypted-data, or authenticated-data content type.5 Signed-data Content Type
The signed-data content type consists of a content of any type and zero or more signature values. Any number of signers in parallel can sign any type of content. 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 the signer is signing any information other than the content, the message digest of the content and the other information are digested with the signer's message digest algorithm (see Section 5.4), 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 5.3. 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 to verify the signature value. The signer's public key is referenced either by an issuer distinguished name along with an issuer-specific serial number or by a subject key identifier that uniquely identifies the certificate containing the public key. The signer's certificate may be included in the SignedData certificates field.
This section is divided into six parts. The first part describes the top-level type SignedData, the second part describes EncapsulatedContentInfo, the third part describes the per-signer information type SignerInfo, and the fourth, fifth, and sixth parts describe the message digest calculation, signature generation, and signature verification processes, respectively.5.1 SignedData Type
The following object identifier identifies the signed-data content type: 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 CMSVersion, digestAlgorithms DigestAlgorithmIdentifiers, encapContentInfo EncapsulatedContentInfo, 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, the encapsulated content type is id-data, and all of the elements of SignerInfos are version 1, then the value of version shall be 1. Alternatively, if attribute certificates are present, the encapsulated content type is other than id-data, or any of the elements of SignerInfos are version 3, 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 5.4.
encapContentInfo is the signed content, consisting of a content
type identifier and the content itself. Details of the
EncapsulatedContentInfo type are discussed in section 5.2.
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-
level certification authorities. There may also be 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). As discussed above, 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. The
details of the SignerInfo type are discussed in section 5.3.
5.2 EncapsulatedContentInfo Type
The content is represented in the type EncapsulatedContentInfo:
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
ContentType ::= OBJECT IDENTIFIER
The fields of type EncapsulatedContentInfo have the following
meanings:
eContentType is an object identifier that uniquely specifies the
content type.
eContent is the content itself, carried as an octet string. The
eContent need not be DER encoded.
The optional omission of the eContent within the EncapsulatedContentInfo field makes it possible to construct "external signatures." In the case of external signatures, the content being signed is absent from the EncapsulatedContentInfo value included in the signed-data content type. If the eContent value within EncapsulatedContentInfo is absent, then the signatureValue is calculated and the eContentType is assigned as though the eContent value was present. In the degenerate case where there are no signers, the EncapsulatedContentInfo value being "signed" is irrelevant. In this case, the content type within the EncapsulatedContentInfo value being "signed" should be id-data (as defined in section 4), and the content field of the EncapsulatedContentInfo value should be omitted.5.3 SignerInfo Type
Per-signer information is represented in the type SignerInfo: SignerInfo ::= SEQUENCE { version CMSVersion, sid SignerIdentifier, digestAlgorithm DigestAlgorithmIdentifier, signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL, signatureAlgorithm SignatureAlgorithmIdentifier, signature SignatureValue, unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL } SignerIdentifier ::= CHOICE { issuerAndSerialNumber IssuerAndSerialNumber, subjectKeyIdentifier [0] SubjectKeyIdentifier } SignedAttributes ::= SET SIZE (1..MAX) OF Attribute UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute Attribute ::= SEQUENCE { attrType OBJECT IDENTIFIER, attrValues SET OF AttributeValue } AttributeValue ::= ANY SignatureValue ::= OCTET STRING The fields of type SignerInfo have the following meanings: version is the syntax version number. If the SignerIdentifier is the CHOICE issuerAndSerialNumber, then the version shall be 1. If
the SignerIdentifier is subjectKeyIdentifier, then the version
shall be 3.
sid specifies the signer's certificate (and thereby the signer's
public key). The signer's public key is needed by the recipient
to verify the signature. SignerIdentifier provides two
alternatives for specifying the signer's public key. The
issuerAndSerialNumber alternative identifies the signer's
certificate by the issuer's distinguished name and the certificate
serial number; the subjectKeyIdentifier identifies the signer's
certificate by the X.509 subjectKeyIdentifier extension value.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used by the signer. The message digest is
computed on either the content being signed or the content
together with the signed attributes using the process described in
section 5.4. The message digest algorithm should be among those
listed in the digestAlgorithms field of the associated SignerData.
signedAttributes is a collection of attributes that are signed.
The field is optional, but it must be present if the content type
of the EncapsulatedContentInfo value being signed is not id-data.
Each SignedAttribute in the SET must be DER encoded. Useful
attribute types, such as signing time, are defined in Section 11.
If the field is present, it must contain, at a minimum, the
following two attributes:
A content-type attribute having as its value the content type
of the EncapsulatedContentInfo value being signed. Section
11.1 defines the content-type attribute. The content-type
attribute is not required when used as part of a
countersignature unsigned attribute as defined in section 11.4.
A message-digest attribute, having as its value the message
digest of the content. Section 11.2 defines the message-digest
attribute.
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.
unsignedAttributes is a collection of attributes that are not
signed. The field is optional. Useful attribute types, such as
countersignatures, are defined in Section 11.
The fields of type SignedAttribute and UnsignedAttribute have the
following meanings:
attrType indicates the type of attribute. It is an object
identifier.
attrValues is a set of values that comprise the attribute. The
type of each value in the set can be determined uniquely by
attrType.
5.4 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
signed attributes. In either case, the initial input to the message
digest calculation process is the "value" of the encapsulated content
being signed. Specifically, the initial input is the
encapContentInfo eContent OCTET STRING to which the signing process
is applied. Only the octets comprising the value of the eContent
OCTET STRING are input to the message digest algorithm, not the tag
or the length octets.
The result of the message digest calculation process depends on
whether the signedAttributes 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
SignedAttributes value contained in the signedAttributes field.
Since the SignedAttributes value, when present, must contain the
content type and the content message digest attributes, those values
are indirectly included in the result. The content type attribute is
not required when used as part of a countersignature unsigned
attribute as defined in section 11.4. A separate encoding of the
signedAttributes field is performed for message digest calculation.
The IMPLICIT [0] tag in the signedAttributes 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 content octets of the SignedAttributes value.
When the signedAttributes field is absent, then only the octets
comprising the value of the signedData encapContentInfo eContent
OCTET STRING (e.g., the contents of a file) are 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 signature
generation process.
Although the encapContentInfo eContent OCTET STRING tag and 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.5.5 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.5.6 Message Signature Verification Process
The input to the signature verification process includes the result of the message digest calculation process and the signer's public key. The recipient may obtain the correct public key for the signer by any means, but the preferred method is from a certificate obtained from the SignedData certificates field. The selection and validation of the signer's public key may be based on certification path validation (see [PROFILE]) as well as other external context, but is beyond the scope of this document. The details of the signature verification 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 signedAttributes, then the content message digest must be calculated as described in section 5.4. 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 signedAttributes 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 an arbitrary number of recipients using any of the three key management techniques for each recipient.
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
symmetric key-encryption keys: the content-encryption key is
encrypted in a previously distributed symmetric key-encryption
key.
3. For 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 to form an EnvelopedData value
as defined in Section 6.1.
A recipient opens the digital envelope by decrypting one of the
encrypted content-encryption keys and then 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 following object identifier identifies the enveloped-data content type: 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 CMSVersion, originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL, recipientInfos RecipientInfos, encryptedContentInfo EncryptedContentInfo, unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL } OriginatorInfo ::= SEQUENCE { certs [0] IMPLICIT CertificateSet OPTIONAL, crls [1] IMPLICIT CertificateRevocationLists OPTIONAL } RecipientInfos ::= SET OF RecipientInfo EncryptedContentInfo ::= SEQUENCE { contentType ContentType, contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier, encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL } EncryptedContent ::= OCTET STRING UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute 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 other than 0, then the version shall be 2. If unprotectedAttrs is present, then version shall be 2. If originatorInfo is absent, all of the RecipientInfo structures are version 0, and unprotectedAttrs is absent, 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 and CRLs: certs is a collection of certificates. certs may contain originator certificates associated with several different key
management algorithms. certs may also contain attribute
certificates associated with the originator. 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.
recipientInfos is a collection of per-recipient information.
There must be at least one element in the collection.
encryptedContentInfo is the encrypted content information.
unprotectedAttrs is a collection of attributes that are not
encrypted. The field is optional. Useful attribute types are
defined in Section 11.
The fields of type EncryptedContentInfo have the following meanings:
contentType indicates the type of content.
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 content-encryption algorithm and content-encryption
key 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 has a different format for the three key management
techniques that are supported: key transport, key agreement, and
previously distributed symmetric key-encryption keys. Any of the
three key management techniques can be used for each recipient of the
same encrypted content. In all cases, the content-encryption key is
transferred to one or more recipient in encrypted form.
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo }
EncryptedKey ::= OCTET STRING
6.2.1 KeyTransRecipientInfo Type
Per-recipient information using key transport is represented in the
type KeyTransRecipientInfo. Each instance of KeyTransRecipientInfo
transfers the content-encryption key to one recipient.
KeyTransRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0 or 2
rid RecipientIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
RecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
The fields of type KeyTransRecipientInfo have the following meanings:
version is the syntax version number. If the RecipientIdentifier
is the CHOICE issuerAndSerialNumber, then the version shall be 0.
If the RecipientIdentifier is subjectKeyIdentifier, 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. The
RecipientIdentifier provides two alternatives for specifying the
recipient's certificate, and thereby the recipient's public key.
The recipient's certificate must contain a key transport public
key. The content-encryption key is encrypted with the recipient's
public key. The issuerAndSerialNumber alternative identifies the
recipient's certificate by the issuer's distinguished name and the
certificate serial number; the subjectKeyIdentifier identifies the
recipient's certificate by the X.509 subjectKeyIdentifier
extension value.
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.
6.2.2 KeyAgreeRecipientInfo Type
Recipient information using key agreement is represented in the type
KeyAgreeRecipientInfo. Each instance of KeyAgreeRecipientInfo will
transfer the content-encryption key to one or more recipient that
uses the same key agreement algorithm and domain parameters for that
algorithm.
KeyAgreeRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 3
originator [0] EXPLICIT OriginatorIdentifierOrKey,
ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
recipientEncryptedKeys RecipientEncryptedKeys }
OriginatorIdentifierOrKey ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier,
originatorKey [1] OriginatorPublicKey }
OriginatorPublicKey ::= SEQUENCE {
algorithm AlgorithmIdentifier,
publicKey BIT STRING }
RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey
RecipientEncryptedKey ::= SEQUENCE {
rid KeyAgreeRecipientIdentifier,
encryptedKey EncryptedKey }
KeyAgreeRecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
rKeyId [0] IMPLICIT RecipientKeyIdentifier }
RecipientKeyIdentifier ::= SEQUENCE {
subjectKeyIdentifier SubjectKeyIdentifier,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
SubjectKeyIdentifier ::= OCTET STRING
The fields of type KeyAgreeRecipientInfo have the following meanings:
version is the syntax version number. It shall always be 3.
originator is a CHOICE with three alternatives specifying the
sender's key agreement public key. The sender uses the
corresponding private key and the recipient's public key to
generate a pairwise key. The content-encryption key is encrypted
in the pairwise key. The issuerAndSerialNumber alternative
identifies the sender's certificate, and thereby the sender's
public key, by the issuer's distinguished name and the certificate
serial number. The subjectKeyIdentifier alternative identifies
the sender's certificate, and thereby the sender's public key, by
the X.509 subjectKeyIdentifier extension value. The originatorKey
alternative includes the algorithm identifier and sender's key
agreement public key. Permitting originator anonymity since the
public key is not certified.
ukm is optional. With some key agreement algorithms, the sender
provides a User Keying Material (UKM) to ensure that a different
key is generated each time the same two parties generate a
pairwise key.
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content-
encryption key in the key-encryption key. The key-encryption
process is described in Section 6.4.
recipientEncryptedKeys includes a recipient identifier and
encrypted key for one or more recipients. The
KeyAgreeRecipientIdentifier is a CHOICE with two alternatives
specifying the recipient's certificate, and thereby the
recipient's public key, that was used by the sender to generate a
pairwise key-encryption key. The recipient's certificate must
contain a key agreement public key. The content-encryption key is
encrypted in the pairwise key-encryption key. The
issuerAndSerialNumber alternative identifies the recipient's
certificate by the issuer's distinguished name and the certificate
serial number; the RecipientKeyIdentifier is described below. The
encryptedKey is the result of encrypting the content-encryption
key in the pairwise key-encryption key generated using the key
agreement algorithm.
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 previously distributed UKMs was used by the sender.
other is optional. When present, this field contains additional
information used by the recipient to locate the public keying
material used by the sender.
6.2.3 KEKRecipientInfo Type
Recipient information using previously distributed symmetric keys is
represented in the type KEKRecipientInfo. Each instance of
KEKRecipientInfo will transfer the content-encryption key to one or
more recipients who have the previously distributed key-encryption
key.
KEKRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 4
kekid KEKIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KEKIdentifier ::= SEQUENCE {
keyIdentifier OCTET STRING,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
The fields of type KEKRecipientInfo have the following meanings:
version is the syntax version number. It shall always be 4.
kekid specifies a symmetric key-encryption key that was previously
distributed to the sender and one or more recipients.
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content-
encryption key with the key-encryption key. The key-encryption
process is described in Section 6.4.
encryptedKey is the result of encrypting the content-encryption
key in the key-encryption key.
The fields of type KEKIdentifier have the following meanings:
keyIdentifier identifies the key-encryption key that was
previously distributed to the sender and one or more recipients.
date is optional. When present, the date specifies a single key-
encryption key from a set that was previously distributed.
other is optional. When present, this field contains additional
information used by the recipient to determine the key-encryption
key used by the sender.
6.3 Content-encryption Process
The content-encryption key for the desired content-encryption
algorithm is randomly generated. The data to be protected is padded
as described below, then the padded data is encrypted using the
content-encryption key. The encryption operation maps an arbitrary
string of octets (the data) to another string of octets (the
ciphertext) under control of a content-encryption key. The encrypted
data is included in the envelopedData encryptedContentInfo
encryptedContent OCTET STRING.
The input to the content-encryption process is the "value" of the
content being enveloped. Only the value octets of the envelopedData
encryptedContentInfo encryptedContent OCTET STRING are encrypted; the
OCTET STRING tag and length octets are not encrypted.
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-(lth mod k) octets all having value k-(lth mod k), where lth is
the length of the input. In other words, the input is padded at
the trailing end with one of the following strings:
01 -- if lth mod k = k-1
02 02 -- if lth mod k = k-2
.
.
.
k k ... k k -- if lth 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.
Any of the three key management techniques can be used for each
recipient of the same encrypted content.
7 Digested-data Content Type
The digested-data content type consists of content of any type and a message digest of the content. Typically, the digested-data content type is used to provide content integrity, and the result generally becomes an input to the enveloped-data content type. The following steps construct digested-data: 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 following object identifier identifies the digested-data content type: id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 } The digested-data content type shall have ASN.1 type DigestedData: DigestedData ::= SEQUENCE { version CMSVersion, digestAlgorithm DigestAlgorithmIdentifier, encapContentInfo EncapsulatedContentInfo, digest Digest } Digest ::= OCTET STRING The fields of type DigestedData have the following meanings: version is the syntax version number. If the encapsulated content type is id-data, then the value of version shall be 0; however, if the encapsulated content type is other than id-data, then the value of version shall be 2. 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 5.4 in the case when there are no signed attributes.
encapContentInfo is the content that is digested, as defined in
section 5.2.
digest is the result of the message-digesting process.
The ordering of the digestAlgorithm field, the encapContentInfo
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 must 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 following object identifier identifies the encrypted-data content
type:
id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }
The encrypted-data content type shall have ASN.1 type EncryptedData:
EncryptedData ::= SEQUENCE {
version CMSVersion,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
The fields of type EncryptedData have the following meanings:
version is the syntax version number. If unprotectedAttrs is
present, then version shall be 2. If unprotectedAttrs is absent,
then version shall be 0.
encryptedContentInfo is the encrypted content information, as
defined in Section 6.1.
unprotectedAttrs is a collection of attributes that are not
encrypted. The field is optional. Useful attribute types are
defined in Section 11.
9 Authenticated-data Content Type
The authenticated-data content type consists of content of any type, a message authentication code (MAC), and encrypted authentication keys for one or more recipients. The combination of the MAC and one encrypted authentication key for a recipient is necessary for that recipient to verify the integrity of the content. Any type of content can be integrity protected for an arbitrary number of recipients. The process by which authenticated-data is constructed involves the following steps: 1. A message-authentication key for a particular message- authentication algorithm is generated at random. 2. The message-authentication key is encrypted for each recipient. The details of this encryption depend on the key management algorithm used. 3. For each recipient, the encrypted message-authentication key and other recipient-specific information are collected into a RecipientInfo value, defined in Section 6.2. 4. Using the message-authentication key, the originator computes a MAC value on the content. If the originator is authenticating any information in addition to the content (see Section 9.2), a message digest is calculated on the content, the message digest of the content and the other information are authenticated using the message-authentication key, and the result becomes the "MAC value."9.1 AuthenticatedData Type
The following object identifier identifies the authenticated-data content type: id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 2 }
The authenticated-data content type shall have ASN.1 type
AuthenticatedData:
AuthenticatedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
macAlgorithm MessageAuthenticationCodeAlgorithm,
digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
encapContentInfo EncapsulatedContentInfo,
authenticatedAttributes [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthenticatedAttributes [3] IMPLICIT UnauthAttributes OPTIONAL }
AuthAttributes ::= SET SIZE (1..MAX) OF Attribute
UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute
MessageAuthenticationCode ::= OCTET STRING
The fields of type AuthenticatedData have the following meanings:
version is the syntax version number. It 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, attribute certificates,
and CRLs, as defined in Section 6.1.
recipientInfos is a collection of per-recipient information, as
defined in Section 6.1. There must be at least one element in the
collection.
macAlgorithm is a message authentication code (MAC) algorithm
identifier. It identifies the MAC algorithm, along with any
associated parameters, used by the originator. Placement of the
macAlgorithm field facilitates one-pass processing by the
recipient.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used to compute a message digest on the
encapsulated content if authenticated attributes are present. The
message digesting process is described in Section 9.2. Placement
of the digestAlgorithm field facilitates one-pass processing by
the recipient. If the digestAlgorithm field is present, then the
authenticatedAttributes field must also be present.
encapContentInfo is the content that is authenticated, as defined
in section 5.2.
authenticatedAttributes is a collection of authenticated
attributes. The authenticatedAttributes structure is optional,
but it must be present if the content type of the
EncapsulatedContentInfo value being authenticated is not id-data.
If the authenticatedAttributes field is present, then the
digestAlgorithm field must also be present. Each
AuthenticatedAttribute in the SET must be DER encoded. Useful
attribute types are defined in Section 11. If the
authenticatedAttributes field is present, it must contain, at a
minimum, the following two attributes:
A content-type attribute having as its value the content type
of the EncapsulatedContentInfo value being authenticated.
Section 11.1 defines the content-type attribute.
A message-digest attribute, having as its value the message
digest of the content. Section 11.2 defines the message-digest
attribute.
mac is the message authentication code.
unauthenticatedAttributes is a collection of attributes that are
not authenticated. The field is optional. To date, no attributes
have been defined for use as unauthenticated attributes, but other
useful attribute types are defined in Section 11.
9.2 MAC Generation
The MAC calculation process computes a message authentication code
(MAC) on either the message being authenticated or a message digest
of message being authenticated together with the originator's
authenticated attributes.
If authenticatedAttributes field is absent, the input to the MAC
calculation process is the value of the encapContentInfo eContent
OCTET STRING. Only the octets comprising the value of the eContent
OCTET STRING are input to the MAC algorithm; the tag and the length
octets are omitted. This has the advantage that the length of the
content being authenticated need not be known in advance of the MAC
generation process.
If authenticatedAttributes field is present, the content-type
attribute (as described in Section 11.1) and the message-digest
attribute (as described in section 11.2) must be included, and the
input to the MAC calculation process is the DER encoding of
authenticatedAttributes. A separate encoding of the authenticatedAttributes field is performed for message digest calculation. The IMPLICIT [2] 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 [2] tag, is to be included in the message digest calculation along with the length and content octets of the authenticatedAttributes value. The message digest calculation process computes a message digest on the content being authenticated. The initial input to the message digest calculation process is the "value" of the encapsulated content being authenticated. Specifically, the input is the encapContentInfo eContent OCTET STRING to which the authentication process is applied. Only the octets comprising the value of the encapContentInfo eContent OCTET STRING are input to the message digest algorithm, not the tag or the length octets. This has the advantage that the length of the content being authenticated need not be known in advance. Although the encapContentInfo eContent OCTET STRING tag and 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 input to the MAC calculation process includes the MAC input data, defined above, and an authentication key conveyed in a recipientInfo structure. The details of MAC calculation depend on the MAC algorithm employed (e.g., HMAC). The object identifier, along with any parameters, that specifies the MAC algorithm employed by the originator is carried in the macAlgorithm field. The MAC value generated by the originator is encoded as an OCTET STRING and carried in the mac field.9.3 MAC Verification
The input to the MAC verification process includes the input data (determined based on the presence or absence of the authenticatedAttributes field, as defined in 9.2), and the authentication key conveyed in recipientInfo. The details of the MAC verification process depend on the MAC algorithm employed. The recipient may not rely on any MAC values or message digest values computed by the originator. The content is authenticated as described in section 9.2. If the originator includes authenticated attributes, then the content of the authenticatedAttributes is authenticated as described in section 9.2. For authentication to succeed, the message MAC value calculated by the recipient must be
the same as the value of the mac field. Similarly, for authentication to succeed when the authenticatedAttributes field is present, the content message digest value calculated by the recipient must be the same as the message digest value included in the authenticatedAttributes message-digest attribute.10 Useful Types
This section is divided into two parts. The first part defines algorithm identifiers, and the second part defines other useful types.10.1 Algorithm Identifier Types
All of the algorithm identifiers have the same type: AlgorithmIdentifier. The definition of AlgorithmIdentifier is imported from X.509 [X.509-88]. There are many alternatives for each type of algorithm listed. For each of these five types, Section 12 lists the algorithms that must be included in a CMS implementation.10.1.1 DigestAlgorithmIdentifier
The DigestAlgorithmIdentifier type identifies a message-digest algorithm. Examples include SHA-1, MD2, and MD5. A message-digest algorithm maps an octet string (the message) to another octet string (the message digest). DigestAlgorithmIdentifier ::= AlgorithmIdentifier10.1.2 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. SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.3 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. KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier10.1.4 ContentEncryptionAlgorithmIdentifier
The ContentEncryptionAlgorithmIdentifier type identifies a content- encryption algorithm. Examples include Triple-DES and RC2. 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. ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier10.1.5 MessageAuthenticationCodeAlgorithm
The MessageAuthenticationCodeAlgorithm type identifies a message authentication code (MAC) algorithm. Examples include DES-MAC and HMAC. A MAC algorithm supports generation and verification operations. The MAC generation and verification operations use the same symmetric key. Context determines which operation is intended. MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier
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