Cryptography and SSL/TLS Toolkit



EVP_KDF-TLS1_PRF - The TLS1 PRF EVP_KDF implementation


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.


"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.

"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 the "ems_check" is set to 0 and the "extended master secret" test fails. This option is used by the OpenSSL FIPS provider.

"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 is used by the OpenSSL FIPS provider, and breaks FIPS compliance if set to 0.


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.


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);

*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) {


RFC 2246, RFC 5246 and NIST SP 800-135 r1


EVP_KDF(3), EVP_KDF_CTX_new(3), EVP_KDF_CTX_free(3), EVP_KDF_CTX_set_params(3), EVP_KDF_derive(3), "PARAMETERS" in EVP_KDF(3)


This functionality was added in OpenSSL 3.0.

Copyright 2018-2021 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at