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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_KDF\-HKDF \- The HKDF EVP_KDF implementation .SH "DESCRIPTION" .IX Header "DESCRIPTION" Support for computing the \fB\s-1HKDF\s0\fR \s-1KDF\s0 through the \fB\s-1EVP_KDF\s0\fR \s-1API.\s0 .PP The \s-1EVP_KDF\-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). .SS "Identity" .IX Subsection "Identity" \&\*(L"\s-1HKDF\*(R"\s0 is the name for this implementation; it can be used with the \fBEVP_KDF_fetch()\fR function. .SS "Supported parameters" .IX Subsection "Supported parameters" The supported parameters are: .ie n .IP """properties"" (\fB\s-1OSSL_KDF_PARAM_PROPERTIES\s0\fR) <\s-1UTF8\s0 string>" 4 .el .IP "``properties'' (\fB\s-1OSSL_KDF_PARAM_PROPERTIES\s0\fR) <\s-1UTF8\s0 string>" 4 .IX Item "properties (OSSL_KDF_PARAM_PROPERTIES) " .PD 0 .ie n .IP """digest"" (\fB\s-1OSSL_KDF_PARAM_DIGEST\s0\fR) <\s-1UTF8\s0 string>" 4 .el .IP "``digest'' (\fB\s-1OSSL_KDF_PARAM_DIGEST\s0\fR) <\s-1UTF8\s0 string>" 4 .IX Item "digest (OSSL_KDF_PARAM_DIGEST) " .ie n .IP """key"" (\fB\s-1OSSL_KDF_PARAM_KEY\s0\fR) " 4 .el .IP "``key'' (\fB\s-1OSSL_KDF_PARAM_KEY\s0\fR) " 4 .IX Item "key (OSSL_KDF_PARAM_KEY) " .ie n .IP """salt"" (\fB\s-1OSSL_KDF_PARAM_SALT\s0\fR) " 4 .el .IP "``salt'' (\fB\s-1OSSL_KDF_PARAM_SALT\s0\fR) " 4 .IX Item "salt (OSSL_KDF_PARAM_SALT) " .PD These parameters work as described in \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1\fBEVP_KDF\s0\fR\|(3). .ie n .IP """info"" (\fB\s-1OSSL_KDF_PARAM_INFO\s0\fR) " 4 .el .IP "``info'' (\fB\s-1OSSL_KDF_PARAM_INFO\s0\fR) " 4 .IX Item "info (OSSL_KDF_PARAM_INFO) " This parameter sets the info value. The length of the context info buffer cannot exceed 1024 bytes; this should be more than enough for any normal use of \s-1HKDF.\s0 .ie n .IP """mode"" (\fB\s-1OSSL_KDF_PARAM_MODE\s0\fR) <\s-1UTF8\s0 string> or " 4 .el .IP "``mode'' (\fB\s-1OSSL_KDF_PARAM_MODE\s0\fR) <\s-1UTF8\s0 string> or " 4 .IX Item "mode (OSSL_KDF_PARAM_MODE) or " This parameter sets the mode for the \s-1HKDF\s0 operation. There are three modes that are currently defined: .RS 4 .ie n .IP """\s-1EXTRACT_AND_EXPAND""\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND\s0\fR" 4 .el .IP "``\s-1EXTRACT_AND_EXPAND''\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND\s0\fR" 4 .IX Item "EXTRACT_AND_EXPAND or EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND" This is the default mode. Calling \fBEVP_KDF_derive\fR\|(3) on an \s-1EVP_KDF_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 otherwise an error will occur. .ie n .IP """\s-1EXTRACT_ONLY""\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0\fR" 4 .el .IP "``\s-1EXTRACT_ONLY''\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0\fR" 4 .IX Item "EXTRACT_ONLY or EVP_KDF_HKDF_MODE_EXTRACT_ONLY" In this mode calling \fBEVP_KDF_derive\fR\|(3) will just perform the extract operation. The value returned will be the intermediate fixed-length pseudorandom key K. The \fIkeylen\fR parameter must match the size of K, which can be looked up by calling \fBEVP_KDF_CTX_get_kdf_size()\fR after setting the mode and digest. .Sp The digest, key and salt values must be set before a key is derived otherwise an error will occur. .ie n .IP """\s-1EXPAND_ONLY""\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXPAND_ONLY\s0\fR" 4 .el .IP "``\s-1EXPAND_ONLY''\s0 or \fB\s-1EVP_KDF_HKDF_MODE_EXPAND_ONLY\s0\fR" 4 .IX Item "EXPAND_ONLY or EVP_KDF_HKDF_MODE_EXPAND_ONLY" In this mode calling \fBEVP_KDF_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 otherwise an error will occur. .RE .RS 4 .RE .SH "NOTES" .IX Header "NOTES" A context for \s-1HKDF\s0 can be obtained by calling: .PP .Vb 2 \& EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL); \& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf); .Ve .PP The output length of an \s-1HKDF\s0 expand operation is specified via the \fIkeylen\fR parameter to the \fBEVP_KDF_derive\fR\|(3) function. When using \&\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0 the \fIkeylen\fR parameter must equal the size of the intermediate fixed-length pseudorandom key otherwise an error will occur. For that mode, the fixed output size can be looked up by calling \fBEVP_KDF_CTX_get_kdf_size()\fR after setting the mode and digest on the \fB\s-1EVP_KDF_CTX\s0\fR. .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_KDF *kdf; \& EVP_KDF_CTX *kctx; \& unsigned char out[10]; \& OSSL_PARAM params[5], *p = params; \& \& kdf = EVP_KDF_fetch(NULL, "HKDF", NULL); \& kctx = EVP_KDF_CTX_new(kdf); \& EVP_KDF_free(kdf); \& \& *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST, \& SN_sha256, strlen(SN_sha256)); \& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY, \& "secret", (size_t)6); \& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO, \& "label", (size_t)5); \& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT, \& "salt", (size_t)4); \& *p = OSSL_PARAM_construct_end(); \& if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) { \& error("EVP_KDF_derive"); \& } \& \& EVP_KDF_CTX_free(kctx); .Ve .SH "CONFORMING TO" .IX Header "CONFORMING TO" \&\s-1RFC 5869\s0 .SH "SEE ALSO" .IX Header "SEE ALSO" \&\s-1\fBEVP_KDF\s0\fR\|(3), \&\fBEVP_KDF_CTX_new\fR\|(3), \&\fBEVP_KDF_CTX_free\fR\|(3), \&\fBEVP_KDF_CTX_get_kdf_size\fR\|(3), \&\fBEVP_KDF_CTX_set_params\fR\|(3), \&\fBEVP_KDF_derive\fR\|(3), \&\*(L"\s-1PARAMETERS\*(R"\s0 in \s-1\fBEVP_KDF\s0\fR\|(3), \&\s-1\fBEVP_KDF\-TLS13_KDF\s0\fR\|(7) .SH "HISTORY" .IX Header "HISTORY" This functionality was added in OpenSSL 3.0. .SH "COPYRIGHT" .IX Header "COPYRIGHT" Copyright 2016\-2021 The OpenSSL Project Authors. All Rights Reserved. .PP 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 .