S/MIME Working Group R. Housley
Internet Draft RSA Laboratories
expires in six months April 2001
Cryptographic Message Syntax
<draft-ietf-smime-rfc2630bis-00.txt>
Status of this Memo
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Abstract
This document describes the Cryptographic Message Syntax (CMS). This
syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary messages.
The CMS 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.
[*** NEW ***] Once approved, this draft will obsolete RFC 2630. The
discussion of specific cryptographic algorithms has been moved to a
separate document. Separation of the protocol and algorithm
specifications allows the IETF to select different mandatory to
implement algorithms in the future without reissuing this protocol
document. However, implementations of the CMS MUST support the
mandatory to implement algorithms specified in [CMSALG], or its
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successor.
Look for [*** NEW ***]. This string is used to identify text that is
significantly different than RFC 2630. Editorial changes were made
that are not flagged with this string.
This draft is being discussed on the "ietf-smime" mailing list. To
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Table of Contents
Status of this Memo .................................................. 1
Abstract ............................................................. 1
Table of Contents .................................................... 3
1 Introduction ..................................................... 5
2 General Overview ................................................. 5
3 General Syntax ................................................... 6
4 Data Content Type ................................................ 6
5 Signed-data Content Type ......................................... 7
5.1 SignedData Type ............................................. 8
5.2 EncapsulatedContentInfo Type ................................ 9
5.3 SignerInfo Type ............................................. 10
5.4 Message Digest Calculation Process .......................... 12
5.5 Message Signature Generation Process ........................ 13
5.6 Message Signature Verification Process ...................... 13
6 Enveloped-data Content Type ...................................... 14
6.1 EnvelopedData Type .......................................... 15
6.2 RecipientInfo Type .......................................... 17
6.2.1 KeyTransRecipientInfo Type ........................... 17
6.2.2 KeyAgreeRecipientInfo Type ........................... 18
6.2.3 KEKRecipientInfo Type ................................ 20
6.3 Content-encryption Process .................................. 21
6.4 Key-encryption Process ...................................... 22
7 Digested-data Content Type ....................................... 22
8 Encrypted-data Content Type ...................................... 24
9 Authenticated-data Content Type .................................. 24
9.1 AuthenticatedData Type ...................................... 25
9.2 MAC Generation .............................................. 27
9.3 MAC Verification ............................................ 28
10 Useful Types ..................................................... 28
10.1 Algorithm Identifier Types ................................. 28
10.1.1 DigestAlgorithmIdentifier .......................... 29
10.1.2 SignatureAlgorithmIdentifier ....................... 29
10.1.3 KeyEncryptionAlgorithmIdentifier ................... 29
10.1.4 ContentEncryptionAlgorithmIdentifier ............... 29
10.1.5 MessageAuthenticationCodeAlgorithm ................. 30
10.2 Other Useful Types ......................................... 30
10.2.1 CertificateRevocationLists ......................... 30
10.2.2 CertificateChoices ................................. 30
10.2.3 CertificateSet ..................................... 31
10.2.4 IssuerAndSerialNumber .............................. 31
10.2.5 CMSVersion ......................................... 32
10.2.6 UserKeyingMaterial ................................. 32
10.2.7 OtherKeyAttribute .................................. 32
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11 Useful Attributes ................................................ 32
11.1 Content Type ............................................... 33
11.2 Message Digest ............................................. 33
11.3 Signing Time ............................................... 34
11.4 Countersignature ........................................... 35
Appendix A: CMS ASN.1 Module ........................................ 37
Appendix B: Version 1 Attribute Certificate ASN.1 Module 43
References ........................................................... 44
Security Considerations .............................................. 46
Acknowledgments ...................................................... 48
Author Address ....................................................... 49
Full Copyright Statement ............................................. 49
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1 Introduction
This document describes the Cryptographic Message Syntax (CMS). This
syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary messages.
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.
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
described by Scott Bradner 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 if desired.
As a general design philosophy, each content type permits single pass
processing using indefinite-length Basic Encoding Rules (BER)
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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 types 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. The CMS
signed attributes and authenticated attributes MUST be encoded with
DER.
3 General Syntax
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
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(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.
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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:
[*** NEW ***] 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 MUST be 1.
If version 1 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 MUST be 3.
If version 2 attribute certificates are present, then the value of
version MUST be 4.
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
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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 MUST
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. The object identifier MUST
uniquely specify the content type.
eContent is the content itself, carried as an octet string. The
eContent need not be DER encoded.
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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 MUST be 1. If
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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 three
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.
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
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message digest and the signer's private key. [*** NEW ***] This
field MUST be present; however, the details of the signature
depend on the signature algorithm employed.
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 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.
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When the signedAttributes 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 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 MUST be 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 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.
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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
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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:
[*** NEW ***] version is the syntax version number. If
originatorInfo is present, then version MUST be 2, unless the
certificate set in the originatorInfo contains version 2 attribute
certificates, then version MUST be 3. If any of the RecipientInfo
structures included have a version other than 0, then the version
MUST be 2. If unprotectedAttrs is present, then version MUST be
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2. If originatorInfo is absent, all of the RecipientInfo
structures are version 0, and unprotectedAttrs is absent, then
version MUST 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 build
certification paths 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 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. [***
NEW ***] The field MUST be present.
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 MUST be used for all recipients.
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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 supported key
management techniques: 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.
[*** NEW ***] All implementations MUST support the mandatory to
implement key management algorithms are specified in [CMSALG], or its
successor. Use of a companion document enables the IETF to change
the mandatory to implement algorithms without updating this protocol
specification. Since the companion algorithm specification will
undoubtedly change and implementation updates will certainly take
time, all implementations MUST gracefully handle unimplemented
algorithms when they are encountered.
[*** NEW ***] Implementations MUST support all three key management
techniques represented by ktri, kari, and kekri. 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 (and as yet
unspecified) additions to the RecipientInfo CHOICE. Such additions
will have their own tag, permitting unimplemented RecipientInfo
CHOICE alternatives to be ignored while correctly processing the
others.
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.
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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. 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. [*** NEW ***] Implementations MUST support both
alternatives for specifying the recipient's certificate.
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.
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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
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agreement public key. This alternative permits originator
anonymity since the public key is not certified. [*** NEW ***]
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. [*** NEW ***] Implementations SHOULD support UKM
processing. Implementations that do not process 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. 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. [*** NEW ***] 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.
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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.
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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.
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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.
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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
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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 }
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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 MUST 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
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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
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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 MUST 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.
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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].
[*** NEW ***] There are many alternatives for each algorithm type.
For each of these five types, CMS implementations MUST support the
mandatory to implement algorithms specified in [CMSALG], or its
successor.
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 ::= AlgorithmIdentifier
10.1.2 SignatureAlgorithmIdentifier
The SignatureAlgorithmIdentifier type identifies a signature
algorithm. Examples include RSA, DSA, and ECDSA. 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 ::= AlgorithmIdentifier
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10.1.4 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 ::= AlgorithmIdentifier
10.1.5 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
10.2 Other Useful Types
This section defines types that are used other places in the
document. The types are not listed in any particular order.
10.2.1 CertificateRevocationLists
The CertificateRevocationLists type gives a set of certificate
revocation lists (CRLs). It is intended that the set contain
information sufficient to determine whether the certificates and
attribute certificates with which the set is associated are revoked.
However, there MAY be more CRLs than necessary or there MAY be fewer
CRLs than necessary.
The CertificateList MAY contain a CRL, an Authority Revocation List
(ARL), a Delta Revocation List, or an Attribute Certificate
Revocation List. All of these lists share a common syntax.
CRLs are specified in X.509 [X.509-97], and they are profiled for use
in the Internet in RFC 2459 [PROFILE].
The definition of CertificateList is imported from X.509.
CertificateRevocationLists ::= SET OF CertificateList
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10.2.2 CertificateChoices
[*** NEW ***] The CertificateChoices type gives either a PKCS #6
extended certificate [PKCS#6], an X.509 certificate, a version 1
X.509 attribute certificate (ACv1) [X.509-97], or a version 2 X.509
attribute certificate (ACv2) [X.509-00]. The PKCS #6 extended
certificate is obsolete. The PKCS #6 certificate is included for
backward compatibility, and PKCS #6 certificates SHOULD NOT be used.
The ACv1 is also obsolete. ACv1 is included for backward
compatibility, and ACv1 SHOULD NOT be used. The Internet profile of
X.509 certificates is specified in the "Internet X.509 Public Key
Infrastructure: Certificate and CRL Profile" [PROFILE]. The Internet
profile of ACv2 is specified in the "An Internet Attribute
Certificate Profile for Authorization" [ACPROFILE].
The definition of Certificate is imported from X.509.
[*** NEW ***] The definitions of AttributeCertificate are imported
from X.509-1997 and X.509-2000. The definition from X.509-1997 is
assigned to AttributeCertificateV1 (see Appendix B), and the
definition from X.509-2000 is assigned to AttributeCertificateV2.
CertificateChoices ::= CHOICE {
certificate Certificate, -- See X.509
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete
v2AttrCert [2] IMPLICIT AttributeCertificateV2 } -- See X.509
10.2.3 CertificateSet
The CertificateSet type provides a set of certificates. It is
intended that the set be sufficient to contain chains from a
recognized "root" or "top-level certification authority" to all of
the sender certificates with which the set is associated. However,
there MAY be more certificates than necessary, or there MAY be fewer
than necessary.
The precise meaning of a "chain" is outside the scope of this
document. Some applications MAY impose upper limits on the length of
a chain; others MAY enforce certain relationships between the
subjects and issuers of certificates within a chain.
CertificateSet ::= SET OF CertificateChoices
10.2.4 IssuerAndSerialNumber
The IssuerAndSerialNumber type identifies a certificate, and thereby
an entity and a public key, by the distinguished name of the
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certificate issuer and an issuer-specific certificate serial number.
The definition of Name is imported from X.501 [X.501-88], and the
definition of CertificateSerialNumber is imported from X.509
[X.509-97].
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CertificateSerialNumber ::= INTEGER
10.2.5 CMSVersion
The CMSVersion type gives a syntax version number, for compatibility
with future revisions of this specification.
CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4) }
10.2.6 UserKeyingMaterial
The UserKeyingMaterial type gives a syntax for user keying material
(UKM). Some key agreement algorithms require UKMs to ensure that a
different key is generated each time the same two parties generate a
pairwise key. The sender provides a UKM for use with a specific key
agreement algorithm.
UserKeyingMaterial ::= OCTET STRING
10.2.7 OtherKeyAttribute
The OtherKeyAttribute type gives a syntax for the inclusion of other
key attributes that permit the recipient to select the key used by
the sender. The attribute object identifier must be registered along
with the syntax of the attribute itself. Use of this structure
should be avoided since it might impede interoperability.
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
11 Useful Attributes
This section defines attributes that may be used with signed-data,
enveloped-data, encrypted-data, or authenticated-data. The syntax of
Attribute is compatible with X.501 [X.501-88] and RFC 2459 [PROFILE].
Some of the attributes defined in this section were originally
defined in PKCS #9 [PKCS#9]; others were originally defined in a
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previous version of this specification [OLDCMS]. The attributes are
not listed in any particular order.
Additional attributes are defined in many places, notably the S/MIME
Version 3 Message Specification [MSG] and the Enhanced Security
Services for S/MIME [ESS], which also include recommendations on the
placement of these attributes.
11.1 Content Type
[*** NEW ***] The content-type attribute type specifies the content
type of the ContentInfo within signed-data or authenticated-data.
The content-type attribute type MUST be present whenever signed
attributes are present in signed-data or authenticated attributes
present in authenticated-data.
[*** NEW ***] The content-type attribute MUST be a signed attribute
or an authenticated attribute; it MUST NOT be an unsigned attribute,
unauthenticated attribute, or unprotected attribute.
The following object identifier identifies the content-type
attribute:
id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }
Content-type attribute values have ASN.1 type ContentType:
ContentType ::= OBJECT IDENTIFIER
A content-type attribute MUST have a single attribute value, even
though the syntax is defined as a SET OF AttributeValue. There MUST
NOT be zero or multiple instances of AttributeValue present.
The SignedAttributes and AuthAttributes syntaxes are each defined as
a SET OF Attributes. The SignedAttributes in a signerInfo MUST NOT
include multiple instances of the content-type attribute. Similarly,
the AuthAttributes in an AuthenticatedData MUST NOT include multiple
instances of the content-type attribute.
11.2 Message Digest
The message-digest attribute type specifies the message digest of the
encapContentInfo eContent OCTET STRING being signed in signed-data
(see section 5.4) or authenticated in authenticated-data (see section
9.2). For signed-data, the message digest is computed using the
signer's message digest algorithm. For authenticated-data, the
message digest is computed using the originator's message digest
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algorithm.
[*** NEW ***] Within signed-data, the message-digest signed attribute
type MUST be present when there are any signed attributes present.
Within authenticated-data, the message-digest authenticated attribute
type MUST be present when there are any authenticated attributes
present.
[*** NEW ***] The message-digest attribute MUST be a signed attribute
or an authenticated attribute; it MUST NOT be an unsigned attribute,
unauthenticated attribute, or unprotected attribute.
The following object identifier identifies the message-digest
attribute:
id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }
Message-digest attribute values have ASN.1 type MessageDigest:
MessageDigest ::= OCTET STRING
A message-digest attribute MUST have a single attribute value, even
though the syntax is defined as a SET OF AttributeValue. There MUST
NOT be zero or multiple instances of AttributeValue present.
The SignedAttributes syntax is defined as a SET OF Attributes. The
SignedAttributes in a signerInfo MUST NOT include multiple instances
of the message-digest attribute.
11.3 Signing Time
The signing-time attribute type specifies the time at which the
signer (purportedly) performed the signing process. The signing-time
attribute type is intended for use in signed-data.
[*** NEW ***] The signing-time attribute MAY be a signed attribute;
it MUST NOT be an unsigned attribute, authenticated attribute,
unauthenticated attribute, or unprotected attribute.
The following object identifier identifies the signing-time
attribute:
id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }
Signing-time attribute values have ASN.1 type SigningTime:
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SigningTime ::= Time
Time ::= CHOICE {
utcTime UTCTime,
generalizedTime GeneralizedTime }
Note: The definition of Time matches the one specified in the 1997
version of X.509 [X.509-97].
Dates between 1 January 1950 and 31 December 2049 (inclusive) MUST be
encoded as UTCTime. Any dates with year values before 1950 or after
2049 MUST be encoded as GeneralizedTime.
UTCTime values MUST be expressed in Greenwich Mean Time (Zulu) and
MUST include seconds (i.e., times are YYMMDDHHMMSSZ), even where the
number of seconds is zero. Midnight (GMT) MUST be represented as
"YYMMDD000000Z". Century information is implicit, and the century
MUST be determined as follows:
Where YY is greater than or equal to 50, the year MUST be
interpreted as 19YY; and
Where YY is less than 50, the year MUST be interpreted as 20YY.
GeneralizedTime values MUST be expressed in Greenwich Mean Time
(Zulu) and MUST include seconds (i.e., times are YYYYMMDDHHMMSSZ),
even where the number of seconds is zero. GeneralizedTime values
MUST not include fractional seconds.
A signing-time attribute MUST have a single attribute value, even
though the syntax is defined as a SET OF AttributeValue. There MUST
not be zero or multiple instances of AttributeValue present.
The SignedAttributes syntax is defined as a SET OF Attributes. The
SignedAttributes in a signerInfo MUST not include multiple instances
of the signing-time attribute.
No requirement is imposed concerning the correctness of the signing
time, and acceptance of a purported signing time is a matter of a
recipient's discretion. It is expected, however, that some signers,
such as time-stamp servers, will be trusted implicitly.
11.4 Countersignature
The countersignature attribute type specifies one or more signatures
on the contents octets of the DER encoding of the signatureValue
field of a SignerInfo value in signed-data. Thus, the
countersignature attribute type countersigns (signs in serial)
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another signature.
[*** NEW ***] The countersignature attribute MUST be an unsigned
attribute; it MUST NOT be a signed attribute, an authenticated
attribute, an unauthenticated attribute, or an unprotected attribute.
The following object identifier identifies the countersignature
attribute:
id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }
Countersignature attribute values have ASN.1 type Countersignature:
Countersignature ::= SignerInfo
Countersignature values have the same meaning as SignerInfo values
for ordinary signatures, except that:
1. The signedAttributes field MUST contain a message-digest
attribute if it contains any other attributes, but need not
contain a content-type attribute, as there is no content type for
countersignatures.
2. The input to the message-digesting process is the contents
octets of the DER encoding of the signatureValue field of the
SignerInfo value with which the attribute is associated.
A countersignature attribute can have multiple attribute values. The
syntax is defined as a SET OF AttributeValue, and there MUST be one
or more instances of AttributeValue present.
The UnsignedAttributes syntax is defined as a SET OF Attributes. The
UnsignedAttributes in a signerInfo MAY include multiple instances of
the countersignature attribute.
A countersignature, since it has type SignerInfo, can itself contain
a countersignature attribute. Thus, it is possible to construct
arbitrarily long series of countersignatures.
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Appendix A: CMS ASN.1 Module
CryptographicMessageSyntax
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) }
-- [*** NEW ***] A new OID was assigned for this updated module.
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
-- The types and values defined in this module are exported for use in
-- the other ASN.1 modules. Other applications may use them for their
-- own purposes.
IMPORTS
-- Directory Information Framework (X.501)
Name
FROM InformationFramework { joint-iso-itu-t ds(5) modules(1)
informationFramework(1) 3 }
-- Directory Authentication Framework (X.509-2000)
AlgorithmIdentifier, AttributeCertificate, Certificate,
CertificateList, CertificateSerialNumber
FROM AuthenticationFramework { joint-iso-itu-t ds(5)
module(1) authenticationFramework(7) 4 }
-- [*** NEW ***]
-- Indirectly from Directory Authentication Framework (X.509-1997)
AttributeCertificateV1
FROM AttributeCertificateVersion1 { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
modules(0) v1AttrCert(1) } ;
-- Cryptographic Message Syntax
ContentInfo ::= SEQUENCE {
contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType }
ContentType ::= OBJECT IDENTIFIER
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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
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
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
EnvelopedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
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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
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo }
EncryptedKey ::= OCTET STRING
KeyTransRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0 or 2
rid RecipientIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
RecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
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 }
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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
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 }
DigestedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithm DigestAlgorithmIdentifier,
encapContentInfo EncapsulatedContentInfo,
digest Digest }
Digest ::= OCTET STRING
EncryptedData ::= SEQUENCE {
version CMSVersion,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
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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
DigestAlgorithmIdentifier ::= AlgorithmIdentifier
SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier
CertificateRevocationLists ::= SET OF CertificateList
-- [*** OLD ***]
-- CertificateChoices ::= CHOICE {
-- certificate Certificate, -- See X.509
-- extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
-- attrCert [1] IMPLICIT AttributeCertificate } -- See X.509 & X9.57
-- [*** NEW ***]
CertificateChoices ::= CHOICE {
certificate Certificate, -- See X.509
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete
v2AttrCert [2] IMPLICIT AttributeCertificateV2 } -- See X.509
-- [*** NEW ***]
AttributeCertificateV2 ::= AttributeCertificate -- See X.509-2000
CertificateSet ::= SET OF CertificateChoices
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IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4) }
UserKeyingMaterial ::= OCTET STRING
UserKeyingMaterials ::= SET SIZE (1..MAX) OF UserKeyingMaterial
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
-- CMS Attributes
MessageDigest ::= OCTET STRING
SigningTime ::= Time
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
Countersignature ::= SignerInfo
-- Attribute Object Identifiers
id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }
id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }
id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }
id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }
-- Obsolete Extended Certificate syntax from PKCS#6
ExtendedCertificateOrCertificate ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate }
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ExtendedCertificate ::= SEQUENCE {
extendedCertificateInfo ExtendedCertificateInfo,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature Signature }
ExtendedCertificateInfo ::= SEQUENCE {
version CMSVersion,
certificate Certificate,
attributes UnauthAttributes }
Signature ::= BIT STRING
END -- of CryptographicMessageSyntax
Appendix B: Version 1 Attribute Certificate ASN.1 Module
[*** NEW ***]
AttributeCertificateVersion1
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) v1AttrCert(1) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
-- Only one type is defined, and it is exported.
IMPORTS
-- Directory Authentication Framework (X.509-1997)
AttributeCertificate
FROM AuthenticationFramework { joint-iso-itu-t ds(5)
module(1) authenticationFramework(7) 3 } ;
-- Version 1 Attribute Certificate
AttributeCertificateV1 ::= AttributeCertificate
END -- of AttributeCertificateVersion1
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References
3DES American National Standards Institute. ANSI X9.52-1998,
Triple Data Encryption Algorithm Modes of Operation. 1998.
ACPROFILE Farrell, S., and R. Housley. An Internet Attribute
Certificate Profile for Authorization. RFC <TBD>. <Date>.
{draft-ietf-pkix-ac509prof-*.txt}
CMSALG Housley, R. Cryptographic Message Syntax (CMS) Algorithms.
RFC <TBD>. <Date>. {draft-ietf-smime-cmsalg-*.txt}
DES American National Standards Institute. ANSI X3.106,
"American National Standard for Information Systems - Data
Link Encryption". 1983.
DH-X9.42 Rescorla, E. Diffie-Hellman Key Agreement Method.
RFC 2631. June 1999.
DSS National Institute of Standards and Technology.
FIPS Pub 186: Digital Signature Standard. 19 May 1994.
ESS Hoffman, P. Enhanced Security Services for S/MIME.
RFC 2634. June 1999.
HMAC Krawczyk, H. HMAC: Keyed-Hashing for Message Authentication.
RFC 2104. February 1997.
MD5 Rivest, R. The MD5 Message-Digest Algorithm. RFC 1321.
April 1992.
MODES National Institute of Standards and Technology.
FIPS Pub 81: DES Modes of Operation. 2 December 1980.
MSG Ramsdell, B. S/MIME Version 3 Message Specification.
RFC 2633. June 1999.
NEWPKCS#1 Kaliski, B., and J. Staddon. PKCS #1: RSA Encryption,
Version 2.0. RFC 2437. October 1998.
OLDCMS Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
PROFILE Housley, R., W. Ford, W. Polk, and D. Solo. Internet
X.509 Public Key Infrastructure: Certificate and CRL
Profile. RFC 2459. January 1999.
PKCS#1 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5.
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RFC 2313. March 1998.
PKCS#6 RSA Laboratories. PKCS #6: Extended-Certificate Syntax
Standard, Version 1.5. November 1993.
PKCS#7 Kaliski, B. PKCS #7: Cryptographic Message Syntax,
Version 1.5. RFC 2315. March 1998.
PKCS#9 RSA Laboratories. PKCS #9: Selected Attribute Types,
Version 1.1. November 1993.
RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750. December 1994.
RC2 Rivest, R. A Description of the RC2 (r) Encryption Algorithm.
RFC 2268. March 1998.
SHA1 National Institute of Standards and Technology.
FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.
STDWORDS Bradner, S. Key Words for Use in RFCs to Indicate
Requirement Levels. RFC2119. March 1997.
X.208-88 CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
X.209-88 CCITT. Recommendation X.209: Specification of Basic Encoding
Rules for Abstract Syntax Notation One (ASN.1). 1988.
X.501-88 CCITT. Recommendation X.501: The Directory - Models. 1988.
X.509-88 CCITT. Recommendation X.509: The Directory - Authentication
Framework. 1988.
X.509-97 ITU-T. Recommendation X.509: The Directory - Authentication
Framework. 1997.
X.509-00 ITU-T. Recommendation X.509: The Directory - Authentication
Framework. 2000.
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Security Considerations
The Cryptographic Message Syntax provides a method for digitally
signing data, digesting data, encrypting data, and authenticating
data.
Implementations must protect the signer's private key. Compromise of
the signer's private key permits masquerade.
Implementations must protect the key management private key, the key-
encryption key, and the content-encryption key. Compromise of the
key management private key or the key-encryption key may result in
the disclosure of all messages protected with that key. Similarly,
compromise of the content-encryption key may result in disclosure of
the associated encrypted content.
Implementations must protect the key management private key and the
message-authentication key. Compromise of the key management private
key permits masquerade of authenticated data. Similarly, compromise
of the message-authentication key may result in undetectable
modification of the authenticated content.
[*** NEW ***] The key management technique employed to distribute
message-authentication keys must itself provide data origin
authentication, otherwise the message content is delivered with
integrity from an unknown source. Neither RSA [PKCS#1, NEWPKCS#1]
nor Ephemeral-Static Diffie-Hellman [DH-X9.42] provide the necessary
data origin authentication. Static-Static Diffie-Hellman [DH-X9.42]
does provide the necessary data origin authentication when both the
originator and recipient public keys are bound to appropriate
identities in X.509 certificates.
[*** NEW ***] When more than two parties share the same message-
authentication key, data origin authentication is not provided. Any
party that knows the message-authentication key can compute a valid
MAC, therefore the message could originate from any one of the
parties.
Implementations must randomly generate content-encryption keys,
message-authentication keys, initialization vectors (IVs), and
padding. Also, the generation of public/private key pairs relies on
a random numbers. The use of inadequate pseudo-random number
generators (PRNGs) to generate cryptographic keys can result in
little or no security. An attacker may find it much easier to
reproduce the PRNG environment that produced the keys, searching the
resulting small set of possibilities, rather than brute force
searching the whole key space. The generation of quality random
numbers is difficult. RFC 1750 [RANDOM] offers important guidance in
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this area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality
PRNG technique.
When using key agreement algorithms or previously distributed
symmetric key-encryption keys, a key-encryption key is used to
encrypt the content-encryption key. If the key-encryption and
content-encryption algorithms are different, the effective security
is determined by the weaker of the two algorithms. If, for example,
a message content is encrypted with 168-bit Triple-DES and the
Triple-DES content-encryption key is wrapped with a 40-bit RC2 key,
then at most 40 bits of protection is provided. A trivial search to
determine the value of the 40-bit RC2 key can recover Triple-DES key,
and then the Triple-DES key can be used to decrypt the content.
Therefore, implementers must ensure that key-encryption algorithms
are as strong or stronger than content-encryption algorithms.
Section 12.6 specifies key wrap algorithms used to encrypt a Triple-
DES [3DES] content-encryption key with a Triple-DES key-encryption
key or to encrypt a RC2 [RC2] content-encryption key with a RC2 key-
encryption key. The key wrap algorithms make use of CBC mode
[MODES]. These key wrap algorithms have been reviewed for use with
Triple and RC2. They have not been reviewed for use with other
cryptographic modes or other encryption algorithms. Therefore, if a
CMS implementation wishes to support ciphers in addition to Triple-
DES or RC2, then additional key wrap algorithms need to be defined to
support the additional ciphers.
Implementers should be aware that cryptographic algorithms become
weaker with time. As new cryptoanalysis techniques are developed and
computing performance improves, the work factor to break a particular
cryptographic algorithm will reduce. Therefore, cryptographic
algorithm implementations should be modular allowing new algorithms
to be readily inserted. That is, implementers should be prepared for
the set of mandatory to implement algorithms to change over time.
The countersignature unauthenticated attribute includes a digital
signature that is computed on the content signature value, thus the
countersigning process need not know the original signed content.
This structure permits implementation efficiency advantages; however,
this structure may also permit the countersigning of an inappropriate
signature value. Therefore, implementations that perform
countersignatures should either verify the original signature value
prior to countersigning it (this verification requires processing of
the original content), or implementations should perform
countersigning in a context that ensures that only appropriate
signature values are countersigned.
Users of CMS, particularly those employing CMS to support interactive
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applications, should be aware that PKCS #1 Version 1.5 as specified
in RFC 2313 [PKCS#1] is vulnerable to adaptive chosen ciphertext
attacks when applied for encryption purposes. Exploitation of this
identified vulnerability, revealing the result of a particular RSA
decryption, requires access to an oracle which will respond to a
large number of ciphertexts (based on currently available results,
hundreds of thousands or more), which are constructed adaptively in
response to previously-received replies providing information on the
successes or failures of attempted decryption operations. As a
result, the attack appears significantly less feasible to perpetrate
for store-and-forward S/MIME environments than for directly
interactive protocols. Where CMS constructs are applied as an
intermediate encryption layer within an interactive request-response
communications environment, exploitation could be more feasible.
An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
[NEWPKCS#1]. This new document will supersede RFC 2313. PKCS #1
Version 2.0 preserves support for the encryption padding format
defined in PKCS #1 Version 1.5 [PKCS#1], and it also defines a new
alternative. To resolve the adaptive chosen ciphertext
vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
is used to provide confidentiality. Designers of protocols and
systems employing CMS for interactive environments should either
consider usage of OAEP, or should ensure that information which could
reveal the success or failure of attempted PKCS #1 Version 1.5
decryption operations is not provided. Support for OAEP will likely
be added to a future version of the CMS specification.
Acknowledgments
This document is the result of contributions from many professionals.
I appreciate the hard work of all members of the IETF S/MIME Working
Group. I extend a special thanks to Rich Ankney, Simon Blake-Wilson,
Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman,
Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt
Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau,
Jim Schaad, and Dave Solo for their efforts and support.
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Author Address
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
USA
rhousley@rsasecurity.com
Full Copyright Statement
Copyright (C) The Internet Society (date). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. In addition, the
ASN.1 module presented in Appendix A may be used in whole or in part
without inclusion of the copyright notice. However, this document
itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process shall be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. This
document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL
NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY
OR FITNESS FOR A PARTICULAR PURPOSE.
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