.\" $OpenBSD: EVP_PKEY_CTX_set_hkdf_md.3,v 1.4 2024/07/10 07:57:37 tb Exp $ .\" full merge up to: OpenSSL 1cb7eff4 Sep 10 13:56:40 2019 +0100 .\" .\" This file was written by Alessandro Ghedini , .\" Matt Caswell , and Viktor Dukhovni . .\" Copyright (c) 2016 The OpenSSL Project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: July 10 2024 $ .Dt EVP_PKEY_CTX_SET_HKDF_MD 3 .Os .Sh NAME .Nm EVP_PKEY_CTX_set_hkdf_md , .Nm EVP_PKEY_CTX_set1_hkdf_salt , .Nm EVP_PKEY_CTX_set1_hkdf_key , .Nm EVP_PKEY_CTX_add1_hkdf_info , .Nm EVP_PKEY_CTX_hkdf_mode .Nd HMAC-based Extract-and-Expand key derivation algorithm .Sh SYNOPSIS .In openssl/evp.h .In openssl/kdf.h .Ft int .Fo EVP_PKEY_CTX_hkdf_mode .Fa "EVP_PKEY_CTX *pctx" .Fa "int mode" .Fc .Ft int .Fo EVP_PKEY_CTX_set_hkdf_md .Fa "EVP_PKEY_CTX *pctx" .Fa "const EVP_MD *md" .Fc .Ft int .Fo EVP_PKEY_CTX_set1_hkdf_salt .Fa "EVP_PKEY_CTX *pctx" .Fa "unsigned char *salt" .Fa "int saltlen" .Fc .Ft int .Fo EVP_PKEY_CTX_set1_hkdf_key .Fa "EVP_PKEY_CTX *pctx" .Fa "unsigned char *key" .Fa "int keylen" .Fc .Ft int .Fo EVP_PKEY_CTX_add1_hkdf_info .Fa "EVP_PKEY_CTX *pctx" .Fa "unsigned char *info" .Fa "int infolen" .Fc .Sh DESCRIPTION The .Dv EVP_PKEY_HKDF algorithm implements the HKDF key derivation function. HKDF follows the "extract-then-expand" paradigm, where the KDF logically consists of two modules. The first stage takes the input keying material and "extracts" from it a fixed-length pseudorandom key K. The second stage "expands" the key K into several additional pseudorandom keys (the output of the KDF). .Pp .Fn EVP_PKEY_CTX_hkdf_mode sets the mode for the HKDF operation. There are three modes that are currently defined: .Bl -tag -width Ds .It Dv EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND This is the default mode. Calling .Xr EVP_PKEY_derive 3 on an .Vt EVP_PKEY_CTX set up for HKDF will perform an extract followed by an expand operation in one go. The derived key returned will be the result after the expand operation. The intermediate fixed-length pseudorandom key K is not returned. .Pp In this mode the digest, key, salt and info values must be set before a key is derived or an error occurs. .It Dv EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY In this mode calling .Xr EVP_PKEY_derive 3 will just perform the extract operation. The value returned will be the intermediate fixed-length pseudorandom key K. .Pp The digest, key and salt values must be set before a key is derived or an error occurs. .It Dv EVP_PKEY_HKDEF_MODE_EXPAND_ONLY In this mode calling .Xr EVP_PKEY_derive 3 will just perform the expand operation. The input key should be set to the intermediate fixed-length pseudorandom key K returned from a previous extract operation. .Pp The digest, key and info values must be set before a key is derived or an error occurs. .El .Pp .Fn EVP_PKEY_CTX_set_hkdf_md sets the message digest associated with the HKDF. .Pp .Fn EVP_PKEY_CTX_set1_hkdf_salt sets the salt to .Fa saltlen bytes of the buffer .Fa salt . Any existing value is replaced. .Pp .Fn EVP_PKEY_CTX_set1_hkdf_key sets the key to .Fa keylen bytes of the buffer .Fa key . Any existing value is replaced. .Pp .Fn EVP_PKEY_CTX_add1_hkdf_info sets the info value to .Fa infolen bytes of the buffer .Fa info . If a value is already set, it is appended to the existing value. .Sh STRING CTRLS HKDF also supports string based control operations via .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 parameter "mode" accepts "EXTRACT_AND_EXPAND", "EXTRACT_ONLY" and "EXPAND_ONLY" as .Fa value to determine the mode to use. The .Fa type parameters "salt", "key" and "info" use the supplied .Fa value parameter as a seed, key, or info. The names "hexsalt", "hexkey" and "hexinfo" 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 HKDF can be obtained by calling: .Bd -literal EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL); .Ed .Pp The total length of the info buffer cannot exceed 1024 bytes in length: this should be more than enough for any normal use of HKDF. .Pp The output length of an HKDF expand operation is specified via the length parameter to the .Xr EVP_PKEY_derive 3 function. Since the HKDF output length is variable, passing a .Dv NULL buffer as a means to obtain the requisite length is not meaningful with HKDF in any mode that performs an expand operation. Instead, the caller must allocate a buffer of the desired length, and pass that buffer to .Xr EVP_PKEY_derive 3 along with (a pointer initialized to) the desired length. Passing a .Dv NULL buffer to obtain the length is allowed when using .Dv EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY . .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", salt value "salt" and info value "label": .Bd -literal EVP_PKEY_CTX *pctx; unsigned char out[10]; size_t outlen = sizeof(out); if ((pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL)) == NULL) /* Error */ if (EVP_PKEY_derive_init(pctx) <= 0) /* Error */ if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0) /* Error */ if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0) /* Error */ if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0) /* Error */ if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 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 STANDARDS RFC 5869: HMAC-based Extract-and-Expand Key Derivation Function (HKDF) .Sh HISTORY .Fn EVP_PKEY_CTX_set_hkdf_md , .Fn EVP_PKEY_CTX_set1_hkdf_salt , .Fn EVP_PKEY_CTX_set1_hkdf_key , and .Fn EVP_PKEY_CTX_add1_hkdf_info first appeared in OpenSSL 1.1.0 and .Fn EVP_PKEY_CTX_hkdf_mode in OpenSSL 1.1.1. These functions have been available since .Ox 7.2 .