EVP_KDF-SSHKDF(7SSL) OpenSSL EVP_KDF-SSHKDF(7SSL)
NAME
EVP_KDF-SSHKDF - The SSHKDF EVP_KDF implementation
DESCRIPTION
Support for computing the SSHKDF KDF through the EVP_KDF API.
The EVP_KDF-SSHKDF algorithm implements the SSHKDF key derivation
function. It is defined in RFC 4253, section 7.2 and is used by SSH to
derive IVs, encryption keys and integrity keys. Five inputs are
required to perform key derivation: The hashing function (for example
SHA256), the Initial Key, the Exchange Hash, the Session ID, and the
derivation key type.
The output is considered to be keying material.
Identity
"SSHKDF" 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>
"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).
"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.
"type" (OSSL_KDF_PARAM_SSHKDF_TYPE) <UTF8 string>
This parameter sets the type for the SSHKDF operation. There are
six supported types:
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:
"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 SP 800-135r1, the following are approved digest
algorithms: SHA-1, SHA2-224, SHA2-256, SHA2-384, SHA2-512.
"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
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. 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.
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 */
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)
HISTORY
This functionality was added in OpenSSL 3.0.
COPYRIGHT
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>.
3.5.4 2025-09-30 EVP_KDF-SSHKDF(7SSL)
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