119 lines
3.8 KiB
Text
119 lines
3.8 KiB
Text
=pod
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=head1 NAME
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EVP_PKEY_HKDF, EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt,
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EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info -
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HMAC-based Extract-and-Expand key derivation algorithm
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=head1 SYNOPSIS
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#include <openssl/kdf.h>
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int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);
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int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
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int saltlen);
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int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
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int keylen);
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int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
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int infolen);
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=head1 DESCRIPTION
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The EVP_PKEY_HKDF algorithm implements the HKDF key derivation function.
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HKDF follows the "extract-then-expand" paradigm, where the KDF logically
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consists of two modules. The first stage takes the input keying material
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and "extracts" from it a fixed-length pseudorandom key K. The second stage
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"expands" the key K into several additional pseudorandom keys (the output
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of the KDF).
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EVP_PKEY_set_hkdf_md() sets the message digest associated with the HKDF.
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EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to B<saltlen> bytes of the
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buffer B<salt>. Any existing value is replaced.
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EVP_PKEY_CTX_set_hkdf_key() sets the key to B<keylen> bytes of the buffer
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B<key>. Any existing value is replaced.
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EVP_PKEY_CTX_add1_hkdf_info() sets the info value to B<infolen> bytes of the
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buffer B<info>. If a value is already set, it is appended to the existing
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value.
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=head1 STRING CTRLS
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HKDF also supports string based control operations via
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L<EVP_PKEY_CTX_ctrl_str(3)>.
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The B<type> parameter "md" uses the supplied B<value> as the name of the digest
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algorithm to use.
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The B<type> parameters "salt", "key" and "info" use the supplied B<value>
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parameter as a B<seed>, B<key> or B<info> value.
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The names "hexsalt", "hexkey" and "hexinfo" are similar except they take a hex
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string which is converted to binary.
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=head1 NOTES
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All these functions are implemented as macros.
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A context for HKDF can be obtained by calling:
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EVP_PKEY_CTX *pctx = EVP_PKEY_new_id(EVP_PKEY_HKDF, NULL);
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The digest, key, salt and info values must be set before a key is derived or
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an error occurs.
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The total length of the info buffer cannot exceed 1024 bytes in length: this
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should be more than enough for any normal use of HKDF.
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The output length of the KDF is specified via the length parameter to the
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L<EVP_PKEY_derive(3)> function.
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Since the HKDF output length is variable, passing a B<NULL> buffer as a means
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to obtain the requisite length is not meaningful with HKDF.
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Instead, the caller must allocate a buffer of the desired length, and pass that
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buffer to L<EVP_PKEY_derive(3)> along with (a pointer initialized to) the
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desired length.
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Optimised versions of HKDF can be implemented in an ENGINE.
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=head1 RETURN VALUES
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All these functions return 1 for success and 0 or a negative value for failure.
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In particular a return value of -2 indicates the operation is not supported by
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the public key algorithm.
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=head1 EXAMPLE
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This example derives 10 bytes using SHA-256 with the secret key "secret",
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salt value "salt" and info value "label":
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EVP_PKEY_CTX *pctx;
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unsigned char out[10];
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size_t outlen = sizeof(out);
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pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
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if (EVP_PKEY_derive_init(pctx) <= 0)
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/* Error */
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if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
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/* Error */
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if (EVP_PKEY_CTX_set1_salt(pctx, "salt", 4) <= 0)
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/* Error */
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if (EVP_PKEY_CTX_set1_key(pctx, "secret", 6) <= 0)
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/* Error */
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if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 6) <= 0)
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/* Error */
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if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
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/* Error */
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=head1 CONFORMING TO
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RFC 5869
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=head1 SEE ALSO
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L<EVP_PKEY_CTX_new(3)>,
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L<EVP_PKEY_CTX_ctrl_str(3)>,
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L<EVP_PKEY_derive(3)>
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=cut
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