Network Working Group R. Housley
Request for Comments: 5083 Vigil Security
Updates: 3852 November 2007
Category: Standards Track
Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes an additional content type for the
Cryptographic Message Syntax (CMS). The authenticated-enveloped-data
content type is intended for use with authenticated encryption modes.
All of the various key management techniques that are supported in
the CMS enveloped-data content type are also supported by the CMS
authenticated-enveloped-data content type.
1. Introduction
This document describes an additional content type for the
Cryptographic Message Syntax (CMS) [CMS]. The authenticated-
enveloped-data content type is intended for use with authenticated
encryption modes, where an arbitrary content is both authenticated
and encrypted. Also, some associated data in the form of
authenticated attributes can also be authenticated. All of the
various key management techniques that are supported in the CMS
enveloped-data content type are also supported by the CMS
authenticated-enveloped-data content type.
The conventions for using the Advanced Encryption Standard-Counter
with Cipher Block Chaining-Message Authentication Code (AES-CCM) and
the AES-Galois/Counter Mode (GCM) authenticated encryption algorithms
with the CMS authenticated-enveloped-data content type defined in
this document can be found in [AESALGS].
The authenticated-enveloped-data content type, like all of the other
CMS content types, employs ASN.1 [X.208-88], and it uses both the
Basic Encoding Rules (BER) [X.209-88] and the Distinguished Encoding
Rules (DER) [X.509-88].
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1.1. Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
described in [STDWORDS].
1.2. Version Numbers
The major data structure (AuthEnvelopedData) includes a version
number as the first item in the data structure. The version number
is intended to avoid ASN.1 decode errors. Some implementations do
not check the version number prior to attempting a decode, and then
if a decode error occurs, the version number is checked as part of
the error handling routine. This is a reasonable approach; it places
error processing outside of the fast path. This approach is also
forgiving when an incorrect version number is used by the sender.
Whenever the structure is updated, a higher version number will be
assigned. However, to ensure maximum interoperability, the higher
version number is only used when the new syntax feature is employed.
That is, the lowest version number that supports the generated syntax
is used.
2. Authenticated-Enveloped-Data Content Type
The authenticated-enveloped-data content type consists of an
authenticated and encrypted content of any type and encrypted
content-authenticated-encryption keys for one or more recipients.
The combination of the authenticated and encrypted content and one
encrypted content-authenticated-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
supported key management techniques for each recipient. In addition,
authenticated but not encrypted attributes may be provided by the
originator.
The typical application of the authenticated-enveloped-data content
type will represent one or more recipients' digital envelopes on an
encapsulated content.
Authenticated-enveloped-data is constructed by the following steps:
1. A content-authenticated-encryption key for a particular content-
authenticated-encryption algorithm is generated at random.
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2. The content-authenticated-encryption key is encrypted for each
recipient. The details of this encryption depend on the key
management algorithm used, but four general techniques are
supported:
Key Transport: the content-authenticated-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-
encryption key, then the content-authenticated-encryption
key is encrypted in the pairwise symmetric key-encryption
key;
Symmetric Key-Encryption Keys: the content-authenticated-
encryption key is encrypted in a previously distributed
symmetric key-encryption key; and
Passwords: the content-authenticated-encryption key is
encrypted in a key-encryption key that is derived from a
password or other shared secret value.
3. For each recipient, the encrypted content-authenticated-
encryption key and other recipient-specific information are
collected into a RecipientInfo value, defined in Section 6.2 of
[CMS].
4. Any attributes that are to be authenticated but not encrypted are
collected in the authenticated attributes.
5. The attributes collected in step 4 are authenticated and the CMS
content is authenticated and encrypted with the content-
authenticated-encryption key. If the authenticated encryption
algorithm requires either the additional authenticated data (AAD)
or the content to be padded to a multiple of some block size,
then the padding is added as described in Section 6.3 of [CMS].
6. Any attributes that are to be provided without authentication or
encryption are collected in the unauthenticated attributes.
7. The RecipientInfo values for all the recipients, the
authenticated attributes, the unauthenticated attributes, and the
authenticated and encrypted content are collected together to
form an AuthEnvelopedData value as defined in Section 2.1.
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A recipient opens the digital envelope by decrypting one of the
encrypted content-authenticated-encryption keys, and then using the
recovered key to decrypt and verify the integrity of the
authenticated and encrypted content as well as to verify the
integrity of the authenticated attributes.
The recipient MUST verify the integrity of the received content
before releasing any information, especially the plaintext of the
content. If the integrity verification fails, the receiver MUST
destroy all of the plaintext of the content.
This section is divided into three parts. The first part describes
the AuthEnvelopedData content type, the second part describes the
authentication and encryption process, and the third part describes
the key encryption process.
2.1. AuthEnvelopedData Type
The following object identifier identifies the authenticated-
enveloped-data content type:
id-ct-authEnvelopedData OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) ct(1) 23 }
The authenticated-enveloped-data content type MUST have ASN.1 type
AuthEnvelopedData:
AuthEnvelopedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
authEncryptedContentInfo EncryptedContentInfo,
authAttrs [1] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthAttrs [2] IMPLICIT UnauthAttributes OPTIONAL }
The fields of type AuthEnvelopedData have the following meanings:
version is the syntax version number. It MUST be set to 0.
originatorInfo optionally provides information about the
originator. It is present only if required by the key
management algorithm. It may contain certificates and
Certificate Revocation Lists (CRLs), and the OriginatorInfo
type is defined in Section 6.1 of [CMS].
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recipientInfos is a collection of per-recipient information.
There MUST be at least one element in the collection. The
RecipientInfo type is defined in Section 6.2 of [CMS].
authEncryptedContentInfo is the authenticated and encrypted
content. The CMS enveloped-data content type uses the same
type to carry the encrypted content. The EncryptedContentInfo
type is defined in Section 6.1 of [CMS].
authAttrs optionally contains the authenticated attributes. The
CMS authenticated-data content type uses the same type to carry
authenticated attributes. The authAttrs MUST be present if the
content type carried in EncryptedContentInfo is not id-data.
AuthAttributes MUST be DER encoded, even if the rest of the
AuthEnvelopedData structure is BER encoded. The AuthAttributes
type is defined in Section 9.1 of [CMS]; however, in this case,
the message-digest attribute SHOULD NOT be included. Useful
attribute types are defined in Section 11 of [CMS].
mac is the integrity check value (ICV) or message authentication
code (MAC) that is generated by the authenticated encryption
algorithm. The CMS authenticated-data content type uses the
same type to carry a MAC. In this case, the MAC covers the
authenticated attributes and the content directly, and a digest
algorithm is not used. The MessageAuthenticationCode type is
defined in Section 9.1 of [CMS].
unauthAttrs optionally contains the unauthenticated attributes.
The CMS authenticated-data content type uses the same type to
carry unauthenticated attributes. The UnauthAttributes type is
defined in Section 9.1 of [CMS]. Useful attribute types are
defined in Section 11 of [CMS].
2.2. Authentication and Encryption Process
The content-authenticated-encryption key for the desired content-
authenticated-encryption algorithm is randomly generated.
If the authenticated encryption algorithm requires the content to be
padded to a multiple of some block size, then the padding MUST be
added as described in Section 6.3 of [CMS]. This padding method is
well defined if and only if the block size is less than 256 octets.
If optional authenticated attributes are present, then they are DER
encoded. A separate encoding of the authAttrs field is performed to
construct the authenticated associated data (AAD) input to the
authenticated encryption algorithm. For the purposes of constructing
the AAD, the IMPLICIT [1] tag in the authAttrs field is not used for
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the DER encoding: rather a universal SET OF tag is used. That is,
the DER encoding of the SET OF tag, rather than of the IMPLICIT [1]
tag, is to be included in the construction for the AAD along with the
length and content octets of the authAttrs value. If the
authenticated encryption algorithm requires the AAD to be padded to a
multiple of some block size, then the padding MUST be added as
described in Section 6.3 of [CMS]. This padding method is well
defined if and only if the block size is less than 256 octets.
If optional authenticated attributes are absent, then zero bits of
input are provided for the AAD input to the authenticated encryption
algorithm.
The inputs to the authenticated encryption algorithm are the content
(the data, which is padded if necessary), the DER-encoded
authenticated attributes (the AAD, which is padded if necessary), and
the content-authenticated-encryption key. Under control of a
content-authenticated-encryption key, the authenticated encryption
operation maps an arbitrary string of octets (the data) to another
string of octets (the ciphertext) and it computes an authentication
tag over the AAD and the data. The encrypted data is included in the
AuthEnvelopedData authEncryptedContentInfo encryptedContent as an
OCTET STRING, and the authentication tag is included in the
AuthEnvelopedData mac.
2.3. 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-authenticated-encryption key.
Any of the aforementioned key management techniques can be used for
each recipient of the same encrypted content.
3. Security Considerations
This specification defines an additional CMS content type. The
security considerations provided in [CMS] apply to this content type
as well.
Many authenticated encryption algorithms make use of a block cipher
in counter mode to provide encryption. When used properly, counter
mode provides strong confidentiality. Bellare, Desai, Jokipii, and
Rogaway show in [BDJR] that the privacy guarantees provided by
counter mode are at least as strong as those for Cipher Block
Chaining (CBC) mode when using the same block cipher.
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RFC 5083 Authenticated-Enveloped-Data November 2007
Unfortunately, it is easy to misuse counter mode. If counter block
values are ever used for more that one encryption operation with the
same key, then the same key stream will be used to encrypt both
plaintexts, and the confidentiality guarantees are voided.
Fortunately, the CMS authenticated-enveloped-data content type
provides all of the tools needed to avoid misuse of counter mode.
All of the existing key management techniques permit a fresh
content-encryption key to be generated for each content. In
addition, existing authenticated encryption algorithms that make use
of counter mode support the use of an unpredictable nonce value in
the counter block. This unpredictable nonce value (sometimes called
a "salt") should be carried in an algorithm identifier parameter.
Implementations must randomly generate content-authenticated-
encryption keys, padding, and unpredictable nonce values. Also, the
generation of public/private key pairs relies on a random numbers.
The use of inadequate pseudo-random number generators (PRNGs) to
generate cryptographic keys can result in little or no security. An
attacker may find it much easier to reproduce the PRNG environment
that produced the keys, and then 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 4086
[RANDOM] offers important guidance in this area.
If the message-digest attribute is included in the AuthAttributes,
then the attribute value will contain the unencrypted one-way hash
value of the plaintext of the content. Disclosure of this hash value
enables content tracking, and it can be used to determine if the
plaintext matches one or more candidate contents. For these reasons,
the AuthAttributes SHOULD NOT contain the message-digest attribute.
CMS is often used to provide encryption in messaging environments.
In messaging environments, various forms of unsolicited messages
(such as spam and phishing) represent a significant volume of
unwanted traffic. Present mitigation strategies for unwanted message
traffic involve analysis of message plaintext. When recipients
accept unsolicited encrypted messages, they become even more
vulnerable to unwanted traffic since many present mitigation
strategies will be unable to access the plaintext. Therefore,
software that receives messages that have been encrypted using CMS
needs to provide one or more mechanisms to handle the unwanted
message traffic. One approach that does not require disclosure of
keying material to a server is to reject or discard encrypted
messages unless they purport to come from a member of a white list.
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RFC 5083 Authenticated-Enveloped-Data November 2007
4. ASN.1 Module
CMS-AuthEnvelopedData-2007
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) cms-authEnvelopedData(31) }
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
-- Imports from RFC 3852 [CMS], Section 12.1
AuthAttributes, CMSVersion, EncryptedContentInfo,
MessageAuthenticationCode, OriginatorInfo, RecipientInfos,
UnauthAttributes
FROM CryptographicMessageSyntax2004
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
cms-2004(24) } ;
id-ct-authEnvelopedData OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) ct(1) 23 }
AuthEnvelopedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
authEncryptedContentInfo EncryptedContentInfo,
authAttrs [1] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthAttrs [2] IMPLICIT UnauthAttributes OPTIONAL }
END -- of CMS-AuthEnvelopedData-2007
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5. References
5.1. Normative References
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
[X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1).
1988.
[X.509-88] CCITT. Recommendation X.509: The Directory-
Authentication Framework. 1988.
5.2. Informative References
[AESALGS] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, November 2007.
[BDJR] Bellare, M., Desai, A., Jokipii, E., and P. Rogaway, "A
Concrete Security Treatment of Symmetric Encryption:
Analysis of the DES Modes of Operation", Proceedings
38th Annual Symposium on Foundations of Computer
Science, 1997.
[RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.
Author's Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
Housley Standards Track [Page 9]RFC 5083 Authenticated-Enveloped-Data November 2007
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Housley Standards Track [Page 10]
