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📛 NAME

EVP_KDF-TLS13_KDF - The TLS 1.3 EVP_KDF implementation

🚀 Quick Reference

Use Case	Command	Description
🔑 Fetch KDF implementation	EVP_KDF_fetch(NULL, "TLS13-KDF", NULL)	Get the TLS 1.3 KDF provider
🆕 Create KDF context	EVP_KDF_CTX_new(kdf)	Allocate a new KDF context for TLS 1.3
🔧 Set digest algorithm	EVP_KDF_CTX_set_params(ctx, params)	Configure digest (e.g., SHA-256)
🔑 Set key material	EVP_KDF_CTX_set_params(ctx, params)	Provide the input key (IKM) for extract
🧂 Set salt	EVP_KDF_CTX_set_params(ctx, params)	Optional salt for extract operation
🏷️ Set label prefix	OSSL_KDF_PARAM_PREFIX = "tls13 "	Must be ASCII "tls13 " (no trailing zero)
🏷️ Set label	OSSL_KDF_PARAM_LABEL	Label per RFC 8446 §7.1
📦 Set context data	OSSL_KDF_PARAM_DATA	Context data (info) per RFC 8446 §7.1
⚙️ Set mode	OSSL_KDF_PARAM_MODE = EXTRACT_ONLY | EXPAND_ONLY	Choose extract or expand phase
🔐 Derive key (extract)	EVP_KDF_derive(ctx, key, keylen, params)	Perform extract → intermediate pseudorandom key K
🔐 Derive key (expand)	EVP_KDF_derive(ctx, key, keylen, params)	Perform expand using K as input key
📏 Get output size	EVP_KDF_CTX_get_kdf_size(ctx)	Look up fixed output size for extract mode
🧹 Free context	EVP_KDF_CTX_free(kctx)	Release the KDF context

📖 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> — 🔧 Provider properties
• "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string> — 🔧 Message digest algorithm
• "key" (OSSL_KDF_PARAM_KEY) <octet string> — 🔑 Input key material (IKM)
• "salt" (OSSL_KDF_PARAM_SALT) <octet string> — 🧂 Salt value

These parameters work as described in "PARAMETERS" in EVP_KDF(3).

• "prefix" (OSSL_KDF_PARAM_PREFIX) <octet string> — 🏷️ 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> — 🏷️ 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> — ⚙️ 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 — 2026-06-02 — EVP_KDF-TLS13_KDF(7SSL)

📛 NAME 🚀 Quick Reference 📖 DESCRIPTION 📝 NOTES 📜 CONFORMING TO 👁️ SEE ALSO ©️ COPYRIGHT