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Versions: (draft-housley-cms-mix-with-psk) 00 01 02 03 04 05 06

INTERNET-DRAFT                                                R. Housley
Internet Engineering Task Force (IETF)                    Vigil Security
Intended Status: Proposed Standard
Expires: 5 December 2019                                     5 June 2019


  Using Pre-Shared Key (PSK) in the Cryptographic Message Syntax (CMS)
               <draft-ietf-lamps-cms-mix-with-psk-05.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 the new algorithms, if the existing syntax
   does not accommodate 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) 2019 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  ASN.1  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Version Numbers  . . . . . . . . . . . . . . . . . . . . .  4
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  KeyTransPSKRecipientInfo . . . . . . . . . . . . . . . . . . .  6
   4.  KeyAgreePSKRecipientInfo . . . . . . . . . . . . . . . . . . .  7
   5.  Key Derivation . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 15
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 16
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 16
   Appendix A: Key Transport with PSK Example . . . . . . . . . . . . 17
     A.1.  Originator Processing Example  . . . . . . . . . . . . . . 18
     A.2.  ContentInfo and AuthEnvelopedData  . . . . . . . . . . . . 20
     A.3.  Recipient Processing Example . . . . . . . . . . . . . . . 22
   Appendix B: Key Agreement with PSK Example . . . . . . . . . . . . 23
     B.1.  Originator Processing Example  . . . . . . . . . . . . . . 23
     B.2.  ContentInfo and AuthEnvelopedData  . . . . . . . . . . . . 26
     B.3.  Recipient Processing Example . . . . . . . . . . . . . . . 27
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 29
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29

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.




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   The Cryptographic Message Syntax (CMS) [RFC5652][RFC5083] 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 [RFC8017], Diffie-Hellman [RFC2631], and
   Elliptic Curve Diffie-Hellman [RFC5753].  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 the existing syntax does not
   already accommodate the new algorithms.

   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 a strong PSK with the
   output of an existing key transport algorithm, like RSA [RFC8017], 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.

   In addition, there may be other reasons for including a strong PSK
   besides protection against the future invention of a large-scale
   quantum computer.  For example, there is always the possibility of a
   cryptoanalytic breakthrough on one or more of the classic public-key
   algorithm, and there are longstanding concerns about undisclosed
   trapdoors in Diffie-Hellamn parameters.  Inclusion of a strong PSK as
   part of the overall key management offer additional protection
   against these concerns.

   Note that the CMS also supports key management techniques based on
   symmetric key-encryption keys and passwords, but they are not
   discussed in this document because they are already quantum
   resistant.  The symmetric key-encryption key technique is quantum
   resistant when used with an adequate key size.  The password
   technique is quantum resistant when used with a quantum-resistant key
   derivation function and a sufficiently large password.







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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.

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.  No restrictions are imposed on the
   key transport or key agreement public-key algorithms, which means
   that any key transport or key agreement algorithm can be used,
   including algorithms that are specified in the future.  In both
   cases, the sender randomly generates the content-encryption key, and
   then all recipients obtain that key.  All recipients use the sender-
   generated symmetric content-encryption key for decryption.

   This specification defines two quantum-resistant ways to establish a
   symmetric key-encryption key, which is used to encrypt the sender-
   generated content-encryption key.  In both cases, the PSK is used as
   one of the inputs to a key-derivation function to create a quantum-



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   resistant key-encryption key.  The 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
   quantum-resistant, and the sender establishes it using these steps:

      When using a key transport algorithm:

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

      2. The key-derivation key, called KDK, is generated at random.

      3. For each recipient, the KDK 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 KDK to produce the key-
         encryption key, called KEK.

      4. The KEK is used to encrypt the CEK.

      When using a key agreement algorithm:

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

      2. For each recipient, a pairwise key-encryption key, called KEK1,
         is established using the recipient's public key and the
         sender's private key.  Note that KEK1 will be used as a key-
         derivation key.

      3. For each recipient, the key derivation function (KDF) is used
         to mix the pre-shared key (PSK) and the pairwise KEK1, and the
         result is called KEK2.

      4. For each recipient, the pairwise KEK2 is used to encrypt the
         CEK.

   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, and they are each identified by a unique ASN.1
   object identifier.

   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 the key-



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   derivation key is mixed with the PSK using a KDF.  The output of the
   KDF is the key-encryption key, which is used 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 then mixed with the PSK using a KDF to
   produce a second pairwise key-encryption key, which is then used to
   encrypt the content-encryption key or content-authenticated-
   encryption key.

3.  KeyTransPSKRecipientInfo

   Per-recipient information using keyTransPSK is represented in the
   KeyTransPSKRecipientInfo type, which 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

   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].





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      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, which 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 }

   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 }





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   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 their own private key and the recipient's public
      key to generate a pairwise key-encryption key.  A key derivation
      function (KDF) is used to mix the PSK and the pairwise key-
      encryption key to produce a second 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-
      encryption key and the PSK to produce a second key-encryption key
      of the same length as the first one.  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
      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-encryption key.  The encryptedKey is the result of



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      encrypting the content-encryption key or the content-
      authenticated-encryption key with the second pairwise key-
      encryption key.  EncryptedKey is an OCTET STRING.  The
      RecipientEncryptedKeys type is defined in Section 6.2.2 of
      [RFC5652].

5.  Key Derivation

   Many key derivation functions (KDFs) internally employ a one-way hash
   function.  When this is the case, the hash function that is used is
   identified by the KeyDerivationAlgorithmIdentifier.  HKDF [RFC5869]
   is one example of a KDF that make use fo a hash function.

   A KDF has several input values.  This section describes the
   conventions for using the KDF to compute the key-encryption key for
   KeyTransPSKRecipientInfo and KeyAgreePSKRecipientInfo.  For
   simplicity, the terminology used in the HKDF [RFC5869] specification
   is used here.

   The KDF inputs are:

      IKM is the input keying material; it is the symmetric secret input
      to the KDF.  For KeyTransPSKRecipientInfo, it is the key-
      derivation key.  For KeyAgreePSKRecipientInfo, it is the pairwise
      key-encryption key produced by the key agreement algorithm.

      salt is an optional non-secret random value.  The salt is not
      used.

      L is the length of output keying material in octets; the value
      depends on the key-encryption algorithm that will be used.  The
      algorithm is identified by the KeyEncryptionAlgorithmIdentifier.
      In addition, the OBJECT IDENTIFIER portion of the
      KeyEncryptionAlgorithmIdentifier is included in the next input
      value, called info.

      info is optional context and application specific information.
      The DER-encoding of CMSORIforPSKOtherInfo is used as the info
      value, and the PSK is included in this structure.  Note that
      EXPLICIT tagging is used in the ASN.1 module that deines this
      structure.  For KeyTransPSKRecipientInfo, the ENUMERATED value of
      5 is used.  For KeyAgreePSKRecipientInfo, the ENUMERATED value of
      10 is used.  CMSORIforPSKOtherInfo is defined by the following








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      ASN.1 structure:

         CMSORIforPSKOtherInfo ::= SEQUENCE {
           psk                    OCTET STRING,
           keyMgmtAlgType         ENUMERATED {
             keyTrans               (5),
             keyAgree               (10) },
           keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
           pskLength              INTEGER (1..MAX),
           kdkLength              INTEGER (1..MAX) }

   The fields of type CMSORIforPSKOtherInfo have the following meanings:

      psk is an OCTET STRING; it contains the PSK.

      keyMgmtAlgType is either set to 5 or 10.  For
      KeyTransPSKRecipientInfo, the ENUMERATED value of 5 is used.  For
      KeyAgreePSKRecipientInfo, the ENUMERATED value of 10 is used.

      keyEncryptionAlgorithm is the KeyEncryptionAlgorithmIdentifier,
      which identifies the algorithm and provides algorithm parameters,
      if any.

      pskLength is a positive integer; it contains the length of the PSK
      in octets.

      kdkLength is a positive integer; it contains the length of the
      key-derivation key in octets.  For KeyTransPSKRecipientInfo, the
      key-derivation key is generated by the sender.  For
      KeyAgreePSKRecipientInfo, the key-derivation key is the pairwise
      key-encryption key produced by the key agreement algorithm.

   The KDF output is:

      OKM is the output keying material, which is exactly L octets.  The
      OKM is the key-encryption key that is used to encrypt the content-
      encryption key or the content-authenticated-encryption key.

6.  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].








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   <CODE BEGINS>

   CMSORIforPSK-2019
     { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
       smime(16) modules(0) id-mod-cms-ori-psk-2019(TBD0) }

   DEFINITIONS EXPLICIT 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) } ;

   --
   -- 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 }



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   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 }

   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 }

   --
   -- Structure to provide 'info' input to the KDF,
   -- including the Pre-Shared Key
   --

   CMSORIforPSKOtherInfo ::= SEQUENCE {
     psk                    OCTET STRING,
     keyMgmtAlgType         ENUMERATED {
       keyTrans               (5),
       keyAgree               (10) },
     keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
     pskLength              INTEGER (1..MAX),
     kdkLength              INTEGER (1..MAX) }

   END

   <CODE ENDS>




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7.  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 PSK and either the key transport
   private key or the agreement private key may result in the disclosure
   of all contents protected with that combination of keying material.
   Compromise of the PSK and the key-derivation key may result in
   disclosure of all contents protected with that combination of keying
   material.  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 keying materail, which
   in turn may result in the disclosure of the content.

   A large-scale quantum computer will essentially negate the security
   provided by the key transport algorithm or the key agreement
   algorithm, which means that the attacker with a large-scale quantum
   computer can discover the key-derivation key.  In addition a large-
   scale quantum computer effectively cuts the security provided by a
   symmetric key algorithm in half.  Therefore, the PSK needs at least
   256 bits of entropy to provide 128 bits of security.  To match that
   same level of security, the key derivation function needs to be
   quantum-resistant and produce a key-encryption key that is at least
   256 bits in length.  Similarly, the content-encryption key or
   content-authenticated-encryption key needs to be at least 256 bits in
   length.

   When using a PSK with a key transport or 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 different than 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 quantum-resistant key management
   algorithms with their non-quantum-resistant counterparts.  For
   example, the same content should not be protected with
   KeyTransRecipientInfo and KeyTransPSKRecipientInfo.  Likewise, 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.



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   Implementers should not send the same content in different 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.

   This specification does not require that PSK is known only by the
   sender and recipients.  The PSK may be known to a group.  Since
   confidentiality depends on the key transport or key agreement
   algorithm, knowledge of the PSK by other parties does not enable
   eavesdropping.  However, group members can record the traffic of
   other members, and then decrypt it if they ever gain access to a
   large-scale quantum computer.  Also, when many parties know the PSK,
   there are many opportunities for theft of the PSK by an attacker.
   Once an attacker has the PSK, they can decrypt stored traffic if they
   ever gain access to a large-scale quantum computer in the same manner
   as a legitimate group member.

   Sound cryptographic key hygiene is to use a key for one and only one
   purpose.  Use of the recipient's public key for both the traditional
   CMS and the PSK-mixing variation specified in this document would be
   a violation of this principle; however, there is no known way for an
   attacker to take advantage of this situation.  That said, an
   application should enforce separation whenever possible.  For
   example, an purpose identifier for use in the X.509 extended key
   usage certificate extension [RFC5280] could be identified in the
   future to indicate that a public key should only be used in
   conjunction with a PSK, or only without.

   Implementations must randomly generate key-derivation keys as well as
   the content-encryption keys or content-authenticated-encryption keys.
   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.

   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



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   the set of supported algorithms to change over time.

8.  Privacy Considerations

   An observer can see which parties are using each PSK simply by
   watching the PSK key identifiers.  However, the addition of these key
   identifiers is not really making privacy worse.  When key transport
   is used, the RecipientIdentifier is always present, and it clearly
   identifies each recipient to an observer.  When key agreement is
   used, either the IssuerAndSerialNumber or the RecipientKeyIdentifier
   is always present, and these clearly identify each recipient.

9.  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 }

   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 }











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10.  References

10.1.  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.

10.2.  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-04, August 2018.

   [H2019]    Hammell, J., "Re: [lamps] WG Last Call for draft-ietf-
              lamps-cms-mix-with-psk", <https://mailarchive.ietf.org/
              arch/msg/spasm/_6d_4jp3sOprAnbU2fp_yp_-6-k>, 27 May 2019.

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

   [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.




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

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

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [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.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, November 2016.

Appendix A: Key Transport with PSK Example

   This example shows the establishment of an AES-256 content-encryption
   key using:
      - a pre-shared key of 256 bits;
      - key transport using RSA PKCS#1 v1.5 with a 3072-bit key;
      - key derivation using HKDF with SHA-384; and
      - key wrap using AES-256-KEYWRAP.

   In real-world use, the originator would encrypt the key-derivation
   key in their own RSA public key as well as the recipient's public
   key.  This is omited in an attempt to simplify the example.







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A.1.  Originator Processing Example

   The pre-shared key known to Alice and Bob:
      c244cdd11a0d1f39d9b61282770244fb0f6befb91ab7f96cb05213365cf95b15

   The identifier assigned to the pre-shared key is:
      ptf-kmc:13614122112

   Alice obtains Bob's public key:
      -----BEGIN PUBLIC KEY-----
      MIIBojANBgkqhkiG9w0BAQEFAAOCAY8AMIIBigKCAYEA3ocW14cxncPJ47fnEjBZ
      AyfC2lqapL3ET4jvV6C7gGeVrRQxWPDwl+cFYBBR2ej3j3/0ecDmu+XuVi2+s5JH
      Keeza+itfuhsz3yifgeEpeK8T+SusHhn20/NBLhYKbh3kiAcCgQ56dpDrDvDcLqq
      vS3jg/VO+OPnZbofoHOOevt8Q/roahJe1PlIyQ4udWB8zZezJ4mLLfbOA9YVaYXx
      2AHHZJevo3nmRnlgJXo6mE00E/6qkhjDHKSMdl2WG6mO9TCDZc9qY3cAJDU6Ir0v
      SH7qUl8/vN13y4UOFkn8hM4kmZ6bJqbZt5NbjHtY4uQ0VMW3RyESzhrO02mrp39a
      uLNnH3EXdXaV1tk75H3qC7zJaeGWMJyQfOE3YfEGRKn8fxubji716D8UecAxAzFy
      FL6m1JiOyV5acAiOpxN14qRYZdHnXOM9DqGIGpoeY1UuD4Mo05osOqOUpBJHA9fS
      whSZG7VNf+vgNWTLNYSYLI04KiMdulnvU6ds+QPz+KKtAgMBAAE=
      -----END PUBLIC KEY-----

   Bob's RSA public key has the following key identifier:
      9eeb67c9b95a74d44d2f16396680e801b5cba49c

   Alice randomly generates a content-encryption key:
      c8adc30f4a3e20ac420caa76a68f5787c02ab42afea20d19672fd963a5338e83

   Alice randomly generates a key-derivation key:
      df85af9e3cebffde6e9b9d24263db31114d0a8e33a0d50e05eb64578ccde81eb

   Alice encrypts the key-derivation key in Bob's public key:
      4e6200431ed95e0e28f7288dba56d4b90e75959e068884664c43368f3d978f3d
      8179e5837e3c27bf8dc1f6e2827b9ede969be77417516de07d90e37c560add01
      48deb0c9178088ccb72c068d8a9076b6a5e7ecc9093e30fdeaecc9e138d80626
      74fcf685f3082b910839551cd8741beedeee6e87c08ff84f03ba87118730cdf7
      667002316f1a29a6cc596c7ddf95a51e398927d1916bf27929945de080fc7c80
      6af6281aed6492acffa4ef1b4f53e67fca9a417db2350a2277d586ee3cabefd3
      b4a44f04d3c6803d54fe9a7159210dabedda9a94f310d303331da51c0218d92a
      2efb003792259195a9fd4cc403af613fdf1a6265ea70bf702fd1c6f734264c9a
      59196e8e8fd657fa028e272ef741eb7711fd5b3f4ea7da9c33df66bf487da710
      1c9bbfddaf1c073900a3ea99da513d8aa32605db07dc1c47504cab30c9304a85
      d87377f603ec3df4056ddcf3d756fb7ed98254421a4ae151f17ad4e28c5ea077
      63358dfb1ef5f73435f337b21a38c1a3fa697a530dd97e462f6b5fb2052a2d53








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   Alice produces a 256-bit key-encryption key with HKDF using SHA-384;
   the secret value is the key-derivation key; the 'info' is the DER-
   encoded CMSORIforPSKOtherInfo structure with the following values:
       0   56: SEQUENCE {
       2   32:   OCTET STRING
             :     C2 44 CD D1 1A 0D 1F 39 D9 B6 12 82 77 02 44 FB
             :     0F 6B EF B9 1A B7 F9 6C B0 52 13 36 5C F9 5B 15
      36    1:   ENUMERATED 5
      39   11:   SEQUENCE {
      41    9:     OBJECT IDENTIFIER aes256-wrap
             :       { 2 16 840 1 101 3 4 1 45 }
             :     }
      52    1:   INTEGER 32
      55    1:   INTEGER 32
             :   }

   The DER encoding of CMSORIforPSKOtherInfo produces 58 octets:
      30380420c244cdd11a0d1f39d9b61282770244fb0f6befb91ab7f96cb0521336
      5cf95b150a0105300b060960864801650304012d020120020120

   The HKDF output is 256 bits:
      a14d87451dfd11d83cd54ffe2bd38c49a2adfed3ac49f1d3e62bbdc64ae43b32

   Alice uses AES-KEY-WRAP to encrypt the 256-bit content-encryption
   key with the key-encryption key:
      ae4ea1d99e78fcdcea12d9f10d991ac71502939ee0c30ebdcc97dd1fc5ba3566
      c83d0dd5d1b4faa5

   Alice encrypts the content using AES-256-GCM with the content-
   encryption key.  The 12-octet nonce used is:
      cafebabefacedbaddecaf888

   The content plaintext is:
      48656c6c6f2c20776f726c6421

   The resutling ciphertext is:
      9af2d16f21547fcefed9b3ef2d

   The resutling 12-octet authentication tag is:
      a0e5925cc184e0172463c44c











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A.2.  ContentInfo and AuthEnvelopedData

   Alice encodes the AuthEnvelopedData and the ContentInfo, and
   sends the result to Bob.  The resulting structure is:

        0  650: SEQUENCE {
        4   11:  OBJECT IDENTIFIER authEnvelopedData
              :   { 1 2 840 113549 1 9 16 1 23 }
       17  633:  [0] {
       21  629:   SEQUENCE {
       25    1:    INTEGER 0
       28  551:    SET {
       32  547:     [4] {
       36   11:      OBJECT IDENTIFIER ** Placeholder **
              :       { 1 2 840 113549 1 9 16 TBD 1 }
       49  530:      SEQUENCE {
       53    1:       INTEGER 0
       56   19:       OCTET STRING 'ptf-kmc:13614122112'
       77   13:       SEQUENCE {
       79   11:        OBJECT IDENTIFIER ** Placeholder **
              :         { 1 2 840 113549 1 9 16 3 TBD }
              :        }
       92   11:       SEQUENCE {
       94    9:        OBJECT IDENTIFIER aes256-wrap
              :         { 2 16 840 1 101 3 4 1 45 }
              :        }
      105  432:       SEQUENCE {
      109  428:        SEQUENCE {
      113    1:         INTEGER 2
      116   20:         [0]
              :          9E EB 67 C9 B9 5A 74 D4 4D 2F 16 39 66 80 E8 01
              :          B5 CB A4 9C
      138   13:         SEQUENCE {
      140    9:          OBJECT IDENTIFIER rsaEncryption
              :           { 1 2 840 113549 1 1 1 }
      151    0:          NULL
              :          }
      153  384:         OCTET STRING
              :          18 09 D6 23 17 DF 2D 09 55 57 3B FE 75 95 EB 6A
              :          3D 57 84 6C 69 C1 49 0B F1 11 1A BB 40 0C D8 B5
              :          26 5F D3 62 4B E2 D8 E4 CA EC 6A 12 36 CA 38 E3
              :          A0 7D AA E0 5F A1 E3 BC 59 F3 AD A8 8D 95 A1 6B
              :          06 85 20 93 C7 C5 C0 05 62 ED DF 02 1D FE 68 7C
              :          18 A1 3A AB AA 59 92 30 6A 1B 92 73 D5 01 C6 5B
              :          FD 1E BB A9 B9 D2 7F 48 49 7F 3C 4F 3C 13 E3 2B
              :          2A 19 F1 7A CD BC 56 28 EF 7F CA 4F 69 6B 7E 92
              :          66 22 0D 13 B7 23 AD 41 9E 5E 98 2A 80 B7 6C 77
              :          FF 9B 76 B1 04 BA 30 6D 4B 4D F9 25 57 E0 7F 0E



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INTERNET-DRAFT            Using PSK in the CMS                 June 2019


              :          95 9A 43 6D 14 D5 72 3F AA 8F 66 35 40 D0 E3 71
              :          4B 7F 20 9D ED 67 EA 33 79 CD AB 84 16 72 07 D2
              :          AC 8D 3A DA 12 43 B7 2F 3A CF 91 3E F1 D9 58 20
              :          6D F2 9C 09 E1 EC D2 0B 82 BE 5D 69 77 6F FE F7
              :          EB F6 31 C0 D9 B7 15 BF D0 24 F3 05 1F FF 48 76
              :          1D 73 17 19 2C 38 C6 D5 86 BD 67 82 2D B2 61 AA
              :          08 C7 E4 37 34 D1 2D E0 51 32 15 4A AC 6B 2B 28
              :          5B CD FA 7C 65 89 2F A2 63 DB AB 64 88 43 CC 66
              :          27 84 29 AC 15 5F 3B 9E 5B DF 99 AE 4F 1B B2 BC
              :          19 6C 17 A1 99 A5 CF F7 80 32 11 88 F1 9D B3 6F
              :          4B 16 5F 3F 03 F7 D2 04 3D DE 5F 30 CD 8B BB 3A
              :          38 DA 9D EC 16 6C 36 4F 8B 7E 99 AA 99 FB 42 D6
              :          1A FF 3C 85 D7 A2 30 74 2C D3 AA F7 18 2A 25 3C
              :          B5 02 C4 17 62 21 97 F1 E9 81 83 D0 4E BF 5B 5D
              :         }
              :        }
      541   40:       OCTET STRING
              :        AE 4E A1 D9 9E 78 FC DC EA 12 D9 F1 0D 99 1A C7
              :        15 02 93 9E E0 C3 0E BD CC 97 DD 1F C5 BA 35 66
              :        C8 3D 0D D5 D1 B4 FA A5
              :       }
              :      }
              :     }
      583   55:    SEQUENCE {
      585    9:     OBJECT IDENTIFIER data { 1 2 840 113549 1 7 1 }
      596   27:     SEQUENCE {
      598    9:      OBJECT IDENTIFIER aes256-GCM
              :       { 2 16 840 1 101 3 4 1 46 }
      609   14:      SEQUENCE {
      611   12:       OCTET STRING CA FE BA BE FA CE DB AD DE CA F8 88
              :       }
              :      }
      625   13:     [0] 9A F2 D1 6F 21 54 7F CE FE D9 B3 EF 2D
              :     }
      640   12:    OCTET STRING A0 E5 92 5C C1 84 E0 17 24 63 C4 4C
              :    }
              :   }
              :  }













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A.3.  Recipient Processing Example

   Bob's private key:
      -----BEGIN RSA PRIVATE KEY-----
      MIIG5AIBAAKCAYEA3ocW14cxncPJ47fnEjBZAyfC2lqapL3ET4jvV6C7gGeVrRQx
      WPDwl+cFYBBR2ej3j3/0ecDmu+XuVi2+s5JHKeeza+itfuhsz3yifgeEpeK8T+Su
      sHhn20/NBLhYKbh3kiAcCgQ56dpDrDvDcLqqvS3jg/VO+OPnZbofoHOOevt8Q/ro
      ahJe1PlIyQ4udWB8zZezJ4mLLfbOA9YVaYXx2AHHZJevo3nmRnlgJXo6mE00E/6q
      khjDHKSMdl2WG6mO9TCDZc9qY3cAJDU6Ir0vSH7qUl8/vN13y4UOFkn8hM4kmZ6b
      JqbZt5NbjHtY4uQ0VMW3RyESzhrO02mrp39auLNnH3EXdXaV1tk75H3qC7zJaeGW
      MJyQfOE3YfEGRKn8fxubji716D8UecAxAzFyFL6m1JiOyV5acAiOpxN14qRYZdHn
      XOM9DqGIGpoeY1UuD4Mo05osOqOUpBJHA9fSwhSZG7VNf+vgNWTLNYSYLI04KiMd
      ulnvU6ds+QPz+KKtAgMBAAECggGATFfkSkUjjJCjLvDk4aScpSx6+Rakf2hrdS3x
      jwqhyUfAXgTTeUQQBs1HVtHCgxQd+qlXYn3/qu8TeZVwG4NPztyi/Z5yB1wOGJEV
      3k8N/ytul6pJFFn6p48VM01bUdTrkMJbXERe6g/rr6dBQeeItCaOK7N5SIJH3Oqh
      9xYuB5tH4rquCdYLmt17Tx8CaVqU9qPY3vOdQEOwIjjMV8uQUR8rHSO9KkSj8AGs
      Lq9kcuPpvgJc2oqMRcNePS2WVh8xPFktRLLRazgLP8STHAtjT6SlJ2UzkUqfDHGK
      q/BoXxBDu6L1VDwdnIS5HXtL54ElcXWsoOyKF8/ilmhRUIUWRZFmlS1ok8IC5IgX
      UdL9rJVZFTRLyAwmcCEvRM1asbBrhyEyshSOuN5nHJi2WVJ+wSHijeKl1qeLlpMk
      HrdIYBq4Nz7/zXmiQphpAy+yQeanhP8O4O6C8e7RwKdpxe44su4Z8fEgA5yQx0u7
      8yR1EhGKydX5bhBLR5Cm1VM7rT2BAoHBAP/+e5gZLNf/ECtEBZjeiJ0VshszOoUq
      haUQPA+9Bx9pytsoKm5oQhB7QDaxAvrn8/FUW2aAkaXsaj9F+/q30AYSQtExai9J
      fdKKook3oimN8/yNRsKmhfjGOj8hd4+GjX0qoMSBCEVdT+bAjjry8wgQrqReuZnu
      oXU85dmb3jvv0uIczIKvTIeyjXE5afjQIJLmZFXsBm09BG87Ia5EFUKly96BOMJh
      /QWEzuYYXDqOFfzQtkAefXNFW21Kz4Hw2QKBwQDeiGh4lxCGTjECvG7fauMGlu+q
      DSdYyMHif6t6mx57eS16EjvOrlXKItYhIyzW8Kw0rf/CSB2j8ig1GkMLTOgrGIJ1
      0322o50FOr5oOmZPueeR4pOyAP0fgQ8DD1L3JBpY68/8MhYbsizVrR+Ar4jM0f96
      W2bF5Xj3h+fQTDMkx6VrCCQ6miRmBUzH+ZPs5n/lYOzAYrqiKOanaiHy4mjRvlsy
      mjZ6z5CG8sISqcLQ/k3Qli5pOY/v0rdBjgwAW/UCgcEAqGVYGjKdXCzuDvf9EpV4
      mpTWB6yIV2ckaPOn/tZi5BgsmEPwvZYZt0vMbu28Px7sSpkqUuBKbzJ4pcy8uC3I
      SuYiTAhMiHS4rxIBX3BYXSuDD2RD4vG1+XM0h6jVRHXHh0nOXdVfgnmigPGz3jVJ
      B8oph/jD8O2YCk4YCTDOXPEi8Rjusxzro+whvRR+kG0gsGGcKSVNCPj1fNISEte4
      gJId7O1mUAAzeDjn/VaS/PXQovEMolssPPKn9NocbKbpAoHBAJnFHJunl22W/lrr
      ppmPnIzjI30YVcYOA5vlqLKyGaAsnfYqP1WUNgfVhq2jRsrHx9cnHQI9Hu442PvI
      x+c5H30YFJ4ipE3eRRRmAUi4ghY5WgD+1hw8fqyUW7E7l5LbSbGEUVXtrkU5G64T
      UR91LEyMF8OPATdiV/KD4PWYkgaqRm3tVEuCVACDTQkqNsOOi3YPQcm270w6gxfQ
      SOEy/kdhCFexJFA8uZvmh6Cp2crczxyBilR/yCxqKOONqlFdOQKBwFbJk5eHPjJz
      AYueKMQESPGYCrwIqxgZGCxaqeVArHvKsEDx5whI6JWoFYVkFA8F0MyhukoEb/2x
      2qB5T88Dg3EbqjTiLg3qxrWJ2OxtUo8pBP2I2wbl2NOwzcbrlYhzEZ8bJyxZu5i1
      sYILC8PJ4Qzw6jS4Qpm4y1WHz8e/ElW6VyfmljZYA7f9WMntdfeQVqCVzNTvKn6f
      hg6GSpJTzp4LV3ougi9nQuWXZF2wInsXkLYpsiMbL6Fz34RwohJtYA==
      -----END RSA PRIVATE KEY-----

   Bob decrypts the key-derivation key with his RSA private key:
      df85af9e3cebffde6e9b9d24263db31114d0a8e33a0d50e05eb64578ccde81eb






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   Bob produces a 256-bit key-encryption key with HKDF using SHA-384;
   the secret value is the key-derivation key; the 'info' is the DER-
   encoded CMSORIforPSKOtherInfo structure with the same values as
   shown in A.1.  The HKDF output is 256 bits:
      a14d87451dfd11d83cd54ffe2bd38c49a2adfed3ac49f1d3e62bbdc64ae43b32

   Bob uses AES-KEY-WRAP to decrypt the content-encryption key
   with the key-encryption key; the content-encryption key is:
      c8adc30f4a3e20ac420caa76a68f5787c02ab42afea20d19672fd963a5338e83

   Bob decrypts the content using AES-256-GCM with the content-
   encryption key, and checks the received authentication tag.  The
   12-octet nonce used is:
      cafebabefacedbaddecaf888

   The 12-octet authentication tag is:
      a0e5925cc184e0172463c44c

   The received ciphertext content is:
      9af2d16f21547fcefed9b3ef2d

   The resutling plaintext content is:
      48656c6c6f2c20776f726c6421

Appendix B: Key Agreement with PSK Example

   This example shows the establishment of an AES-256 content-encryption
   key using:
      - a pre-shared key of 256 bits;
      - key agreement using ECDH on curve P-384 and X9.63 KDF
        with SHA-384;
      - key derivation using HKDF with SHA-384; and
      - key wrap using AES-256-KEYWRAP.

   In real-world use, the originator would treat themselves as an
   additional recipient by performing key agreement with their own
   static public key and the ephemeral private key generated for this
   message.  This is omited in an attempt to simplify the example.

B.1.  Originator Processing Example

   The pre-shared key known to Alice and Bob:
      4aa53cbf500850dd583a5d9821605c6fa228fb5917f87c1c078660214e2d83e4

   The identifier assigned to the pre-shared key is:
      ptf-kmc:216840110121





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   Alice randomly generates a content-encryption key:
      937b1219a64d57ad81c05cc86075e86017848c824d4e85800c731c5b7b091033

   Alice obtains Bob's static ECDH public key:
      -----BEGIN PUBLIC KEY-----
      MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEScGPBO9nmUwGrgbGEoFY9HR/bCo0WyeY
      /dePQVrwZmwN2yMJmO2d1kWCvLTz8U7atinxyIRe9CV54yau1KWU/wbkhPDnzuSM
      YkcpxMGo32z3JetEloW5aFOja13vv/W5
      -----END PUBLIC KEY-----

   It has a key identifier of:
      e8218b98b8b7d86b5e9ebdc8aeb8c4ecdc05c529

   Alice generates an ephemeral ECDH key pair on the same curve:
      -----BEGIN EC PRIVATE KEY-----
      MIGkAgEBBDCMiWLG44ik+L8cYVvJrQdLcFA+PwlgRF+Wt1Ab25qUh8OB7OePWjxp
      /b8P6IOuI6GgBwYFK4EEACKhZANiAAQ5G0EmJk/2ks8sXY1kzbuG3Uu3ttWwQRXA
      LFDJICjvYfr+yTpOQVkchm88FAh9MEkw4NKctokKNgpsqXyrT3DtOg76oIYENpPb
      GE5lJdjPx9sBsZQdABwlsU0Zb7P/7i8=
      -----END EC PRIVATE KEY-----

   Alice computes a shared secret, called Z, using the Bob's static
   ECDH public key and her ephemeral ECDH private key; Z is:
      3f015ed0ff4b99523a95157bbe77e9cc0ee52fcffeb7e41eac79d1c11b6cc556
      19cf8807e6d800c2de40240fe0e26adc

   Alice computes the pairwise key-encryption key, called KEK1, from Z
   using the X9.63 KDF with the ECC-CMS-SharedInfo structure with the
   following values:
       0   21: SEQUENCE {
       2   11:   SEQUENCE {
       4    9:     OBJECT IDENTIFIER aes256-wrap
             :       { 2 16 840 1 101 3 4 1 45 }
             :     }
      15    6:   [2] {
      17    4:     OCTET STRING 00 00 00 20
             :     }
             :   }

   The DER encoding of ECC-CMS-SharedInfo produces 23 octets:
      3015300b060960864801650304012da206040400000020

   The X9.63 KDF output is the 256-bit KEK1:
      27dc25ddb0b425f7a968ceada80a8f73c6ccaab115baafcce4a22a45d6b8f3da







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   Alice produces the 256-bit KEK2 with HKDF using SHA-384; the secret
   value is KEK1; the 'info' is the DER-encoded CMSORIforPSKOtherInfo
   structure with the following values:
       0   56: SEQUENCE {
       2   32:   OCTET STRING
             :     4A A5 3C BF 50 08 50 DD 58 3A 5D 98 21 60 5C 6F
             :     A2 28 FB 59 17 F8 7C 1C 07 86 60 21 4E 2D 83 E4
      36    1:   ENUMERATED 10
      39   11:   SEQUENCE {
      41    9:     OBJECT IDENTIFIER aes256-wrap
             :       { 2 16 840 1 101 3 4 1 45 }
             :     }
      52    1:   INTEGER 32
      55    1:   INTEGER 32
             :   }

   The DER encoding of CMSORIforPSKOtherInfo produces 58 octets:
      303804204aa53cbf500850dd583a5d9821605c6fa228fb5917f87c1c07866021
      4e2d83e40a010a300b060960864801650304012d020120020120

   The HKDF output is the 256-bit KEK2:
      7de693ee30ae22b5f8f6cd026c2164103f4e1430f1ab135dc1fb98954f9830bb

   Alice uses AES-KEY-WRAP to encrypt the content-encryption key
   with the KEK2; the wrapped key is:
      229fe0b45e40003e7d8244ec1b7e7ffb2c8dca16c36f5737222553a71263a92b
      de08866a602d63f4

   Alice encrypts the content using AES-256-GCM with the content-
   encryption key.  The 12-octet nonce used is:
      dbaddecaf888cafebabeface

   The resulting ciphertext is:
      fc6d6f823e3ed2d209d0c6ffcf

   The resulting 12-octet authentication tag is:
      550260c42e5b29719426c1ff














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B.2.  ContentInfo and AuthEnvelopedData

   Alice encodes the AuthEnvelopedData and the ContentInfo, and
   sends the result to Bob.  The resulting structure is:

        0  327: SEQUENCE {
        4   11:  OBJECT IDENTIFIER authEnvelopedData
              :   { 1 2 840 113549 1 9 16 1 23 }
       17  310:  [0] {
       21  306:   SEQUENCE {
       25    1:    INTEGER 0
       28  229:    SET {
       31  226:     [4] {
       34   11:      OBJECT IDENTIFIER ** Placeholder **
              :       { 1 2 840 113549 1 9 16 TBD 2 }
       47  210:      SEQUENCE {
       50    1:       INTEGER 0
       53   20:       OCTET STRING 'ptf-kmc:216840110121'
       75   85:       [0] {
       77   83:        [1] {
       79   19:         SEQUENCE {
       81    6:          OBJECT IDENTIFIER
              :           dhSinglePass-stdDH-sha256kdf-scheme
              :           { 1 3 132 1 11 1 }
       89    9:          OBJECT IDENTIFIER aes256-wrap
              :           { 2 16 840 1 101 3 4 1 45 }
              :          }
      100   60:         BIT STRING, encapsulates {
      103   57:          OCTET STRING
              :          1B 41 26 26 4F F6 92 CF 2C 5D 8D 64 CD BB 86 DD
              :          4B B7 B6 D5 B0 41 15 C0 2C 50 C9 20 28 EF 61 FA
              :          FE C9 3A 4E 41 59 1C 86 6F 3C 14 08 7D 30 49 30
              :          E0 D2 9C B6 89 0A 36 0A 6C
              :          }
              :         }
              :        }
      162   13:       SEQUENCE {
      164   11:        OBJECT IDENTIFIER ** Placeholder **
              :         { 1 2 840 113549 1 9 16 3 TBD }
              :        }
      177   11:       SEQUENCE {
      179    9:        OBJECT IDENTIFIER aes256-wrap
              :         { 2 16 840 1 101 3 4 1 45 }
              :        }
      190   68:       SEQUENCE {
      192   66:        SEQUENCE {
      194   22:         [0] {
      196   20:          OCTET STRING



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              :          E8 21 8B 98 B8 B7 D8 6B 5E 9E BD C8 AE B8 C4 EC
              :          DC 05 C5 29
              :          }
      218   40:         OCTET STRING
              :         22 9F E0 B4 5E 40 00 3E 7D 82 44 EC 1B 7E 7F FB
              :         2C 8D CA 16 C3 6F 57 37 22 25 53 A7 12 63 A9 2B
              :         DE 08 86 6A 60 2D 63 F4
              :         }
              :        }
              :       }
              :      }
              :     }
      260   55:    SEQUENCE {
      262    9:     OBJECT IDENTIFIER data { 1 2 840 113549 1 7 1 }
      273   27:     SEQUENCE {
      275    9:      OBJECT IDENTIFIER aes256-GCM
              :       { 2 16 840 1 101 3 4 1 46 }
      286   14:      SEQUENCE {
      288   12:       OCTET STRING DB AD DE CA F8 88 CA FE BA BE FA CE
              :       }
              :      }
      302   13:     [0] FC 6D 6F 82 3E 3E D2 D2 09 D0 C6 FF CF
              :     }
      317   12:    OCTET STRING 55 02 60 C4 2E 5B 29 71 94 26 C1 FF
              :    }
              :   }
              :  }

B.3.  Recipient Processing Example

   Bob obtains Alice's ephemeral ECDH public key from the message:
      -----BEGIN PUBLIC KEY-----
      MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEORtBJiZP9pLPLF2NZM27ht1Lt7bVsEEV
      wCxQySAo72H6/sk6TkFZHIZvPBQIfTBJMODSnLaJCjYKbKl8q09w7ToO+qCGBDaT
      2xhOZSXYz8fbAbGUHQAcJbFNGW+z/+4v
      -----END PUBLIC KEY-----

   Bob's static ECDH private key:
      -----BEGIN EC PRIVATE KEY-----
      MIGkAgEBBDAnJ4hB+tTUN9X03/W0RsrYy+qcptlRSYkhaDIsQYPXfTU0ugjJEmRk
      NTPj4y1IRjegBwYFK4EEACKhZANiAARJwY8E72eZTAauBsYSgVj0dH9sKjRbJ5j9
      149BWvBmbA3bIwmY7Z3WRYK8tPPxTtq2KfHIhF70JXnjJq7UpZT/BuSE8OfO5Ixi
      RynEwajfbPcl60SWhbloU6NrXe+/9bk=
      -----END EC PRIVATE KEY-----







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   Bob computes a shared secret, called Z, using the Alice's ephemeral
   ECDH public key and his static ECDH private key; Z is:
      3f015ed0ff4b99523a95157bbe77e9cc0ee52fcffeb7e41eac79d1c11b6cc556
      19cf8807e6d800c2de40240fe0e26adc

   Bob computes the pairwise key-encryption key, called KEK1, from Z
   using the X9.63 KDF with the ECC-CMS-SharedInfo structure with the
   values shown in B.1.  The X9.63 KDF output is the 256-bit KEK1:
      27dc25ddb0b425f7a968ceada80a8f73c6ccaab115baafcce4a22a45d6b8f3da

   Bob produces the 256-bit KEK2 with HKDF using SHA-384; the secret
   value is KEK1; the 'info' is the DER-encoded CMSORIforPSKOtherInfo
   structure with the values shown in B.1.  The HKDF output is the
   256-bit KEK2:
      7de693ee30ae22b5f8f6cd026c2164103f4e1430f1ab135dc1fb98954f9830bb

   Bob uses AES-KEY-WRAP to decrypt the content-encryption key
   with the KEK2; the content-encryption key is:
      937b1219a64d57ad81c05cc86075e86017848c824d4e85800c731c5b7b091033

   Bob decrypts the content using AES-256-GCM with the content-
   encryption key, and checks the received authentication tag.  The
   12-octet nonce used is:
      dbaddecaf888cafebabeface

   The 12-octet authentication tag is:
      550260c42e5b29719426c1ff

   The received ciphertext content is:
      fc6d6f823e3ed2d209d0c6ffcf

   The resutling plaintext content is:
      48656c6c6f2c20776f726c6421


















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Acknowledgements

   Many thanks to Burt Kaliski, Panos Kampanakis, Jim Schaad, Sean
   Turner, and Daniel Van Geest for their review and insightful
   comments.  They have greatly improved the design, clarity, and
   implementation guidance.

   The security properties provided by the mechanisms specified in this
   document can be validated using formal methods.  A ProVerif proof in
   [H2019] shows that an attacker with a large-scale quantum computer
   that is capable of breaking the Diffie-Hellman key agreement
   algorithm cannot disrupt the delivery of the content-encryption key
   to the recipient and the attacker cannot learn the content-encryption
   key from the protocol exchange.

Author's Address

   Russ Housley
   Vigil Security, LLC
   516 Dranesville Road
   Herndon, VA 20170
   USA
   EMail: housley@vigilsec.com




























Housley                                                        [Page 29]


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