EVP_KDF-SSHKDF(7SSL)

📦
version 4.0.3 · 13 versions
, in ubuntu stonking

💖 EVP_KDF-SSHKDF 💖

okay bestie, pull up a chair because this is EVP_KDF-SSHKDF(7SSL), straight outta the OpenSSL manual, and it's literally the page that tells you how to do SSH key derivation the ✨correct✨ way. no cap.

✨ NAME ✨

EVP_KDF-SSHKDF — the whole SSHKDF EVP_KDF implementation, hi hello welcome 🫶

🌸 DESCRIPTION 🌸

so the tea is: this page is all about computing the SSHKDF KDF through the EVP_KDF API. slay.

The EVP_KDF-SSHKDF algorithm implements the SSHKDF key derivation function. It's defined in RFC 4253, section 7.2 and it's what SSH uses to derive IVs, encryption keys and integrity keys. iconic behaviour, honestly. 🌈

To actually do a key derivation you need five inputs, like a little squad:

  • the hashing function (for example SHA256)
  • the Initial Key
  • the Exchange Hash
  • the Session ID
  • the derivation key type

The output is considered to be keying material. that's it, that's the product 💅

🦋 Identity 🦋

"SSHKDF" is the name for this implementation, and you can use it with the EVP_KDF_fetch() function. iykyk.

🎀 Supported parameters 🎀

The supported parameters are, bestie, as follows:

  • "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
  • "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>
  • "key" (OSSL_KDF_PARAM_KEY) <octet string>

These parameters work as described in "PARAMETERS" in EVP_KDF(3). yes, go read that one too, it's giving homework but make it cute 📚

  • "xcghash" (OSSL_KDF_PARAM_SSHKDF_XCGHASH) <octet string>
  • "session_id" (OSSL_KDF_PARAM_SSHKDF_SESSION_ID) <octet string>

These parameters set the respective values for the KDF. If a value is already set, the contents are replaced — no merge, no vibes, just overwrite 💀

  • "type" (OSSL_KDF_PARAM_SSHKDF_TYPE) <UTF8 string>

This parameter sets the type for the SSHKDF operation. There are six supported types, and they are literally just the letters A through F:

  • EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV — The Initial IV from client to server. A single char of value 65 (ASCII char 'A'). 🅰️
  • EVP_KDF_SSHKDF_TYPE_INITIAL_IV_SRV_TO_CLI — The Initial IV from server to client A single char of value 66 (ASCII char 'B'). 🅱️
  • EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_CLI_TO_SRV — The Encryption Key from client to server A single char of value 67 (ASCII char 'C'). 🔑
  • EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_SRV_TO_CLI — The Encryption Key from server to client A single char of value 68 (ASCII char 'D'). 🔑
  • EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_CLI_TO_SRV — The Integrity Key from client to server A single char of value 69 (ASCII char 'E'). 🛡️
  • EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_SRV_TO_CLI — The Integrity Key from client to server A single char of value 70 (ASCII char 'F'). 🛡️

The OpenSSL FIPS provider also supports the following parameters, and yes they have opinions:

  • "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. ma'am. 😭

According to SP 800-135r1, the following are approved digest algorithms: SHA-1, SHA2-224, SHA2-256, SHA2-384, SHA2-512. those five, periodt.

  • "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. again, bestie, be careful ✋

💅 NOTES 💅

A context for SSHKDF can be obtained by calling:

EVP_KDF *kdf = EVP_KDF_fetch(NULL, "SSHKDF", NULL);
EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

The output length of the SSHKDF derivation is specified via the keylen parameter to the EVP_KDF_derive(3) function. And here's the little plot twist: since the SSHKDF output length is variable, calling EVP_KDF_CTX_get_kdf_size(3) to obtain the requisite length is not meaningful. The caller must allocate a buffer of the desired length, and pass that buffer to the EVP_KDF_derive(3) function along with the desired length.

so yeah — you're the main character here, you allocate the buffer. the library is not doing it for you. no skips 🫶

⭐ EXAMPLES ⭐

This example derives an 8 byte IV using SHA-256 with a 1K "key" and appropriate "xcghash" and "session_id" values:

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
char type = EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV;
unsigned char key[1024] = "01234...";
unsigned char xcghash[32] = "012345...";
unsigned char session_id[32] = "012345...";
unsigned char out[8];
size_t outlen = sizeof(out);
OSSL_PARAM params[6], *p = params;

kdf = EVP_KDF_fetch(NULL, "SSHKDF", NULL);
kctx = EVP_KDF_CTX_new(kdf);
EVP_KDF_free(kdf);

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                        SN_sha256, strlen(SN_sha256));
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                         key, (size_t)1024);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SSHKDF_XCGHASH,
                                         xcghash, (size_t)32);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SSHKDF_SESSION_ID,
                                         session_id, (size_t)32);
*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_SSHKDF_TYPE,
                                        &type, sizeof(type));
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
    /* Error */

ate. that's the whole recipe, copy it and go off 👑

💜 CONFORMING TO 💜

RFC 4253

🌈 SEE ALSO 🌈

EVP_KDF(3), EVP_KDF_CTX_new(3), EVP_KDF_CTX_free(3), EVP_KDF_CTX_set_params(3), EVP_KDF_CTX_get_kdf_size(3), EVP_KDF_derive(3), "PARAMETERS" in EVP_KDF(3)

like, go read the friends too, they're all connected 💫

🧚 HISTORY 🧚

This functionality was added in OpenSSL 3.0. vintage but make it fresh 🎀

Copyright 2016-2024 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 whole page, bestie 🫶 this was EVP_KDF-SSHKDF(7SSL), version 4.0.3, dated 2026-09-29. go derive some keys, you absolute icon 💖✨🌸

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