1EVP_KDF_HKDF(7) OpenSSL EVP_KDF_HKDF(7)
2
6 EVP_KDF_HKDF - The HKDF EVP_KDF implementation
7
9 Support for computing the HKDF KDF through the EVP_KDF API.
10 The EVP_KDF_HKDF algorithm implements the HKDF key derivation function.
12 HKDF follows the "extract-then-expand" paradigm, where the KDF
13 logically consists of two modules. The first stage takes the input
14 keying material and "extracts" from it a fixed-length pseudorandom key
15 K. The second stage "expands" the key K into several additional
16 pseudorandom keys (the output of the KDF).
17 Numeric identity
19 EVP_KDF_HKDF is the numeric identity for this implementation; it can be
20 used with the EVP_KDF_CTX_new_id() function.
21 Supported controls
23 The supported controls are:
24 EVP_KDF_CTRL_SET_SALT
26 EVP_KDF_CTRL_SET_MD
27 EVP_KDF_CTRL_SET_KEY
28 These controls work as described in "CONTROLS" in EVP_KDF_CTX(3).
29 EVP_KDF_CTRL_RESET_HKDF_INFO
31 This control does not expect any arguments.
32 Resets the context info buffer to zero length.
34 EVP_KDF_CTRL_ADD_HKDF_INFO
36 This control expects two arguments: "unsigned char *info", "size_t
37 infolen"
38 Sets the info value to the first infolen bytes of the buffer info.
40 If a value is already set, the contents of the buffer are appended
41 to the existing value.
42 The total length of the context info buffer cannot exceed 1024
44 bytes; this should be more than enough for any normal use of HKDF.
45 EVP_KDF_ctrl_str() takes two type strings for this control:
47 "info"
49 The value string is used as is.
50 "hexinfo"
52 The value string is expected to be a hexadecimal number, which
53 will be decoded before being passed on as the control value.
54 EVP_KDF_CTRL_SET_HKDF_MODE
56 This control expects one argument: "int mode"
57 Sets the mode for the HKDF operation. There are three modes that
59 are currently defined:
60 EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND
62 This is the default mode. Calling EVP_KDF_derive(3) on an
63 EVP_KDF_CTX set up for HKDF will perform an extract followed by
64 an expand operation in one go. The derived key returned will
65 be the result after the expand operation. The intermediate
66 fixed-length pseudorandom key K is not returned.
67 In this mode the digest, key, salt and info values must be set
69 before a key is derived otherwise an error will occur.
70 EVP_KDF_HKDF_MODE_EXTRACT_ONLY
72 In this mode calling EVP_KDF_derive(3) will just perform the
73 extract operation. The value returned will be the intermediate
74 fixed-length pseudorandom key K. The "keylen" parameter must
75 match the size of K, which can be looked up by calling
76 EVP_KDF_size() after setting the mode and digest.
77 The digest, key and salt values must be set before a key is
79 derived otherwise an error will occur.
80 EVP_KDF_HKDF_MODE_EXPAND_ONLY
82 In this mode calling EVP_KDF_derive(3) will just perform the
83 expand operation. The input key should be set to the
84 intermediate fixed-length pseudorandom key K returned from a
85 previous extract operation.
86 The digest, key and info values must be set before a key is
88 derived otherwise an error will occur.
89 EVP_KDF_ctrl_str() type string: "mode"
91 The value string is expected to be one of: "EXTRACT_AND_EXPAND",
93 "EXTRACT_ONLY" or "EXPAND_ONLY".
94
96 A context for HKDF can be obtained by calling:
97 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_HKDF);
99 The output length of an HKDF expand operation is specified via the
101 "keylen" parameter to the EVP_KDF_derive(3) function. When using
102 EVP_KDF_HKDF_MODE_EXTRACT_ONLY the "keylen" parameter must equal the
103 size of the intermediate fixed-length pseudorandom key otherwise an
104 error will occur. For that mode, the fixed output size can be looked
105 up by calling EVP_KDF_size() after setting the mode and digest on the
106 "EVP_KDF_CTX".
107
109 This example derives 10 bytes using SHA-256 with the secret key
110 "secret", salt value "salt" and info value "label":
111 EVP_KDF_CTX *kctx;
113 unsigned char out[10];
114 kctx = EVP_KDF_CTX_new_id(EVP_KDF_HKDF);
116 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_MD, EVP_sha256()) <= 0) {
118 error("EVP_KDF_CTRL_SET_MD");
119 }
120 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SALT, "salt", (size_t)4) <= 0) {
121 error("EVP_KDF_CTRL_SET_SALT");
122 }
123 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KEY, "secret", (size_t)6) <= 0) {
124 error("EVP_KDF_CTRL_SET_KEY");
125 }
126 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_ADD_HKDF_INFO, "label", (size_t)5) <= 0) {
127 error("EVP_KDF_CTRL_ADD_HKDF_INFO");
128 }
129 if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0) {
130 error("EVP_KDF_derive");
131 }
132 EVP_KDF_CTX_free(kctx);
134
136 RFC 5869
137
139 EVP_KDF_CTX, EVP_KDF_CTX_new_id(3), EVP_KDF_CTX_free(3),
140 EVP_KDF_ctrl(3), EVP_KDF_size(3), EVP_KDF_derive(3), "CONTROLS" in
141 EVP_KDF_CTX(3)
142
144 Copyright 2016-2018 The OpenSSL Project Authors. All Rights Reserved.
145 Licensed under the Apache License 2.0 (the "License"). You may not use
147 this file except in compliance with the License. You can obtain a copy
148 in the file LICENSE in the source distribution or at
149 <https://www.openssl.org/source/license.html>.
1501.1.1c 2019-06-03 EVP_KDF_HKDF(7)