=pod =head1 NAME EVP_KDF_HKDF - The HKDF EVP_KDF implementation =head1 DESCRIPTION Support for computing the B KDF through the B 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). =head2 Numeric identity B is the numeric identity for this implementation; it can be used with the EVP_KDF_CTX_new_id() function. =head2 Supported controls The supported controls are: =over 4 =item B =item B =item B These controls work as described in L. =item B This control does not expect any arguments. Resets the context info buffer to zero length. =item B This control expects two arguments: C, C Sets the info value to the first B bytes of the buffer B. If a value is already set, the contents of the buffer are appended to the existing value. The total length of the context info buffer cannot exceed 1024 bytes; this should be more than enough for any normal use of HKDF. EVP_KDF_ctrl_str() takes two type strings for this control: =over 4 =item "info" The value string is used as is. =item "hexinfo" The value string is expected to be a hexadecimal number, which will be decoded before being passed on as the control value. =back =item B This control expects one argument: C Sets the mode for the HKDF operation. There are three modes that are currently defined: =over 4 =item EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND This is the default mode. Calling L on an EVP_KDF_CTX set up for HKDF 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. In this mode the digest, key, salt and info values must be set before a key is derived otherwise an error will occur. =item EVP_KDF_HKDF_MODE_EXTRACT_ONLY In this mode calling L will just perform the extract operation. The value returned will be the intermediate fixed-length pseudorandom key K. The C parameter must match the size of K, which can be looked up by calling EVP_KDF_size() after setting the mode and digest. The digest, key and salt values must be set before a key is derived otherwise an error will occur. =item EVP_KDF_HKDF_MODE_EXPAND_ONLY In this mode calling L 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. The digest, key and info values must be set before a key is derived otherwise an error will occur. =back EVP_KDF_ctrl_str() type string: "mode" The value string is expected to be one of: "EXTRACT_AND_EXPAND", "EXTRACT_ONLY" or "EXPAND_ONLY". =back =head1 NOTES A context for HKDF can be obtained by calling: EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_HKDF); The output length of an HKDF expand operation is specified via the C parameter to the L function. When using EVP_KDF_HKDF_MODE_EXTRACT_ONLY the C 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_size() after setting the mode and digest on the C. =head1 EXAMPLE This example derives 10 bytes using SHA-256 with the secret key "secret", salt value "salt" and info value "label": EVP_KDF_CTX *kctx; unsigned char out[10]; kctx = EVP_KDF_CTX_new_id(EVP_KDF_HKDF); if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_MD, EVP_sha256()) <= 0) { error("EVP_KDF_CTRL_SET_MD"); } if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SALT, "salt", (size_t)4) <= 0) { error("EVP_KDF_CTRL_SET_SALT"); } if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KEY, "secret", (size_t)6) <= 0) { error("EVP_KDF_CTRL_SET_KEY"); } if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_ADD_HKDF_INFO, "label", (size_t)5) <= 0) { error("EVP_KDF_CTRL_ADD_HKDF_INFO"); } if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0) { error("EVP_KDF_derive"); } EVP_KDF_CTX_free(kctx); =head1 CONFORMING TO RFC 5869 =head1 SEE ALSO L, L, L, L, L, L, L =head1 COPYRIGHT Copyright 2016-2018 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 L. =cut