Using Commercial National Security Algorithm Suite Algorithms in Secure/&wj;Multipurpose Internet Mail Extensions
National Security Agency
mjjenki@nsa.gov
Security
Internet Engineering Task Force
NSA
CNSA
NSS
smime
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.
Introduction
This document specifies the conventions for using the United States National Security Agency's Commercial National Security Algorithm (CNSA) Suite algorithms in Secure/Multipurpose Internet Mail Extensions (S/MIME) . 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.
S/MIME makes use of the Cryptographic Message Syntax (CMS) . 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.
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
when, and only when, they appear in all capitals, as shown here.
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.
Requirements and Assumptions
CMS values are generated using ASN.1 , the Basic Encoding Rules (BER) , and the Distinguished Encoding Rules (DER) .
The elliptic curve used in the CNSA Suite is specified in 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 |

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 .
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 ; 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 and , 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 .
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 , 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 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 and , respectively.
SHA-384 Message Digest Algorithm
SHA-384 is the sole CNSA Suite message digest algorithm. specifies the conventions for using SHA-384 with the Cryptographic Message Syntax (CMS). CNSA Suite-compliant S/MIME implementations MUST follow the conventions in .
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 . The algorithm identifier for SHA-384 is defined in as follows:
For SHA-384, the AlgorithmIdentifier parameters field is OPTIONAL, and if present, the parameters field MUST contain a NULL. As specified in , implementations MUST generate SHA-384 AlgorithmIdentifiers with absent parameters. Implementations MUST accept SHA-384 AlgorithmIdentifiers with absent parameters or with NULL parameters.
Digital Signature
ECDSA Signature
The Elliptic Curve Digital Signature Algorithm (ECDSA) is the CNSA Suite digital signature algorithm based on ECC. specifies the conventions for using ECDSA with the Cryptographic Message Syntax (CMS). CNSA Suite-compliant S/MIME implementations MUST follow the conventions in .
defines the signature algorithm identifier used in CMS for ECDSA with SHA-384 as follows:
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 :
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 .
RSA-PKCS1-v1_5
defines the signature
algorithm identifier used in CMS for an RSA signature with SHA-384 as follows:
When the sha384WithRSAEncryption algorithm identifier is used, the parameters MUST be NULL. Implementations MUST accept the parameters being absent as well as present.
RSA-PSS
defines the signature
algorithm identifier used in CMS for an RSA-PSS signature as follows (presented here in expanded form):
The parameters field of an AlgorithmIdentifier that identifies RSASSA-PSS is defined in as follows:
The AlgorithmIdentifier parameters field MUST contain RSASSA-PSS-params with the following values:
- The hash algorithm MUST be id-sha384 as defined in ;
- The mask generation function MUST use the algorithm identifier mfg1SHA384Identifier as defined in ;
- The salt length MUST be 48 octets (the same length as the SHA-384 output); and
- The trailerField MUST have value 1.

Key Establishment
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 .
When a key agreement algorithm is used, the following steps are performed:
- A content-encryption key (CEK) for a particular content-encryption algorithm is generated at random.
- The recipient's public key and sender's private key are used with a key agreement scheme to generate a shared secret (Z).
- The shared secret is used with a key derivation function (KDF) to produce a key-encryption key (KEK).
- The KEK is used with a key wrap algorithm to encrypt the CEK.

Key derivation is discussed in . Key wrapping is discussed in .
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 , is (presented here in expanded form):
The keyEncryptionAlgorithm parameter field MUST be constructed as described in .
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 and in 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 , the ANSI-X9.63-KDF described in Section 3.6.1 of and based on SHA-384 MUST be used.
As specified in , 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:
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 . 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 . Note that ECC-CMS-SharedInfo differs from the OtherInfo specified in . Here, a counter value is not included in the keyInfo field because the KDF specified in ensures that sufficient keying data is provided.
The KDF specified in Section 3.6.1 of 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:
The KM blocks are concatenated left to right as they are
generated, and the first (leftmost) L bits are used as the KEK:
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:
Note that the only source of secret entropy in this computation is Z.
AES Key Wrap
The AES Key Wrap with Padding key-encryption algorithm, as specified in and , is used to encrypt the content-encryption key with a pairwise key-encryption key that is generated using ephemeral-static ECDH. 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 , is:
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 , 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 . These certificates are suitable for use with either RSAES-OAEP or RSAES-PKCS1-v1_5.
RSAES-PKCS1-v1_5
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:
The AlgorithmIdentifier parameters field MUST be present, and the parameters field MUST contain NULL.
RSAES-OAEP
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:
The parameters field of an AlgorithmIdentifier that identifies RSAES-OAEP is defined in as follows:
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 ;
- The mask generation function must use the algorithm identifier mfg1SHA384Identifier as defined in ;
- 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 , with the sequence representing the rSAES-OAEP-SHA384-Identifier.
Content Encryption
AES-GCM is the preferred mode for CNSA Suite applications, as described in the Security Considerations. AES-CBC is acceptable where AES-GCM is not yet available.
AES-GCM Content Encryption
CNSA Suite-compliant S/MIME implementations using the
authenticated-enveloped-data content type MUST use AES in Galois Counter Mode (GCM) as the content-authenticated encryption algorithm and MUST follow the conventions for using AES-GCM with the CMS defined in .
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 and . The algorithm identifier for AES-256 in GCM mode is:
The AlgorithmIdentifier parameters field MUST be present, and the parameters field must contain GCMParameters:
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 . 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.
AES-CBC Content Encryption
CNSA Suite-compliant S/MIME implementations using the
enveloped-data content type MUST use AES-256 in Cipher Block Chaining (CBC)
mode as the content-encryption algorithm and MUST follow the conventions for using AES with the CMS defined in .
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 and . The algorithm identifier for AES-256 in CBC mode is:
The AlgorithmIdentifier parameters field MUST be present, and the parameters field must contain AES-IV:
The 16-octet initialization vector is generated at random by the originator. See for guidance on generation of random values.
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 for guidance on generation of random values.
The security considerations in discuss the CMS as a method for digitally signing data and encrypting data.
The security considerations in discuss cryptographic algorithm implementation concerns in the context of the CMS.
The security considerations in discuss the use of elliptic curve cryptography (ECC) in the CMS.
The security considerations in discuss the use of AES in the CMS.
The security considerations in 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).
IANA Considerations
This document has no IANA actions.
References
Normative References
Use of Public Standards for Secure Information Sharing
Committee for National Security Systems
Advanced Encryption Standard (AES)
National Institute of Standards and Technology
Recommendation for Block Cipher Modes of Operation: Methods
and Techniques
Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC
Recommendation for Block Cipher Modes of Operation: Methods
for Key Wrapping
Secure Hash Standard (SHS)
National Institute of Standards and Technology
Digital Signature Standard (DSS)
National Institute of Standards and Technology
SEC1: Elliptic Curve Cryptography
Standards for Efficient Cryptography Group
Specification of Abstract Syntax Notation One (ASN.1)
CCITT
Specification of Basic Encoding Rules
for Abstract Syntax Notation One (ASN.1)
CCITT
The Directory - Authentication Framework
CCITT
Informative References
Guideline for Identifying an Information System as a
National Security System