RFC3369 - Cryptographic Message Syntax (CMS)

Network Working Group R. Housley
Request for Comments: 3369 RSA Laboratories
Obsoletes: 2630, 3211 August 2002
Category: Standards Track
Cryptographic Message Syntax (CMS)
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 (2002). All Rights Reserved.
Abstract
This document describes the Cryptographic Message Syntax (CMS). This
syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary message content.
Table of Contents
1. IntrodUCtion ................................................ 3
1.1 Changes Since RFC2630 ...................................... 3
1.2 Terminology ................................................. 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 ................................ 9
5.2.1 Compatibility with PKCS #7 ................................ 9
5.3 SignerInfo Type ............................................. 11
5.4 Message Digest Calculation Process .......................... 13
5.5 Signature Generation Process ................................ 14
5.6 Signature Verification Process .............................. 14
6. Enveloped-data Content Type ................................. 14
6.1 EnvelopedData Type .......................................... 16
6.2 RecipientInfo Type .......................................... 18
6.2.1 KeyTransRecipientInfo Type ................................ 19
6.2.2 KeyAgreeRecipientInfo Type ................................ 20
6.2.3 KEKRecipientInfo Type ..................................... 22
6.2.4 PassWordRecipientInfo Type ................................ 23
6.2.5 OtherRecipientInfo Type ................................... 24
6.3 Content-encryption Process .................................. 24
6.4 Key-encryption Process ...................................... 25
7. Digested-data Content Type .................................. 25
8. Encrypted-data Content Type ................................. 26
9. Authenticated-data Content Type ............................. 27
9.1 AuthenticatedData Type ...................................... 28
9.2 MAC Generation .............................................. 29
9.3 MAC Verification ............................................ 31
10. Useful Types ................................................ 31
10.1 Algorithm Identifier Types ................................. 31
10.1.1 DigestAlgorithmIdentifier ................................ 31
10.1.2 SignatureAlgorithmIdentifier ............................. 32
10.1.3 KeyEncryptionAlgorithmIdentifier ......................... 32
10.1.4 ContentEncryptionAlgorithmIdentifier ..................... 32
10.1.5 MessageAuthenticationCodeAlgorithm ....................... 32
10.1.6 KeyDerivationAlgorithmIdentifier ......................... 33
10.2 Other Useful Types ......................................... 33
10.2.1 CertificateRevocationLists ............................... 33
10.2.2 CertificateChoices ....................................... 33
10.2.3 CertificateSet ........................................... 34
10.2.4 IssuerAndSerialNumber .................................... 34
10.2.5 CMSVersion ............................................... 35
10.2.6 UserKeyingMaterial ....................................... 35
10.2.7 OtherKeyAttribute ........................................ 35
11. Useful Attributes ........................................... 35
11.1 Content Type ............................................... 36
11.2 Message Digest ............................................. 36
11.3 Signing Time ............................................... 37
11.4 Countersignature ........................................... 39
12. ASN.1 Modules ............................................... 40
12.1 CMS ASN.1 Module ........................................... 40
12.2 Version 1 Attribute Certificate ASN.1 Module ............... 46
13. References .................................................. 47
14. Security Considerations ..................................... 48
15. Acknowledgments ............................................. 50
16. Author Address .............................................. 50
17. Full Copyright Statement .................................... 51
1. Introduction
This document describes the Cryptographic Message Syntax (CMS). This
syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary message content.
The CMS describes an encapsulation syntax for data protection. It
supports digital signatures and encryption. The syntax allows
multiple encapsulations; 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 CMS can support a variety of architectures for certificate-based
key management, such as the one defined by the PKIX working group
[PROFILE].
The CMS 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.
The CMS is derived from PKCS #7 version 1.5 as specified in RFC2315
[PKCS#7]. Wherever possible, backward compatibility is preserved;
however, changes were necessary to accommodate version 1 attribute
certificate transfer, key agreement and symmetric key-encryption key
techniques for key management.
1.1 Changes Since RFC2630
This document obsoletes RFC2630 [OLDCMS] and RFC3211 [PWRI].
Password-based key management is included in the CMS specification,
and an extension mechanism to support new key management schemes
without further changes to the CMS is specified. Backward
compatibility with RFC2630 and RFC3211 is preserved; however,
version 2 attribute certificate transfer is added. The use of
version 1 attribute certificates is deprecated.
S/MIME v2 signatures [OLDMSG], which are based on PKCS#7 version 1.5,
are compatible with S/MIME v3 signatures [MSG], which are based on
RFC2630. However, there are some suBTle compatibility issues with
signatures using PKCS#7 version 1.5 and the CMS. These issues are
discussed in section 5.2.1.
Specific cryptographic algorithms are not discussed in this document,
but they were discussed in RFC2630. The discussion of specific
cryptographic algorithms has been moved to a separate document
[CMSALG]. Separation of the protocol and algorithm specifications
allows the IETF to update each document independently. This
specification does not require the implementation of any particular
algorithms. Rather, protocols that rely on the CMS are eXPected to
choose appropriate algorithms for their environment. The algorithms
may be selected from [CMSALG] or elsewhere.
1.2 Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
described in [STDWORDS].
2 General Overview
The 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.
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 need to 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 data types used
in the CMS that require DER encoding.
3 General Syntax
The following object identifier identifies the content information
type:
id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 6 }
The CMS associates a content type identifier with a content. The
syntax MUST 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).
S/MIME uses id-data to identify MIME encoded content. The use of
this content identifier is specified in RFC2311 for S/MIME v2
[OLDMSG] and RFC2633 for S/MIME v3 [MSG].
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 can 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. The appropriate value
depends on certificates, eContentType, and SignerInfo. The
version MUST be assigned as follows:
IF (certificates is present) AND
(any version 2 attribute certificates are present)
THEN version MUST be 4
ELSE
IF ((certificates is present) AND
(any version 1 attribute certificates are present)) OR
(encapContentInfo eContentType is other than id-data) OR
(any SignerInfo structures are version 3)
THEN version MUST be 3
ELSE version MUST be 1
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.
Implementations MAY fail to validate signatures that use a digest
algorithm that is not included in this set. 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). The signer"s certificate
MAY be included. The use of version 1 attribute certificates is
strongly discouraged.
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.
Since each signer can employ a digital signature technique and
future specifications could update the syntax, all implementations
MUST gracefully handle unimplemented versions of SignerInfo.
Further, since all implementations will not support every possible
signature algorithm, all implementations MUST gracefully handle
unimplemented signature algorithms when they are encountered.
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. The object identifier
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" MUST be id-data (as defined in section 4), and the content
field of the EncapsulatedContentInfo value MUST be omitted.
5.2.1 Compatibility with PKCS #7
This section contains a word of warning to implementers that wish to
support both the CMS and PKCS #7 [PKCS#7] SignedData content types.
Both the CMS and PKCS #7 identify the type of the encapsulated
content with an object identifier, but the ASN.1 type of the content
itself is variable in PKCS #7 SignedData content type.
PKCS #7 defines content as:
content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL
The CMS defines eContent as:
eContent [0] EXPLICIT OCTET STRING OPTIONAL
The CMS definition is much easier to use in most applications, and it
is compatible with both S/MIME v2 and S/MIME v3. S/MIME signed
messages using the CMS and PKCS #7 are compatible because identical
signed message formats are specified in RFC2311 for S/MIME v2
[OLDMSG] and RFC2633 for S/MIME v3 [MSG]. S/MIME v2 encapsulates
the MIME content in a Data type (that is, an OCTET STRING) carried in
the SignedData contentInfo content ANY field, and S/MIME v3 carries
the MIME content in the SignedData encapContentInfo eContent OCTET
STRING. Therefore, in both S/MIME v2 and S/MIME v3, the MIME content
is placed in an OCTET STRING and the message digest is computed over
the identical portions of the content. That is, the message digest
is computed over the octets comprising the value of the OCTET STRING,
neither the tag nor length octets are included.
There are incompatibilities between the CMS and PKCS #7 signedData
types when the encapsulated content is not formatted using the Data
type. For example, when an RFC2634 [ESS] signed receipt is
encapsulated in the CMS signedData type, then the Receipt SEQUENCE is
encoded in the signedData encapContentInfo eContent OCTET STRING and
the message digest is computed using the entire Receipt SEQUENCE
encoding (including tag, length and value octets). However, if an
RFC2634 signed receipt is encapsulated in the PKCS #7 signedData
type, then the Receipt SEQUENCE is DER encoded [X.509-88] in the
SignedData contentInfo content ANY field (a SEQUENCE, not an OCTET
STRING). Therefore, the message digest is computed using only the
value octets of the Receipt SEQUENCE encoding.
The following strategy can be used to achieve backward compatibility
with PKCS #7 when processing SignedData content types. If the
implementation is unable to ASN.1 decode the signedData type using
the CMS signedData encapContentInfo eContent OCTET STRING syntax,
then the implementation MAY attempt to decode the signedData type
using the PKCS #7 SignedData contentInfo content ANY syntax and
compute the message digest accordingly.
The following strategy can be used to achieve backward compatibility
with PKCS #7 when creating a SignedData content type in which the
encapsulated content is not formatted using the Data type.
Implementations MAY examine the value of the eContentType, and then
adjust the expected DER encoding of eContent based on the object
identifier value. For example, to support Microsoft AuthentiCode,
the following information MAY be included:
eContentType Object Identifier is set to { 1 3 6 1 4 1 311 2 1 4 }
eContent contains DER encoded AuthentiCode signing information
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 MUST be 1. If
the SignerIdentifier is subjectKeyIdentifier, then the version
MUST 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.
Implementations MUST support the reception of the
issuerAndSerialNumber and subjectKeyIdentifier forms of
SignerIdentifier. When generating a SignerIdentifier,
implementations MAY support one of the forms (either
issuerAndSerialNumber or subjectKeyIdentifier) and always use it,
or implementations MAY arbitrarily mix the two forms.
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.
Implementations MAY fail to validate signatures that use a digest
algorithm that is not included in the SignedData digestAlgorithms
set.
signedAttrs 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.
SignedAttributes MUST be DER encoded, even if the rest of the
structure is BER 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. However, the
content-type attribute MUST NOT be 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. The details of the
signature depend on the signature algorithm employed.
unsignedAttrs 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. The attrType can impose restrictions on the number of
items in the set.
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 signedAttrs 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 SignedAttrs value
contained in the signedAttrs field. Since the SignedAttrs value,
when present, must contain the content-type and the message-digest
attributes, those values are indirectly included in the result. The
content-type attribute MUST NOT be included in a countersignature
unsigned attribute as defined in section 11.4. A separate encoding
of the signedAttrs field is performed for message digest calculation.
The IMPLICIT [0] tag in the signedAttrs is not used for the DER
encoding, rather an EXPLICIT SET OF tag is used. That is, the DER
encoding of the EXPLICIT SET OF tag, rather than of the IMPLICIT [0]
tag, MUST be included in the message digest calculation along with
the length and content octets of the SignedAttributes value.
When the signedAttrs field is absent, 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
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 message contents of any length
that have the same message digest.
5.5 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 MUST be encoded as an OCTET STRING and
carried in the signature field.
5.6 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 MUST 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.
If the SignedData signerInfo includes signedAttributes, then the
content-type attribute value MUST match the SignedData
encapContentInfo eContentType value.
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 four 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;
symmetric key-encryption keys: the content-encryption key is
encrypted in a previously distributed symmetric key-encryption
key; and
passwords: the content-encryption key is encrypted in a key-
encryption key that is derived from a password or other shared
secret value.
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 SIZE (1..MAX) 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. The appropriate value
depends on originatorInfo, RecipientInfo, and unprotectedAttrs.
The version MUST be assigned as follows:
IF ((originatorInfo is present) AND
(any version 2 attribute certificates are present)) OR
(any RecipientInfo structures include pwri) OR
(any RecipientInfo structures include ori)
THEN version is 3
ELSE
IF (originatorInfo is present) OR
(unprotectedAttrs is present) OR
(any RecipientInfo structures are a version other than 0)
THEN version is 2
ELSE version is 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 for all
recipients to build certification paths from a recognized
"root" or "top-level certification authority." However, certs
may contain more certificates than necessary, and there may be
certificates sufficient to make certification paths 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 are 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 each of the supported key
management techniques. Any of the key management techniques can be
used for each recipient of the same encrypted content. In all cases,
the encrypted content-encryption key is transferred to one or more
recipients.
Since all implementations will not support every possible key
management algorithm, all implementations MUST gracefully handle
unimplemented algorithms when they are encountered. For example, if
a recipient receives a content-encryption key encrypted in their RSA
public key using RSA-OAEP and the implementation only supports RSA
PKCS #1 v1.5, then a graceful failure must be implemented.
Implementations MUST support key transport, key agreement, and
previously distributed symmetric key-encryption keys, as represented
by ktri, kari, and kekri, respectively. Implementations MAY support
the password-based key management as represented by pwri.
Implementations MAY support any other key management technique as
represented by ori. Since each recipient can employ a different key
management technique and future specifications could define
additional key management techniques, all implementations MUST
gracefully handle unimplemented alternatives within the RecipientInfo
CHOICE, all implementations MUST gracefully handle unimplemented
versions of otherwise supported alternatives within the RecipientInfo
CHOICE, and all implementations MUST gracefully handle unimplemented
or unknown ori alternatives.
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo,
pwri [3] PasswordRecipientinfo,
ori [4] OtherRecipientInfo }
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 MUST be 0.
If the RecipientIdentifier is subjectKeyIdentifier, then the
version MUST 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. Therefore, a recipient X.509 version 3 certificate that
contains a key usage extension MUST assert the keyEncipherment
bit. 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. For recipient processing, implementations MUST
support both of these alternatives for specifying the recipient"s
certificate; and for sender processing, implementations MUST
support at least one of these alternatives.
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 recipients that
use 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 MUST 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. This alternative permits originator
anonymity since the public key is not certified. Implementations
MUST support all three alternatives for specifying the sender"s
public key.
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. Implementations MUST support recipient processing
of a KeyAgreeRecipientInfo SEQUENCE that includes a ukm field.
Implementations that do not support key agreement algorithms that
make use of UKMs MUST gracefully handle the presence of UKMs.
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.
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. Therefore, a recipient X.509
version 3 certificate that contains a key usage extension MUST
assert the keyAgreement bit. 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. Implementations MUST support both
alternatives for specifying the recipient"s certificate.
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 MUST 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.
6.2.4 PasswordRecipientInfo Type
Recipient information using a password or shared secret value is
represented in the type PasswordRecipientInfo. Each instance of
PasswordRecipientInfo will transfer the content-encryption key to one
or more recipients who possess the password or shared secret value.
The PasswordRecipientInfo Type is specified in RFC3211 [PWRI]. The
PasswordRecipientInfo structure is repeated here for completeness.
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- Always set to 0
keyDerivationAlgorithm [0] KeyDerivationAlgorithmIdentifier
OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
The fields of type PasswordRecipientInfo have the following meanings:
version is the syntax version number. It MUST always be 0.
keyDerivationAlgorithm identifies the key-derivation algorithm,
and any associated parameters, used to derive the key-encryption
key from the password or shared secret value. If this field is
absent, the key-encryption key is supplied from an external
source, for example a hardware crypto token such as a smart card.
keyEncryptionAlgorithm identifies the encryption algorithm, and
any associated parameters, used to encrypt the content-encryption
key with the key-encryption key.
encryptedKey is the result of encrypting the content-encryption
key with the key-encryption key.
6.2.5 OtherRecipientInfo Type
Recipient information for additional key management techniques are
represented in the type OtherRecipientInfo. The OtherRecipientInfo
type allows key management techniques beyond key transport, key
agreement, previously distributed symmetric key-encryption keys, and
password-based key management to be specified in future documents.
An object identifier uniquely identifies such key management
techniques.
OtherRecipientInfo ::= SEQUENCE {
oriType OBJECT IDENTIFIER,
oriValue ANY DEFINED BY oriType }
The fields of type OtherRecipientInfo have the following meanings:
oriType identifies the key management technique.
oriValue contains the protocol data elements needed by a recipient
using the identified key management technique.
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.
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 aforementioned 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 MUST be 0; however, if
the encapsulated content type is other than id-data, then the
value of version MUST 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 derived from 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 MUST be 2. If unprotectedAttrs is absent,
then version MUST 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,
authAttrs [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthAttrs [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. The version MUST be
assigned as follows:
IF ((originatorInfo is present) AND
(any version 2 attribute certificates are present))
THEN version is 1
ELSE version is 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
authAttrs field MUST also be present.
encapContentInfo is the content that is authenticated, as defined
in section 5.2.
authAttrs is a collection of authenticated attributes. The
authAttrs structure is optional, but it MUST be present if the
content type of the EncapsulatedContentInfo value being
authenticated is not id-data. If the authAttrs field is present,
then the digestAlgorithm field MUST also be present. The
AuthAttributes structure MUST be DER encoded, even if the rest of
the structure is BER encoded. Useful attribute types are defined
in Section 11. If the authAttrs 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.
unauthAttrs 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 content being authenticated or a message digest
of content being authenticated together with the originator"s
authenticated attributes.
If authAttrs 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 authAttrs 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 authAttrs. A separate
encoding of the authAttrs field is performed for message digest
calculation. The IMPLICIT [2] tag in the authAttrs 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 authAttrs 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 encapsu