DH_generate_key, DH_compute_key, DH_compute_key_padded - perform Diffie-Hellman key exchange
#include <openssl/dh.h> int DH_generate_key(DH *dh); int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh); int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh);
DH_generate_key() performs the first step of a Diffie-Hellman key exchange by generating private and public DH values. By calling DH_compute_key() or DH_compute_key_padded(), these are combined with the other party's public value to compute the shared key.
DH_generate_key() expects dh to contain the shared parameters dh->p and dh->g. It generates a random private DH value unless dh->priv_key is already set, and computes the corresponding public value dh->pub_key, which can then be published.
DH_compute_key() computes the shared secret from the private DH value in dh and the other party's public value in pub_key and stores it in key. key must point to DH_size(dh) bytes of memory. The padding style is RFC 5246 (8.1.2) that strips leading zero bytes. It is not constant time due to the leading zero bytes being stripped. The return value should be considered public.
DH_compute_key_padded() is similar but stores a fixed number of bytes. The padding style is NIST SP 800-56A (C.1) that retains leading zero bytes. It is constant time due to the leading zero bytes being retained. The return value should be considered public.
DH_generate_key() returns 1 on success, 0 otherwise.
DH_compute_key() returns the size of the shared secret on success, -1 on error.
DH_compute_key_padded() returns DH_size(dh) on success, -1 on error.
The error codes can be obtained by ERR_get_error(3).
DH_compute_key_padded() was added in OpenSSL 1.0.2.
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