DH_SIZE(3) | Library Functions Manual | DH_SIZE(3)

# NAME

`DH_size`, `DH_bits` — get Diffie-Hellman prime size

# SYNOPSIS

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

int
`DH_size`(const DH *dh);

int
`DH_bits`(const DH *dh);

# DESCRIPTION

`DH_size`() returns the Diffie-Hellman prime size in bytes. It can be used to determine how much memory must be allocated for the shared secret computed by DH_compute_key(3).

`DH_bits`() returns the number of significant bits in the key.

dh and dh->p must not be `NULL`.

# SEE ALSO

BN_num_bytes(3), DH_generate_key(3), DH_get0_key(3), DH_new(3), DH_security_bits(3)

# HISTORY

`DH_size`() first appeared in SSLeay 0.5.1 and has been available since OpenBSD 2.4.

`DH_bits`() first appeared in OpenSSL 1.1.0 and has been available since OpenBSD 6.3.

June 8, 2025 | Debian