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Network Working Group
Request for Comments: 3185
Category: Standards Track
S. Farrell
Baltimore Technologies
S. Turner
IECA
October 2001

Reuse of CMS Content Encryption Keys

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.

Copyright Notice

Copyright © The Internet Society (2001). All Rights Reserved.

Abstract

This document describes a way to include a key identifier in a CMS (Cryptographic Message Syntax) enveloped data structure, so that the content encryption key can be re-used for further enveloped data packets.

Table Of Contents

   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
   Authors' Addresses................................................  6
   Appendix A: ASN.1 Module..........................................  7
   Full Copyright Statement.......................................... 10

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 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.
  1. 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), 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
   
   If CEKMaxDecrypts is missing, or has the value one, then each CEK
   will be re-used once as the KEK for the next message.  This means
   that MSG[n].KEK is the byte-reversal of MSG[n-1].CEK; subsequently
   MSG[n+1].KEK will be the byte-reversal of MSG[n].CEK.  Note that
   MSG[n-1].CEK has no impact whatsoever to MSG[n+1], so long as CEKs
   are generated randomly (and not e.g., derived from KEKs somehow).

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
   }

kekAlg 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 since modification of keyAlg only represents a denial-of-service attack.

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.

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.

Implementations should be very careful with this scheme if MSG[n].KEK is used to derive MSG[n].CEK, e.g., if MSG[n].CEK were the byte- reversal of MSG[n].KEK, then this scheme could result in a fixed key being unexpectedly used.

7. References

   [CMS]     Housley, R., "Cryptographic Message Syntax", RFC 2630, June
             1999.
   
   [CMS-MSG] Ramsdell, B. "S/MIME Version 3 Message Specification", RFC
             2633, June 1999.
   
   [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", BCP 9, RFC 2026, October 1996.
   
   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

Authors' Addresses

Stephen Farrell,
Baltimore Technologies,
39 Parkgate Street,
Dublin 8
IRELAND 

   Phone: +353-1-881-6000
   EMail: stephen.farrell@baltimore.ie

Sean Turner
IECA, Inc.
9010 Edgepark Road
Vienna, VA 22182
USA 

   Phone: +1.703.628.3180
   EMail: turners@ieca.com

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
       -- 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

Full Copyright Statement

Copyright © The Internet Society (2001). All Rights Reserved.

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Acknowledgement

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