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Using Commercial National Security Algorithm Suite Algorithms in Secure/Multipurpose Internet Mail ExtensionsNational Security Agencymjjenki@nsa.gov
Security
Internet Engineering Task ForceThe United States government has published the 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.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) . 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 suite; 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.
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 USG 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. NSA has established the Commercial National Security Algorithm (CNSA) Suite to provide vendors and IT users near-term flexibility in meeting their IA interoperability requirements. The purpose behind this flexibility is to avoid vendors and customers making two major transitions in a relatively short timeframe, as we anticipate a need to shift to quantum-resistant cryptography in the near future.
Transition to post-quantum algorithms will occur after NIST has completed their evaluation and standardization. In the meantime, NSA is publishing a set of RFCs, including this one, to provide updated guidance concerning the use of certain commonly available commercial algorithms [CNSSP15] 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.
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.
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:
For CNSA Suite applications, public key certificates used to verify S/MIME signatures MUST be compliant with the CNSA Suite Certificate and CRL Profile specified in .
Within the CMS signed-data content type, signature algorithm identifiers are located in the SignerInfo signatureAlgorithm field of SignedData. In addition, signature algorithm identifiers are located in the SignerInfo signatureAlgorithm field of countersignature attributes.
ECC based implementations also require specification of schemes for key derivation and key wrap. Requirements for these schemes are in sections and repectively.
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 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 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.
The Elliptic Curve Digital Signature Algorithm (ECDSA) is the CNSA Suite digital signature algorithm based on Elliptic Curve Cryptography (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 :
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 .
defines the signature algorithm identifier used in CMS for 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.
defines the signature algorithm identifier used in CMS for RSA-PSS signature as follows:
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; andthe trailerField must have value 1.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, a key-encryption algorithm is also needed. In the CNSA Suite for S/MIME, the Advanced Encryption Standard (AES) Key Wrap with Padding Algorithm, as specified in and , MUST be used as the key-encryption algorithm. AES Key Wrap is discussed further in . The key-encryption key used with the AES Key Wrap algorithm is obtained from a key derivation function (KDF). In the CNSA Suite for S/MIME, the KDF described in -- based on SHA-384 -- MUST be used.
Section 3.1 of 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 MUST be dhSinglePass-stdDH-sha384kdf-scheme, and the keyEncryptionAlgorithm parameter MUST be a KeyWrapAlgorithm containing id-aes256-wrap-pad (see ). The key wrap algorithm denotes the symmetric encryption algorithm used to encrypt the randomly generated content-encryption key, employing the pairwise key-encryption key generated using the ephemeral-static ECDH key agreement algorithm (see ).
The algorithm identifier for the dhSinglePass-stdDH-sha384kdf-scheme, repeated from Section 7.1.4 of , is:
with KeyWrapAlgorithm as the type for its parameter:
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. Section 8 of 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:
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 of 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.
The KDF based on SHA-384 MUST be used.
As specified in Section 7.2 of , 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 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 the user keying material (ukm) is present, 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 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 provides an algorithm for generating an essentially arbitrary amount of keying material (KM) from a shared secret, Z, produced by ephemeral-static ECDH. The KDF generates successive blocks of keying material, KM(1), KM(2), and so on, using:
To generate an L-bit key-encryption key (KEK), one or more KM blocks are generated, 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 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 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:
In CNSA Suite for S/MIME, the key-encryption key MUST be the most significant 256 bits of the SHA-384 output value.
Note that the only source of secret entropy in this computation is Z.
RSA encryption (RSA) is the CNSA Suite key transport algorithm. The RSA key transport algorithm is the RSA encryption scheme defined in , block type is 02, 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.
Section 4.2 of 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 content type, 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.
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 content type, 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.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 , with the sequence representing the rSAES-OAEP-SHA384-Identifier.
CNSA Suite compliant S/MIME implementations MUST use AES-256 in Cipher Block Chaining (CBC) mode as the content encryption algorithm when using the enveloped-data content type, 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.
CNSA Suite compliant S/MIME implementations MUST use AES in Galois Counter Mode (GCM) as the content-authenticated encryption algorithm when using the authenticated-enveloped-data content type , and MUST follow the conventions for using AES-GCM with the CMS defined in .
In CNSA Suite for S/MIME AES-256 in GCM mode MUST be used.
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 [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.
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.
This document has no IANA actions.
Advanced Encryption Standard (AES)National Institute of Standards and TechnologyRecommendation for Block Cipher Modes of Operation: Methods and TechniquesNational Institute of Standards and TechnologyRecommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMACNational Institute of Standards and TechnologyRecommendation for Block Cipher Modes of Operation: Methods for Key WrappingNational Institute of Standards and TechnologySecure Hash Standard (SHS)National Institute of Standards and TechnologyDigital Signature StandardNational Institute of Standards and TechnologySEC1: Elliptic Curve CryptographyStandards for Efficient Cryptography Group
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&rfc3560;
&rfc3565;
&rfc4055;
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&rfc5083;
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Commercial National Security Algorithms (CNSA) Suite Certificate and Certificate Revocation List (CRL) ProfileWork in progress.Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message SpecificationWork in progress.Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1)CCITTRecommendation X.209: Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1)CCITTRecommendation X.509: The Directory - Authentication FrameworkCCITTCommercial National Security Algorithm (CNSA) Suite Fact SheetCommittee for National Security SystemsGuideline for Identifying an Information System as a National Security SystemNational Institute of Standards and Technology
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