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.
They set the respective values to the first length bytes of the buffer buffer. If a value is already set, the contents are replaced.
EVP_KDF_ctrl_str() takes two type strings for these controls:
Sets the type for the SSHHKDF operation. There are six supported types:
EVP_KDF_ctrl_str() type string: ``type''
The value is a string of length one character. The only valid values are the numerical values of the ASCII caracters: ``A'' (65) to ``F'' (70).
EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_SSHKDF);
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_size() 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.
EVP_KDF_CTX *kctx; unsigned char key = "01234..."; unsigned char xcghash = "012345..."; unsigned char session_id = "012345..."; unsigned char out; size_t outlen = sizeof(out); kctx = EVP_KDF_CTX_new_id(EVP_KDF_SSHKDF); if (EVP_KDF_CTX_set_md(kctx, EVP_sha256()) <= 0) /* Error */ if (EVP_KDF_CTX_set1_key(kctx, key, 1024) <= 0) /* Error */ if (EVP_KDF_CTX_set1_sshkdf_xcghash(kctx, xcghash, 32) <= 0) /* Error */ if (EVP_KDF_CTX_set1_sshkdf_session_id(kctx, session_id, 32) <= 0) /* Error */ if (EVP_KDF_CTX_set_sshkdf_type(kctx, EVP_KDF_SSHKDF_TYPE_ININITAL_IV_CLI_TO_SRV) <= 0) /* Error */ if (EVP_KDF_derive(kctx, out, &outlen) <= 0) /* Error */
Licensed under the OpenSSL license (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>.