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Editing: EVP_PKEY_CTX_hkdf_mode.3
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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" 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_hkdf_mode \- HMAC\-based Extract\-and\-Expand key derivation algorithm .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 1 \& #include <openssl/kdf.h> \& \& int EVP_PKEY_CTX_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 .SH "DESCRIPTION" .IX 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). .PP \&\fBEVP_PKEY_CTX_hkdf_mode()\fR sets the mode for the \s-1HKDF\s0 operation. There are three modes that are currently defined: .IP "\s-1EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND\s0" 4 .IX Item "EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND" This is the default mode. Calling \fBEVP_PKEY_derive\fR\|(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. .IP "\s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY\s0" 4 .IX Item "EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY" In this mode calling \fBEVP_PKEY_derive\fR\|(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. .IP "\s-1EVP_PKEY_HKDEF_MODE_EXPAND_ONLY\s0" 4 .IX Item "EVP_PKEY_HKDEF_MODE_EXPAND_ONLY" In this mode calling \fBEVP_PKEY_derive\fR\|(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. .PP \&\fBEVP_PKEY_CTX_set_hkdf_md()\fR sets the message digest associated with the \s-1HKDF.\s0 .PP \&\fBEVP_PKEY_CTX_set1_hkdf_salt()\fR sets the salt to \fBsaltlen\fR bytes of the buffer \fBsalt\fR. Any existing value is replaced. .PP \&\fBEVP_PKEY_CTX_set1_hkdf_key()\fR sets the key to \fBkeylen\fR bytes of the buffer \&\fBkey\fR. Any existing value is replaced. .PP \&\fBEVP_PKEY_CTX_add1_hkdf_info()\fR sets the info value to \fBinfolen\fR bytes of the buffer \fBinfo\fR. If a value is already set, it is appended to the existing value. .SH "STRING CTRLS" .IX Header "STRING CTRLS" \&\s-1HKDF\s0 also supports string based control operations via \&\fBEVP_PKEY_CTX_ctrl_str\fR\|(3). The \fBtype\fR parameter \*(L"md\*(R" uses the supplied \fBvalue\fR as the name of the digest algorithm to use. The \fBtype\fR 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 \fBtype\fR parameters \*(L"salt\*(R", \*(L"key\*(R" and \*(L"info\*(R" use the supplied \fBvalue\fR parameter as a \fBseed\fR, \fBkey\fR or \fBinfo\fR 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. .SH "NOTES" .IX Header "NOTES" All these functions are implemented as macros. .PP A context for \s-1HKDF\s0 can be obtained by calling: .PP .Vb 1 \& EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL); .Ve .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 \s-1HKDF.\s0 .PP The output length of an \s-1HKDF\s0 expand operation is specified via the length parameter to the \fBEVP_PKEY_derive\fR\|(3) function. Since the \s-1HKDF\s0 output length is variable, passing a \fB\s-1NULL\s0\fR 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 \fBEVP_PKEY_derive\fR\|(3) along with (a pointer initialized to) the desired length. Passing a \fB\s-1NULL\s0\fR buffer to obtain the length is allowed when using \s-1EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.\s0 .PP Optimised versions of \s-1HKDF\s0 can be implemented in an \s-1ENGINE.\s0 .SH "RETURN VALUES" .IX 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. .SH "EXAMPLES" .IX 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": .PP .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 .SH "CONFORMING TO" .IX Header "CONFORMING TO" \&\s-1RFC 5869\s0 .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fBEVP_PKEY_CTX_new\fR\|(3), \&\fBEVP_PKEY_CTX_ctrl_str\fR\|(3), \&\fBEVP_PKEY_derive\fR\|(3) .SH "COPYRIGHT" .IX Header "COPYRIGHT" Copyright 2016\-2019 The OpenSSL Project Authors. All Rights Reserved. .PP Licensed under the OpenSSL license (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>.
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