INTERNET-DRAFT S. Farrell
Expires in six months Baltimore Technologies
S. Turner
IECA
May 2001
Reuse of CMS Content Encryption Keys
<draft-ietf-smime-rcek-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC2026].
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Abstract
This note describes a way to include a key identifier in a CMS
enveloped data structure, so that the content encryption key can be
re-used for further enveloped data packets.
Table Of Contents
Status of this Memo.............................................1
Abstract........................................................1
Table Of Contents...............................................1
1. Introduction.................................................2
2. Applicability................................................2
3. How to do it.................................................3
4. Using different CEK and KEK algorithms.......................4
5. Conformance..................................................5
6. Security Considerations......................................5
7. References...................................................6
Author's Addresses..............................................6
Full Copyright Statement........................................6
Appendix A: ASN.1 Module........................................7
Appendix B: Revision History....................................9
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1. Introduction
CMS [CMS] specifies EnvelopedData. EnvelopedData supports data
encryption using either symmetric or asymmetric key management
techniques. Since asymmetric key establishment is relatively
expensive, it is desirable in some environments to re-use a shared
content-encryption key established using asymmetric mechanisms for
encryption operations in subsequent messages.
The basic idea here is to reuse the content encryption key (CEK)
from a message (say MSG1) to derive the key encryption key (KEK) for
a later message, (MSG2), by including a reference value for the CEK
in message 1, and that same value as the KEKIdentifier for message
2. The CEK from message 1 is called the "referenced CEK".
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
in this document are to be interpreted as described in [RFC2119].
2. Applicability
This specification is intended to be used to provide more efficient
selective field confidentiality between communicating peers, in
particular in the cases where:
- The originator is a client that wishes to send a number of fields
to a server (the recipient) in a single transaction, where the
referenced CEK is used for the separate encryption of each field.
- The originator and recipient are servers that communicate very
frequently and use this specification purely for efficiency.
This specification is not intended to be applicable in all cases. It
is suited for use where:
- Its use is further scoped: that is, this specification doesn't
define a protocol but merely a trick that can be used in a larger
context and additional specification will be needed for each such
case. In particular, in order to use this specification, it is
REQUIRED to define the originators' and recipients' behavior where
a referenced CEK has been "lost".
- This specification is not suitable for general group key
management.
- The underlying cryptographic API is suitable: it is very likely
that any cryptographic API that completely "hides" the bits of
cryptographic keys from the CMS layer will prevent reuse of a
referenced CEK (since they won't have a primitive that allows
MSG1.CEK to be transformed to MSG2.KEK).
- The algorithms for content and key encryption have compatible key
values and strengths, that is, if MSG1.contentEncryptionAlgorithm
is a 40bit cipher and MSG2.keyEncryptionAlgorithm requires 168
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bits of keying material, then this specification SHOULD NOT be
used.
There are other ways that could be envisaged to establish the
required symmetric keying material, e.g. by leveraging a group
keying scheme or by defining a content type that contains a KEK
value. Although this scheme is much simpler than generic group key
management, if an implementation already supports group key
management then this scheme doesn't add value. This scheme is also
suitable for inclusion in CMS libraries (though the addition of new
state might be a problem for some implementations), which can offer
some advantages over application layer schemes (e.g. where the
content includes MSG2.KEK).
3. How to do it
In order to reference the content-encryption key (CEK) used in an
EnvelopedData, a key identifier can be included in the
unprotectedAttrs field of MSG1. This key can then be used to derive
the key-encryption key (KEK) for other instances of EnvelopedData or
for other purposes. If the CEK from MSG1 is to be used to derive the
KEK for MSG2 then MSG1 MUST contain an unprotectedAttrs Attribute of
type id-aa-CEKReference with a single value using the CEKReference
syntax.
MSG2.KEK is to be derived by reversing the bytes of MSG1.CEK. The
byte reversal is to avoid an attack where the attacker has a known
plaintext and the related ciphertext (encrypted with MSG1.CEK) that
(otherwise) could be directly used as a MSG2.KEK.
The application MUST ensure that the relevant algorithms are
compatible. That is, a CEKReference attribute alone can only be used
where the content-encryption algorithm from MSG1 employs the same
type of symmetric key as the key-encryption algorithm from MSG2.
Notes:
1) There is nothing to prevent inclusion of a CEKReference attribute
in MSG2 as well as in MSG1. That is, an originator could "roll"
the referenced CEK with every message.
2) The CEKReference attribute can occur with any of the choices for
RecipientInfo: ktri, kari or kekri. Implementors MUST NOT assume
that CEKReference can only occur where ktri or kari is used.
id-aa-CEKReference OBJECT IDENTIFIER ::= { id-aa 30 }
CEKReference ::= OCTET STRING
id-aa is an object identifier defined in [CMS-MSG].
In order to allow the originator of MSG1 to indicate the "lifetime"
of the CEK, the originator MAY include a CEKMaxDecrypts attribute,
also in the unprotectedAttrs field of EnvelopedData. This attribute
has an INTEGER syntax (the value MUST be >=1 and maximally 2^31),
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and indicates to the recipient the maximum number of messages
(excluding MSG1) that will use the referenced CEK. This Attribute
MUST only be sent when a CEKReference attribute is also included.
The recipient SHOULD maintain the CEKReference information
(minimally the key identifier and the CEK value) while less than
maxDecrypt messages have been successfully received. Recipients
SHOULD delete the CEKReference information after some locally
configured period.
When this attribute is not present, originators and recipients
SHOULD behave as if a value of one had been sent.
id-aa-CEKMaxDecrypts OBJECT IDENTIFIER ::= { id-aa 31 }
CEKMaxDecrypts ::= INTEGER
4. Using different CEK and KEK algorithms
Where MSG1.content-encryption algorithm and MSG2.key-encryption
algorithm are the same then the MSG2.KEK is the byte-reverse of
MSG1.CEK. However, in general, these algorithms MAY differ, e.g.
requiring different key lengths. This section specifies a generic
way to derive MSG2.KEK for such cases.
Note: In some sense, the CEK and KEK algorithms are never the
"same", e.g. id-alg-CMS3DESwrap and des-ede3-cbc differ. However,
for the purposes of this specification, all we care about is that
the algorithms use the same format and size of keying material (see
also security considerations) and that they do not differ
significantly in terms of the resulting cryptographic "strength". In
that sense the two algorithms in the example above are the "same."
Implementations MAY include this functionality.
The basic approach is to use the PBKDF2 key derivation function
defined in PKCS#5 [RFC2898], but using MSG1.CEK as input instead of
a password. The output of the PBKDF2 function is MSG2.KEK. To this
end, a new attribute type is defined which allows passing of the
required parameters.
id-aa-KEKDerivationAlg OBJECT IDENTIFIER ::= { id-aa 32 }
KEKDerivationAlgorithm ::= SEQUENCE {
kekAlg AlgorithmIdentifier,
pbkdf2Param PBKDF2-params
}
keyAlg is the algorithm identifier (and associated parameters, if
any), for the MSG2 key encryption algorithm. Note that it is not
necessary to protect this field MSG.KEK is only used by the
originator.
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The PBKDF2 algorithm parameters are to be handled as follows:
- The salt MUST use the "specified" element of the CHOICE.
- The message originator selects the iterationCount.
- The value of keyLength is determined by the kekAlg and MUST be
present.
- The prf field MUST use the default algorithm specified in
[RFC2898] which is algid-hmacWithSHA1 (and so the prf field MUST
be omitted).
5. Conformance
This specification only applies to messages where the CEKReference
attribute is present. All attributes specified here SHOULD be
ignored unless they are present in a message containing a valid, new
or recognized, existing value of CEKReference. The CEKMaxDecrypts
attribute is to be treated by the recipient as a hint, but MUST be
honored by the originator.
The optional to implement KEKDerivationAlgorithm attribute MUST only
be present when MSG1.content-encryption-algorithm differs from
MSG2.key-encryption-algorithm, in which case it MUST be present.
Implementations that recognize this attribute, but do not support
the functionality SHOULD ignore the attribute.
Ignoring attributes as discussed above, will lead to decryption
failures. CMS implementations SHOULD be able to signal the
particular reason for this failure to the calling application.
6. Security Considerations
Encryption does not provide authentication, for example, if the
encryptedContent is essentially random then recipients MUST NOT
assume that "knowing" a CEKReference value proves anything - anyone
could have created the EnvelopedData. This is relevant both for
security (the recovered plaintext should not be entirely random) and
for avoiding denial of service (the recipient MUST NOT assume that
using the right CEKReference means that message originator is
genuine).
Similarly, using the correct CEKReference does not mean that a
message has not been replayed or inserted, and recipients MUST NOT
assume that replay has been avoided.
The maxDecrypts field presents a potential denial-of-service attack
if a very large value is included by an originator in an attempt to
get a recipient to consume memory by storing the referenced CEKs for
a long period or if the originator never sends the indicated number
of ciphertexts. Recipients SHOULD therefore drop referenced CEKs
where the maxDecrypts value is too large (according to local
configuration) or the referenced CEK has been held for too long a
period.
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Suppose MSG1 is sent to a set S1 of users. In the case where MSG2 is
sent to only a subset of users in S1, all users from S1 will still
be able to decrypt MSG2 (since MSG2.KEK is computed only from
MSG1.CEK). Implementers should be aware that in such cases, all
members of the original set of recipients (S1) can access the
plaintext of MSG2 and subsequent messages.
The reason for the byte reversal is as follows: without the byte
reversal, an attacker knowing some of MSG1.plaintext (a prefix in a
field for instance) can use the corresponding ciphertext block as
the next encrypted CEK, i.e. as MSG2.KEKRecipientInfo.encryptedKey.
Now the attacker knows the next CEK. This attacks something this
note is not claiming to protect (origin authentication), but is
easily avoided using the byte reversal. Byte-reversal was chosen
over bit-reversal since bit-reversal would cause parity bits from
MSG1.CEK to be used as keying bits for MSG2.KEK for DES-based
algorithms. Note that byte reversal would similarly affect parity if
parity checks spanned more than one octet, however no well-known
algorithms operate in this way.
7. References
[CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630.
[CMS-MSG] Ramsdell, B. "S/MIME Version 3 Message Specification",
RFC 2633.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC 2026, BCP 9, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119.
Author's Addresses
Stephen Farrell,
Baltimore Technologies,
39 Parkgate Street,
Dublin 8
IRELAND
tel: +353-1-881-6000
email: stephen.farrell@baltimore.ie
Sean Turner
IECA, Inc.
9010 Edgepark Road
Vienna, VA 22182
USA
tel: +1.703.628.3180
email: turners@ieca.com
Full Copyright Statement
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Copyright (C) The Internet Society (date). All Rights Reserved.
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are included on all such copies and derivative works. In addition,
the ASN.1 module presented in Appendix B may be used in whole or in
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Appendix A: ASN.1 Module
SMIMERcek
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) rcek(13) }
-- This module contains the definitions of the attributes
-- used for re-using the content encryption key from a
-- message in further messages.
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
AlgorithmIdentifier FROM
AuthenticationFramework { joint-iso-itu-t ds(5)
module(1) authenticationFramework(7) 3 } ;
-- [RFC2898] uses 1993 ASN.1 to define PBKDF2-params. Since
-- this specification only uses 1988 ASN.1, the definition is
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-- repeated here for completeness.
-- The DEFAULT prf field value, MUST be used for this
-- specification
digestAlgorithm OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) us(840) rsadsi(113549) 2}
id-hmacWithSHA1 OBJECT IDENTIFIER ::= {digestAlgorithm 7}
-- [RFC2898] defines PBKDF2-params using 1993 ASN.1, which
-- results in the same encoding as produced by the definition
-- below. See [RFC2898] for that definition.
PBKDF2-params ::= SEQUENCE {
salt CHOICE {
specified OCTET STRING, -- MUST BE USED
otherSource AlgorithmIdentifier -- DO NOT USE THIS FIELD
},
iterationCount INTEGER (1..MAX),
keyLength INTEGER (1..MAX) OPTIONAL,
}
-- id-aa is the arc with all new authenticated and
-- unauthenticated attributes produced the by S/MIME
-- Working Group. It is also defined in [CMS-MSG]
id-aa OBJECT IDENTIFIER ::=
{iso(1) member-body(2) usa(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) attributes(2)}
-- This attribute contains what will be the key identifier
-- for subsequent messages
id-aa-CEKReference OBJECT IDENTIFIER ::= { id-aa 30 }
CEKReference ::= OCTET STRING
-- This attribute contains a "hint" to the recipient
-- indicating how many times the originator will use
-- the re-used CEK
id-aa-CEKMaxDecrypts OBJECT IDENTIFIER ::= { id-aa 31 }
CEKMaxDecrypts ::= INTEGER
-- This attribute specifies the key derivation function
-- to be used when the default byte reversal operation cannot
-- be used.
id-aa-KEKDerivationAlg OBJECT IDENTIFIER ::= { id-aa 32 }
KEKDerivationAlgorithm ::= SEQUENCE {
kekAlg AlgorithmIdentifier,
pbkdf2Param PBKDF2-params }
END
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Appendix B: Revision History
Note to RFC editor: Please delete this section.
Changes from -00 to -01:
- Removed error flag attribute, since this is the responsibility of
a consuming protocol
- Change the key derivation from home-grown to use pkcs#5 scheme
- Added compilable ASN.1 module
Changes from -01 to -02:
- Changed default KDF from bit to byte reversal to avoid parity-bit
problems
- Added allocated OIDs for module and attributes
- Added more justification text to section 2
- Added conformance text (new section 5)
- Added security consideration about subset of recipients
- Added security consideration describing reason for byte reversal
- Changed from unidirectional since Diameter may need bi-directional
- Copied kdf params stuff from rfc2898 since it uses '93 ASN.1
- Changed so that max decrypts=1, implies that one more message can
re-use the CEK (used to be silly where a value of 1 meant no more)
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