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EVP_KDF-TLS13_KDF(7SSL)                        OpenSSL                       EVP_KDF-TLS13_KDF(7SSL)



NAME
       EVP_KDF-TLS13_KDF - The TLS 1.3 EVP_KDF implementation

DESCRIPTION
       Support for computing the TLS 1.3 version of the HKDF KDF through the EVP_KDF API.

       The EVP_KDF-TLS13_KDF algorithm implements the HKDF key derivation function as used by TLS
       1.3.

   Identity
       "TLS13-KDF" is the name for this implementation; it can be used with the EVP_KDF_fetch()
       function.

   Supported parameters
       The supported parameters are:

       "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
       "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>
       "key" (OSSL_KDF_PARAM_KEY) <octet string>
       "salt" (OSSL_KDF_PARAM_SALT) <octet string>
           These parameters work as described in "PARAMETERS" in EVP_KDF(3).

       "prefix" (OSSL_KDF_PARAM_PREFIX) <octet string>
           This parameter sets the label prefix on the specified TLS 1.3 KDF context.  For TLS 1.3
           this should be set to the ASCII string "tls13 " without a trailing zero byte.  Refer to
           RFC 8446 section 7.1 "Key Schedule" for details.

       "label" (OSSL_KDF_PARAM_LABEL) <octet string>
           This parameter sets the label on the specified TLS 1.3 KDF context.  Refer to RFC 8446
           section 7.1 "Key Schedule" for details.

       "data" (OSSL_KDF_PARAM_DATA) <octet string>
           This parameter sets the context data on the specified TLS 1.3 KDF context.  Refer to RFC
           8446 section 7.1 "Key Schedule" for details.

       "mode" (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>
           This parameter sets the mode for the TLS 1.3 KDF operation.  There are two modes that are
           currently defined:

           "EXTRACT_ONLY" or EVP_KDF_HKDF_MODE_EXTRACT_ONLY
               In this mode calling EVP_KDF_derive(3) will just perform the extract operation. The
               value returned will be the intermediate fixed-length pseudorandom key K.  The keylen
               parameter must match the size of K, which can be looked up by calling
               EVP_KDF_CTX_get_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.

           "EXPAND_ONLY" or EVP_KDF_HKDF_MODE_EXPAND_ONLY
               In this mode calling 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.

NOTES
       This KDF is intended for use by the TLS 1.3 implementation in libssl.  It does not support
       all the options and capabilities that HKDF does.

       The OSSL_PARAM array passed to EVP_KDF_derive(3) or EVP_KDF_CTX_set_params(3) must specify
       all of the parameters required.  This KDF does not support a piecemeal approach to providing
       these.

       A context for a TLS 1.3 KDF can be obtained by calling:

        EVP_KDF *kdf = EVP_KDF_fetch(NULL, "TLS13-KDF", NULL);
        EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

       The output length of a TLS 1.3 KDF 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.

CONFORMING TO
       RFC 8446

SEE ALSO
       EVP_KDF(3), EVP_KDF_CTX_new(3), EVP_KDF_CTX_free(3), EVP_KDF_CTX_get_kdf_size(3),
       EVP_KDF_CTX_set_params(3), EVP_KDF_derive(3), "PARAMETERS" in EVP_KDF(3), EVP_KDF-HKDF(7)

COPYRIGHT
       Copyright 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>.



3.0.2                                        2026-04-07                      EVP_KDF-TLS13_KDF(7SSL)