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SSL_CTX_set_tmp_rsa_callback(3)	    OpenSSL    SSL_CTX_set_tmp_rsa_callback(3)

NAME
       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

SYNOPSIS
	#include <openssl/ssl.h>

	void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx,
		   RSA *(*tmp_rsa_callback)(SSL	*ssl, int is_export, int keylength));
	long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa);
	long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx);

	void SSL_set_tmp_rsa_callback(SSL_CTX *ctx,
		   RSA *(*tmp_rsa_callback)(SSL	*ssl, int is_export, int keylength));
	long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa)
	long SSL_need_tmp_rsa(SSL *ssl)

	RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength);

DESCRIPTION
       SSL_CTX_set_tmp_rsa_callback() sets the callback	function for ctx to be
       used when a temporary/ephemeral RSA key is required to
       tmp_rsa_callback.  The callback is inherited by all SSL objects newly
       created from ctx	with <SSL_new(3)|SSL_new(3)>. Already created SSL
       objects are not affected.

       SSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA key to be	used
       to be rsa. The key is inherited by all SSL objects newly	created	from
       ctx with	<SSL_new(3)|SSL_new(3)>. Already created SSL objects are not
       affected.

       SSL_CTX_need_tmp_rsa() returns 1, if a temporary/ephemeral RSA key is
       needed for RSA-based strength-limited 'exportable' ciphersuites because
       a RSA key with a	keysize	larger than 512	bits is	installed.

       SSL_set_tmp_rsa_callback() sets the callback only for ssl.

       SSL_set_tmp_rsa() sets the key only for ssl.

       SSL_need_tmp_rsa() returns 1, if	a temporary/ephemeral RSA key is
       needed, for RSA-based strength-limited 'exportable' ciphersuites
       because a RSA key with a	keysize	larger than 512	bits is	installed.

       These functions apply to	SSL/TLS	servers	only.

NOTES
       When using a cipher with	RSA authentication, an ephemeral RSA key
       exchange	can take place.	In this	case the session data are negotiated
       using the ephemeral/temporary RSA key and the RSA key supplied and
       certified by the	certificate chain is only used for signing.

       Under previous export restrictions, ciphers with	RSA keys shorter (512
       bits) than the usual key	length of 1024 bits were created. To use these
       ciphers with RSA	keys of	usual length, an ephemeral key exchange	must
       be performed, as	the normal (certified) key cannot be directly used.

       Using ephemeral RSA key exchange	yields forward secrecy,	as the
       connection can only be decrypted, when the RSA key is known. By
       generating a temporary RSA key inside the server	application that is
       lost when the application is left, it becomes impossible	for an
       attacker	to decrypt past	sessions, even if he gets hold of the normal
       (certified) RSA key, as this key	was used for signing only. The
       downside	is that	creating a RSA key is computationally expensive.

       Additionally, the use of	ephemeral RSA key exchange is only allowed in
       the TLS standard, when the RSA key can be used for signing only,	that
       is for export ciphers. Using ephemeral RSA key exchange for other
       purposes	violates the standard and can break interoperability with
       clients.	 It is therefore strongly recommended to not use ephemeral RSA
       key exchange and	use DHE	(Ephemeral Diffie-Hellman) key exchange
       instead in order	to achieve forward secrecy (see
       SSL_CTX_set_tmp_dh_callback(3)).

       An application may either directly specify the key or can supply	the
       key via a callback function. The	callback approach has the advantage,
       that the	callback may generate the key only in case it is actually
       needed. As the generation of a RSA key is however costly, it will lead
       to a significant	delay in the handshake procedure.  Another advantage
       of the callback function	is that	it can supply keys of different	size
       while the explicit setting of the key is	only useful for	key size of
       512 bits	to satisfy the export restricted ciphers and does give away
       key length if a longer key would	be allowed.

       The tmp_rsa_callback is called with the keylength needed	and the
       is_export information. The is_export flag is set, when the ephemeral
       RSA key exchange	is performed with an export cipher.

EXAMPLES
       Generate	temporary RSA keys to prepare ephemeral	RSA key	exchange. As
       the generation of a RSA key costs a lot of computer time, they saved
       for later reuse.	For demonstration purposes, two	keys for 512 bits and
       1024 bits respectively are generated.

	...
	/* Set up ephemeral RSA	stuff */
	RSA *rsa_512 = NULL;
	RSA *rsa_1024 =	NULL;

	rsa_512	= RSA_generate_key(512,RSA_F4,NULL,NULL);
	if (rsa_512 == NULL)
	    evaluate_error_queue();

	rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
	if (rsa_1024 ==	NULL)
	  evaluate_error_queue();

	...

	RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
	{
	   RSA *rsa_tmp=NULL;

	   switch (keylength) {
	   case	512:
	     if	(rsa_512)
	       rsa_tmp = rsa_512;
	     else { /* generate	on the fly, should not happen in this example */
	       rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
	       rsa_512 = rsa_tmp; /* Remember for later	reuse */
	     }
	     break;
	   case	1024:
	     if	(rsa_1024)
	       rsa_tmp=rsa_1024;
	     else
	       should_not_happen_in_this_example();
	     break;
	   default:
	     /*	Generating a key on the	fly is very costly, so use what	is there */
	     if	(rsa_1024)
	       rsa_tmp=rsa_1024;
	     else
	       rsa_tmp=rsa_512;	/* Use at least	a shorter key */
	   }
	   return(rsa_tmp);
	}

RETURN VALUES
       SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_callback() do	not
       return diagnostic output.

       SSL_CTX_set_tmp_rsa() and SSL_set_tmp_rsa() do return 1 on success and
       0 on failure. Check the error queue to find out the reason of failure.

       SSL_CTX_need_tmp_rsa() and SSL_need_tmp_rsa() return 1 if a temporary
       RSA key is needed and 0 otherwise.

SEE ALSO
       ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3),
       SSL_CTX_set_tmp_dh_callback(3), SSL_new(3), ciphers(1)

1.0.2l				  2017-05-25   SSL_CTX_set_tmp_rsa_callback(3)

NAME | SYNOPSIS | DESCRIPTION | NOTES | EXAMPLES | RETURN VALUES | SEE ALSO

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