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Title "EVP_KDF-TLS13_KDF 7" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
EVP_KDF-TLS13_KDF - The TLS 1.3 EVP_KDF implementation Header "DESCRIPTION" Support for computing the \s-1TLS 1.3\s0 version of the \s-1HKDF\s0 \s-1KDF\s0 through the \s-1EVP_KDF\s0 \s-1API.\s0

The \s-1EVP_KDF-TLS13_KDF\s0 algorithm implements the \s-1HKDF\s0 key derivation function as used by \s-1TLS 1.3.\s0

Subsection "Identity" \*(L"\s-1TLS13-KDF\*(R"\s0 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>"

0 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 \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1EVP_KDF\s0\|(3). Item "prefix (OSSL_KDF_PARAM_PREFIX) <octet string>" This parameter sets the label prefix on the specified \s-1TLS 1.3 KDF\s0 context. For \s-1TLS 1.3\s0 this should be set to the \s-1ASCII\s0 string \*(L"tls13 \*(R" without a trailing zero byte. Refer to \s-1RFC 8446\s0 section 7.1 \*(L"Key Schedule\*(R" for details. Item "label (OSSL_KDF_PARAM_LABEL) <octet string>" This parameter sets the label on the specified \s-1TLS 1.3 KDF\s0 context. Refer to \s-1RFC 8446\s0 section 7.1 \*(L"Key Schedule\*(R" for details. Item "data (OSSL_KDF_PARAM_DATA) <octet string>" This parameter sets the context data on the specified \s-1TLS 1.3 KDF\s0 context. Refer to \s-1RFC 8446\s0 section 7.1 \*(L"Key Schedule\*(R" for details. Item "mode (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>" This parameter sets the mode for the \s-1TLS 1.3 KDF\s0 operation. There are two modes that are currently defined:

Header "NOTES" This \s-1KDF\s0 is intended for use by the \s-1TLS 1.3\s0 implementation in libssl. It does not support all the options and capabilities that \s-1HKDF\s0 does.

The \s-1OSSL_PARAM\s0 array passed to EVP_KDF_derive\|(3) or \fBEVP_KDF_CTX_set_params\|(3) must specify all of the parameters required. This \s-1KDF\s0 does not support a piecemeal approach to providing these.

A context for a \s-1TLS 1.3 KDF\s0 can be obtained by calling:

.Vb 2 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "TLS13-KDF", NULL); EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf); .Ve

The output length of a \s-1TLS 1.3 KDF\s0 expand operation is specified via the \fIkeylen parameter to the EVP_KDF_derive\|(3) function. When using \s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0 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 \fBEVP_KDF_CTX_get_kdf_size() after setting the mode and digest on the \fB\s-1EVP_KDF_CTX\s0.

Header "CONFORMING TO" \s-1RFC 8446\s0 Header "SEE ALSO" \s-1EVP_KDF\s0\|(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), \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1EVP_KDF\s0\|(3), \s-1EVP_KDF-HKDF\s0\|(7) Header "HISTORY" This functionality was added in OpenSSL 3.0. Header "COPYRIGHT" Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.