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Title "EVP_KDF-HKDF 7ossl" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
EVP_KDF-HKDF - The HKDF EVP_KDF implementation Header "DESCRIPTION" Support for computing the \s-1HKDF\s0 \s-1KDF\s0 through the \s-1EVP_KDF\s0 \s-1API.\s0

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).

Subsection "Identity" \*(L"\s-1HKDF\*(R"\s0 is the name for this implementation; it can be used with the EVP_KDF_fetch() function. Subsection "Supported parameters" The supported parameters are: Item "properties (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>"

0 Item "digest (OSSL_KDF_PARAM_DIGEST) <UTF8 string>" Item "key (OSSL_KDF_PARAM_KEY) <octet string>" Item "salt (OSSL_KDF_PARAM_SALT) <octet string>"

These parameters work as described in \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1EVP_KDF\s0\|(3). Item "info (OSSL_KDF_PARAM_INFO) <octet string>" 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 Item "mode (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>" This parameter sets the mode for the \s-1HKDF\s0 operation. There are three modes that are currently defined:

Header "NOTES" A context for \s-1HKDF\s0 can be obtained by calling:

.Vb 2 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL); EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf); .Ve

The output length of an \s-1HKDF\s0 expand operation is specified via the keylen parameter to the EVP_KDF_derive\|(3) function. When using \s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0 the keylen 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 EVP_KDF_CTX_get_kdf_size() after setting the mode and digest on the \s-1EVP_KDF_CTX\s0.

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_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

Header "CONFORMING TO" \s-1RFC 5869\s0 Header "SEE ALSO" \s-1EVP_KDF\s0\|(3), \fBEVP_KDF_CTX_new\|(3), \fBEVP_KDF_CTX_free\|(3), \fBEVP_KDF_CTX_get_kdf_size\|(3), \fBEVP_KDF_CTX_set_params\|(3), \fBEVP_KDF_derive\|(3), \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1EVP_KDF\s0\|(3), \s-1EVP_KDF-TLS13_KDF\s0\|(7) Header "HISTORY" This functionality was added in OpenSSL 3.0. Header "COPYRIGHT" Copyright 2016-2021 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>.