312 lines
11 KiB
Groff
312 lines
11 KiB
Groff
.\" $NetBSD: SSL_CTX_set_tmp_rsa_callback.3,v 1.9 2005/04/24 00:10:03 wiz Exp $
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.\" ========================================================================
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.\"
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.IX Title "SSL_CTX_set_tmp_rsa_callback 3"
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.TH SSL_CTX_set_tmp_rsa_callback 3 "2002-06-09" "0.9.7g" "OpenSSL"
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.SH "NAME"
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SSL_CTX_set_tmp_rsa_callback, SSL_CTX_set_tmp_rsa, SSL_CTX_need_tmp_rsa, SSL_set_tmp_rsa_callback, SSL_set_tmp_rsa, SSL_need_tmp_rsa \- handle RSA 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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.PP
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.Vb 4
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\& void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx,
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\& RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
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\& long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa);
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\& long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx);
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.Ve
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.PP
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.Vb 4
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\& void SSL_set_tmp_rsa_callback(SSL_CTX *ctx,
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\& RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
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\& long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa)
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\& long SSL_need_tmp_rsa(SSL *ssl)
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.Ve
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.PP
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.Vb 1
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\& RSA *(*tmp_rsa_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_rsa_callback()\fR sets the callback function for \fBctx\fR to be
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used when a temporary/ephemeral \s-1RSA\s0 key is required to \fBtmp_rsa_callback\fR.
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The callback is inherited by all \s-1SSL\s0 objects newly created from \fBctx\fR
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with <\fISSL_new\fR\|(3)|\fISSL_new\fR\|(3)>. Already created \s-1SSL\s0 objects are not affected.
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.PP
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\&\fISSL_CTX_set_tmp_rsa()\fR sets the temporary/ephemeral \s-1RSA\s0 key to be used to be
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\&\fBrsa\fR. The key is inherited by all \s-1SSL\s0 objects newly created from \fBctx\fR
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with <\fISSL_new\fR\|(3)|\fISSL_new\fR\|(3)>. Already created \s-1SSL\s0 objects are not affected.
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.PP
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\&\fISSL_CTX_need_tmp_rsa()\fR returns 1, if a temporary/ephemeral \s-1RSA\s0 key is needed
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for RSA-based strength-limited 'exportable' ciphersuites because a \s-1RSA\s0 key
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with a keysize larger than 512 bits is installed.
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.PP
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\&\fISSL_set_tmp_rsa_callback()\fR sets the callback only for \fBssl\fR.
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.PP
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\&\fISSL_set_tmp_rsa()\fR sets the key only for \fBssl\fR.
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.PP
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\&\fISSL_need_tmp_rsa()\fR returns 1, if a temporary/ephemeral \s-1RSA\s0 key is needed,
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for RSA-based strength-limited 'exportable' ciphersuites because a \s-1RSA\s0 key
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with a keysize larger than 512 bits is installed.
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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-1RSA\s0 key exchange
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can take place. In this case the session data are negotiated using the
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ephemeral/temporary \s-1RSA\s0 key and the \s-1RSA\s0 key supplied and certified
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by the certificate chain is only used for signing.
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.PP
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Under previous export restrictions, ciphers with \s-1RSA\s0 keys shorter (512 bits)
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than the usual key length of 1024 bits were created. To use these ciphers
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with \s-1RSA\s0 keys of usual length, an ephemeral key exchange must be performed,
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as the normal (certified) key cannot be directly used.
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.PP
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Using ephemeral \s-1RSA\s0 key exchange yields forward secrecy, as the connection
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can only be decrypted, when the \s-1RSA\s0 key is known. By generating a temporary
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\&\s-1RSA\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) \s-1RSA\s0 key, as this key was
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used for signing only. The downside is that creating a \s-1RSA\s0 key is
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computationally expensive.
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.PP
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Additionally, the use of ephemeral \s-1RSA\s0 key exchange is only allowed in
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the \s-1TLS\s0 standard, when the \s-1RSA\s0 key can be used for signing only, that is
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for export ciphers. Using ephemeral \s-1RSA\s0 key exchange for other purposes
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violates the standard and can break interoperability with clients.
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It is therefore strongly recommended to not use ephemeral \s-1RSA\s0 key
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exchange and use \s-1EDH\s0 (Ephemeral Diffie\-Hellman) key exchange instead
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in order to achieve forward secrecy (see
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\&\fISSL_CTX_set_tmp_dh_callback\fR\|(3)).
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.PP
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On OpenSSL servers ephemeral \s-1RSA\s0 key exchange is therefore disabled by default
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and must be explicitly enabled using the \s-1SSL_OP_EPHEMERAL_RSA\s0 option of
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\&\fISSL_CTX_set_options\fR\|(3), violating the \s-1TLS/SSL\s0
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standard. When ephemeral \s-1RSA\s0 key exchange is required for export ciphers,
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it will automatically be used without this option!
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.PP
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An application may either directly specify the key or can supply the key via
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a callback function. The callback approach has the advantage, that the
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callback may generate the key only in case it is actually needed. As the
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generation of a \s-1RSA\s0 key is however costly, it will lead to a significant
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delay in the handshake procedure. Another advantage of the callback function
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is that it can supply keys of different size (e.g. for \s-1SSL_OP_EPHEMERAL_RSA\s0
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usage) while the explicit setting of the key is only useful for key size of
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512 bits to satisfy the export restricted ciphers and does give away key length
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if a longer key would be allowed.
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.PP
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The \fBtmp_rsa_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-1RSA\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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Generate temporary \s-1RSA\s0 keys to prepare ephemeral \s-1RSA\s0 key exchange. As the
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generation of a \s-1RSA\s0 key costs a lot of computer time, they saved for later
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reuse. For demonstration purposes, two keys for 512 bits and 1024 bits
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respectively are generated.
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.PP
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.Vb 4
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\& ...
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\& /* Set up ephemeral RSA stuff */
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\& RSA *rsa_512 = NULL;
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\& RSA *rsa_1024 = NULL;
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.Ve
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.PP
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.Vb 3
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\& rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
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\& if (rsa_512 == NULL)
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\& evaluate_error_queue();
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.Ve
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.PP
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.Vb 3
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\& rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
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\& if (rsa_1024 == NULL)
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\& evaluate_error_queue();
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.Ve
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.PP
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.Vb 1
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\& ...
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.Ve
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.PP
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.Vb 3
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\& RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
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\& {
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\& RSA *rsa_tmp=NULL;
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.Ve
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.PP
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.Vb 24
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\& switch (keylength) {
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\& case 512:
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\& if (rsa_512)
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\& rsa_tmp = rsa_512;
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\& else { /* generate on the fly, should not happen in this example */
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\& rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
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\& rsa_512 = rsa_tmp; /* Remember for later reuse */
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\& }
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\& break;
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\& case 1024:
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\& if (rsa_1024)
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\& rsa_tmp=rsa_1024;
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\& else
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\& should_not_happen_in_this_example();
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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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\& if (rsa_1024)
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\& rsa_tmp=rsa_1024;
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\& else
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\& rsa_tmp=rsa_512; /* Use at least a shorter key */
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\& }
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\& return(rsa_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_rsa_callback()\fR and \fISSL_set_tmp_rsa_callback()\fR do not return
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diagnostic output.
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.PP
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\&\fISSL_CTX_set_tmp_rsa()\fR and \fISSL_set_tmp_rsa()\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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.PP
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\&\fISSL_CTX_need_tmp_rsa()\fR and \fISSL_need_tmp_rsa()\fR return 1 if a temporary
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\&\s-1RSA\s0 key is needed and 0 otherwise.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
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\&\fISSL_CTX_set_options\fR\|(3),
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\&\fISSL_CTX_set_tmp_dh_callback\fR\|(3),
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\&\fISSL_new\fR\|(3), \fIciphers\fR\|(1)
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