| 1 | =pod
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| 2 |
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| 3 | =head1 NAME
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| 4 |
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| 5 | EVP_KDF-KRB5KDF - The RFC3961 Krb5 KDF EVP_KDF implementation
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| 6 |
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| 7 | =head1 DESCRIPTION
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| 8 |
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| 9 | Support for computing the B<KRB5KDF> KDF through the B<EVP_KDF> API.
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| 10 |
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| 11 | The EVP_KDF-KRB5KDF algorithm implements the key derivation function defined
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| 12 | in RFC 3961, section 5.1 and is used by Krb5 to derive session keys.
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| 13 | Three inputs are required to perform key derivation: a cipher, (for example
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| 14 | AES-128-CBC), the initial key, and a constant.
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| 15 |
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| 16 | =head2 Identity
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| 17 |
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| 18 | "KRB5KDF" is the name for this implementation;
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| 19 | it can be used with the EVP_KDF_fetch() function.
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| 20 |
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| 21 | =head2 Supported parameters
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| 22 |
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| 23 | The supported parameters are:
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| 24 |
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| 25 | =over 4
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| 26 |
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| 27 | =item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>
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| 28 |
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| 29 | =item "cipher" (B<OSSL_KDF_PARAM_CIPHER>) <UTF8 string>
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| 30 |
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| 31 | =item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string>
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| 32 |
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| 33 | These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
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| 34 |
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| 35 | =item "constant" (B<OSSL_KDF_PARAM_CONSTANT>) <octet string>
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| 36 |
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| 37 | This parameter sets the constant value for the KDF.
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| 38 | If a value is already set, the contents are replaced.
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| 39 |
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| 40 | =back
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| 41 |
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| 42 | =head1 NOTES
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| 43 |
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| 44 | A context for KRB5KDF can be obtained by calling:
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| 45 |
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| 46 | EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
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| 47 | EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
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| 48 |
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| 49 | The output length of the KRB5KDF derivation is specified via the I<keylen>
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| 50 | parameter to the L<EVP_KDF_derive(3)> function, and MUST match the key
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| 51 | length for the chosen cipher or an error is returned. Moreover, the
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| 52 | constant's length must not exceed the block size of the cipher.
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| 53 | Since the KRB5KDF output length depends on the chosen cipher, calling
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| 54 | L<EVP_KDF_CTX_get_kdf_size(3)> to obtain the requisite length returns the correct length
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| 55 | only after the cipher is set. Prior to that B<EVP_MAX_KEY_LENGTH> is returned.
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| 56 | The caller must allocate a buffer of the correct length for the chosen
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| 57 | cipher, and pass that buffer to the L<EVP_KDF_derive(3)> function along
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| 58 | with that length.
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| 59 |
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| 60 | =head1 EXAMPLES
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| 61 |
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| 62 | This example derives a key using the AES-128-CBC cipher:
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| 63 |
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| 64 | EVP_KDF *kdf;
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| 65 | EVP_KDF_CTX *kctx;
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| 66 | unsigned char key[16] = "01234...";
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| 67 | unsigned char constant[] = "I'm a constant";
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| 68 | unsigned char out[16];
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| 69 | size_t outlen = sizeof(out);
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| 70 | OSSL_PARAM params[4], *p = params;
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| 71 |
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| 72 | kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
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| 73 | kctx = EVP_KDF_CTX_new(kdf);
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| 74 | EVP_KDF_free(kdf);
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| 75 |
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| 76 | *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_CIPHER,
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| 77 | SN_aes_128_cbc,
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| 78 | strlen(SN_aes_128_cbc));
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| 79 | *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
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| 80 | key, (size_t)16);
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| 81 | *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_CONSTANT,
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| 82 | constant, strlen(constant));
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| 83 | *p = OSSL_PARAM_construct_end();
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| 84 | if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
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| 85 | /* Error */
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| 86 |
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| 87 | EVP_KDF_CTX_free(kctx);
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| 88 |
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| 89 | =head1 CONFORMING TO
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| 90 |
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| 91 | RFC 3961
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| 92 |
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| 93 | =head1 SEE ALSO
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| 94 |
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| 95 | L<EVP_KDF(3)>,
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| 96 | L<EVP_KDF_CTX_free(3)>,
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| 97 | L<EVP_KDF_CTX_get_kdf_size(3)>,
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| 98 | L<EVP_KDF_derive(3)>,
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| 99 | L<EVP_KDF(3)/PARAMETERS>
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| 100 |
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| 101 | =head1 HISTORY
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| 102 |
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| 103 | This functionality was added to OpenSSL 3.0.
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| 104 |
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| 105 | =head1 COPYRIGHT
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| 106 |
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| 107 | Copyright 2016-2021 The OpenSSL Project Authors. All Rights Reserved.
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| 108 |
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| 109 | Licensed under the Apache License 2.0 (the "License"). You may not use
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| 110 | this file except in compliance with the License. You can obtain a copy
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| 111 | in the file LICENSE in the source distribution or at
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| 112 | L<https://www.openssl.org/source/license.html>.
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| 113 |
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| 114 | =cut
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| 115 |
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