EVP_KDF-TLS13_KDF(7ossl)

📦
version 3.5.9 · 8 versions
, in alpine 3.23, alpine 3.24

💖 NAME 💖

EVP_KDF-TLS13_KDF — the TLS 1.3 flavor of the EVP_KDF implementation, bestie. It's a 7ossl manual page and honestly it's kind of a baddie.

✨ DESCRIPTION ✨

Okay so picture this: we've got support for computing the TLS 1.3 version of the HKDF KDF, all through the EVP_KDF API. Slay.

The EVP_KDF-TLS13_KDF algorithm implements the HKDF key derivation function as used by TLS 1.3. That's the whole vibe.

The output is considered to be keying material. Like, that's what it is. Periodt.

🦋 Identity 🦋

"TLS13-KDF" is the name for this implementation — that's its whole government name — and it can be used with the EVP_KDF_fetch() function. iykyk.

🌸 Supported parameters 🌸

The supported parameters are, in order, no skipping:

  • "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 exactly as described in "PARAMETERS" in EVP_KDF(3). No notes, they just do their thing.

  • "prefix" (OSSL_KDF_PARAM_PREFIX) <octet string> — this one 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 (yes, the space is load-bearing, do not come for it 💅). Refer to RFC 8446 section 7.1 "Key Schedule" for details.
  • "label" (OSSL_KDF_PARAM_LABEL) <octet string> — 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> — 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 sets the mode for the TLS 1.3 KDF operation. There are two modes currently defined, and they are:
    • "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. Not optional, bestie.
    • "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. Same deal, no shortcuts.

The OpenSSL FIPS provider also supports the following parameters, and yes they're getting their own little section:

  • "fips-indicator" (OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR) <integer> — a getter that returns 1 if the operation is FIPS approved, or 0 otherwise. This may be used after calling EVP_KDF_derive. It returns 0 if any "***-check" related parameter is set to 0 and the check fails.
  • "digest-check" (OSSL_KDF_PARAM_FIPS_DIGEST_CHECK) <integer> — the default value of 1 causes an error during EVP_KDF_CTX_set_params() if used digest is not approved. Setting this to zero will ignore the error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0. According to RFC 8446, the following are approved digest algorithms: SHA2-256, SHA2-384.
  • "key-check" (OSSL_KDF_PARAM_FIPS_KEY_CHECK) <integer> — the default value of 1 causes an error during EVP_KDF_CTX_set_params() if the length of used key-derivation key (OSSL_KDF_PARAM_KEY) is shorter than 112 bits. Setting this to zero will ignore the error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0.

🎀 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. It's giving specialized. It's giving main character who doesn't do everything.

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 — no drip-feeding, it wants the whole fit in one go 🙈.

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.

⚠️ Genuinely though — mismatched keylen is an error, not a vibe. Check your sizes.

🧚 CONFORMING TO 🧚

RFC 8446. That's it, that's the section. Iconic.

🔥 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)

Go read them, they're all connected, it's a whole universe 🌈.

⭐ HISTORY ⭐

This functionality was added in OpenSSL 3.0. A classic, a legend, an original.

Copyright 2021-2024 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>.

k that's the whole page babes 💜 — brought to you by OpenSSL 3.5.9, dated 2026-09-29, and this EVP_KDF-TLS13_KDF(7ossl) manual page. mwah 🫶

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