261 lines
9.3 KiB
Groff
Executable File
261 lines
9.3 KiB
Groff
Executable File
.\" Automatically generated by Pod::Man 2.25 (Pod::Simple 3.16)
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.\" ========================================================================
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.\"
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.IX Title "SSL_CTX_SET_TMP_DH_CALLBACK 3"
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.TH SSL_CTX_SET_TMP_DH_CALLBACK 3 "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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SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh \- handle DH keys for ephemeral key exchange
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/ssl.h>
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\&
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\& void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
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\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
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\& int keylength));
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\& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
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\&
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\& void SSL_set_tmp_dh_callback(SSL *ctx,
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\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
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\& int keylength));
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\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
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used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
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The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
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.PP
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\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
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The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
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.PP
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\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
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.PP
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\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
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.PP
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These functions apply to \s-1SSL/TLS\s0 servers only.
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.SH "NOTES"
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.IX Header "NOTES"
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When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
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can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
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In these cases, the session data are negotiated using the
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ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
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by the certificate chain is only used for signing.
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Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
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.PP
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Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
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can only be decrypted, when the \s-1DH\s0 key is known. By generating a temporary
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\&\s-1DH\s0 key inside the server application that is lost when the application
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is left, it becomes impossible for an attacker to decrypt past sessions,
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even if he gets hold of the normal (certified) key, as this key was
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only used for signing.
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.PP
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In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group
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(\s-1DH\s0 parameters) and generate a \s-1DH\s0 key. The server will always generate
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a new \s-1DH\s0 key during the negotiation.
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.PP
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As generating \s-1DH\s0 parameters is extremely time consuming, an application
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should not generate the parameters on the fly but supply the parameters.
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\&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
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the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
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may specialize on a very often used \s-1DH\s0 group. Applications should therefore
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generate their own \s-1DH\s0 parameters during the installation process using the
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openssl \fIopenssl\-dhparam\fR\|(1) application. This application
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guarantees that \*(L"strong\*(R" primes are used.
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.PP
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Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
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version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
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which use safe primes and were generated verifiably pseudo-randomly.
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These files can be converted into C code using the \fB\-C\fR option of the
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\&\fIopenssl\-dhparam\fR\|(1) application. Generation of custom \s-1DH\s0
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parameters during installation should still be preferred to stop an
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attacker from specializing on a commonly used group. File dh1024.pem
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contains old parameters that must not be used by applications.
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.PP
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An application may either directly specify the \s-1DH\s0 parameters or
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can supply the \s-1DH\s0 parameters via a callback function.
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.PP
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Previous versions of the callback used \fBis_export\fR and \fBkeylength\fR
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parameters to control parameter generation for export and non-export
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cipher suites. Modern servers that do not support export cipher suites
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are advised to either use \fISSL_CTX_set_tmp_dh()\fR or alternatively, use
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the callback but ignore \fBkeylength\fR and \fBis_export\fR and simply
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supply at least 2048\-bit parameters in the callback.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
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diagnostic output.
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.PP
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\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
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on failure. Check the error queue to find out the reason of failure.
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.SH "EXAMPLES"
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.IX Header "EXAMPLES"
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Setup \s-1DH\s0 parameters with a key length of 2048 bits. (Error handling
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partly left out.)
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.PP
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Command-line parameter generation:
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.PP
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.Vb 1
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\& $ openssl dhparam \-out dh_param_2048.pem 2048
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.Ve
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.PP
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Code for setting up parameters during server initialization:
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.PP
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.Vb 1
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\& SSL_CTX ctx = SSL_CTX_new();
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\&
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\& DH *dh_2048 = NULL;
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\& FILE *paramfile = fopen("dh_param_2048.pem", "r");
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\&
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\& if (paramfile) {
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\& dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
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\& fclose(paramfile);
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\& } else {
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\& /* Error. */
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\& }
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\& if (dh_2048 == NULL)
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\& /* Error. */
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\& if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1)
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\& /* Error. */
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\& ...
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.Ve
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fIssl\fR\|(7), \fISSL_CTX_set_cipher_list\fR\|(3),
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\&\fISSL_CTX_set_options\fR\|(3),
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\&\fIopenssl\-ciphers\fR\|(1), \fIopenssl\-dhparam\fR\|(1)
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.SH "COPYRIGHT"
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.IX Header "COPYRIGHT"
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Copyright 2001\-2016 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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