Update KDF documentation (section 7)

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9662)

1 files changed, 154 insertions, 0 deletions

diff --git a/doc/man7/EVP_KDF-HKDF.pod b/doc/man7/EVP_KDF-HKDF.pod
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+++ b/doc/man7/EVP_KDF-HKDF.pod
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+=pod
+
+=head1 NAME
+
+EVP_KDF-HKDF - The HKDF EVP_KDF implementation
+
+=head1 DESCRIPTION
+
+Support for computing the B<HKDF> KDF through the B<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).
+
+=head2 Identity
+
+"HKDF" is the name for this implementation; it
+can be used with the EVP_KDF_fetch() function.
+
+=head2 Supported parameters
+
+The supported parameters are:
+
+=over 4
+
+=item B<OSSL_KDF_PARAM_PROPERTIES> ("properties") <UTF8 string>
+
+=item B<OSSL_KDF_PARAM_DIGEST> ("digest") <UTF8 string>
+
+=item B<OSSL_KDF_PARAM_KEY> ("key") <octet string>
+
+=item B<OSSL_KDF_PARAM_SALT> ("salt") <octet string>
+
+These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
+
+=item B<OSSL_KDF_PARAM_INFO> ("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 B<OSSL_KDF_PARAM_MODE> ("mode") <UTF8 string> or <int>
+
+This parameter sets the mode for the HKDF operation.
+There are three modes that are currently defined:
+
+=over 4
+
+=item B<EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND> "EXTRACT_AND_EXPAND"
+
+This is the default mode.  Calling L<EVP_KDF-derive(3)> 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 B<EVP_KDF_HKDF_MODE_EXTRACT_ONLY> "EXTRACT_ONLY"
+
+In this mode calling L<EVP_KDF-derive(3)> will just perform the extract
+operation. The value returned will be the intermediate fixed-length pseudorandom
+key K.  The C<keylen> 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 B<EVP_KDF_HKDF_MODE_EXPAND_ONLY> "EXPAND_ONLY"
+
+In this mode calling L<EVP_KDF-derive(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.
+
+The digest, key and info values must be set before a key is derived otherwise
+an error will occur.
+
+=back
+
+=back
+
+=head1 NOTES
+
+A context for HKDF can be obtained by calling:
+
+ EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
+ EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
+
+The output length of an HKDF expand operation is specified via the C<keylen>
+parameter to the L<EVP_KDF-derive(3)> function.  When using
+EVP_KDF_HKDF_MODE_EXTRACT_ONLY the C<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_size()
+after setting the mode and digest on the C<EVP_KDF_CTX>.
+
+=head1 EXAMPLES
+
+This example derives 10 bytes using SHA-256 with the secret key "secret",
+salt value "salt" and info value "label":
+
+ 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_set_params(kctx, params) <= 0) {
+     error("EVP_KDF_set_params");
+ }
+ 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<EVP_KDF>,
+L<EVP_KDF-CTX_new_id(3)>,
+L<EVP_KDF-CTX_free(3)>,
+L<EVP_KDF-ctrl(3)>,
+L<EVP_KDF-size(3)>,
+L<EVP_KDF-derive(3)>,
+L<EVP_KDF-CTX(3)/PARAMETERS>
+
+=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<https://www.openssl.org/source/license.html>.
+
+=cut