EVP_KDF-TLS13_KDF(7ossl)

📦
version 15.1 · 3 versions
, in freebsd 15.1

💖 EVP_KDF-TLS13_KDF — the TLS 1.3 EVP_KDF implementation 💖

okay so this is a page from the OpenSSL manual, section 7ossl, and it's all about the EVP_KDF-TLS13_KDF thing. buckle up bestie, it's actually not that deep once you vibe with it 🦋

🌸 NAME 🌸

EVP_KDF-TLS13_KDF — literally the TLS 1.3 EVP_KDF implementation, that's the whole name, no notes 💅

✨ DESCRIPTION ✨

So basically there's support for computing the TLS 1.3 version of the HKDF KDF through the EVP_KDF API. iconic.

The EVP_KDF-TLS13_KDF algorithm implements the HKDF key derivation function as used by TLS 1.3. no cap.

The output is considered to be keying material. 🎀

🧚 Identity 🧚

"TLS13-KDF" is the name for this implementation — that's its whole government name, and you can use it with the EVP_KDF_fetch() function. periodt.

🔥 Supported parameters 🔥

These are the supported parameters, bestie, all of them, let's go:

  • "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 four work exactly as described in "PARAMETERS" in EVP_KDF(3). nothing new here, slay.

  • "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, note the space inside the quotes, that's not a typo, that's the whole point 💀). 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. iykyk.
  • "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> — 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 you can look up by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest. also: the digest, key and salt values must be set before a key is derived otherwise an error will occur. so don't skip steps 🙈
    • "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. and again: the digest, key and info values must be set before a key is derived otherwise an error will occur. no cap.
🌈 heads up! the OpenSSL FIPS provider also supports the following extra parameters — these are FIPS-specific so pay attention 🫶
  • "fips-indicator" (OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR) <integer> — a getter that returns 1 if the operation is FIPS approved, or 0 otherwise. You can use this 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 the 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 the 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 🦋

okay so real talk: 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. so don't come for it expecting HKDF's whole wardrobe, she has a specific job 😭

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. it's all or nothing, main character energy 👑

You can get a context for a TLS 1.3 KDF 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 — that's the one, bestie, we stan a standard ⭐

🎀 SEE ALSO 🎀

go read these too, they're all linked up like a little group chat:

⭐ HISTORY ⭐

This functionality was added in OpenSSL 3.0. so it's a 3.x girlie, no going back 💅

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>.

🫶 and that's the tea! this page is from OpenSSL 3.5.6, dated 2026-04-07, manual EVP_KDF-TLS13_KDF(7ossl) — go forth and derive those keys, bestie, you got this 💖✨

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