man > EVP_KDF-SSHKDF(7ssl)

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.

   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').

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 2016-2022 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>.

3.0.13                                       2026-07-29                         EVP_KDF-SSHKDF(7SSL)