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Title "EVP_PKEY_CTX_SET_HKDF_MD 3ossl" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt, EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info, EVP_PKEY_CTX_set_hkdf_mode - HMAC-based Extract-and-Expand key derivation algorithm Header "SYNOPSIS" .Vb 1 #include <openssl/kdf.h> \& int EVP_PKEY_CTX_set_hkdf_mode(EVP_PKEY_CTX *pctx, int mode); \& int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md); \& int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt, int saltlen); \& int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key, int keylen); \& int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info, int infolen); .Ve Header "DESCRIPTION" The \s-1EVP_PKEY_HKDF\s0 algorithm implements the \s-1HKDF\s0 key derivation function. \s-1HKDF\s0 follows the \*(L"extract-then-expand\*(R" paradigm, where the \s-1KDF\s0 logically consists of two modules. The first stage takes the input keying material and \*(L"extracts\*(R" from it a fixed-length pseudorandom key K. The second stage \*(L"expands\*(R" the key K into several additional pseudorandom keys (the output of the \s-1KDF\s0).

\fBEVP_PKEY_CTX_set_hkdf_mode() sets the mode for the \s-1HKDF\s0 operation. There are three modes that are currently defined:

Item "EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND" This is the default mode. Calling EVP_PKEY_derive\|(3) on an \s-1EVP_PKEY_CTX\s0 set up for \s-1HKDF\s0 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. .Sp In this mode the digest, key, salt and info values must be set before a key is derived or an error occurs. Item "EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY" In this mode calling EVP_PKEY_derive\|(3) will just perform the extract operation. The value returned will be the intermediate fixed-length pseudorandom key K. .Sp The digest, key and salt values must be set before a key is derived or an error occurs. Item "EVP_PKEY_HKDEF_MODE_EXPAND_ONLY" In this mode calling 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. .Sp The digest, key and info values must be set before a key is derived or an error occurs.

\fBEVP_PKEY_CTX_set_hkdf_md() sets the message digest associated with the \s-1HKDF.\s0

\fBEVP_PKEY_CTX_set1_hkdf_salt() sets the salt to saltlen bytes of the buffer salt. Any existing value is replaced.

\fBEVP_PKEY_CTX_set1_hkdf_key() sets the key to keylen bytes of the buffer \fBkey. Any existing value is replaced.

\fBEVP_PKEY_CTX_add1_hkdf_info() sets the info value to infolen bytes of the buffer info. If a value is already set, it is appended to the existing value.

Header "STRING CTRLS" \s-1HKDF\s0 also supports string based control operations via \fBEVP_PKEY_CTX_ctrl_str\|(3). The type parameter \*(L"md\*(R" uses the supplied value as the name of the digest algorithm to use. The type parameter \*(L"mode\*(R" uses the values \*(L"\s-1EXTRACT_AND_EXPAND\*(R", \*(L"EXTRACT_ONLY\*(R"\s0 and \*(L"\s-1EXPAND_ONLY\*(R"\s0 to determine the mode to use. The type parameters \*(L"salt\*(R", \*(L"key\*(R" and \*(L"info\*(R" use the supplied value parameter as a seed, key or info value. The names \*(L"hexsalt\*(R", \*(L"hexkey\*(R" and \*(L"hexinfo\*(R" are similar except they take a hex string which is converted to binary. Header "NOTES" A context for \s-1HKDF\s0 can be obtained by calling:

.Vb 1 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL); .Ve

The total length of the info buffer cannot exceed 2048 bytes in length: this should be more than enough for any normal use of \s-1HKDF.\s0

The output length of an \s-1HKDF\s0 expand operation is specified via the length parameter to the EVP_PKEY_derive\|(3) function. Since the \s-1HKDF\s0 output length is variable, passing a \s-1NULL\s0 buffer as a means to obtain the requisite length is not meaningful with \s-1HKDF\s0 in any mode that performs an expand operation. Instead, the caller must allocate a buffer of the desired length, and pass that buffer to EVP_PKEY_derive\|(3) along with (a pointer initialized to) the desired length. Passing a \s-1NULL\s0 buffer to obtain the length is allowed when using \s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.\s0

Optimised versions of \s-1HKDF\s0 can be implemented in an \s-1ENGINE.\s0

Header "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. Header "EXAMPLES" This example derives 10 bytes using \s-1SHA-256\s0 with the secret key \*(L"secret\*(R", salt value \*(L"salt\*(R" and info value \*(L"label\*(R":

.Vb 4 EVP_PKEY_CTX *pctx; unsigned char out[10]; size_t outlen = sizeof(out); pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL); \& 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 */ .Ve

Header "CONFORMING TO" \s-1RFC 5869\s0 Header "SEE ALSO" \fBEVP_PKEY_CTX_new\|(3), \fBEVP_PKEY_CTX_ctrl_str\|(3), \fBEVP_PKEY_derive\|(3) Header "HISTORY" All of the functions described here were converted from macros to functions in OpenSSL 3.0. Header "COPYRIGHT" Copyright 2016-2022 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.