.\" $OpenBSD: EVP_PKEY_CTX_set_tls1_prf_md.3,v 1.2 2024/07/10 10:22:03 tb Exp $
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.Dd $Mdocdate: July 10 2024 $
.Dt EVP_PKEY_CTX_SET_TLS1_PRF_MD 3
.Os
.Sh NAME
.Nm EVP_PKEY_CTX_set_tls1_prf_md ,
.Nm EVP_PKEY_CTX_set1_tls1_prf_secret ,
.Nm EVP_PKEY_CTX_add1_tls1_prf_seed
.Nd TLS PRF key derivation algorithm
.Sh SYNOPSIS
.In openssl/evp.h
.In openssl/kdf.h
.Ft int
.Fo EVP_PKEY_CTX_set_tls1_prf_md
.Fa "EVP_PKEY_CTX *pctx"
.Fa "const EVP_MD *md"
.Fc
.Ft int
.Fo EVP_PKEY_CTX_set1_tls1_prf_secret
.Fa "EVP_PKEY_CTX *pctx"
.Fa "unsigned char *sec"
.Fa "int seclen"
.Fc
.Ft int
.Fo EVP_PKEY_CTX_add1_tls1_prf_seed
.Fa "EVP_PKEY_CTX *pctx"
.Fa "unsigned char *seed"
.Fa "int seedlen"
.Fc
.Sh DESCRIPTION
The
.Dv 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
.Xr EVP_PKEY_derive 3 .
.Pp
.Fn EVP_PKEY_set_tls1_prf_md
sets the message digest associated with the TLS PRF.
.Xr EVP_md5_sha1 3
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.
.Pp
.Fn EVP_PKEY_CTX_set_tls1_prf_secret
sets the secret value of the TLS PRF to
.Fa seclen
bytes of the buffer
.Fa sec .
Any existing secret value is replaced and any seed is reset.
.Pp
.Fn EVP_PKEY_CTX_add1_tls1_prf_seed
sets the seed to
.Fa seedlen
bytes of
.Fa seed .
If a seed is already set it is appended to the existing value.
.Sh STRING CTRLS
The TLS PRF also supports string based control operations using
.Xr EVP_PKEY_CTX_ctrl_str 3 .
The
.Fa type
parameter "md" uses the supplied
.Fa value
as the name of the digest algorithm to use.
The
.Fa type
parameters "secret" and "seed" use the supplied
.Fa 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.
.Sh NOTES
All these functions are implemented as macros.
.Pp
A context for the TLS PRF can be obtained by calling:
.Bd -literal
 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_TLS1_PRF, NULL);
.Ed
.Pp
The digest, secret value and seed must be set before a key is derived or
an error occurs.
.Pp
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.
.Pp
The output length of the PRF is specified by the length parameter in the
.Xr EVP_PKEY_derive 3
function.
Since the output length is variable, setting the buffer to
.Dv NULL
is not meaningful for the TLS PRF.
.Sh 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.
.Sh EXAMPLES
This example derives 10 bytes using SHA-256 with the secret key "secret"
and seed value "seed":
.Bd -literal
 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 */
.Ed
.Sh SEE ALSO
.Xr EVP_PKEY_CTX_ctrl_str 3 ,
.Xr EVP_PKEY_CTX_new 3 ,
.Xr EVP_PKEY_derive 3
.Sh HISTORY
These functions first appeared in OpenSSL 1.1.0 and have been available since
.Ox 7.6 .