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