rsa.c

#include "rsa.h"


BIGNUM* get_rsa_priv_key(BIGNUM* p, BIGNUM* q, BIGNUM* e)
{
	/*
		given two large prime numbers, compute a private key
		using the modulo inverse of the totatives of the product p*q
	*/
	BN_CTX *ctx = BN_CTX_new();
	BIGNUM* p_minus_one = BN_new();
	BIGNUM* q_minus_one = BN_new();
	BIGNUM* one = BN_new();
	BIGNUM* tt = BN_new();

	BN_dec2bn(&one, "1");
	BN_sub(p_minus_one, p, one);
	BN_sub(q_minus_one, q, one);
	BN_mul(tt, p_minus_one, q_minus_one, ctx);

	BIGNUM* res = BN_new();
	BN_mod_inverse(res, e, tt, ctx);
	BN_CTX_free(ctx);
	return res;
}

BIGNUM* rsa_encrypt(BIGNUM* message, BIGNUM* mod, BIGNUM* pub_key)
{
	/*
		compute the RSA cipher on message
		the ciphertext is congruent to: message^mod (modulo pub_key)
	*/
	BN_CTX *ctx = BN_CTX_new();
	BIGNUM* enc = BN_new();
	BN_mod_exp(enc, message, mod, pub_key, ctx);
	BN_CTX_free(ctx);
	return enc;
}

BIGNUM* rsa_decrypt(BIGNUM* enc, BIGNUM* priv_key, BIGNUM* pub_key)
{
	/*
		compute the original message: (message ^ mod) ^ pub_key
	*/
	BN_CTX *ctx = BN_CTX_new();
	BIGNUM* dec = BN_new();
	BN_mod_exp(dec, enc, priv_key, pub_key, ctx);
	BN_CTX_free(ctx);
	return dec;
}