



Independent Submission                                        M. Jenkins
Request for Comments: 8755                                           NSA
Category: Informational                                       March 2020
ISSN: 2070-1721


    Using Commercial National Security Algorithm Suite Algorithms in
              Secure/Multipurpose Internet Mail Extensions

Abstract

   The United States Government has published the National Security
   Agency (NSA) Commercial National Security Algorithm (CNSA) Suite,
   which defines cryptographic algorithm policy for national security
   applications.  This document specifies the conventions for using the
   United States National Security Agency's CNSA Suite algorithms in
   Secure/Multipurpose Internet Mail Extensions (S/MIME) as specified in
   RFC 8551.  It applies to the capabilities, configuration, and
   operation of all components of US National Security Systems that
   employ S/MIME messaging.  US National Security Systems are described
   in NIST Special Publication 800-59.  It is also appropriate for all
   other US Government systems that process high-value information.  It
   is made publicly available for use by developers and operators of
   these and any other system deployments.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8755.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://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.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  The Commercial National Security Algorithm Suite
   3.  Requirements and Assumptions
   4.  SHA-384 Message Digest Algorithm
   5.  Digital Signature
     5.1.  ECDSA Signature
     5.2.  RSA Signature
   6.  Key Establishment
     6.1.  Elliptic Curve Key Agreement
     6.2.  RSA Key Transport
   7.  Content Encryption
     7.1.  AES-GCM Content Encryption
     7.2.  AES-CBC Content Encryption
   8.  Security Considerations
   9.  IANA Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Author's Address

1.  Introduction

   This document specifies the conventions for using the United States
   National Security Agency's Commercial National Security Algorithm
   (CNSA) Suite algorithms [CNSA] in Secure/Multipurpose Internet Mail
   Extensions (S/MIME) [RFC8551].  It applies to the capabilities,
   configuration, and operation of all components of US National
   Security Systems that employ S/MIME messaging.  US National Security
   Systems are described in NIST Special Publication 800-59 [SP80059].
   It is also appropriate for all other US Government systems that
   process high-value information.  It is made publicly available for
   use by developers and operators of these and any other system
   deployments.

   S/MIME makes use of the Cryptographic Message Syntax (CMS) [RFC5652]
   [RFC5083].  In particular, the signed-data, enveloped-data, and
   authenticated-enveloped-data content types are used.  This document
   only addresses CNSA Suite compliance for S/MIME.  Other applications
   of CMS are outside the scope of this document.

   This document does not define any new cryptographic algorithm suites;
   instead, it defines a CNSA-compliant profile of S/MIME.  Since many
   of the CNSA Suite algorithms enjoy uses in other environments as
   well, the majority of the conventions needed for these algorithms are
   already specified in other documents.  This document references the
   source of these conventions, with some relevant details repeated to
   aid developers that choose to support the CNSA Suite.  Where details
   have been repeated, the cited documents are authoritative.

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.

2.  The Commercial National Security Algorithm Suite

   The National Security Agency (NSA) profiles commercial cryptographic
   algorithms and protocols as part of its mission to support secure,
   interoperable communications for US Government National Security
   Systems.  To this end, it publishes guidance both to assist with the
   US Government transition to new algorithms and to provide vendors --
   and the Internet community in general -- with information concerning
   their proper use and configuration.

   Recently, cryptographic transition plans have become overshadowed by
   the prospect of the development of a cryptographically relevant
   quantum computer.  The NSA has established the Commercial National
   Security Algorithm (CNSA) Suite to provide vendors and IT users near-
   term flexibility in meeting their cybersecurity interoperability
   requirements.  The purpose behind this flexibility is to avoid having
   vendors and customers make two major transitions in a relatively
   short timeframe, as we anticipate a need to shift to quantum-
   resistant cryptography in the near future.

   The NSA is authoring a set of RFCs, including this one, to provide
   updated guidance concerning the use of certain commonly available
   commercial algorithms in IETF protocols.  These RFCs can be used in
   conjunction with other RFCs and cryptographic guidance (e.g., NIST
   Special Publications) to properly protect Internet traffic and data-
   at-rest for US Government National Security Systems.

3.  Requirements and Assumptions

   CMS values are generated using ASN.1 [X208], the Basic Encoding Rules
   (BER) [X209], and the Distinguished Encoding Rules (DER) [X509].

   The elliptic curve used in the CNSA Suite is specified in [FIPS186]
   and appears in the literature under two different names.  For the
   sake of clarity, we list both names below:

   +----------+-----------+-----------+---------------+
   | Curve    | NIST Name | SECG Name | OID [FIPS186] |
   +==========+===========+===========+===============+
   | nistp384 | P-384     | secp384r1 | 1.3.132.0.34  |
   +----------+-----------+-----------+---------------+

                         Table 1

   For CNSA Suite applications, public key certificates used to verify
   S/MIME signatures MUST be compliant with the CNSA Suite Certificate
   and Certificate Revocation List (CRL) profile specified in [RFC8603].

   Within the CMS signed-data content type, signature algorithm
   identifiers are located in the signatureAlgorithm field of SignerInfo
   structures contained within the SignedData.  In addition, signature
   algorithm identifiers are located in the SignerInfo
   signatureAlgorithm field of countersignature attributes.  Specific
   requirements for digital signatures are given in Section 5; compliant
   implementations MUST consider signatures not meeting these
   requirements as invalid.

   Implementations based on Elliptic Curve Cryptography (ECC) also
   require specification of schemes for key derivation and key wrap.
   Requirements for these schemes are in Sections 6.1.1 and 6.1.2,
   respectively.

   RSA key pairs (public, private) are identified by the modulus size
   expressed in bits; RSA-3072 and RSA-4096 are computed using moduli of
   3072 bits and 4096 bits, respectively.

   RSA signature key pairs used in CNSA Suite-compliant implementations
   are either RSA-3072 or RSA-4096.  The RSA exponent e MUST satisfy
   2^(16) < e < 2^(256) and be odd per [FIPS186].

   It is recognized that, while the vast majority of RSA signatures are
   currently made using the RSASSA-PKCS1-v1_5 algorithm, the preferred
   RSA signature scheme for new applications is RSASSA-PSS.  CNSA Suite-
   compliant X.509 certificates will be issued in accordance with
   [RFC8603], and while those certificates must be signed and validated
   using RSASSA-PKCS1-v1_5, the subject's RSA key pair can be used to
   generate and validate signatures appropriate for either signing
   scheme.  Where use of RSASSA-PSS is indicated in this document, the
   parameters in Section 5.2.2 apply.

   This document assumes that the required trust anchors have been
   securely provisioned to the client.

   All implementations use SHA-384 for hashing and either AES-CBC or
   AES-GCM for encryption, the requirements for which are given in
   Section 4 and Section 7, respectively.

4.  SHA-384 Message Digest Algorithm

   SHA-384 is the sole CNSA Suite message digest algorithm.  [RFC5754]
   specifies the conventions for using SHA-384 with the Cryptographic
   Message Syntax (CMS).  CNSA Suite-compliant S/MIME implementations
   MUST follow the conventions in [RFC5754].

   Within the CMS signed-data content type, message digest algorithm
   identifiers are located in the SignedData digestAlgorithms field and
   the SignerInfo digestAlgorithm field.

   The SHA-384 message digest algorithm is defined in FIPS Pub 180
   [FIPS180].  The algorithm identifier for SHA-384 is defined in
   [RFC5754] as follows:

         id-sha384  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
             country(16) us(840) organization(1) gov(101) csor(3)
             nistalgorithm(4) hashalgs(2) 2 }

   For SHA-384, the AlgorithmIdentifier parameters field is OPTIONAL,
   and if present, the parameters field MUST contain a NULL.  As
   specified in [RFC5754], implementations MUST generate SHA-384
   AlgorithmIdentifiers with absent parameters.  Implementations MUST
   accept SHA-384 AlgorithmIdentifiers with absent parameters or with
   NULL parameters.

5.  Digital Signature

5.1.  ECDSA Signature

   The Elliptic Curve Digital Signature Algorithm (ECDSA) is the CNSA
   Suite digital signature algorithm based on ECC.  [RFC5753] specifies
   the conventions for using ECDSA with the Cryptographic Message Syntax
   (CMS).  CNSA Suite-compliant S/MIME implementations MUST follow the
   conventions in [RFC5753].

   [RFC5480] defines the signature algorithm identifier used in CMS for
   ECDSA with SHA-384 as follows:

         ecdsa-with-SHA384  OBJECT IDENTIFIER  ::=  { iso(1)
            member-body(2) us(840) ansi-X9-62(10045) signatures(4)
            ecdsa-with-sha2(3) 3 }

   When the ecdsa-with-SHA384 algorithm identifier is used, the
   AlgorithmIdentifier parameters field MUST be absent.

   When signing, the ECDSA algorithm generates two values, commonly
   called r and s.  These two values MUST be encoded using the ECDSA-
   Sig-Value type specified in [RFC5480]:

         ECDSA-Sig-Value  ::=  SEQUENCE {
            r  INTEGER,
            s  INTEGER }

5.2.  RSA Signature

   The RSA signature generation process and the encoding of the result
   is either RSASSA-PKCS1-v1_5 or RSA-PSS, as described in detail in
   PKCS #1 version 2.2 [RFC8017].

5.2.1.  RSA-PKCS1-v1_5

   [RFC5754] defines the signature algorithm identifier used in CMS for
   an RSA signature with SHA-384 as follows:

         sha384WithRSAEncryption  OBJECT IDENTIFIER  ::= { iso(1)
           member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 12 }

   When the sha384WithRSAEncryption algorithm identifier is used, the
   parameters MUST be NULL.  Implementations MUST accept the parameters
   being absent as well as present.

5.2.2.  RSA-PSS

   [RFC4056] defines the signature algorithm identifier used in CMS for
   an RSA-PSS signature as follows (presented here in expanded form):

         RSASSA-PSS  OBJECT IDENTIFIER  ::= { iso(1)
           member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 10 }

   The parameters field of an AlgorithmIdentifier that identifies
   RSASSA-PSS is defined in [RFC4055] as follows:

          RSASSA-PSS-params  ::=  SEQUENCE  {
             hashAlgorithm      [0] HashAlgorithm DEFAULT
                                       sha1Identifier,
             maskGenAlgorithm   [1] MaskGenAlgorithm DEFAULT
                                       mgf1SHA1Identifier,
             saltLength         [2] INTEGER DEFAULT 20,
             trailerField       [3] INTEGER DEFAULT 1  }

   The AlgorithmIdentifier parameters field MUST contain RSASSA-PSS-
   params with the following values:

   *  The hash algorithm MUST be id-sha384 as defined in [RFC8017];

   *  The mask generation function MUST use the algorithm identifier
      mfg1SHA384Identifier as defined in [RFC4055];

   *  The salt length MUST be 48 octets (the same length as the SHA-384
      output); and

   *  The trailerField MUST have value 1.

6.  Key Establishment

6.1.  Elliptic Curve Key Agreement

   Elliptic Curve Diffie-Hellman (ECDH) is the CNSA Suite key agreement
   algorithm.  Since S/MIME is used in store-and-forward communications,
   ephemeral-static ECDH is always employed.  This means that the
   message originator possesses an ephemeral ECDH key pair and that the
   message recipient possesses a static ECDH key pair whose public key
   is provided in an X.509 certificate.  The certificate used to obtain
   the recipient's public key MUST be compliant with [RFC8603].

   When a key agreement algorithm is used, the following steps are
   performed:

   1.  A content-encryption key (CEK) for a particular content-
       encryption algorithm is generated at random.

   2.  The recipient's public key and sender's private key are used with
       a key agreement scheme to generate a shared secret (Z).

   3.  The shared secret is used with a key derivation function (KDF) to
       produce a key-encryption key (KEK).

   4.  The KEK is used with a key wrap algorithm to encrypt the CEK.

   Key derivation is discussed in Section 6.1.1.  Key wrapping is
   discussed in Section 6.1.2.

   Section 3.1 of [RFC5753] specifies the conventions for using ECDH
   with the CMS.  CNSA Suite-compliant S/MIME implementations MUST
   follow these conventions.

   Within the CMS enveloped-data and authenticated-enveloped-data
   content types, key agreement algorithm identifiers are located in the
   EnvelopedData RecipientInfos KeyAgreeRecipientInfo
   keyEncryptionAlgorithm field.

   The keyEncryptionAlgorithm field comprises two fields, an algorithm
   field and a parameter field.  The algorithm field MUST identify
   dhSinglePass-stdDH-sha384kdf-scheme.  The algorithm identifier for
   the dhSinglePass-stdDH-sha384kdf-scheme, repeated from Section 7.1.4
   of [RFC5753], is (presented here in expanded form):

         dhSinglePass-stdDH-sha384kdf-scheme  OBJECT IDENTIFIER  ::=
             { iso(1) identified-organization(3) certicom(132)
               schemes(1) 11 2 }

   The keyEncryptionAlgorithm parameter field MUST be constructed as
   described in Section 6.1.2.

6.1.1.  Key Derivation Functions

   KDFs based on SHA-384 are used to derive a pairwise key-encryption
   key from the shared secret produced by ephemeral-static ECDH.
   Sections 7.1.8 and 7.2 in [RFC5753] specify the CMS conventions for
   using a KDF with the shared secret generated during ephemeral-static
   ECDH.  CNSA Suite-compliant S/MIME implementations MUST follow these
   conventions.

   As specified in Section 7.1.8 of [RFC5753], the ANSI-X9.63-KDF
   described in Section 3.6.1 of [SEC1] and based on SHA-384 MUST be
   used.

   As specified in Section 7.2 of [RFC5753], when using ECDH with the
   CMS enveloped-data or authenticated-enveloped-data content type, the
   derivation of key-encryption keys makes use of the ECC-CMS-SharedInfo
   type:

         ECC-CMS-SharedInfo  ::=  SEQUENCE {
            keyInfo      AlgorithmIdentifier,
            entityUInfo  [0] EXPLICIT OCTET STRING OPTIONAL,
            suppPubInfo  [2] EXPLICIT OCTET STRING }

   In the CNSA Suite for S/MIME, the fields of ECC-CMS-SharedInfo are
   used as follows:

   *  keyInfo contains the object identifier of the key-encryption
      algorithm used to wrap the content-encryption key.  If AES-256 Key
      Wrap is used, then the keyInfo will contain id-aes256-wrap-pad,
      and the parameters will be absent.

   *  entityUInfo optionally contains a random value provided by the
      message originator.  If user keying material (ukm) is included in
      the KeyAgreeRecipientInfo, then the entityUInfo MUST be present,
      and it MUST contain the ukm value.  If the ukm is not present,
      then the entityUInfo MUST be absent.

   *  suppPubInfo contains the length of the generated key-encryption
      key in bits, represented as a 32-bit unsigned number, as described
      in [RFC2631].  When a 256-bit AES key is used, the length MUST be
      0x00000100.

   ECC-CMS-SharedInfo is DER encoded and is used as input to the key
   derivation function, as specified in Section 3.6.1 of [SEC1].  Note
   that ECC-CMS-SharedInfo differs from the OtherInfo specified in
   [RFC2631].  Here, a counter value is not included in the keyInfo
   field because the KDF specified in [SEC1] ensures that sufficient
   keying data is provided.

   The KDF specified in Section 3.6.1 of [SEC1] describes how to
   generate an essentially arbitrary amount of keying material from a
   shared secret, Z, produced by ephemeral-static ECDH.  To generate an
   L-bit key-encryption key (KEK), blocks of key material (KM) are
   computed by incrementing Counter appropriately until enough material
   has been generated:

         KM(Counter) = Hash ( Z || Counter || ECC-CMS-SharedInfo )

   The KM blocks are concatenated left to right as they are generated,
   and the first (leftmost) L bits are used as the KEK:

         KEK = the leftmost L bits of
                  [KM ( counter=1 ) || KM ( counter=2 ) ...]

   In the CNSA Suite for S/MIME, the elements of the KDF are defined as
   follows:

   *  Hash is a one-way hash function.  The SHA-384 hash MUST be used.

   *  Z is the shared secret value generated during ephemeral-static
      ECDH.  Z MUST be exactly 384 bits, i.e., leading zero bits MUST be
      preserved.

   *  Counter is a 32-bit unsigned number represented in network byte
      order.  Its initial value MUST be 0x00000001 for any key
      derivation operation.

   *  ECC-CMS-SharedInfo is composed as described above.  It MUST be DER
      encoded.

   In the CNSA Suite for S/MIME, exactly one iteration is needed; the
   Counter is not incremented.  The key-encryption key (KEK) MUST be the
   first (leftmost) 256 bits of the SHA-384 output value:

         KEK = the leftmost 256 bits of
                  SHA-384 ( Z || 0x00000001 || ECC-CMS-SharedInfo )

   Note that the only source of secret entropy in this computation is Z.

6.1.2.  AES Key Wrap

   The AES Key Wrap with Padding key-encryption algorithm, as specified
   in [RFC5649] and [SP80038F], is used to encrypt the content-
   encryption key with a pairwise key-encryption key that is generated
   using ephemeral-static ECDH.  Section 8 of [RFC5753] specifies the
   CMS conventions for using AES Key Wrap with a pairwise key generated
   through ephemeral-static ECDH.  CNSA Suite-compliant S/MIME
   implementations MUST follow these conventions.

   Within the CMS enveloped-data content type, key wrap algorithm
   identifiers are located in the KeyWrapAlgorithm parameters within the
   EnvelopedData RecipientInfos KeyAgreeRecipientInfo
   keyEncryptionAlgorithm field.

   The KeyWrapAlgorithm MUST be id-aes256-wrap-pad.  The required
   algorithm identifier, specified in [RFC5649], is:

         id-aes256-wrap-pad  OBJECT IDENTIFIER ::=  { joint-iso-itu-t(2)
            country(16) us(840) organization(1) gov(101) csor(3)
            nistAlgorithm(4) aes(1) 48 }

6.2.  RSA Key Transport

   RSA encryption (RSA) is the CNSA Suite key transport algorithm.  The
   RSA key transport algorithm is the RSA encryption scheme defined in
   [RFC8017], where the message to be encrypted is the content-
   encryption key.

   The recipient of an S/MIME message possesses an RSA key pair whose
   public key is represented by an X.509 certificate.  The certificate
   used to obtain the recipient's public key MUST be compliant with
   [RFC8603].  These certificates are suitable for use with either
   RSAES-OAEP or RSAES-PKCS1-v1_5.

6.2.1.  RSAES-PKCS1-v1_5

   Section 4.2 of [RFC3370] specifies the conventions for using RSAES-
   PKCS1-v1_5 with the CMS.  S/MIME implementations employing this form
   of key transport MUST follow these conventions.

   Within the CMS enveloped-data and authenticated-enveloped-data
   content types, key transport algorithm identifiers are located in the
   EnvelopedData RecipientInfos KeyTransRecipientInfo
   keyEncryptionAlgorithm field.

   The algorithm identifier for RSA (PKCS #1 v1.5) is:

         rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
             us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

   The AlgorithmIdentifier parameters field MUST be present, and the
   parameters field MUST contain NULL.

6.2.2.  RSAES-OAEP

   [RFC3560] specifies the conventions for using RSAES-OAEP with the
   CMS.  CNSA Suite-compliant S/MIME implementations employing this form
   of key transport MUST follow these conventions.

   Within the CMS enveloped-data and authenticated-enveloped-data
   content types, key transport algorithm identifiers are located in the
   EnvelopedData RecipientInfos KeyTransRecipientInfo
   keyEncryptionAlgorithm field.

   The algorithm identifier for RSA (OAEP) is:

         id-RSAES-OAEP  OBJECT IDENTIFIER  ::=  { iso(1) member-body(2)
             us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 7 }

   The parameters field of an AlgorithmIdentifier that identifies RSAES-
   OAEP is defined in [RFC4055] as follows:

          RSAES-OAEP-params  ::=  SEQUENCE  {
             hashFunc          [0] AlgorithmIdentifier DEFAULT
                                      sha1Identifier,
             maskGenFunc       [1] AlgorithmIdentifier DEFAULT
                                      mgf1SHA1Identifier,
             pSourceFunc       [2] AlgorithmIdentifier DEFAULT
                                      pSpecifiedEmptyIdentifier  }

          pSpecifiedEmptyIdentifier  AlgorithmIdentifier  ::=
                               { id-pSpecified, nullOctetString }

          nullOctetString  OCTET STRING (SIZE (0))  ::=  { ''H }

   The AlgorithmIdentifier parameters field MUST be present, and the
   parameters field MUST contain RSAES-OAEP-params with values as
   follows:

   *  The hashFunc algorithm must be id-sha384 as defined in [RFC8017];

   *  The mask generation function must use the algorithm identifier
      mfg1SHA384Identifier as defined in [RFC4055];

   *  The pSourceFunc field must be absent.

   The SMIMECapabilities signed attribute is used to specify a partial
   list of algorithms that the software announcing the SMIMECapabilities
   can support.  If the SMIMECapabilities signed attribute is included
   to announce support for the RSAES-OAEP algorithm, it MUST be
   constructed as defined in Section 5 of [RFC3560], with the sequence
   representing the rSAES-OAEP-SHA384-Identifier.

7.  Content Encryption

   AES-GCM is the preferred mode for CNSA Suite applications, as
   described in the Security Considerations (Section 8).  AES-CBC is
   acceptable where AES-GCM is not yet available.

7.1.  AES-GCM Content Encryption

   CNSA Suite-compliant S/MIME implementations using the authenticated-
   enveloped-data content type [RFC5083] MUST use AES [FIPS197] in
   Galois Counter Mode (GCM) [SP80038D] as the content-authenticated
   encryption algorithm and MUST follow the conventions for using AES-
   GCM with the CMS defined in [RFC5084].

   Within the CMS authenticated-enveloped-data content type, content-
   authenticated encryption algorithm identifiers are located in the
   AuthEnvelopedData EncryptedContentInfo contentEncryptionAlgorithm
   field.  The content-authenticated encryption algorithm is used to
   encipher the content located in the AuthEnvelopedData
   EncryptedContentInfo encryptedContent field.

   The AES-GCM content-authenticated encryption algorithm is described
   in [FIPS197] and [SP80038D].  The algorithm identifier for AES-256 in
   GCM mode is:

            id-aes256-GCM  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
               country(16) us(840) organization(1) gov(101) csor(3)
               nistAlgorithm(4) aes(1) 46 }

   The AlgorithmIdentifier parameters field MUST be present, and the
   parameters field must contain GCMParameters:

         GCMParameters ::= SEQUENCE {
           aes-nonce        OCTET STRING,
           aes-ICVlen       AES-GCM-ICVlen DEFAULT 12 }

   The authentication tag length (aes-ICVlen) SHALL be 16 (indicating a
   tag length of 128 bits).

   The initialization vector (aes-nonce) MUST be generated in accordance
   with Section 8.2 of [SP80038D].  AES-GCM loses security
   catastrophically if a nonce is reused with a given key on more than
   one distinct set of input data.  Therefore, a fresh content-
   authenticated encryption key MUST be generated for each message.

7.2.  AES-CBC Content Encryption

   CNSA Suite-compliant S/MIME implementations using the enveloped-data
   content type MUST use AES-256 [FIPS197] in Cipher Block Chaining
   (CBC) mode [SP80038A] as the content-encryption algorithm and MUST
   follow the conventions for using AES with the CMS defined in
   [RFC3565].

   Within the CMS enveloped-data content type, content-encryption
   algorithm identifiers are located in the EnvelopedData
   EncryptedContentInfo contentEncryptionAlgorithm field.  The content-
   encryption algorithm is used to encipher the content located in the
   EnvelopedData EncryptedContentInfo encryptedContent field.

   The AES-CBC content-encryption algorithm is described in [FIPS197]
   and [SP80038A].  The algorithm identifier for AES-256 in CBC mode is:

         id-aes256-CBC  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
            country(16) us(840) organization(1) gov(101) csor(3)
            nistAlgorithm(4) aes(1) 42 }

   The AlgorithmIdentifier parameters field MUST be present, and the
   parameters field must contain AES-IV:

         AES-IV  ::=  OCTET STRING (SIZE(16))

   The 16-octet initialization vector is generated at random by the
   originator.  See [RFC4086] for guidance on generation of random
   values.

8.  Security Considerations

   This document specifies the conventions for using the NSA's CNSA
   Suite algorithms in S/MIME.  All of the algorithms and algorithm
   identifiers have been specified in previous documents.

   See [RFC4086] for guidance on generation of random values.

   The security considerations in [RFC5652] discuss the CMS as a method
   for digitally signing data and encrypting data.

   The security considerations in [RFC3370] discuss cryptographic
   algorithm implementation concerns in the context of the CMS.

   The security considerations in [RFC5753] discuss the use of elliptic
   curve cryptography (ECC) in the CMS.

   The security considerations in [RFC3565] discuss the use of AES in
   the CMS.

   The security considerations in [RFC8551] apply to this profile,
   particularly the recommendation to use authenticated encryption modes
   (i.e., use authenticated-enveloped-data with AES-GCM rather than
   enveloped-data with AES-CBC).

9.  IANA Considerations

   This document has no IANA actions.

10.  References

10.1.  Normative References

   [CNSA]     Committee for National Security Systems, "Use of Public
              Standards for Secure Information Sharing", CNSS Policy 15,
              October 2016,
              <https://www.cnss.gov/CNSS/Issuances/Policies.cfm>.

   [FIPS180]  National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", Federal Information Processing
              Standard 180-4, August 2015,
              <https://csrc.nist.gov/publications/detail/fips/180/4/
              final>.

   [FIPS186]  National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", DOI 10.6028/NIST.FIPS.186-4,
              FIPS PUB 186-4, July 2013,
              <https://csrc.nist.gov/publications/detail/fips/186/4/
              final>.

   [FIPS197]  National Institute of Standards and Technology, "Advanced
              Encryption Standard (AES)", DOI 10.6028/NIST.FIPS.197,
              FIPS PUB 197, November 2001,
              <https://csrc.nist.gov/publications/detail/fips/197/
              final>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2631]  Rescorla, E., "Diffie-Hellman Key Agreement Method",
              RFC 2631, DOI 10.17487/RFC2631, June 1999,
              <https://www.rfc-editor.org/info/rfc2631>.

   [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
              Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,
              <https://www.rfc-editor.org/info/rfc3370>.

   [RFC3560]  Housley, R., "Use of the RSAES-OAEP Key Transport
              Algorithm in Cryptographic Message Syntax (CMS)",
              RFC 3560, DOI 10.17487/RFC3560, July 2003,
              <https://www.rfc-editor.org/info/rfc3560>.

   [RFC3565]  Schaad, J., "Use of the Advanced Encryption Standard (AES)
              Encryption Algorithm in Cryptographic Message Syntax
              (CMS)", RFC 3565, DOI 10.17487/RFC3565, July 2003,
              <https://www.rfc-editor.org/info/rfc3565>.

   [RFC4055]  Schaad, J., Kaliski, B., and R. Housley, "Additional
              Algorithms and Identifiers for RSA Cryptography for use in
              the Internet X.509 Public Key Infrastructure Certificate
              and Certificate Revocation List (CRL) Profile", RFC 4055,
              DOI 10.17487/RFC4055, June 2005,
              <https://www.rfc-editor.org/info/rfc4055>.

   [RFC4056]  Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
              Cryptographic Message Syntax (CMS)", RFC 4056,
              DOI 10.17487/RFC4056, June 2005,
              <https://www.rfc-editor.org/info/rfc4056>.

   [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
              Authenticated-Enveloped-Data Content Type", RFC 5083,
              DOI 10.17487/RFC5083, November 2007,
              <https://www.rfc-editor.org/info/rfc5083>.

   [RFC5084]  Housley, R., "Using AES-CCM and AES-GCM Authenticated
              Encryption in the Cryptographic Message Syntax (CMS)",
              RFC 5084, DOI 10.17487/RFC5084, November 2007,
              <https://www.rfc-editor.org/info/rfc5084>.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
              <https://www.rfc-editor.org/info/rfc5480>.

   [RFC5649]  Housley, R. and M. Dworkin, "Advanced Encryption Standard
              (AES) Key Wrap with Padding Algorithm", RFC 5649,
              DOI 10.17487/RFC5649, September 2009,
              <https://www.rfc-editor.org/info/rfc5649>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC5753]  Turner, S. and D. Brown, "Use of Elliptic Curve
              Cryptography (ECC) Algorithms in Cryptographic Message
              Syntax (CMS)", RFC 5753, DOI 10.17487/RFC5753, January
              2010, <https://www.rfc-editor.org/info/rfc5753>.

   [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
              Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
              2010, <https://www.rfc-editor.org/info/rfc5754>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", RFC 8551, DOI 10.17487/RFC8551,
              April 2019, <https://www.rfc-editor.org/info/rfc8551>.

   [RFC8603]  Jenkins, M. and L. Zieglar, "Commercial National Security
              Algorithm (CNSA) Suite Certificate and Certificate
              Revocation List (CRL) Profile", RFC 8603,
              DOI 10.17487/RFC8603, May 2019,
              <https://www.rfc-editor.org/info/rfc8603>.

   [SEC1]     Standards for Efficient Cryptography Group, "SEC1:
              Elliptic Curve Cryptography", May 2009,
              <https://www.secg.org/sec1-v2.pdf>.

   [SP80038A] Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Methods and Techniques",
              DOI 10.6028/NIST.SP.800-38A, Special Publication 800-38A,
              December 2001, <https://csrc.nist.gov/publications/detail/
              sp/800-38a/final>.

   [SP80038D] Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Galois/Counter Mode (GCM) and GMAC",
              DOI 10.6028/NIST.SP.800-38D, Special Publication 800-38D,
              November 2007, <https://csrc.nist.gov/publications/detail/
              sp/800-38d/final>.

   [SP80038F] Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Methods for Key Wrapping",
              DOI 10.6028/NIST.SP.800-38F, Special Publication 800-38F,
              December 2012, <https://csrc.nist.gov/publications/detail/
              sp/800-38f/final>.

   [X208]     CCITT, "Specification of Abstract Syntax Notation One
              (ASN.1)", CCITT Recommendation X.208, 1988,
              <https://www.itu.int/rec/T-REC-X.208-198811-W/en>.

   [X209]     CCITT, "Specification of Basic Encoding Rules for Abstract
              Syntax Notation One (ASN.1)", CCITT Recommendation X.209,
              1988, <https://www.itu.int/rec/T-REC-X.209-198811-W/en>.

   [X509]     CCITT, "The Directory - Authentication Framework", CCITT
              Recommendation X.509, 1988,
              <https://www.itu.int/rec/T-REC-X.509-198811-S>.

10.2.  Informative References

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [SP80059]  Barker, W., "Guideline for Identifying an Information
              System as a National Security System",
              DOI 10.6028/NIST.SP.800-59, Special Publication 800-59,
              August 2003, <https://csrc.nist.gov/publications/detail/
              sp/800-59/final>.

Author's Address

   Michael Jenkins
   National Security Agency

   Email: mjjenki@nsa.gov
