EVP_KDF-TLS1_PRF7ssl

The tls1 prf evp_kdf implementation

Overview

Support for computing the TLS1 PRF through the EVP_KDF API.

The EVP_KDF-TLS1_PRF algorithm implements the PRF used by TLS versions up to and including TLS 1.2.

The output is considered to be keying material.

Identity

"TLS1-PRF" is the name for this implementation; it can be used with the EVP_KDF_fetch() function.

Supported parameters

The supported parameters are:

"properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>

"digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>

These parameters work as described in "PARAMETERS" in EVP_KDF(3).

The OSSL_KDF_PARAM_DIGEST parameter is used to set the message digest associated with the TLS PRF. EVP_md5_sha1() is treated as a special case which uses the PRF algorithm using both MD5 and SHA1 as used in TLS 1.0 and 1.1.

"secret" (OSSL_KDF_PARAM_SECRET) <octet string>: This parameter sets the secret value of the TLS PRF. Any existing secret value is replaced.

"seed" (OSSL_KDF_PARAM_SEED) <octet string>: This parameter sets the context seed. The length of the context seed cannot exceed 1024 bytes; this should be more than enough for any normal use of the TLS PRF.

The OpenSSL FIPS provider also supports the following parameters:

"fips-indicator" (OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR) <integer>: A getter that returns 1 if the operation is FIPS approved, or 0 otherwise. This may be used after calling EVP_KDF_derive. It returns 0 if any "***-check" related parameter is set to 0 and the check fails.

"ems_check" (OSSL_KDF_PARAM_FIPS_EMS_CHECK) <integer>: The default value of 1 causes an error during EVP_KDF_derive() if "master secret" is used instead of "extended master secret" Setting this to zero will ignore the error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0.

"digest-check" (OSSL_KDF_PARAM_FIPS_DIGEST_CHECK) <integer>

The default value of 1 causes an error during EVP_KDF_CTX_set_params() if used digest is not approved. Setting this to zero will ignore the error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0.

According to SP 800-135r1, the following are approved digest algorithms: SHA2-256, SHA2-384, SHA2-512.

"key-check" (OSSL_KDF_PARAM_FIPS_KEY_CHECK) <integer>: The default value of 1 causes an error during EVP_KDF_CTX_set_params() if the length of used key-derivation key (OSSL_KDF_PARAM_SECRET) is shorter than 112 bits. Setting this to zero will ignore the error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0.

Notes

A context for the TLS PRF can be obtained by calling:

 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "TLS1-PRF", NULL);
 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

The digest, secret value and seed must be set before a key is derived otherwise an error will occur.

The output length of the PRF is specified by the keylen parameter to the EVP_KDF_derive() function.

Examples

This example derives 10 bytes using SHA-256 with the secret key "secret" and seed value "seed":

 EVP_KDF *kdf;
 EVP_KDF_CTX *kctx;
 unsigned char out[10];
 OSSL_PARAM params[4], *p = params;
 kdf = EVP_KDF_fetch(NULL, "TLS1-PRF", NULL);
 kctx = EVP_KDF_CTX_new(kdf);
 EVP_KDF_free(kdf);
 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                         SN_sha256, strlen(SN_sha256));
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SECRET,
                                          "secret", (size_t)6);
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SEED,
                                          "seed", (size_t)4);
 *p = OSSL_PARAM_construct_end();
 if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
     error("EVP_KDF_derive");
 }
 EVP_KDF_CTX_free(kctx);

Releases

3 releases since Debian 12 (Bookworm); the text changed 2 times.