278 lines
10 KiB
Groff
Executable File
278 lines
10 KiB
Groff
Executable File
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.\" ========================================================================
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.\"
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.IX Title "EVP_KDF-HKDF 7"
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.TH EVP_KDF-HKDF 7 "2020-03-02" "3.0.0-dev" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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EVP_KDF\-HKDF \- The HKDF EVP_KDF implementation
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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Support for computing the \fB\s-1HKDF\s0\fR \s-1KDF\s0 through the \fB\s-1EVP_KDF\s0\fR \s-1API\s0.
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.PP
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The \s-1EVP_KDF\-HKDF\s0 algorithm implements the \s-1HKDF\s0 key derivation function.
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\&\s-1HKDF\s0 follows the \*(L"extract-then-expand\*(R" paradigm, where the \s-1KDF\s0 logically
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consists of two modules. The first stage takes the input keying material
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and \*(L"extracts\*(R" from it a fixed-length pseudorandom key K. The second stage
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\&\*(L"expands\*(R" the key K into several additional pseudorandom keys (the output
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of the \s-1KDF\s0).
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.SS "Identity"
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.IX Subsection "Identity"
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\&\*(L"\s-1HKDF\s0\*(R" is the name for this implementation; it
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can be used with the \fIEVP_KDF_fetch()\fR function.
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.SS "Supported parameters"
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.IX Subsection "Supported parameters"
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The supported parameters are:
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.ie n .IP """properties"" (\fB\s-1OSSL_KDF_PARAM_PROPERTIES\s0\fR) <\s-1UTF8\s0 string>" 4
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.el .IP "``properties'' (\fB\s-1OSSL_KDF_PARAM_PROPERTIES\s0\fR) <\s-1UTF8\s0 string>" 4
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.IX Item "properties (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>"
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.PD 0
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.ie n .IP """digest"" (\fB\s-1OSSL_KDF_PARAM_DIGEST\s0\fR) <\s-1UTF8\s0 string>" 4
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.el .IP "``digest'' (\fB\s-1OSSL_KDF_PARAM_DIGEST\s0\fR) <\s-1UTF8\s0 string>" 4
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.IX Item "digest (OSSL_KDF_PARAM_DIGEST) <UTF8 string>"
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.ie n .IP """key"" (\fB\s-1OSSL_KDF_PARAM_KEY\s0\fR) <octet string>" 4
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.el .IP "``key'' (\fB\s-1OSSL_KDF_PARAM_KEY\s0\fR) <octet string>" 4
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.IX Item "key (OSSL_KDF_PARAM_KEY) <octet string>"
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.ie n .IP """salt"" (\fB\s-1OSSL_KDF_PARAM_SALT\s0\fR) <octet string>" 4
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.el .IP "``salt'' (\fB\s-1OSSL_KDF_PARAM_SALT\s0\fR) <octet string>" 4
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.IX Item "salt (OSSL_KDF_PARAM_SALT) <octet string>"
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.PD
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These parameters work as described in \*(L"\s-1PARAMETERS\s0\*(R" in \s-1\fIEVP_KDF\s0\fR\|(3).
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.ie n .IP """info"" (\fB\s-1OSSL_KDF_PARAM_INFO\s0\fR) <octet string>" 4
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.el .IP "``info'' (\fB\s-1OSSL_KDF_PARAM_INFO\s0\fR) <octet string>" 4
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.IX Item "info (OSSL_KDF_PARAM_INFO) <octet string>"
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This parameter sets the info value.
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The length of the context info buffer cannot exceed 1024 bytes;
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this should be more than enough for any normal use of \s-1HKDF\s0.
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.ie n .IP """mode"" (\fB\s-1OSSL_KDF_PARAM_MODE\s0\fR) <\s-1UTF8\s0 string> or <integer>" 4
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.el .IP "``mode'' (\fB\s-1OSSL_KDF_PARAM_MODE\s0\fR) <\s-1UTF8\s0 string> or <integer>" 4
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.IX Item "mode (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>"
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This parameter sets the mode for the \s-1HKDF\s0 operation.
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There are three modes that are currently defined:
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.RS 4
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.ie n .IP "\fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND\s0\fR ""\s-1EXTRACT_AND_EXPAND\s0""" 4
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.el .IP "\fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND\s0\fR ``\s-1EXTRACT_AND_EXPAND\s0''" 4
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.IX Item "EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND EXTRACT_AND_EXPAND"
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This is the default mode. Calling \fIEVP_KDF_derive\fR\|(3) on an \s-1EVP_KDF_CTX\s0 set
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up for \s-1HKDF\s0 will perform an extract followed by an expand operation in one go.
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The derived key returned will be the result after the expand operation. The
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intermediate fixed-length pseudorandom key K is not returned.
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.Sp
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In this mode the digest, key, salt and info values must be set before a key is
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derived otherwise an error will occur.
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.ie n .IP "\fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0\fR ""\s-1EXTRACT_ONLY\s0""" 4
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.el .IP "\fB\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0\fR ``\s-1EXTRACT_ONLY\s0''" 4
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.IX Item "EVP_KDF_HKDF_MODE_EXTRACT_ONLY EXTRACT_ONLY"
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In this mode calling \fIEVP_KDF_derive\fR\|(3) will just perform the extract
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operation. The value returned will be the intermediate fixed-length pseudorandom
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key K. The \fIkeylen\fR parameter must match the size of K, which can be looked
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up by calling \fIEVP_KDF_size()\fR after setting the mode and digest.
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.Sp
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The digest, key and salt values must be set before a key is derived otherwise
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an error will occur.
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.ie n .IP "\fB\s-1EVP_KDF_HKDF_MODE_EXPAND_ONLY\s0\fR ""\s-1EXPAND_ONLY\s0""" 4
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.el .IP "\fB\s-1EVP_KDF_HKDF_MODE_EXPAND_ONLY\s0\fR ``\s-1EXPAND_ONLY\s0''" 4
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.IX Item "EVP_KDF_HKDF_MODE_EXPAND_ONLY EXPAND_ONLY"
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In this mode calling \fIEVP_KDF_derive\fR\|(3) will just perform the expand
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operation. The input key should be set to the intermediate fixed-length
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pseudorandom key K returned from a previous extract operation.
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.Sp
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The digest, key and info values must be set before a key is derived otherwise
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an error will occur.
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.RE
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.RS 4
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.RE
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.SH "NOTES"
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.IX Header "NOTES"
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A context for \s-1HKDF\s0 can be obtained by calling:
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.PP
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.Vb 2
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\& EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
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\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
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.Ve
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.PP
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The output length of an \s-1HKDF\s0 expand operation is specified via the \fIkeylen\fR
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parameter to the \fIEVP_KDF_derive\fR\|(3) function. When using
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\&\s-1EVP_KDF_HKDF_MODE_EXTRACT_ONLY\s0 the \fIkeylen\fR parameter must equal the size of
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the intermediate fixed-length pseudorandom key otherwise an error will occur.
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For that mode, the fixed output size can be looked up by calling \fIEVP_KDF_size()\fR
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after setting the mode and digest on the \fB\s-1EVP_KDF_CTX\s0\fR.
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.SH "EXAMPLES"
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.IX Header "EXAMPLES"
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This example derives 10 bytes using \s-1SHA\-256\s0 with the secret key \*(L"secret\*(R",
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salt value \*(L"salt\*(R" and info value \*(L"label\*(R":
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.PP
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.Vb 4
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\& EVP_KDF *kdf;
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\& EVP_KDF_CTX *kctx;
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\& unsigned char out[10];
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\& OSSL_PARAM params[5], *p = params;
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\&
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\& kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
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\& kctx = EVP_KDF_CTX_new(kdf);
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\& EVP_KDF_free(kdf);
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\&
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\& *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
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\& SN_sha256, strlen(SN_sha256));
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\& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
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\& "secret", (size_t)6);
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\& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
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\& "label", (size_t)5);
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\& *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
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\& "salt", (size_t)4);
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\& *p = OSSL_PARAM_construct_end();
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\& if (EVP_KDF_CTX_set_params(kctx, params) <= 0) {
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\& error("EVP_KDF_CTX_set_params");
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\& }
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\& if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0) {
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\& error("EVP_KDF_derive");
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\& }
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\&
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\& EVP_KDF_CTX_free(kctx);
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.Ve
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.SH "CONFORMING TO"
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.IX Header "CONFORMING TO"
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\&\s-1RFC\s0 5869
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\s-1\fIEVP_KDF\s0\fR\|(3),
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\&\fIEVP_KDF_CTX_new\fR\|(3),
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\&\fIEVP_KDF_CTX_free\fR\|(3),
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\&\fIEVP_KDF_size\fR\|(3),
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\&\fIEVP_KDF_CTX_set_params\fR\|(3),
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\&\fIEVP_KDF_derive\fR\|(3),
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\&\*(L"\s-1PARAMETERS\s0\*(R" in \s-1\fIEVP_KDF\s0\fR\|(3)
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.SH "COPYRIGHT"
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.IX Header "COPYRIGHT"
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Copyright 2016\-2019 The OpenSSL Project Authors. All Rights Reserved.
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.PP
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Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file \s-1LICENSE\s0 in the source distribution or at
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<https://www.openssl.org/source/license.html>.
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