$NetBSD: EVP_KDF-HKDF.7,v 1.5 2024/09/08 13:08:21 christos Exp $

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Title "EVP_KDF-HKDF 7" 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 HKDF KDF through the EVP_KDF API.

The EVP_KDF-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).

Subsection "Identity" "HKDF" 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>"

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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 "PARAMETERS" in EVP_KDF\|(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 HKDF. Item """mode"" (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>" This parameter sets the mode for the HKDF operation. There are three modes that are currently defined: Header "NOTES" A context for HKDF 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 HKDF expand operation is specified via the keylen parameter to the EVP_KDF_derive\|(3) function. When using EVP_KDF_HKDF_MODE_EXTRACT_ONLY 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 EVP_KDF_CTX.

Header "EXAMPLES" This example derives 10 bytes using SHA-256 with the secret key "secret", salt value "salt" and info value "label":

.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" RFC 5869 Header "SEE ALSO" \fBEVP_KDF\|(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), "PARAMETERS" in EVP_KDF\|(3), \fBEVP_KDF-TLS13_KDF\|(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 "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.