DH_GENERATE_PARAMETERS(3) | Library Functions Manual | DH_GENERATE_PARAMETERS(3)

# NAME

`DH_generate_parameters_ex`, `DH_check`, `DH_check_pub_key`, `DH_generate_parameters` — generate and check Diffie-Hellman parameters

# SYNOPSIS

library “libcrypto”
`#include <openssl/dh.h>`

int
`DH_generate_parameters_ex`(DH *dh, int prime_len, int generator, BN_GENCB *cb);

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

int
`DH_check_pub_key`(const DH *dh, const BIGNUM *pub_key, int *codes);

Deprecated:

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

# DESCRIPTION

`DH_generate_parameters_ex`() generates Diffie-Hellman parameters that can be shared among a group of users, and stores them in the provided DH structure.

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 cb is not `NULL`, it will be called as described in BN_generate_prime(3) while a random prime number is generated, and when a prime has been found, `BN_GENCB_call`(cb, 3, 0) is called; see BN_GENCB_call(3).

`DH_check`() validates Diffie-Hellman parameters. If no problems are found, *codes is set to zero. Otherwise, one or more of the following bits are set:

`DH_CHECK_P_NOT_PRIME`
  The parameter dh->p is not prime.
`DH_CHECK_P_NOT_SAFE_PRIME`
  The parameter dh->p is not a safe prime.
`DH_UNABLE_TO_CHECK_GENERATOR`
  The generator dh->g cannot be checked for suitability: it is neither 2 nor 5.
`DH_NOT_SUITABLE_GENERATOR`
  The generator dh->g is not suitable.

`DH_check_pub_key`() checks whether pub_key is a valid public key when using the domain parameters contained in dh. If no problems are found, *codes is set to zero. Otherwise, one or more of the following bits are set:

`DH_CHECK_PUBKEY_TOO_SMALL`
  pub_key is less than or equal to 1.
`DH_CHECK_PUBKEY_TOO_LARGE`
  pub_key is greater than or equal to dh->p - 1.
DH_CHECK_PUBKEY_INVALID
  dh->q is set but pub_key to the power of dh->q is not 1 modulo dh->p.

# RETURN VALUES

`DH_generate_parameters_ex`(), `DH_check`(), and `DH_check_pub_key`() return 1 if the check could be performed or 0 otherwise.

`DH_generate_parameters`() (deprecated) returns a pointer to the DH structure, or `NULL` if the parameter generation fails.

The error codes can be obtained by ERR_get_error(3).

# SEE ALSO

DH_get0_pqg(3), DH_new(3)

# HISTORY

`DH_check`() and `DH_generate_parameters`() first appeared in SSLeay 0.5.1 and have been available since OpenBSD 2.4.

The cb_arg argument to `DH_generate_parameters`() was added in SSLeay 0.9.0.

`DH_check_pub_key`() first appeared in OpenSSL 0.9.8a and has been available since OpenBSD 4.0.

`DH_generate_parameters_ex`() first appeared in OpenSSL 0.9.8 and has been available since OpenBSD 4.5.

# CAVEATS

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

The parameters generated by `DH_generate_parameters_ex`() and `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.

June 8, 2025 | Debian