INTERNET-DRAFT S. Farrell
Expires in six months Baltimore Technologies
S. Turner
IECA
September 2000
Reuse of CMS Content Encryption Keys
<draft-ietf-smime-rcek-00.txt>
Status of this Memo
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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. Security Considerations......................................5
6. References...................................................6
Author's Addresses..............................................6
Full Copyright Statement........................................6
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1. Introduction
<<Editorial comments are in angle-brackets, like this.>>
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 message 1) to derive the key encryption key
(KEK) for a later message, (message 2), 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 system 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".
- Key encryption is "unidirectional": that is, the referenced CEK is
only used by the originator for encryption and the recipient for
decryption, recipients must not expect originators to be able to
decrypt using (anything derived from) the referenced CEK. This
means that the referenced CEK MUST NOT be considered to be a
shared secret between many parties (i.e. this specification is not
sufficient for group keying schemes). This also means that
originators may have discarded the referenced CEK by the time the
recipient receives the first message containing the reference.
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Recipients MUST NOT use the referenced CEK when replying to the
originator. <<Is this too restrictive?>>
- 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
bits of keying material, then this specification SHOULD NOT be
used.
In particular, this specification is not intended to be a general
specification for group key management.
3. How to do it
In order to reference the content-encryption key (CEK) used in an
EnvelopedData (call this MSG1) 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 (say MSG2) or for other purposes. If the CEK from MSG1
is to be used to dervie the KEK for MSG2 then MSG1 MUST contain an
unprotectedAttrs Attribute of type id-cek-reference with a single
value using the CEKReference syntax.
MSG2.KEK is to be derived by reversing the bits of MSG1.CEK. The bit
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 in 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-cek-reference ::= OBJECT IDENTIFIER { TBD }
CEKReference ::= OCTET STRING
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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 indicates to the
recipient the maximum number of messages (including this one) 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.
id-cek-maxDecrypts ::= OBJECT IDENTIFIER { TBD }
CEKMaxDecrypts ::= INTEGER
<<The unknown reference attribute is probably not needed, since the
consuming application probably has to do the signalling at the
application layer and not the "CMS" layer, but if we did need
it... Unless there's a specific reason, this will disappear in the
next draft.>>
When a recipient receives a message that contains a CEKReference
that it cannot use (for whatever reason), then the recipient MAY
respond to the originator with a message containing an
UnknownReference attribute. This attribute SHOULD be in either
signedAttrs or authenticatedAttrs, but MAY be in unsignedAttrs in an
otherwise empty SignedData.
id-kek-unknownReference::= OBJECT IDENTIFIER { TBD }
UnknownReference ::= OCTET STRING
The value of this MUST be the KEKIdentifier from the message that
caused the problem.
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 bit-reversal 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.
Implementations MAY include this functionality.
In this case the originator MUST include a KEKDerivationAlgorithm
attribute in MSG1, which indicates how to derive MSG2.KEK from
MSG1.CEK. This attribute has the following syntax:
id-kek-derivation-algorithm ::= OBJECT IDENTIFIER { TBD }
KEKDerivationAlgorithm ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
iv OCTET STRING OPTIONAL,
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padding OCTET STRING OPTIONAL,
kekAlg AlgorithmIdentifier }
<<It's not clear that this specification needs to define its own key
derivation scheme for this case (since itÆs a MAY, not a SHOULD or
MUST). If we have to, it would certainly be better if we could
reference an existing derivation algorithm. In other words, this
section is likely to change, a lot.>>
In order to derive MSG2.KEK from MSG1.CEK the following algorithm is
used:
1. Randomly pick an IV if necessary and some padding bytes
2. DER encode the kekAlg value, producing kek-alg-d. kek-alg-d does
include the full DER encoding, that is, it begins with the '30'H
from the ASN.1 SEQUENCE.
3. Catenate the following values to produce CEK-input:
CEK-input = "padding-bytes||CEK||CEKReference||key-alg-d"
Note: the padding and CEKReference are both transmitted in the
KEKDerivationAlgorithm structure as ASN.1 OCTET STRINGS, but
CEK-input MUST NOT include OCTET STRING tags or lengths for
either.
4. Encrypt CEK-input using MSG1.content-encryption algorithm and
the CEK as key, with the IV generated in step 1, (if an IV is
necessary), giving KEK-input.
5. The KEK is the rightmost bits of KEK-input, as required by
MSG2.key-encryption-algorithm.
5. 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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<<More TBS no doubt.>>
6. References
[CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630.
[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,
61/62 Fitzwilliam Lane,
Dublin 2,
IRELAND
tel: +353-1-647-3000
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
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