Ubuntu Manpage: SSL_CTX_set_tmp_rsa_callback, SSL_CTX_set_tmp_rsa, SSL_CTX_need_tmp

Provided by: libssl-doc_1.0.1f-1ubuntu2.27_all

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 EDH (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