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Title "EVP_KDF-SSHKDF 7" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
EVP_KDF-SSHKDF - The SSHKDF EVP_KDF implementation Header "DESCRIPTION" Support for computing the \s-1SSHKDF\s0 \s-1KDF\s0 through the \s-1EVP_KDF\s0 \s-1API.\s0

The \s-1EVP_KDF-SSHKDF\s0 algorithm implements the \s-1SSHKDF\s0 key derivation function. It is defined in \s-1RFC 4253,\s0 section 7.2 and is used by \s-1SSH\s0 to derive IVs, encryption keys and integrity keys. Five inputs are required to perform key derivation: The hashing function (for example \s-1SHA256\s0), the Initial Key, the Exchange Hash, the Session \s-1ID,\s0 and the derivation key type.

Subsection "Identity" \*(L"\s-1SSHKDF\*(R"\s0 is the name for this implementation; it can be used with the EVP_KDF_fetch() function. Subsection "Supported parameters" The supported parameters are: Item "properties (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>"

0 Item "digest (OSSL_KDF_PARAM_DIGEST) <UTF8 string>" Item "key (OSSL_KDF_PARAM_KEY) <octet string>"

These parameters work as described in \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1EVP_KDF\s0\|(3). Item "xcghash (OSSL_KDF_PARAM_SSHKDF_XCGHASH) <octet string>"

0 Item "session_id (OSSL_KDF_PARAM_SSHKDF_SESSION_ID) <octet string>"

These parameters set the respective values for the \s-1KDF.\s0 If a value is already set, the contents are replaced. Item "type (OSSL_KDF_PARAM_SSHKDF_TYPE) <UTF8 string>" This parameter sets the type for the \s-1SSHKDF\s0 operation. There are six supported types:

Header "NOTES" A context for \s-1SSHKDF\s0 can be obtained by calling:

.Vb 2 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "SSHKDF", NULL); EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf); .Ve

The output length of the \s-1SSHKDF\s0 derivation is specified via the keylen parameter to the EVP_KDF_derive\|(3) function. Since the \s-1SSHKDF\s0 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 \fBEVP_KDF_derive\|(3) function along with the desired length.

Header "EXAMPLES" This example derives an 8 byte \s-1IV\s0 using \s-1SHA-256\s0 with a 1K \*(L"key\*(R" and appropriate \*(L"xcghash\*(R" and \*(L"session_id\*(R" values:

.Vb 9 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 */ .Ve

Header "CONFORMING TO" \s-1RFC 4253\s0 Header "SEE ALSO" \s-1EVP_KDF\s0\|(3), \fBEVP_KDF_CTX_new\|(3), \fBEVP_KDF_CTX_free\|(3), \fBEVP_KDF_CTX_set_params\|(3), \fBEVP_KDF_CTX_get_kdf_size\|(3), \fBEVP_KDF_derive\|(3), \*(L"\s-1PARAMETERS\*(R"\s0 in \s-1EVP_KDF\s0\|(3) Header "HISTORY" This functionality was added in OpenSSL 3.0. Header "COPYRIGHT" Copyright 2016-2022 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.