EVP_PKEY_CTX_SET_TLS1_PRF_MD(3) | Library Functions Manual | EVP_PKEY_CTX_SET_TLS1_PRF_MD(3)

# NAME

`EVP_PKEY_CTX_set_tls1_prf_md`, `EVP_PKEY_CTX_set1_tls1_prf_secret`, `EVP_PKEY_CTX_add1_tls1_prf_seed` — TLS PRF key derivation algorithm

# SYNOPSIS

library “libcrypto”
`#include <openssl/evp.h>`
`#include <openssl/kdf.h>`

int
`EVP_PKEY_CTX_set_tls1_prf_md`(EVP_PKEY_CTX *pctx, const EVP_MD *md);

int
`EVP_PKEY_CTX_set1_tls1_prf_secret`(EVP_PKEY_CTX *pctx, unsigned char *sec, int seclen);

int
`EVP_PKEY_CTX_add1_tls1_prf_seed`(EVP_PKEY_CTX *pctx, unsigned char *seed, int seedlen);

# DESCRIPTION

The `EVP_PKEY_TLS1_PRF` algorithm implements the PRF key derivation function for TLS. It has no associated private key and only implements key derivation using EVP_PKEY_derive(3).

`EVP_PKEY_set_tls1_prf_md`() sets the message digest associated with the TLS PRF. EVP_md5_sha1(3) is treated as a special case which uses the PRF algorithm using both MD5 and SHA-1 as used in TLS 1.0 and 1.1.

`EVP_PKEY_CTX_set_tls1_prf_secret`() sets the secret value of the TLS PRF to seclen bytes of the buffer sec. Any existing secret value is replaced and any seed is reset.

`EVP_PKEY_CTX_add1_tls1_prf_seed`() sets the seed to seedlen bytes of seed. If a seed is already set it is appended to the existing value.

# STRING CTRLS

The TLS PRF also supports string based control operations using EVP_PKEY_CTX_ctrl_str(3). The type parameter "md" uses the supplied value as the name of the digest algorithm to use. The type parameters "secret" and "seed" use the supplied value parameter as a secret or seed value. The names "hexsecret" and "hexseed" are similar except they take a hex string which is converted to binary.

# NOTES

All these functions are implemented as macros.

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

     EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_TLS1_PRF, NULL);

The digest, secret value and seed must be set before a key is derived or an error occurs.

The total length of all seeds cannot exceed 1024 bytes in length: this should be more than enough for any normal use of the TLS PRF.

The output length of the PRF is specified by the length parameter in the EVP_PKEY_derive(3) function. Since the output length is variable, setting the buffer to `NULL` is not meaningful for the TLS PRF.

# RETURN VALUES

All these functions return 1 for success and 0 or a negative value for failure. In particular a return value of -2 indicates the operation is not supported by the public key algorithm.

# EXAMPLES

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

     EVP_PKEY_CTX *pctx;
     unsigned char out[10];
     size_t outlen = sizeof(out);

     pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_TLS1_PRF, NULL);
     if (EVP_PKEY_derive_init(pctx) <= 0)
         /* Error */
     if (EVP_PKEY_CTX_set_tls1_prf_md(pctx, EVP_sha256()) <= 0)
         /* Error */
     if (EVP_PKEY_CTX_set1_tls1_prf_secret(pctx, "secret", 6) <= 0)
         /* Error */
     if (EVP_PKEY_CTX_add1_tls1_prf_seed(pctx, "seed", 4) <= 0)
         /* Error */
     if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
         /* Error */

# SEE ALSO

EVP_PKEY_CTX_ctrl_str(3), EVP_PKEY_CTX_new(3), EVP_PKEY_derive(3)

# HISTORY

These functions first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 7.6.

June 8, 2025 | Debian