EVP_KDF_SSHKDF.7ssl - Man Page

The SSHKDF EVP_KDF implementation


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.

Numeric identity

EVP_KDF_SSHKDF is the numeric identity for this implementation; it can be used with the EVP_KDF_CTX_new_id() function.

Supported controls

The supported controls are:


These controls work as described in “CONTROLS” in EVP_KDF_CTX(3).


These controls expect two arguments: unsigned char *buffer, size_t length

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:


The value string is used as is.


The value string is expected to be a hexadecimal number, which will be decoded before being passed on as the control value.


This control expects one argument: int mode

Sets the type for the SSHHKDF operation. There are six supported types:


The Initial IV from client to server. A single char of value 65 (ASCII char 'A').


The Initial IV from server to client A single char of value 66 (ASCII char 'B').


The Encryption Key from client to server A single char of value 67 (ASCII char 'C').


The Encryption Key from server to client A single char of value 68 (ASCII char 'D').


The Integrity Key from client to server A single char of value 69 (ASCII char 'E').


The Integrity Key from client to server A single char of value 70 (ASCII char 'F').

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).


A context for SSHKDF can be obtained by calling:


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.


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

 EVP_KDF_CTX *kctx;
 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);
 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 */

Conforming to

RFC 4253

See Also

EVP_KDF_CTX, EVP_KDF_CTX_new_id(3), EVP_KDF_CTX_free(3), EVP_KDF_ctrl(3), EVP_KDF_size(3), EVP_KDF_derive(3), “CONTROLS” in EVP_KDF_CTX(3)


2021-03-26 1.1.1k OpenSSL