Using Pre-Shared Key (PSK) in the Cryptographic Message Syntax (CMS)
draft-housley-cms-mix-with-psk-03

INTERNET-DRAFT                                                R. Housley
Intended Status: Proposed Standard                        Vigil Security
Expires: 5 September 2018                                   5 March 2018

  Using Pre-Shared Key (PSK) in the Cryptographic Message Syntax (CMS)
                <draft-housley-cms-mix-with-psk-03.txt>

Abstract

   The invention of a large-scale quantum computer would pose a serious
   challenge for the cryptographic algorithms that are widely deployed
   today.  The Cryptographic Message Syntax (CMS) supports key transport
   and key agreement algorithms that could be broken by the invention of
   such a quantum computer.  By storing communications that are
   protected with the CMS today, someone could decrypt them in the
   future when a large-scale quantum computer becomes available.  Once
   quantum-secure key management algorithms are available, the CMS will
   be extended to support them, if current syntax the does not
   accommodated them.  In the near-term, this document describes a
   mechanism to protect today's communication from the future invention
   of a large-scale quantum computer by mixing the output of key
   transport and key agreement algorithms with a pre-shared key.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  ASN.1  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Version Numbers  . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  KeyTransPSKRecipientInfo . . . . . . . . . . . . . . . . . . .  5
   4.  KeyAgreePSKRecipientInfo . . . . . . . . . . . . . . . . . . .  6
   5.  ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 12
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 12
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.  Introduction

   The invention of a large-scale quantum computer would pose a serious
   challenge for the cryptographic algorithms that are widely deployed
   today.  It is an open question whether or not it is feasible to build
   a large-scale quantum computer, and if so, when that might happen.
   However, if such a quantum computer is invented, many of the
   cryptographic algorithms and the security protocols that use them
   would become vulnerable.

   The Cryptographic Message Syntax (CMS) [RFC5652][RFC5803] supports
   key transport and key agreement algorithms that could be broken by
   the invention of a large-scale quantum computer [C2PQ].  These
   algorithms include RSA [RFC4055], Diffie-Hellman [RFC2631], and
   Elliptic Curve Diffie-Hellman.  As a result, an adversary that stores
   CMS-protected communications today, could decrypt those
   communications in the future when a large-scale quantum computer
   becomes available.

   Once quantum-secure key management algorithms are available, the CMS
   will be extended to support them, if current syntax the does not

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   accommodated them.

   In the near-term, this document describes a mechanism to protect
   today's communication from the future invention of a large-scale
   quantum computer by mixing the output of existing key transport and
   key agreement algorithms with a pre-shared key (PSK).  Secure
   communication can be achieved today by mixing with a strong PSK with
   the output of an existing key transport algorithm, like RSA, or an
   existing key agreement algorithm, like Diffie-Hellman [RFC2631] or
   Elliptic Curve Diffie-Hellman [RFC5753].  A security solution that is
   believed to be quantum resistant can be achieved by using a PSK with
   sufficient entropy along with a quantum resistant key derivation
   function (KDF), like HKDF [RFC5869], and a quantum resistant
   encryption algorithm, like 256-bit AES [AES].  In this way, today's
   CMS-protected communication can be invulnerable to an attacker with a
   large-scale quantum computer.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  ASN.1

   CMS values are generated using ASN.1 [X680], which uses the Basic
   Encoding Rules (BER) and the Distinguished Encoding Rules (DER)
   [X690].

1.3.  Version Numbers

   The major data structures include 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.

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2.  Overview

   The CMS enveloped-data content type [RFC5652] and the CMS
   authenticated-enveloped-data content type [RFC5083] support both key
   transport and key agreement public-key algorithms to establish the
   key used to encrypt the content.  In both cases, the sender randomly
   generates the key, and then all recipient obtain that key.  For
   enveloped-data, a content-encryption key is established.  For
   authenticated-enveloped-data, a content-authenticated-encryption key
   is established.  All recipients use the sender-generated key for
   decryption.

   This specification defines two quantum-resistant ways to establish
   these keys.  In both cases, a PSK MUST be distributed to the sender
   and all of the recipients by some out-of-band means that does not
   make it vulnerable to the future invention of a large-scale quantum
   computer, and an identifier MUST be assigned to the PSK.

   The content-encryption key or content-authenticated-encryption key is
   established by following these steps:

   1. The content-encryption key or the content-authenticated-encryption
      key is generated at random.

   2. The key-derivation key is generated at random.

   3. The key-encryption key is established for each recipient.  The
      details depend on the key management algorithm used:

         key transport:  the key-derivation key is encrypted in the
         recipient's public key, then the key derivation  function (KDF)
         is used to mix the pre-shared key (PSK) and the key-derivation
         key to produce the key-encryption key; or

         key agreement:  the recipient's public key and the sender's
         private key are used to generate a pairwise symmetric key, then
         the key derivation function (KDF) is used to mix the pre-shared
         key (PSK) and the pairwise symmetric key to produce the key-
         encryption key.

   4. The key-encryption key is used to encrypt the content-encryption
      key or content-authenticated-encryption key.

   As specified in Section 6.2.5 of [RFC5652], recipient information for
   additional key management techniques are represented in the
   OtherRecipientInfo type.  Two key management techniques are specified
   in this document.  Each of these is identified by a unique ASN.1
   object identifier.

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   The first key management technique, called keyTransPSK, see Section
   3, uses a key transport algorithm to transfer the key-derivation key
   from the sender to the recipient, and then key-derivation key is
   mixed with the PSK using a KDF.  The output of the KDF is the key-
   encryption key for the encryption of the content-encryption key or
   content-authenticated-encryption key.

   The second key management technique, called keyAgreePSK, see Section
   4, uses a key agreement algorithm to establish a pairwise key-
   encryption key, which is used to encrypt the key-derivation key, and
   then key-derivation key is mixed with the PSK using a KDF.  The
   output of the KDF is the key-encryption key for the encryption of the
   content-encryption key or content-authenticated-encryption key.

3.  KeyTransPSKRecipientInfo

   Per-recipient information using keyTransPSK is represented in the
   KeyTransPSKRecipientInfo type, and its use is indicated by the id-
   ori-keyTransPSK object identifier.  Each instance of
   KeyTransPSKRecipientInfo establishes the content-encryption key or
   content-authenticated-encryption key for one or more recipients that
   have access to the same PSK.

   The id-ori-keyTransPSK object identifier is:

      id-ori OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
        rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) TBD1 }

      id-ori-keyTransPSK OBJECT IDENTIFIER ::= { id-ori 1 }

   The KeyTransPSKRecipientInfo type is:

      KeyTransPSKRecipientInfo ::= SEQUENCE {
        version CMSVersion,  -- always set to 0
        pskid PreSharedKeyIdentifier,
        kdfAlgorithm KeyDerivationAlgorithmIdentifier,
        keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
        ktris KeyTransRecipientInfos,
        encryptedKey EncryptedKey }

      PreSharedKeyIdentifier ::= OCTET STRING

      KeyTransRecipientInfos ::= SEQUENCE OF KeyTransRecipientInfo

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   The fields of the KeyTransPSKRecipientInfo type have the following
   meanings:

      version is the syntax version number.  The version MUST be 0.  The
      CMSVersion type is described in Section 10.2.5 of [RFC5652].

      pskid is the identifier of the PSK used by the sender.  The
      identifier is an OCTET STRING, and it need not be human readable.

      kdfAlgorithm identifies the key-derivation algorithm, and any
      associated parameters, used by the sender to mix the key-
      derivation key and the PSK to generate the key-encryption key.
      The KeyDerivationAlgorithmIdentifier is described in Section
      10.1.6 of [RFC5652].

      keyEncryptionAlgorithm identifies a key-encryption algorithm used
      to encrypt the content-encryption key.  The
      KeyEncryptionAlgorithmIdentifier is described in Section 10.1.3 of
      [RFC5652].

      ktris contains one KeyTransRecipientInfo type for each recipient;
      it uses a key transport algorithm to establish the key-derivation
      key.  KeyTransRecipientInfo is described in Section 6.2.1 of
      [RFC5652].

      encryptedKey is the result of encrypting the content-encryption
      key or the content-authenticated-encryption key with the key-
      encryption key.  EncryptedKey is an OCTET STRING.

4.  KeyAgreePSKRecipientInfo

   Per-recipient information using keyAgreePSK is represented in the
   KeyAgreePSKRecipientInfo type, and its use is indicated by the id-
   ori-keyAgreePSK object identifier.  Each instance of
   KeyAgreePSKRecipientInfo establishes the content-encryption key or
   content-authenticated-encryption key for one or more recipients that
   have access to the same PSK.

   The id-ori-keyAgreePSK object identifier is:

      id-ori-keyAgreePSK OBJECT IDENTIFIER ::= { id-ori 2 }

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   The KeyAgreePSKRecipientInfo type is:

      KeyAgreePSKRecipientInfo ::= SEQUENCE {
        version CMSVersion,  -- always set to 0
        pskid PreSharedKeyIdentifier,
        originator [0] EXPLICIT OriginatorIdentifierOrKey,
        ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
        kdfAlgorithm KeyDerivationAlgorithmIdentifier,
        keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
        recipientEncryptedKeys RecipientEncryptedKeys }

   The fields of the KeyAgreePSKRecipientInfo type have the following
   meanings:

      version is the syntax version number.  The version MUST be 0.  The
      CMSVersion type is described in Section 10.2.5 of [RFC5652].

      pskid is the identifier of the PSK used by the sender.  The
      identifier is an OCTET STRING, and it need not be human readable.

      originator is a CHOICE with three alternatives specifying the
      sender's key agreement public key.  Implementations MUST support
      all three alternatives for specifying the sender's public key.
      The sender uses the corresponding private key and the recipient's
      public key to generate a pairwise key.  A key derivation function
      (KDF) is used to mix the PSK and the pairwise key to produce a
      key-encryption key.  The OriginatorIdentifierOrKey type is
      described in Section 6.2.2 of [RFC5652].

      ukm is optional.  With some key agreement algorithms, the sender
      provides a User Keying Material (UKM) to ensure that a different
      key is generated each time the same two parties generate a
      pairwise key.  Implementations MUST accept a
      KeyAgreePSKRecipientInfo SEQUENCE that includes a ukm field.
      Implementations that do not support key agreement algorithms that
      make use of UKMs MUST gracefully handle the presence of UKMs.  The
      UserKeyingMaterial type is described in Section 10.2.6 of
      [RFC5652].

      kdfAlgorithm identifies the key-derivation algorithm, and any
      associated parameters, used by the sender to mix the pairwise key
      and the PSK.  The KeyDerivationAlgorithmIdentifier is described in
      Section 10.1.6 of [RFC5652].

      keyEncryptionAlgorithm identifies a key-encryption algorithm used
      to encrypt the content-encryption key or the content-
      authenticated-encryption key.  The
      KeyEncryptionAlgorithmIdentifier type is described in Section

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      10.1.3 of [RFC5652].

      recipientEncryptedKeys includes a recipient identifier and
      encrypted key for one or more recipients.  The
      KeyAgreeRecipientIdentifier is a CHOICE with two alternatives
      specifying the recipient's certificate, and thereby the
      recipient's public key, that was used by the sender to generate a
      pairwise key.  The encryptedKey is the result of encrypting the
      content-encryption key or the content-authenticated-encryption key
      with the key-encryption key.  EncryptedKey is an OCTET STRING.
      The RecipientEncryptedKeys type is defined in Section 6.2.2 of
      [RFC5652].

5.  ASN.1 Module

   This section contains the ASN.1 module for the two key management
   techniques defined in this document.  This module imports types from
   other ASN.1 modules that are defined in [RFC5911] and [RFC5912].

   CMSORIforPSK-2017
     { iso(1) member-body(2) us(840) rsadsi(113549)
       pkcs(1) pkcs-9(9) smime(16) modules(0)
       id-mod-cms-ori-psk-2017(TBD0) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   -- EXPORTS All

   IMPORTS

   AlgorithmIdentifier{}, KEY-DERIVATION
     FROM AlgorithmInformation-2009  -- [RFC5912]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-algorithmInformation-02(58) }

   OTHER-RECIPIENT, OtherRecipientInfo, CMSVersion,
   KeyTransRecipientInfo, OriginatorIdentifierOrKey,
   UserKeyingMaterial, RecipientEncryptedKeys, EncryptedKey,
   KeyDerivationAlgorithmIdentifier, KeyEncryptionAlgorithmIdentifier
     FROM CryptographicMessageSyntax-2009  -- [RFC5911]
       { iso(1) member-body(2) us(840) rsadsi(113549)
         pkcs(1) pkcs-9(9) smime(16) modules(0)
         id-mod-cms-2004-02(41) } ;

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   --
   -- OtherRecipientInfo Types (ori-)
   --

   SupportedOtherRecipInfo OTHER-RECIPIENT ::= {
     ori-keyTransPSK |
     ori-keyAgreePSK,
     ... }

   --
   -- Key Transport with Pre-Shared Key
   --

   ori-keyTransPSK OTHER-RECIPIENT ::= {
     KeyTransPSKRecipientInfo IDENTIFIED BY id-ori-keyTransPSK }

   id-ori OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
     rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) TBD1 }

   id-ori-keyTransPSK OBJECT IDENTIFIER ::= { id-ori 1 }

   KeyTransPSKRecipientInfo ::= SEQUENCE {
     version CMSVersion,  -- always set to 0
     pskid PreSharedKeyIdentifier,
     kdfAlgorithm KeyDerivationAlgorithmIdentifier,
     keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
     ktris KeyTransRecipientInfos,
     encryptedKey EncryptedKey }

   PreSharedKeyIdentifier ::= OCTET STRING

   KeyTransRecipientInfos ::= SEQUENCE OF KeyTransRecipientInfo

   --
   -- Key Agreement with Pre-Shared Key
   --

   ori-keyAgreePSK ORI-TYPE ::= {
     KeyAgreePSKRecipientInfo IDENTIFIED BY id-ori-keyAgreePSK }

   id-ori-keyAgreePSK OBJECT IDENTIFIER ::= { id-ori 2 }

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   KeyAgreePSKRecipientInfo ::= SEQUENCE {
     version CMSVersion,  -- always set to 0
     pskid PreSharedKeyIdentifier,
     originator [0] EXPLICIT OriginatorIdentifierOrKey,
     ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
     kdfAlgorithm KeyDerivationAlgorithmIdentifier,
     keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
     recipientEncryptedKeys RecipientEncryptedKeys }

   END

6.  Security Considerations

   Implementations must protect the pre-shared key (PSK), key transport
   private key, the agreement private key, the key-derivation key, and
   the key-encryption key.  Compromise of the PSK will make the
   encrypted content vulnerable to the future invention of a large-scale
   quantum computer.  Compromise of the key transport private key or the
   agreement private key may result in the disclosure of all contents
   protected with that key.  Compromise of the key-derivation key that
   is established with the key transport private key or the agreement
   private key may result in disclosure of the associated encrypted
   content.  Compromise of the key-encryption key may result in the
   disclosure of all content-encryption keys or content-authenticated-
   encryption keys that were protected with that key, which in turn may
   result in the disclosure of the content.

   A large-scale quantum computer will effectively cut the security
   provided by a symmetric key algorithm in half.  As a result, the PSK,
   the key-derivation key, and the key-encryption key need at least 256
   bits of entropy to provide 128 bits of security.  For this reason,
   these symmetric keys SHOULD be at least 256 bits in length.

   Implementations must randomly generate key-derivation key as well as
   the content-encryption key or content-authenticated-encryption key.
   Also, the generation of public/private key pairs for the key
   transport and key agreement algorithms rely 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, searching the resulting small set of
   possibilities, rather than brute force searching the whole key space.
   The generation of quality random numbers is difficult.  [RFC4086]
   offers important guidance in this area.

   When using a key agreement algorithm, a key-encryption key is
   produced to encrypt the content-encryption key or content-
   authenticated-encryption key.  If the key-encryption algorithm is

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   different that the algorithm used to protect the content, then the
   effective security is determined by the weaker of the two algorithms.
   If, for example, content is encrypted with 256-bit AES, and the key
   is wrapped with 128-bit AES, then at most 128 bits of protection is
   provided.  Implementers must ensure that the key-encryption algorithm
   is as strong or stronger than the content-encryption algorithm or
   content-authenticated-encryption algorithm.

   Implementers should not mix the quantum-resistant key management
   algorithms with their non-quantum-resistant counterparts.  That is,
   the same content should not be protected with KeyTransRecipientInfo
   and KeyTransPSKRecipientInfo, and the same content should not be
   protected with KeyAgreeRecipientInfo and KeyAgreePSKRecipientInfo.
   Doing so would make the content vulnerable to the future invention of
   a large-scale quantum computer.

   Implementer should not send the same content in different in separate
   messages, one using a quantum-resistant key management algorithm and
   the other using a non-quantum-resistant key management algorithm,
   even if the content-encryption key is generated independently.  Doing
   so may allow an eavesdropper to correlate the messages, making the
   content vulnerable to the future invention of a large-scale quantum
   computer.

   Implementers should be aware that cryptographic algorithms become
   weaker with time.  As new cryptoanalysis techniques are developed and
   computing performance improves, the work factor to break a particular
   cryptographic algorithm will be reduced.  Therefore, cryptographic
   algorithm implementations should be modular, allowing new algorithms
   to be readily inserted.  That is, implementors should be prepared for
   the set of supported algorithms to change over time.

7.  IANA Considerations

   One object identifier for the ASN.1 module in the Section 5 was
   assigned in the SMI Security for S/MIME Module Identifiers
   (1.2.840.113549.1.9.16.0) [IANA-MOD] registry:

      id-mod-cms-ori-psk-2017 OBJECT IDENTIFIER ::= {
         iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
         pkcs-9(9) smime(16) mod(0) TBD0 }

   One object identifier for an arc to assign Other Recipient Info
   Identifiers was assigned in the SMI Security for S/MIME Mail Security
   (1.2.840.113549.1.9.16) [IANA-SMIME] registry:

      id-ori OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
        rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) TBD1 }

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   This assignment created the new SMI Security for Other Recipient Info
   Identifiers (1.2.840.113549.1.9.16.TBD1) [IANA-ORI] registry with the
   following two entries with references to this document:

      id-ori-keyTransPSK OBJECT IDENTIFIER ::= {
         iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
         pkcs-9(9) smime(16) id-ori(TBD1) 1 }

      id-ori-keyAgreePSK OBJECT IDENTIFIER ::= {
         iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
         pkcs-9(9) smime(16) id-ori(TBD1) 2 }

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
              Authenticated-Enveloped-Data Content Type", RFC 5083,
              November 2007.

   [RFC5652]   Housley, R., "Cryptographic Message Syntax (CMS)", RFC
              5652, September 2009.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, May 2017.

   [X680]     ITU-T, "Information technology -- Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, 2015.

   [X690]     ITU-T, "Information technology -- ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, 2015.

9.  Informative References

   [AES]      National Institute of Standards and Technology, FIPS Pub
              197: Advanced Encryption Standard (AES), 26 November 2001.

   [C2PQ]     Hoffman, P., "The Transition from Classical to Post-
              Quantum Cryptography", work-in-progress, draft-hoffman-
              c2pq-02, August 2017.

   [IANA-MOD] https://www.iana.org/assignments/smi-numbers/smi-
              numbers.xhtml#security-smime-0.

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   [IANA-SMIME] https://www.iana.org/assignments/smi-numbers/smi-
              numbers.xhtml#security-smime.

   [IANA-ORI] https://www.iana.org/assignments/smi-numbers/smi-
              numbers.xhtml#security-smime-13.

   [RFC2631]  Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
              2631, June 1999.

   [RFC3560]  Housley, R., "Use of the RSAES-OAEP Key Transport
              Algorithm in Cryptographic Message Syntax (CMS)", RFC
              3560, July 2003.

   [RFC4086]  D. Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", RFC 4086, June
              2005.

   [RFC5753]  Turner, S., and D. Brown, "Use of Elliptic Curve
              Cryptography (ECC) Algorithms in Cryptographic Message
              Syntax (CMS)", RFC 5753, January 2010.

   [RFC5869]  Krawczyk, H., and P. Eronen, "HMAC-based Extract-and-
              Expand Key Derivation Function (HKDF)", RFC 5869, May
              2010.

   [RFC5911]  Hoffman, P., and J. Schaad, "New ASN.1 Modules for
              Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
              June 2010.

   [RFC5912]  Hoffman, P., and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)" RFC 5912,
              June 2010.

Acknowledgements

   Many thanks to Burt Kaliski, Jim Schaad, and Sean Turner for their
   review and insightful comments.  They have greatly improved the
   design.

Author's Address

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA
   EMail: housley@vigilsec.com

Housley                                                        [Page 13]