- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- NOTES
- BUGS
- SEE ALSO
- HISTORY

# NAME

DH_generate_parameters, DH_check - generate and check Diffie-Hellman parameters

# SYNOPSIS

     #include <openssl/dh.h>

     DH *DH_generate_parameters(int prime_len, int generator,
         void (*callback)(int, int, void *), void *cb_arg);

     int DH_check(DH *dh, int *codes);

# DESCRIPTION

`DH_generate_parameters()` generates Diffie-Hellman parameters that can be shared among a group of users, and returns them in a newly allocated DH structure. The pseudo-random number generator must be seeded prior to calling DH_generate_parameters().

prime_len is the length in bits of the safe prime to be generated. generator is a small number > 1, typically 2 or 5.

A callback function may be used to provide feedback about the progress of the key generation. If callback is not NULL, it will be called as described in BN_generate_prime while a random prime number is generated, and when a prime has been found, callback(3, 0, cb_arg) is called.

`DH_check()` validates Diffie-Hellman parameters. It checks that p is a safe prime, and that g is a suitable generator. In the case of an error, the bit flags DH_CHECK_P_NOT_SAFE_PRIME or DH_NOT_SUITABLE_GENERATOR are set in *codes. DH_UNABLE_TO_CHECK_GENERATOR is set if the generator cannot be checked, i.e. it does not equal 2 or 5.

# RETURN VALUES

`DH_generate_parameters()` returns a pointer to the DH structure, or NULL if the parameter generation fails. The error codes can be obtained by ERR_get_error.

`DH_check()` returns 1 if the check could be performed, 0 otherwise.

# NOTES

`DH_generate_parameters()` may run for several hours before finding a suitable prime.

The parameters generated by `DH_generate_parameters()` are not to be used in signature schemes.

# BUGS

If generator is not 2 or 5, dh->g=generator is not a usable generator.

# SEE ALSO

dh, ERR_get_error, rand, DH_free

# HISTORY

`DH_check()` is available in all versions of SSLeay and OpenSSL. The cb_arg argument to `DH_generate_parameters()` was added in SSLeay 0.9.0.

In versions before OpenSSL 0.9.5, DH_CHECK_P_NOT_STRONG_PRIME is used instead of DH_CHECK_P_NOT_SAFE_PRIME.