1EVP_KDF-SSHKDF(7ossl)               OpenSSL              EVP_KDF-SSHKDF(7ossl)
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NAME

6       EVP_KDF-SSHKDF - The SSHKDF EVP_KDF implementation
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DESCRIPTION

9       Support for computing the SSHKDF KDF through the EVP_KDF API.
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11       The EVP_KDF-SSHKDF algorithm implements the SSHKDF key derivation
12       function.  It is defined in RFC 4253, section 7.2 and is used by SSH to
13       derive IVs, encryption keys and integrity keys.  Five inputs are
14       required to perform key derivation: The hashing function (for example
15       SHA256), the Initial Key, the Exchange Hash, the Session ID, and the
16       derivation key type.
17
18   Identity
19       "SSHKDF" is the name for this implementation; it can be used with the
20       EVP_KDF_fetch() function.
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22   Supported parameters
23       The supported parameters are:
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25       "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
26       "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>
27       "key" (OSSL_KDF_PARAM_KEY) <octet string>
28           These parameters work as described in "PARAMETERS" in EVP_KDF(3).
29
30       "xcghash" (OSSL_KDF_PARAM_SSHKDF_XCGHASH) <octet string>
31       "session_id" (OSSL_KDF_PARAM_SSHKDF_SESSION_ID) <octet string>
32           These parameters set the respective values for the KDF.  If a value
33           is already set, the contents are replaced.
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35       "type" (OSSL_KDF_PARAM_SSHKDF_TYPE) <UTF8 string>
36           This parameter sets the type for the SSHKDF operation.  There are
37           six supported types:
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39           EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV
40               The Initial IV from client to server.  A single char of value
41               65 (ASCII char 'A').
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43           EVP_KDF_SSHKDF_TYPE_INITIAL_IV_SRV_TO_CLI
44               The Initial IV from server to client A single char of value 66
45               (ASCII char 'B').
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47           EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_CLI_TO_SRV
48               The Encryption Key from client to server A single char of value
49               67 (ASCII char 'C').
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51           EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_SRV_TO_CLI
52               The Encryption Key from server to client A single char of value
53               68 (ASCII char 'D').
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55           EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_CLI_TO_SRV
56               The Integrity Key from client to server A single char of value
57               69 (ASCII char 'E').
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59           EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_SRV_TO_CLI
60               The Integrity Key from client to server A single char of value
61               70 (ASCII char 'F').
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NOTES

64       A context for SSHKDF can be obtained by calling:
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66        EVP_KDF *kdf = EVP_KDF_fetch(NULL, "SSHKDF", NULL);
67        EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
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69       The output length of the SSHKDF derivation is specified via the keylen
70       parameter to the EVP_KDF_derive(3) function.  Since the SSHKDF output
71       length is variable, calling EVP_KDF_CTX_get_kdf_size(3) to obtain the
72       requisite length is not meaningful. The caller must allocate a buffer
73       of the desired length, and pass that buffer to the EVP_KDF_derive(3)
74       function along with the desired length.
75

EXAMPLES

77       This example derives an 8 byte IV using SHA-256 with a 1K "key" and
78       appropriate "xcghash" and "session_id" values:
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80        EVP_KDF *kdf;
81        EVP_KDF_CTX *kctx;
82        char type = EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV;
83        unsigned char key[1024] = "01234...";
84        unsigned char xcghash[32] = "012345...";
85        unsigned char session_id[32] = "012345...";
86        unsigned char out[8];
87        size_t outlen = sizeof(out);
88        OSSL_PARAM params[6], *p = params;
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90        kdf = EVP_KDF_fetch(NULL, "SSHKDF", NULL);
91        kctx = EVP_KDF_CTX_new(kdf);
92        EVP_KDF_free(kdf);
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94        *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
95                                                SN_sha256, strlen(SN_sha256));
96        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
97                                                 key, (size_t)1024);
98        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SSHKDF_XCGHASH,
99                                                 xcghash, (size_t)32);
100        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SSHKDF_SESSION_ID,
101                                                 session_id, (size_t)32);
102        *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_SSHKDF_TYPE,
103                                                &type, sizeof(type));
104        *p = OSSL_PARAM_construct_end();
105        if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
106            /* Error */
107

CONFORMING TO

109       RFC 4253
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SEE ALSO

112       EVP_KDF(3), EVP_KDF_CTX_new(3), EVP_KDF_CTX_free(3),
113       EVP_KDF_CTX_set_params(3), EVP_KDF_CTX_get_kdf_size(3),
114       EVP_KDF_derive(3), "PARAMETERS" in EVP_KDF(3)
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HISTORY

117       This functionality was added in OpenSSL 3.0.
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120       Copyright 2016-2022 The OpenSSL Project Authors. All Rights Reserved.
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122       Licensed under the Apache License 2.0 (the "License").  You may not use
123       this file except in compliance with the License.  You can obtain a copy
124       in the file LICENSE in the source distribution or at
125       <https://www.openssl.org/source/license.html>.
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1293.0.9                             2023-07-27             EVP_KDF-SSHKDF(7ossl)
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