317 lines
11 KiB
Groff
317 lines
11 KiB
Groff
.\" $NetBSD: SSL_CTX_set_tmp_dh_callback.3,v 1.6 2003/07/24 14:16:45 itojun Exp $
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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 "0.9.7b" "2001-09-07" "OpenSSL"
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.UC
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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 \s-1DH\s0 keys for ephemeral key exchange
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.SH "LIBRARY"
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libcrypto, -lcrypto
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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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.Ve
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.Vb 3
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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, int keylength));
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\& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
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.Ve
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.Vb 3
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\& void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
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\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
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\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
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.Ve
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.Vb 1
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\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
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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 a new
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\&\s-1DH\s0 key during the negotiation, when the \s-1DH\s0 parameters are supplied via
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callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of
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SSL_CTX_set_options(3) is set. It will
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immediately create a \s-1DH\s0 key, when \s-1DH\s0 parameters are supplied via
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\&\fISSL_CTX_set_tmp_dh()\fR and \s-1SSL_OP_SINGLE_DH_USE\s0 is not set. In this case,
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it may happen that a key is generated on initialization without later
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being needed, while on the other hand the computer time during the
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negotiation is being saved.
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.PP
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If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
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necessary to generate a new key for each handshake but it does improve forward
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secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
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the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
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in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
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has an impact on the computer time needed during negotiation, but it is not
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very large, so application authors/users should consider to always enable
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this option.
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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 openssl_dhparam(1) application. In order to reduce the computer
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time needed for this generation, it is possible to use \s-1DSA\s0 parameters
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instead (see openssl_dhparam(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
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is mandatory.
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.PP
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Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
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dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of 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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openssl_dhparam(1) application.
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Authors may also generate their own set of parameters using
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openssl_dhparam(1), but a user may not be sure how the parameters were
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generated. The generation of \s-1DH\s0 parameters during installation is therefore
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recommended.
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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. The callback approach
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has the advantage, that the callback may supply \s-1DH\s0 parameters for different
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key lengths.
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.PP
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The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and
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the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
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ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
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.SH "EXAMPLES"
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.IX Header "EXAMPLES"
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Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
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partly left out.)
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.PP
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.Vb 5
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\& ...
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\& /* Set up ephemeral DH stuff */
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\& DH *dh_512 = NULL;
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\& DH *dh_1024 = NULL;
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\& FILE *paramfile;
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.Ve
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.Vb 14
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\& ...
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\& /* "openssl dhparam -out dh_param_512.pem -2 512" */
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\& paramfile = fopen("dh_param_512.pem", "r");
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\& if (paramfile) {
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\& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
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\& fclose(paramfile);
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\& }
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\& /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
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\& paramfile = fopen("dh_param_1024.pem", "r");
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\& if (paramfile) {
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\& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
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\& fclose(paramfile);
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\& }
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\& ...
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.Ve
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.Vb 3
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\& /* "openssl dhparam -C -2 512" etc... */
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\& DH *get_dh512() { ... }
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\& DH *get_dh1024() { ... }
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.Ve
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.Vb 3
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\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
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\& {
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\& DH *dh_tmp=NULL;
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.Ve
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.Vb 17
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\& switch (keylength) {
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\& case 512:
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\& if (!dh_512)
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\& dh_512 = get_dh512();
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\& dh_tmp = dh_512;
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\& break;
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\& case 1024:
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\& if (!dh_1024)
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\& dh_1024 = get_dh1024();
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\& dh_tmp = dh_1024;
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\& break;
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\& default:
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\& /* Generating a key on the fly is very costly, so use what is there */
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\& setup_dh_parameters_like_above();
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\& }
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\& return(dh_tmp);
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\& }
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.Ve
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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 "SEE ALSO"
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.IX Header "SEE ALSO"
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ssl(3), SSL_CTX_set_cipher_list(3),
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SSL_CTX_set_tmp_rsa_callback(3),
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SSL_CTX_set_options(3),
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openssl_ciphers(1), openssl_dhparam(1)
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