S/MIME Working Group R. Housley
Internet Draft SPYRUS
expires in six months October 1998
Cryptographic Message Syntax
<draft-ietf-smime-cms-07.txt>
Status of this Memo
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Abstract
This document describes the Cryptographic Message Syntax. This
syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary messages.
The Cryptographic Message Syntax is derived from PKCS #7 version 1.5
[RFC 2315]. Wherever possible, backward compatibility is preserved;
however, changes were necessary to accommodate attribute certificate
transfer and key agreement techniques for key management.
This draft is being discussed on the "ietf-smime" mailing list. To
join the list, send a message to <ietf-smime-request@imc.org> with
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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, Tim Dean, Steve
Dusse, Paul Hoffman, Scott Hollenbeck, Burt Kaliski, John Pawling,
Blake Ramsdell, Jim Schaad, and Dave Solo for their efforts and
support.
1 Introduction
This document describes the Cryptographic Message Syntax. This
syntax is used to digitally sign or encrypt arbitrary messages.
The Cryptographic Message Syntax describes an encapsulation syntax
for data protection. It supports digital signatures and encryption.
The syntax allows multiple encapsulation, so one encapsulation
envelope can be nested inside another. Likewise, one party can
digitally sign some previously encapsulated data. It also allows
arbitrary attributes, such as signing time, to be signed along with
the message content, and provides for other attributes such as
countersignatures to be associated with a signature.
The Cryptographic Message Syntax can support a variety of
architectures for certificate-based key management, such as the one
defined by the PKIX working group.
The Cryptographic Message Syntax values are generated using ASN.1,
using BER-encoding. Values are typically represented as octet
strings. While many systems are capable of transmitting arbitrary
octet strings reliably, it is well known that many electronic-mail
systems are not. This document does not address mechanisms for
encoding octet strings for reliable transmission in such
environments.
2 General Overview
The Cryptographic Message Syntax (CMS) is general enough to support
many different content types. This document defines one protection
content, ContentInfo. ContentInfo encapsulates one or more
protection content type. 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 type and the data, signed-data, and
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enveloped-data
content types. The other content types may be implemented if
desired.
As a general design philosophy, content types permit single pass
processing using indefinite-length Basic Encoding Rules (BER)
encoding. Single-pass operation is especially helpful if content is
large, stored on tapes, or is "piped" from another process. Single-
pass operation has one significant drawback: it is difficult to
perform encode operations using the Distinguished Encoding Rules
(DER) encoding in a single pass since the lengths of the various
components may not be known in advance. However, signed attributes
within the signed-data content type and authenticated attributes
within the authenticated-data content type require DER encoding.
Signed attributes and authenticated attributes must be transmitted in
DER form to ensure that recipients can validate a content that
contains an unrecognized attribute.
3 General Syntax
The Cryptographic Message Syntax (CMS) associates a protection
content type with a protection content. The syntax shall have ASN.1
type ContentInfo:
ContentInfo ::= SEQUENCE {
contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType }
ContentType ::= OBJECT IDENTIFIER
The fields of ContentInfo have the following meanings:
contentType indicates the type of protection 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 protection content. The type of protection content
can be determined uniquely by contentType. Protection content
types for signed-data, enveloped-data, digested-data, encrypted-
data, and authenticated-data are defined in this document. If
additional protection content types are defined in other
documents, the ASN.1 type defined along with the object identifier
should not be a CHOICE type.
4 Data Content Type
The following object identifier identifies the data content type:
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id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
The data content type is intended to refer to arbitrary octet
strings, such as ASCII text files; the interpretation is left to the
application. Such strings need not have any internal structure
(although they could have their own ASN.1 definition or other
structure).
The data content type is generally encapsulated in the signed-data,
enveloped-data, digested-data, encrypted-data, or authenticated-data
content type. Object identifiers other than id-data may be used to
identify the specific type of encapsulated content, but such usage is
outside the scope of this specification.
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 two signers employ the same message digest algorithm, then the
message digest need be computed for only one of them. 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
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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 validate the signature
value. The signer's public key is referenced by an issuer
distinguished name and an issuer-specific serial number that uniquely
identify the certificate containing the public key. The signer's
certificate may be included in the SignedData certificates field.
This section is divided into 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 validation processes, respectively.
5.1 SignedData Type
The following object identifier identifies the signed-data content
type:
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
The signed-data content type shall have ASN.1 type SignedData:
SignedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithms DigestAlgorithmIdentifiers,
encapContentInfo EncapsulatedContentInfo,
certificates [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT CertificateRevocationLists OPTIONAL,
signerInfos SignerInfos }
DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier
SignerInfos ::= SET OF SignerInfo
The fields of type SignedData have the following meanings:
version is the syntax version number. If no attribute
certificates are present in the certificates field and the
encapsulated content type is id-data, then the value of version
shall be 1; however, if attribute certificates are present or the
encapsulated content type is other than id-data, then the value of
version shall be 3.
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digestAlgorithms is a collection of message digest algorithm
identifiers. There may be any number of elements in the
collection, including zero. Each element identifies the message
digest algorithm, along with any associated parameters, used by
one or more signer. The collection is intended to list the
message digest algorithms employed by all of the signers, in any
order, to facilitate one-pass signature verification. The message
digesting process is described in Section 5.4.
encapContentInfo is the signed content, consisting of a content
type identifier and the content itself. Details of the
EncapsulatedContentInfo type are discussed in section 5.2.
certificates is a collection of certificates. It is intended that
the set of certificates be sufficient to contain chains from a
recognized "root" or "top-level certification authority" to all of
the signers in the signerInfos field. There may be more
certificates than necessary, and there may be certificates
sufficient to contain chains from two or more independent top-
level certification authorities. There may also be fewer
certificates than necessary, if it is expected that recipients
have an alternate means of obtaining necessary certificates (e.g.,
from a previous set of certificates). If no attribute
certificates are present in the collection, then the value of
version shall be 1; however, if attribute certificates are
present, then the value of version shall be 3.
crls is a collection of certificate revocation lists (CRLs). It
is intended that the set contain information sufficient to
determine whether or not the certificates in the certificates
field are valid, but such correspondence is not necessary. There
may be more CRLs than necessary, and there may also be fewer CRLs
than necessary.
signerInfos is a collection of per-signer information. There may
be any number of elements in the collection, including zero. The
details of the SignerInfo type are discussed in section 5.3.
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
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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.
5.2 EncapsulatedContentInfo Type
The content is represented in the type EncapsulatedContentInfo:
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
ContentType ::= OBJECT IDENTIFIER
The fields of type EncapsulatedContentInfo have the following
meanings:
eContentType is an object identifier that uniquely specifies the
content type.
eContent is the content itself, carried as an octet string. The
eContent need not be DER encoded.
5.3 SignerInfo Type
Per-signer information is represented in the type SignerInfo:
SignerInfo ::= SEQUENCE {
version CMSVersion,
issuerAndSerialNumber IssuerAndSerialNumber,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
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
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SignatureValue ::= OCTET STRING
The fields of type SignerInfo have the following meanings:
version is the syntax version number; it shall be 1.
issuerAndSerialNumber specifies the signer's certificate (and
thereby the signer's public key) by issuer distinguished name and
issuer-specific serial number.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used by the signer. The message digest is
computed over the encapsulated content and signed attributes, if
present. The message digest algorithm should be among those
listed in the digestAlgorithms field of the associated SignerData.
The message digesting process is described in Section 5.4.
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.
A message-digest attribute, having as its value the message
digest of the content. Section 11.2 defines the message-digest
attribute.
signatureAlgorithm identifies the signature algorithm, and any
associated parameters, used by the signer to generate the digital
signature.
signature is the result of digital signature generation, using the
message digest and the signer's private key.
unsignedAttributes is a collection of attributes that are not
signed. The field is optional. Useful attribute types, such as
countersignatures, are defined in Section 11.
The fields of type SignedAttribute and UnsignedAttribute have the
following meanings:
attrType indicates the type of attribute. It is an object
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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. A separate encoding of the
signedAttributes field is performed for message digest calculation.
The IMPLICIT [0] tag in the signedAttributes field is not used for
the DER encoding, rather an EXPLICIT SET OF tag is used. That is,
the DER encoding of the SET OF tag, rather than of the IMPLICIT [0]
tag, is to be included in the message digest calculation along with
the length and content octets of the SignedAttributes value.
When the signedAttributes field is absent, then only the octets
comprising the value of the signedData encapContentInfo eContent
OCTET STRING (e.g., the contents of a file) are input to the message
digest calculation. This has the advantage that the length of the
content being signed need not be known in advance of the signature
generation process.
Although the encapContentInfo eContent OCTET STRING tag and length
octets are not included in the message digest calculation, they are
still protected by other means. The length octets are protected by
the nature of the message digest algorithm since it is
computationally infeasible to find any two distinct messages of any
length that have the same message digest.
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5.5 Message Signature Generation Process
The input to the signature generation process includes the result of
the message digest calculation process and the signer's private key.
The details of the signature generation depend on the signature
algorithm employed. The object identifier, along with any
parameters, that specifies the signature algorithm employed by the
signer is carried in the signatureAlgorithm field. The signature
value generated by the signer is encoded as an OCTET STRING and
carried in the signature field.
5.6 Message Signature Validation Process
The input to the signature validation process includes the result of
the message digest calculation process and the signer's public key.
The details of the signature validation depend on the signature
algorithm employed.
The recipient may not rely on any message digest values computed by
the originator. If the signedData signerInfo includes
signedAttributes, then the content message digest must be calculated
as described in section 5.4. For the signature to be valid, the
message digest value calculated by the recipient must be the same as
the value of the messageDigest attribute included in the
signedAttributes of the signedData signerInfo.
6 Enveloped-data Content Type
The enveloped-data content type consists of an encrypted content of
any type and encrypted content-encryption keys for one or more
recipients. The combination of the encrypted content and one
encrypted content-encryption key for a recipient is a "digital
envelope" for that recipient. Any type of content can be enveloped
for an arbitrary number of recipients using any of the three key
management techniques for each recipient.
The typical application of the enveloped-data content type will
represent one or more recipients' digital envelopes on content of the
data or signed-data content types.
Enveloped-data is constructed by the following steps:
1. A content-encryption key for a particular content-encryption
algorithm is generated at random.
2. The content-encryption key is encrypted for each recipient.
The details of this encryption depend on the key management
algorithm used, but three general techniques are supported:
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key transport: the content-encryption key is encrypted in the
recipient's public key;
key agreement: the recipient's public key and the sender's
private key are used to generate a pairwise symmetric key, then
the content-encryption key is encrypted in the pairwise
symmetric key; and
mail list keys: the content-encryption key is encrypted in a
previously distributed symmetric key.
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 }
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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
The fields of type EnvelopedData have the following meanings:
version is the syntax version number. If originatorInfo is
present, then version shall be 2. If any of the RecipientInfo
structures included have a version other than 0, then the version
shall be 2. If originatorInfo is absent and all of the
RecipientInfo structures are version 0, then version shall be 0.
originatorInfo optionally provides information about the
originator. It is present only if required by the key management
algorithm. It may contain certificates and CRLs:
certs is a collection of certificates. certs may contain
originator certificates associated with several different key
management algorithms. certs may also contain attribute
certificates associated with the originator. The certificates
contained in certs are intended to be sufficient to make chains
from a recognized "root" or "top-level certification authority"
to all recipients. However, certs may contain more
certificates than necessary, and there may be certificates
sufficient to make chains from two or more independent top-
level certification authorities. Alternatively, certs may
contain fewer certificates than necessary, if it is expected
that recipients have an alternate means of obtaining necessary
certificates (e.g., from a previous set of certificates).
crls is a collection of CRLs. It is intended that the set
contain information sufficient to determine whether or not the
certificates in the certs field are valid, but such
correspondence is not necessary. There may be more CRLs than
necessary, and there may also be fewer CRLs than necessary.
recipientInfos is a collection of per-recipient information.
There must be at least one element in the collection.
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encryptedContentInfo is the encrypted content information.
The fields of type EncryptedContentInfo have the following meanings:
contentType indicates the type of content.
contentEncryptionAlgorithm identifies the content-encryption
algorithm, and any associated parameters, used to encrypt the
content. The content-encryption process is described in Section
6.3. The same content-encryption algorithm and content-encryption
key is used for all recipients.
encryptedContent is the result of encrypting the content. The
field is optional, and if the field is not present, its intended
value must be supplied by other means.
The recipientInfos field comes before the encryptedContentInfo field
so that an EnvelopedData value may be processed in a single pass.
6.2 RecipientInfo Type
Per-recipient information is represented in the type RecipientInfo.
RecipientInfo has a different format for the three key management
techniques that are supported: key transport, key agreement, and
previously distributed mail list keys. Any of the three key
management techniques can be used for each recipient of the same
encrypted content. In all cases, the content-encryption key is
transferred to one or more recipient in encrypted form.
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
mlri [2] MailListRecipientInfo }
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 }
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RecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
The fields of type KeyTransRecipientInfo have the following meanings:
version is the syntax version number. If the RecipientIdentifier
is the CHOICE issuerAndSerialNumber, then the version shall be 0.
If the RecipientIdentifier is subjectKeyIdentifier, then the
version shall be 2.
rid specifies the recipient's certificate or key that was used by
the sender to protect the content-encryption key. The
RecipientIdentifier provides two alternatives for specifying the
recipient's certificate, and thereby the recipient's public key.
The recipient's certificate must contain a key transport public
key. The content-encryption key is encrypted with the recipient's
public key. The issuerAndSerialNumber alternative identifies the
recipient's certificate by the issuer's distinguished name and the
certificate serial number; the subjectKeyIdentifier identifies the
recipient's certificate by the X.509 subjectKeyIdentifier
extension value.
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content-
encryption key for the recipient. The key-encryption process is
described in Section 6.4.
encryptedKey is the result of encrypting the content-encryption
key for the recipient.
6.2.2 KeyAgreeRecipientInfo Type
Recipient information using key agreement is represented in the type
KeyAgreeRecipientInfo. Each instance of KeyAgreeRecipientInfo will
transfer the content-encryption key to one or more recipient.
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 }
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OriginatorPublicKey ::= SEQUENCE {
algorithm AlgorithmIdentifier,
publicKey BIT STRING }
RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey
RecipientEncryptedKey ::= SEQUENCE {
rid KeyAgreeRecipientIdentifier,
encryptedKey EncryptedKey }
KeyAgreeRecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
rKeyId [0] IMPLICIT RecipientKeyIdentifier }
RecipientKeyIdentifier ::= SEQUENCE {
subjectKeyIdentifier SubjectKeyIdentifier,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
SubjectKeyIdentifier ::= OCTET STRING
The fields of type KeyAgreeRecipientInfo have the following meanings:
version is the syntax version number. It shall always be 3.
originator is a CHOICE with three alternatives specifying the
sender's key agreement public key. The sender uses the
corresponding private key and the recipient's public key to
generate a pairwise key. The content-encryption key is encrypted
in the pairwise key. The issuerAndSerialNumber alternative
identifies the sender's certificate, and thereby the sender's
public key, by the issuer's distinguished name and the certificate
serial number. The subjectKeyIdentifier alternative identifies
the sender's certificate, and thereby the sender's public key, by
the X.509 subjectKeyIdentifier extension value. The originatorKey
alternative includes the algorithm identifier and sender's key
agreement public key. Permitting originator anonymity since the
public key is not certified.
ukm is optional. With some key agreement algorithms, the sender
provides a User Keying Material (UKM) to ensure that a different
key is generated each time the same two parties generate a
pairwise key.
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content-
encryption key in the key-encryption key. The key-encryption
process is described in Section 6.4.
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recipientEncryptedKeys includes a recipient identifier and the
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. The recipient's certificate must contain a key
agreement public key. The content-encryption key is encrypted in
the pairwise 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 generated using the key agreement algorithm.
The fields of type RecipientKeyIdentifier have the following
meanings:
subjectKeyIdentifier identifies the recipient's certificate by the
X.509 subjectKeyIdentifier extension value.
date is optional. When present, the date specifies which of the
recipient's previously distributed UKMs was used by the sender.
other is optional. When present, this field contains additional
information used by the recipient to locate the public keying
material used by the sender.
6.2.3 MailListRecipientInfo Type
Recipient information using previously distributed symmetric keys is
represented in the type MailListRecipientInfo. Each instance of
MailListRecipientInfo will transfer the content-encryption key to one
or more recipients who have the previously distributed key-encryption
key.
MailListRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 4
mlkid MailListKeyIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
MailListKeyIdentifier ::= SEQUENCE {
kekIdentifier OCTET STRING,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
The fields of type MailListRecipientInfo have the following meanings:
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version is the syntax version number. It shall always be 4.
mlkid 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 in 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 MailListKeyIdentifier have the following meanings:
kekIdentifier identifies the key-encryption key that was
previously distributed to the sender and one or more recipients.
date is optional. When present, the date specifies a single key-
encryption key from a set that was previously distributed.
other is optional. When present, this field contains additional
information used by the recipient to determine the key-encryption
key used by the sender.
6.3 Content-encryption Process
The content-encryption key for the desired content-encryption
algorithm is randomly generated. The data to be protected is padded
as described below, then the padded data is encrypted using the
content-encryption key. The encryption operation maps an arbitrary
string of octets (the data) to another string of octets (the
ciphertext) under control of a content-encryption key. The encrypted
data is included in the envelopedData encryptedContentInfo
encryptedContent OCTET STRING.
The input to the content-encryption process is the "value" of the
content being enveloped. Only the value octets of the envelopedData
encryptedContentInfo encryptedContent OCTET STRING are encrypted; the
OCTET STRING tag and length octets are not encrypted.
Some content-encryption algorithms assume the input length is a
multiple of k octets, where k is greater than one. For such
algorithms, the input shall be padded at the trailing end with
k-(l mod k) octets all having value k-(l mod k), where l is the
length of the input. In other words, the input is padded at the
trailing end with one of the following strings:
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01 -- if l mod k = k-1
02 02 -- if l mod k = k-2
.
.
.
k k ... k k -- if l mod k = 0
The padding can be removed unambiguously since all input is padded,
including input values that are already a multiple of the block size,
and no padding string is a suffix of another. This padding method is
well defined if and only if k is less than 256.
6.4 Key-encryption Process
The input to the key-encryption process -- the value supplied to the
recipient's key-encryption algorithm --is just the "value" of the
content-encryption key.
Any of the three key management techniques can be used for each
recipient of the same encrypted content.
7 Digested-data Content Type
The digested-data content type consists of content of any type and a
message digest of the content.
Typically, the digested-data content type is used to provide content
integrity, and the result generally becomes an input to the
enveloped-data content type.
The following steps construct digested-data:
1. A message digest is computed on the content with a message-
digest algorithm.
2. The message-digest algorithm and the message digest are
collected together with the content into a DigestedData value.
A recipient verifies the message digest by comparing the message
digest to an independently computed message digest.
The following object identifier identifies the digested-data content
type:
id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }
The digested-data content type shall have ASN.1 type DigestedData:
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DigestedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithm DigestAlgorithmIdentifier,
encapContentInfo EncapsulatedContentInfo,
digest Digest }
Digest ::= OCTET STRING
The fields of type DigestedData have the following meanings:
version is the syntax version number. If the encapsulated content
type is id-data, then the value of version shall be 0; however, if
the encapsulated content type is other than id-data, then the
value of version shall be 2.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, under which the content is digested. The
message-digesting process is the same as in Section 5.4 in the
case when there are no signed attributes.
encapContentInfo is the content that is digested, as defined in
section 5.2.
digest is the result of the message-digesting process.
The ordering of the digestAlgorithm field, the encapContentInfo
field, and the digest field makes it possible to process a
DigestedData value in a single pass.
8 Encrypted-data Content Type
The encrypted-data content type consists of encrypted content of any
type. Unlike the enveloped-data content type, the encrypted-data
content type has neither recipients nor encrypted content-encryption
keys. Keys must be managed by other means.
The typical application of the encrypted-data content type will be to
encrypt the content of the data content type for local storage,
perhaps where the encryption key is a password.
The following object identifier identifies the encrypted-data content
type:
id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }
The encrypted-data content type shall have ASN.1 type EncryptedData:
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EncryptedData ::= SEQUENCE {
version CMSVersion,
encryptedContentInfo EncryptedContentInfo }
The fields of type EncryptedData have the following meanings:
version is the syntax version number. It shall be 0.
encryptedContentInfo is the encrypted content information, as
defined in Section 6.1.
9 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 validate 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), the
MAC value of the content and the other information are generated
using the same message authentication code algorithm and 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)
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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,
encapContentInfo EncapsulatedContentInfo,
authenticatedAttributes [1] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthenticatedAttributes [2] IMPLICIT UnauthAttributes OPTIONAL }
AuthAttributes ::= SET SIZE (1..MAX) OF Attribute
UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute
MessageAuthenticationCode ::= OCTET STRING
The fields of type AuthenticatedData have the following meanings:
version is the syntax version number. It shall be 0.
originatorInfo optionally provides information about the
originator. It is present only if required by the key management
algorithm. It may contain certificates, attribute certificates,
and CRLs, as defined in Section 6.1.
recipientInfos is a collection of per-recipient information, as
defined in Section 6.1. There must be at least one element in the
collection.
macAlgorithm is a message authentication code algorithm
identifier. It identifies the message authentication code
algorithm, along with any associated parameters, used by the
originator. Placement of the macAlgorithm field facilitates one-
pass processing by the recipient.
encapContentInfo is the content that is authenticated, as defined
in section 5.2.
authenticatedAttributes is a collection of attributes that are
authenticated. The field is optional, but it must be present if
the content type of the EncapsulatedContentInfo value being
authenticated is not id-data. Each AuthenticatedAttribute in the
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SET
must be DER encoded. Useful attribute types 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.
A mac-value attribute, having as its value the message
authentication code of the content. Section 11.5 defines the
mac-value 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 the 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. Although the encapContentInfo eContent OCTET
STRING tag and length octets are not included in the MAC calculation,
they are still protected by other means. The length octets are
protected by the nature of the MAC algorithm since it is
computationally infeasible to find any two distinct messages of any
length that have the same MAC.
If authenticatedAttributes field is present, the content-type
attribute (as described in Section 11.1) and the mac-value attribute
(as described in section 11.5) 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 MAC calculation. The
IMPLICIT [0] tag in the authenticatedAttributes field is not used for
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the
DER encoding, rather an EXPLICIT SET OF tag is used. The DER
encoding of the SET OF tag, rather than of the IMPLICIT [0] tag, is
to be included in the MAC calculation along with the length and
content octets of the authenticatedAttributes value.
The fact that the MAC is computed on part of a DER encoding does not
mean that DER is the required method of representing that part for
data transfer. Indeed, it is expected that some implementations will
store objects in forms other than their DER encodings, but such
practices do not affect MAC computation.
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., DES-MAC and 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 Validation
The input to the MAC validation process includes the input data
(determined based on the presence or absence of authenticated
attributes, as defined in 9.2), and the authentication key conveyed
in recipientInfo. The details of the MAC validation process depend
on the MAC algorithm employed.
The recipient may not rely on any MAC values computed by the
originator. If the originator includes authenticated attributes,
then the content of the authenticatedAttributes must be authenticated
as described in section 9.2. For the MAC to be valid, the message
MAC value calculated by the recipient must be the same as the value
of the macValue attribute included in the authenticatedAttributes.
Likewise, the attribute MAC value calculated by the recipient must be
the same as the value of the mac field included in the
authenticatedData.
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.
There are many alternatives for each type of algorithm listed. For
each of these five types, Section 12 lists the algorithms that must
be included in a CMS implementation.
10.1.1 DigestAlgorithmIdentifier
The DigestAlgorithmIdentifier type identifies a message-digest
algorithm. Examples include SHA-1, MD2, and MD5. A message-digest
algorithm maps an octet string (the message) to another octet string
(the message digest).
DigestAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.2 SignatureAlgorithmIdentifier
The SignatureAlgorithmIdentifier type identifies a signature
algorithm. Examples include DSS and RSA. A signature algorithm
supports signature generation and verification operations. The
signature generation operation uses the message digest and the
signer's private key to generate a signature value. The signature
verification operation uses the message digest and the signer's
public key to determine whether or not a signature value is valid.
Context determines which operation is intended.
SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.3 KeyEncryptionAlgorithmIdentifier
The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption
algorithm used to encrypt a content-encryption key. The encryption
operation maps an octet string (the key) to another octet string (the
encrypted key) under control of a key-encryption key. The decryption
operation is the inverse of the encryption operation. Context
determines which operation is intended.
The details of encryption and decryption depend on the key management
algorithm used. Key transport, key agreement, and previously
distributed symmetric key-encrypting keys are supported.
KeyEncryptionAlgorithmIdentifier ::= 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
or not. 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.
The definition of CertificateList is imported from X.509.
CertificateRevocationLists ::= SET OF CertificateList
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10.2.2 CertificateChoices
The CertificateChoices type gives either a PKCS #6 extended
certificate [PKCS #6], an X.509 certificate, or an X.509 attribute
certificate. The PKCS #6 extended certificate is obsolete. It is
included for backward compatibility, and its use should be avoided.
The definitions of Certificate and AttributeCertificate are imported
from X.509.
CertificateChoices ::= CHOICE {
certificate Certificate, -- See X.509
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
attrCert [1] IMPLICIT AttributeCertificate } -- See X.509 and X9.57
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
certificate issuer and an issuer-specific certificate serial number.
The definition of Name is imported from X.501, and the definition of
CertificateSerialNumber is imported from X.509.
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CertificateSerialNumber ::= INTEGER
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10.2.5 CMSVersion
The Version type gives a syntax version number, for compatibility
with future revisions of this document.
CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4) }
10.2.6 UserKeyingMaterial
The UserKeyingMaterial type gives a syntax 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 may impede interoperability.
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
11 Useful Attributes
This section defines attributes that may used with signed-data or
authenticated-data. Some of the attributes defined in this section
were originally defined in PKCS #9 [PKCS #9], others were not
previously defined. 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
The content-type attribute type specifies the content type of the
ContentInfo value being signed in signed-data. The content-type
attribute type is required if there are any authenticated attributes
present.
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The content-type attribute must be a signed attribute or an
authenticated attribute; it cannot be an unsigned attribute or
unauthenticated 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), where the message digest is computed using the
signer's message digest algorithm.
Within signed-data, the message-digest signed attribute type is
required if there are any attributes present.
The message-digest attribute must be a signed attribute; it cannot be
an unsigned attribute, an authenticated attribute, or unauthenticated
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
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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.
The signing-time attribute may be a signed attribute; it cannot be an
unsigned attribute, an authenticated attribute, or an unauthenticated
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:
SigningTime ::= Time
Time ::= CHOICE {
utcTime UTCTime,
generalizedTime GeneralizedTime }
Note: The definition of Time matches the one specified in the 1997
version of X.509.
Dates through the year 2049 must be encoded as UTCTime, and dates in
the year 2050 or later must be encoded as GeneralizedTime.
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 shall be
interpreted as 19YY; and
Where YY is less than 50, the year shall be interpreted as 20YY.
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GeneralizedTime values shall 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)
another signature.
The countersignature attribute must be an unsigned attribute; it
cannot be a signed attribute, an authenticated attribute, or an
unauthenticated 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.
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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.
11.5 Message Authentication Code (MAC) Value
The MAC-value attribute type specifies the MAC of the
encapContentInfo eContent OCTET STRING being authenticated in
authenticated-data (see section 9), where the MAC value is computed
using the originator's MAC algorithm and the data-authentication key.
Within authenticated-data, the MAC-value attribute type is required
if there are any authenticated attributes present.
The MAC-value attribute must be a authenticated attribute; it cannot
be an signed attribute, an unsigned attribute, or unauthenticated
attribute.
The following object identifier identifies the MAC-value attribute:
id-macValue OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) aa(2) 8 }
MAC-value attribute values have ASN.1 type MACValue:
MACValue ::= OCTET STRING
A MAC-value 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 AuthAttributes syntax is defined as a SET OF Attributes. The
AuthAttributes in an AuthenticatedData must not include multiple
instances of the MAC-value attribute.
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12 Supported Algorithms
This section lists the algorithms that must be implemented.
Additional algorithms that should be implemented are also included.
12.1 Digest Algorithms
CMS implementations must include SHA-1. CMS implementations may
include MD5.
Digest algorithm identifiers are located in the SignedData
digestAlgorithms field, the SignerInfo digestAlgorithm field, and the
DigestedData digestAlgorithm field.
Digest values are located in the DigestedData digest field, and
digest values are located in the Message Digest authenticated
attribute. In addition, digest values are input to signature
algorithms.
12.1.1 SHA-1
The SHA-1 digest algorithm is defined in FIPS Pub 180-1 [SHA1]. The
algorithm identifier for SHA-1 is:
sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 26 }
The AlgorithmIdentifier parameters field is optional. If present,
the parameters field must contain an ASN.1 NULL. Implementations
should accept SHA-1 AlgorithmIdentifiers with absent parameters as
well as NULL parameters. Implementations should generate SHA-1
AlgorithmIdentifiers with NULL parameters.
12.1.2 MD5
The MD5 digest algorithm is defined in RFC 1321 [RFC 1321]. The
algorithm identifier for MD5 is:
md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) digestAlgorithm(2) 5 }
The AlgorithmIdentifier parameters field must be present, and the
parameters field must contain NULL. Implementations may accept the
MD5 AlgorithmIdentifiers with absent parameters as well as NULL
parameters.
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12.2 Signature Algorithms
CMS implementations must include DSA. CMS implementations may
include RSA.
Signature algorithm identifiers are located in the SignerInfo
signatureAlgorithm field. Also, signature algorithm identifiers are
located in the SignerInfo signatureAlgorithm field of
countersignature attributes.
Signature values are located in the SignerInfo signature field.
Also, signature values are located in the SignerInfo signature field
of countersignature attributes.
12.2.1 DSA
The DSA signature algorithm is defined in FIPS Pub 186 [DSS]. DSA is
always used with the SHA-1 message digest algorithm. The algorithm
identifier for DSA is:
id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57 (10040) x9cm(4) 3 }
The AlgorithmIdentifier parameters field must not be present.
12.2.2 RSA
The RSA signature algorithm is defined in RFC 2313 [RFC 2313]. RFC
2313 specifies the use of the RSA signature algorithm with the MD5
message digest algorithm. That definition is extended here to
include support for the SHA-1 message digest algorithm as well. The
algorithm identifier for RSA is:
rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
The AlgorithmIdentifier parameters field must be present, and the
parameters field must contain NULL.
This specification modifies RFC 2313 to include SHA-1 as an
additional message digest algorithm. Section 10.1.2 of RFC 2313 is
modified to list SHA-1 in the bullet item about digestAlgorithm. The
following object identifier is added to the list in section 10.1.2 of
RFC 2313:
sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 26 }
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12.3 Key Management Algorithms
CMS accommodates three general key management techniques: key
agreement, key transport, and mail list keys.
12.3.1 Key Agreement Algorithms
CMS implementations must include key agreement using X9.42
Ephemeral-Static Diffie-Hellman. CMS implementations must include
key agreement of Triple-DES pairwise key-encryption keys and Triple-
DES wrapping Triple-DES content-encryption keys. CMS implementations
should include key agreement of RC2 pairwise key-encryption keys and
RC2 wrapping RC2 content-encryption keys. The key wrap algorithm is
described in section 12.6.
Key agreement algorithm identifiers are located in the EnvelopedData
RecipientInfo KeyAgreeRecipientInfo keyEncryptionAlgorithm field.
Wrapped content-encryption keys are located in the EnvelopedData
RecipientInfo KeyAgreeRecipientInfo recipientEncryptedKeys
encryptedKey field.
12.3.1.1 X9.42 Ephemeral-Static Diffie-Hellman with Triple-DES
Ephemeral-Static Diffie-Hellman key agreement is defined in RFC TBD1
[RFC TBD1]. When using Ephemeral-Static Diffie-Hellman with Triple-
DES, the EnvelopedData RecipientInfo KeyAgreeRecipientInfo fields are
used as follows:
version must be 3.
originator must be the originatorKey alternative. The
originatorKey algorithm fields must contain the dh-public-number
object identifier with absent parameters. The originatorKey
publicKey field must contain the sender's ephemeral public key.
The dh-public-number object identifier is:
dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }
ukm must be absent.
keyEncryptionAlgorithm must be the id-alg-ESDHwith3DES algorithm
identifier with absent parameters. The id-alg-ESDHwith3DES
algorithm identifier is:
id-alg-ESDHwith3DES OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 1 }
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recipientEncryptedKeys contains an identifier and an encrypted key
for each recipient. The RecipientEncryptedKey
KeyAgreeRecipientIdentifier must contain either the
issuerAndSerialNumber identifying the recipient's certificate or
the RecipientKeyIdentifier containing the subject key identifier
from the recipient's certificate. In both cases, the recipient's
certificate contains the recipient's static public key.
RecipientEncryptedKey EncryptedKey must contain the content-
encryption Triple-DES key wrapped in the pairwise key agreement
Triple-DES key.
12.3.1.1 X9.42 Ephemeral-Static Diffie-Hellman with RC2
Ephemeral-Static Diffie-Hellman key agreement is defined in RFC TBD1
[RFC TBD1]. When using Ephemeral-Static Diffie-Hellman with RC2, the
EnvelopedData RecipientInfo KeyAgreeRecipientInfo fields are used as
follows:
version must be 3.
originator must be the originatorKey alternative. The
originatorKey algorithm fields must contain the dh-public-number
object identifier with absent parameters. The originatorKey
publicKey field must contain the sender's ephemeral public key.
The dh-public-number object identifier is:
dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }
ukm must be absent.
keyEncryptionAlgorithm must be the id-alg-ESDHwithRC2 algorithm
identifier with absent parameters. The id-alg-ESDHwithRC2
algorithm identifier is:
id-alg-ESDHwithRC2 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 2 }
recipientEncryptedKeys contains an identifier and an encrypted key
for each recipient. The RecipientEncryptedKey
KeyAgreeRecipientIdentifier must contain either the
issuerAndSerialNumber identifying the recipient's certificate or
the RecipientKeyIdentifier containing the subject key identifier
from the recipient's certificate. In both cases, the recipient's
certificate contains the recipient's static public key.
RecipientEncryptedKey EncryptedKey must contain the content-
encryption RC2 key wrapped in the pairwise key agreement RC2 key.
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12.3.2 Key Transport Algorithms
CMS implementations should include key transport using RSA. RSA
implementations must include key transport of Triple-DES content-
encryption keys. RSA implementations should include key transport of
RC2 content-encryption keys.
Key transport algorithm identifiers are located in the EnvelopedData
RecipientInfo KeyTransRecipientInfo keyEncryptionAlgorithm field.
Key transport encrypted content-encryption keys are located in the
EnvelopedData RecipientInfo KeyTransRecipientInfo EncryptedKey field.
12.3.2.1 RSA
The RSA key transport algorithm is defined in RFC 2313 [RFC 2313].
RFC 2313 specifies the transport of content-encryption keys,
including Triple-DES and RC2 keys. The algorithm identifier for RSA
is:
rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
The AlgorithmIdentifier parameters field must be present, and the
parameters field must contain NULL.
12.3.3 Mail List Key Algorithms
CMS implementations may include mail list key management. Mail list
key management implementations must include Triple-DES mail list keys
wrapping Triple-DES content-encryption keys. Mail list key
management implementations should include key transport of RC2
content-encryption keys. The key wrap algorithm is specified in
section 12.6.
Key mail list key algorithm identifiers are located in the
EnvelopedData RecipientInfo MailListRecipientInfo
keyEncryptionAlgorithm field.
Wrapped content-encryption keys are located in the EnvelopedData
RecipientInfo MailListRecipientInfo encryptedKey field.
12.3.3.1 Triple-DES Key Wrap
Mail list key encryption with Triple-DES has the algorithm
identifier:
id-alg-3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
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us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 3 }
The AlgorithmIdentifier parameter field must be NULL.
Distribution of the Triple-DES mail list keying material used to
encrypt the content-encryption key is out of the scope of this
document.
12.3.3.2 RC2 Key Wrap
Mail list key encryption with RC2 has the algorithm identifier:
id-alg-RC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 4 }
The AlgorithmIdentifier parameter field must be RC2wrapParameter:
RC2wrapParameter ::= RC2ParameterVersion
RC2ParameterVersion ::= INTEGER
The RC2 effective-key-bits (key size) greater than 32 and less than
256 is encoded in the RC2ParameterVersion. For the effective-key-
bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
and 58 respectively. These values are not simply the RC2 key length.
Note that the value 160 must be encoded as two octets (00 A0),
because the one octet (A0) encoding represents a negative number.
Distribution of the RC2 mail list keying material used to encrypt the
content-encryption key is out of the scope of this document.
12.4 Content Encryption Algorithms
CMS implementations must include Triple-DES in CBC mode. CMS
implementations should include RC2 in CBC mode.
Content encryption algorithms identifiers are located in the
EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm field
and the EncryptedData EncryptedContentInfo contentEncryptionAlgorithm
field.
Content encryption algorithms are used to encipher the content
located in the EnvelopedData EncryptedContentInfo encryptedContent
field and the EncryptedData EncryptedContentInfo encryptedContent
field.
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12.4.1 Triple-DES CBC
The Triple-DES algorithm is described in [3DES]. The algorithm
identifier for Triple-DES is:
des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }
The AlgorithmIdentifier parameters field must be present and contain
a CBCParameter:
CBCParameter ::= IV
IV ::= OCTET STRING -- exactly 8 octets
12.4.2 RC2 CBC
The RC2 algorithm is described in RFC 2268 [RFC 2268]. The algorithm
identifier for RC2 in CBC mode is:
RC2-CBC OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) encryptionAlgorithm(3) 2 }
The AlgorithmIdentifier parameters field must be present and contain
a RC2-CBC:
RC2-CBC parameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING -- exactly 8 octets -- }
The RC2 effective-key-bits (key size) greater than 32 and less than
256 is encoded in the rc2ParameterVersion. For the effective-key-
bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
and 58 respectively. These values are not simply the RC2 key length.
Note that the value 160 must be encoded as two octets (00 A0), since
the one octet (A0) encoding represents a negative number.
12.5 Message Authentication Code Algorithms
CMS implementations that support authenticatedData must include HMAC
with SHA-1. CMS implementations may also include DES MAC.
MAC algorithm identifiers are located in the AuthenticatedData
macAlgorithm field.
MAC values are located in the AuthenticatedData mac field. MAC
values are also located in the mac-value authenticated attribute.
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12.5.1 HMAC with SHA-1
The HMAC with SHA-1 algorithm is described in RFC 2104 [RFC 2104].
The algorithm identifier for HMAC with SHA-1 is:
HMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }
The AlgorithmIdentifier parameters field must be absent.
12.5.2 DES MAC
The DES MAC algorithm is described in FIPS Pub 113 [DES MAC]. CMS
implementations choosing to implement DES MAC must support 32 bit MAC
values. CMS implementations should also support 64 bit MAC values.
The algorithm identifier for DES MAC is:
DES-MAC OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 10 }
The AlgorithmIdentifier parameters field must be present. The
parameters contain an INTEGER identifying the length in bits of the
MAC value, constrained to multiples of eight between 16 and 64:
DESMACLength ::= INTEGER -- may be 16, 24, 32, 40, 48, 56, or 64
12.6 CMS Key Wrap Algorithm
CMS implementations must implement the key wrap algorithm specified
in this section.
Key Transport algorithms allow for the content-encryption key to be
directly encrypted; however, key agreement and mail list key
algorithms encrypt the content-encryption key with a second (possibly
different) symmetric encryption algorithm. This section describes
how the content-encryption key is formatted and encrypted.
Key agreement algorithms generate a pairwise key-encryption key, and
this key wrap algorithm is used to encrypt the content-encryption key
in that pairwise key-encryption key. Similarly, this key wrap
algorithm is used to encrypt the content-encryption key in a mail
list key.
The key-encryption key is generated by the key agreement algorithm or
distributed as a mail list key. With key agreement, the minimum
number of bits needed to form the key-encryption key must be used.
As an example, only the first 40 bits of Diffie-Hellman generated
keying material are used for a RC2/40 key-encryption key.
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The block size of the key-encryption algorithm must be implicitly
determined from the KeyEncryptionAlgorithmIdentifier field.
Likewise, the size of the content-encryption key must be implicitly
determined from the ContentEncryptionAlgorithmIdentifier field.
Since the same algorithm identifier is used for both 2-key and 3-key
Triple DES, three keys are always wrapped for Triple-DES. Thus, 2-
key Triple-DES provides three keys where the first and third keys are
the same.
12.6.1 Sum of Sums Key Checksum
The Sum of Sums [SUM] key checksum algorithm is:
1. Initialize two 16 bit integers, sum1 and sum2, to zero.
2. Loop through the octets of the content-encryption key, most
significant octet to least significant octet.
2a. Create a 16 bit integer, called temp, by concatenating
eight zero bits and the key octet.
2b. sum1 = sum1 + temp.
2c. sum2 = sum2 + sum1.
3. Use sum2 as the checksum value.
12.6.2 Key Wrap
1. Modify the content-encryption key to meet any restrictions on the key.
For example, adjust the parity bits for DES and Triple-DES keys.
2. Compute a 16-bit key checksum value on the content-encryption key as
described above.
3. Generate a 32-bit random salt value.
4. Concatenate the salt, content-encryption key, and key checksum value.
5. Randomly generate the number of pad octets necessary to make the result
a multiple of block size of the key-encryption algorithm (the Triple-DES
block size is 8 bytes), then append them to the result.
6. Encrypt the result with the key-encryption algorithm key. Use an IV
with each octet equal to 'A5' hexadecimal.
Some key-encryption algorithm identifiers include an IV as part of
the parameters. The IV must still be the constant above.
12.6.3 Key Unwrap
The key unwrap algorithm is:
1. Decrypt the ciphertext using the key-encryption key. Use an IV
with each octet equal to 'A5' hexadecimal.
2. Decompose the result into the content-encryption key and key checksum
values. The salt and pad values are discarded.
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3. Compute a 16-bit key checksum value on the content-encryption key
as
described above. 4. If computed key checksum value does not
match the decrypted key checksum
value, then there is an error. 5. If there are restrictions on
keys, then check if the content-encryption
key meets these restrictions. For example, check for odd parity
of each
octet in a DES or Triple-DES key. If any restriction is
incorrect then
there is an error.
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Appendix A: ASN.1 Module
CryptographicMessageSyntax
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- 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)
AlgorithmIdentifier, AttributeCertificate, Certificate,
CertificateList, CertificateSerialNumber
FROM AuthenticationFramework { joint-iso-itu-t ds(5)
module(1) authenticationFramework(7) 3 } ;
-- Cryptographic Message Syntax
ContentInfo ::= SEQUENCE {
contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL }
ContentType ::= OBJECT IDENTIFIER
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
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EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
ContentType ::= OBJECT IDENTIFIER
SignerInfo ::= SEQUENCE {
version CMSVersion,
issuerAndSerialNumber IssuerAndSerialNumber,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
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 }
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
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
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kari [1] KeyAgreeRecipientInfo,
mlri [2] MailListRecipientInfo }
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 }
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
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MailListRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 4
mlkid MailListKeyIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
MailListKeyIdentifier ::= SEQUENCE {
kekIdentifier 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 }
AuthenticatedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
macAlgorithm MessageAuthenticationCodeAlgorithm,
encapContentInfo EncapsulatedContentInfo,
authenticatedAttributes [1] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthenticatedAttributes [2] 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
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CertificateRevocationLists ::= SET OF CertificateList
CertificateChoices ::= CHOICE {
certificate Certificate, -- See X.509
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
attrCert [1] IMPLICIT AttributeCertificate } -- See X.509 & X9.57
CertificateSet ::= SET OF CertificateChoices
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
MACValue ::= OCTET STRING
-- Object Identifiers
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
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us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }
id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }
id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
ct(1) 2 }
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 }
id-macValue OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) aa(2) 8 }
-- Obsolete Extended Certificate syntax from PKCS#6
ExtendedCertificateOrCertificate ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate }
ExtendedCertificate ::= SEQUENCE {
extendedCertificateInfo ExtendedCertificateInfo,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature Signature }
ExtendedCertificateInfo ::= SEQUENCE {
version CMSVersion,
certificate Certificate,
attributes UnauthAttributes }
Signature ::= BIT STRING
-- Algorithm Identifiers and Parameters
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sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 26 }
END -- of CryptographicMessageSyntax
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References
3DES Tuchman, W. "Hellman Presents No Shortcut Solutions To DES".
IEEE Spectrum, v. 16, n. 7, pp40-41. July 1979.
DES American National Standards Institute. ANSI X3.106,
"American National Standard for Information Systems - Data
Link Encryption". 1983.
DES MAC National Institute of Standards and Technology. FIPS Pub 113:
Computer Data Authentication. May 1985.
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.
Internet draft, draft-ietf-smime-ess-*.txt.
MSG Ramsdell, B. S/MIME Version 3 Message Specification.
Internet Draft, draft-ietf-smime-msg-*.txt.
PKCS #6 RSA Laboratories. PKCS #6: Extended-Certificate Syntax
Standard, Version 1.5. November 1993.
PKCS #9 RSA Laboratories. PKCS #9: Selected Attribute Types,
Version 1.1. November 1993.
RFC 1321 Rivest, R. The MD5 Message-Digest Algorithm. April 1992.
RFC 1750 Eastlake, D.; S. Crocker; J. Schiller. Randomness
Recommendations for Security. December 1994.
RFC 2104 Krawczyk, H. HMAC: Keyed-Hashing for Message Authentication.
February 1997.
RFC 2268 Rivest, R. A Description of the RC2 (r) Encryption Algorithm.
March 1998.
RFC 2313 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5.
March 1998.
RFC 2315 Kaliski, B. PKCS #7: Cryptographic Message Syntax,
Version 1.5. March 1998.
RFC TBD1 Rescorla, E. Ephemeral-Static Diffie-Hellman Key
Agreement Method. (currently draft-ietf-smime-x942).
SHA1 National Institute of Standards and Technology.
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FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.
SUM Fletcher, J. An Arithmetic Checksum for Serial
Transmissions. Reprint UCRL-82569, Lawrence Livermore
Laboraory, University of California. May 1979.
X.208 CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
X.209 CCITT. Recommendation X.209: Specification of Basic
Encoding
Rules for Abstract Syntax Notation One (ASN.1). 1988.
X.501 CCITT. Recommendation X.501: The Directory - Models.
1988.
X.509 CCITT. Recommendation X.509: The Directory -
Authentication
Framework. 1988.
Security Considerations
The Cryptographic Message Syntax provides a method for digitally
signing data, digesting data, 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 mail
list key, and the content-encryption key. Compromise of the key
management private key or the mail list 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.
Implementations must randomly generate content-encryption keys,
message-authentication keys, initialization vectors (Ivs), salt
values, 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
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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 offers important guidance in this
area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality PRNG
technique.
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 validate the original signature value
prior to countersigning it (this validation 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
applications, should be aware that PKCS #1 [RFC 2313] 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. This new document may succeed RFC 2313. To resolve the
adaptive chosen ciphertext vulnerability, the new document specifies
and recommends use of Optimal Asymmetric Encryption Padding (OAEP)
when RSA encryption is applied to provide secrecy. 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 decryption operations is not provided. Support for OAEP may
be added to a future version of the CMS specification.
Housley [Page 51]
INTERNET DRAFT October 1998
Author Address
Russell Housley
SPYRUS
381 Elden Street
Suite 1120
Herndon, VA 20170
USA
housley@spyrus.com
Housley [Page 52]
