Ubuntu Manpage: SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh

name
synopsis
description
notes
examples
return values
see also

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

NAME

       SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh - handle DH keys
       for ephemeral key exchange

SYNOPSIS

        #include <openssl/ssl.h>

        void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
                   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
        long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);

        void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
                   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
        long SSL_set_tmp_dh(SSL *ssl, DH *dh)

        DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));

DESCRIPTION

       SSL_CTX_set_tmp_dh_callback() sets the callback function for ctx to be used when a DH parameters are
       required to tmp_dh_callback.  The callback is inherited by all ssl objects created from ctx.

       SSL_CTX_set_tmp_dh() sets DH parameters to be used to be dh.  The key is inherited by all ssl objects
       created from ctx.

       SSL_set_tmp_dh_callback() sets the callback only for ssl.

       SSL_set_tmp_dh() sets the parameters only for ssl.

       These functions apply to SSL/TLS servers only.

NOTES

When using a cipher with RSA authentication, an ephemeral DH key exchange can take place. Ciphers with
DSA keys always use ephemeral DH keys as well. In these cases, the session data are negotiated using the
ephemeral/temporary DH key and the key supplied and certified by the certificate chain is only used for
signing. Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.

Using ephemeral DH key exchange yields forward secrecy, as the connection can only be decrypted, when the
DH key is known. By generating a temporary DH 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) key, as this key was only used for signing.

In order to perform a DH key exchange the server must use a DH group (DH parameters) and generate a DH
key. The server will always generate a new DH key during the negotiation, when the DH parameters are
supplied via callback and/or when the SSL_OP_SINGLE_DH_USE option of SSL_CTX_set_options(3) is set. It
will immediately create a DH key, when DH parameters are supplied via SSL_CTX_set_tmp_dh() and
SSL_OP_SINGLE_DH_USE is not set. In this case, it may happen that a key is generated on initialization
without later being needed, while on the other hand the computer time during the negotiation is being
saved.

If "strong" primes were used to generate the DH parameters, it is not strictly necessary to generate a
new key for each handshake but it does improve forward secrecy. If it is not assured that "strong" primes
were used, SSL_OP_SINGLE_DH_USE must be used in order to prevent small subgroup attacks. Always using
SSL_OP_SINGLE_DH_USE has an impact on the computer time needed during negotiation, but it is not very
large, so application authors/users should consider always enabling this option. The option is required
to implement perfect forward secrecy (PFS).

As generating DH parameters is extremely time consuming, an application should not generate the
parameters on the fly but supply the parameters. DH parameters can be reused, as the actual key is newly
generated during the negotiation. The risk in reusing DH parameters is that an attacker may specialize on
a very often used DH group. Applications should therefore generate their own DH parameters during the
installation process using the openssl dhparam(1) application. This application guarantees that "strong"
primes are used.

Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current version of the OpenSSL
distribution contain the 'SKIP' DH parameters, which use safe primes and were generated verifiably
pseudo-randomly. These files can be converted into C code using the -C option of the dhparam(1)
application. Generation of custom DH parameters during installation should still be preferred to stop an
attacker from specializing on a commonly used group. Files dh1024.pem and dh512.pem contain old
parameters that must not be used by applications.

An application may either directly specify the DH parameters or can supply the DH parameters via a
callback function.

Previous versions of the callback used is_export and keylength parameters to control parameter generation
for export and non-export cipher suites. Modern servers that do not support export ciphersuites are
advised to either use SSL_CTX_set_tmp_dh() in combination with SSL_OP_SINGLE_DH_USE, or alternatively,
use the callback but ignore keylength and is_export and simply supply at least 2048-bit parameters in the
callback.

EXAMPLES

       Setup DH parameters with a key length of 2048 bits. (Error handling partly left out.)

        Command-line parameter generation:
        $ openssl dhparam -out dh_param_2048.pem 2048

        Code for setting up parameters during server initialization:

        ...
        SSL_CTX ctx = SSL_CTX_new();
        ...

        /* Set up ephemeral DH parameters. */
        DH *dh_2048 = NULL;
        FILE *paramfile;
        paramfile = fopen("dh_param_2048.pem", "r");
        if (paramfile) {
          dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
          fclose(paramfile);
        } else {
          /* Error. */
        }
        if (dh_2048 == NULL) {
         /* Error. */
        }
        if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
          /* Error. */
        }
        SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE);
        ...

RETURN VALUES

       SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return diagnostic output.

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