.\"
.\"	$OpenBSD: SSL_CTX_set_tmp_rsa_callback.3,v 1.2 2014/12/02 14:11:01 jmc Exp $
.\"
.Dd $Mdocdate: December 2 2014 $
.Dt SSL_CTX_SET_TMP_RSA_CALLBACK.POD 3
.Os
.Sh NAME
.Nm SSL_CTX_set_tmp_rsa_callback ,
.Nm SSL_CTX_set_tmp_rsa ,
.Nm SSL_CTX_need_tmp_rsa ,
.Nm SSL_set_tmp_rsa_callback ,
.Nm SSL_set_tmp_rsa ,
.Nm SSL_need_tmp_rsa
.Nd handle RSA keys for ephemeral key exchange
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft void
.Fo SSL_CTX_set_tmp_rsa_callback
.Fa "SSL_CTX *ctx"
.Fa "RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)"
.Fc
.Ft long
.Fn SSL_CTX_set_tmp_rsa "SSL_CTX *ctx" "RSA *rsa"
.Ft long
.Fn SSL_CTX_need_tmp_rsa "SSL_CTX *ctx"
.Ft void
.Fo SSL_set_tmp_rsa_callback
.Fa "SSL_CTX *ctx"
.Fa "RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)"
.Fc
.Ft long
.Fn SSL_set_tmp_rsa "SSL *ssl" "RSA *rsa"
.Ft long
.Fn SSL_need_tmp_rsa "SSL *ssl"
.Ft RSA *
.Fn "(*tmp_rsa_callback)" "SSL *ssl" "int is_export" "int keylength"
.Sh DESCRIPTION
.Fn SSL_CTX_set_tmp_rsa_callback
sets the callback function for
.Fa ctx
to be used when a temporary/ephemeral RSA key is required to
.Fa tmp_rsa_callback .
The callback is inherited by all
.Vt SSL
objects newly created from
.Fa ctx
with
.Xr SSL_new 3 .
Already created SSL objects are not affected.
.Pp
.Fn SSL_CTX_set_tmp_rsa
sets the temporary/ephemeral RSA key to be used to be
.Fa rsa .
The key is inherited by all
.Vt SSL
objects newly created from
.Fa ctx
with
.Xr SSL_new 3 .
Already created SSL objects are not affected.
.Pp
.Fn SSL_CTX_need_tmp_rsa
returns 1,
if a temporary/ephemeral RSA key is needed for RSA-based strength-limited
.Sq exportable
ciphersuites because a RSA key with a keysize larger than 512 bits is installed.
.Pp
.Fn SSL_set_tmp_rsa_callback
sets the callback only for
.Fa ssl .
.Pp
.Fn SSL_set_tmp_rsa
sets the key only for
.Fa ssl .
.Pp
.Fn SSL_need_tmp_rsa
returns 1,
if a temporary/ephemeral RSA key is needed for RSA-based strength-limited
.Sq exportable
ciphersuites because a RSA key with a keysize larger than 512 bits is installed.
.Pp
These functions apply to SSL/TLS servers only.
.Sh 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.
.Pp
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.
.Pp
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.
.Pp
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 EDH (Ephemeral Diffie-Hellman) key exchange instead in order to achieve
forward secrecy (see
.Xr SSL_CTX_set_tmp_dh_callback 3 ) .
.Pp
On OpenSSL servers ephemeral RSA key exchange is therefore disabled by default
and must be explicitly enabled using the
.Dv SSL_OP_EPHEMERAL_RSA
option of
.Xr SSL_CTX_set_options 3 ,
violating the TLS/SSL
standard.
When ephemeral RSA key exchange is required for export ciphers,
it will automatically be used without this option!
.Pp
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.
However, as the generation of a RSA key is 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 (e.g., for
.Dv SSL_OP_EPHEMERAL_RSA
usage) 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.
.Pp
The
.Fa tmp_rsa_callback
is called with the
.Fa keylength
needed and the
.Fa is_export
information.
The
.Fa is_export
flag is set when the ephemeral RSA key exchange is performed with an export
cipher.
.Sh RETURN VALUES
.Fn SSL_CTX_set_tmp_rsa_callback
and
.Fn SSL_set_tmp_rsa_callback
do not return diagnostic output.
.Pp
.Fn SSL_CTX_set_tmp_rsa
and
.Fn SSL_set_tmp_rsa
return 1 on success and 0 on failure.
Check the error queue to find out the reason of failure.
.Pp
.Fn SSL_CTX_need_tmp_rsa
and
.Fn SSL_need_tmp_rsa
return 1 if a temporary RSA key is needed and 0 otherwise.
.Sh 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 are saved for later reuse.
For demonstration purposes, two keys for 512 bits and 1024 bits
respectively are generated.
.Bd -literal
\&...

/* 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
			/* Use at least a shorter key */
			rsa_tmp = rsa_512;
	}
	return rsa_tmp;
}
.Ed
.Sh SEE ALSO
.Xr openssl 1 ,
.Xr ssl 3 ,
.Xr SSL_CTX_set_cipher_list 3 ,
.Xr SSL_CTX_set_options 3 ,
.Xr SSL_CTX_set_tmp_dh_callback 3 ,
.Xr SSL_new 3