pqcrypto/crypto_kem/kyber768/clean/kem.c

#include "api.h"
#include "fips202.h"
#include "indcpa.h"
#include "params.h"
#include "randombytes.h"
#include "verify.h"

/*************************************************
 * Name:        crypto_kem_keypair
 *
 * Description: Generates public and private key
 *              for CCA-secure Kyber key encapsulation mechanism
 *
 * Arguments:   - unsigned char *pk: pointer to output public key (an already
 *allocated array of CRYPTO_PUBLICKEYBYTES bytes)
 *              - unsigned char *sk: pointer to output private key (an already
 *allocated array of CRYPTO_SECRETKEYBYTES bytes)
 *
 * Returns 0 (success)
 **************************************************/
int crypto_kem_keypair(unsigned char *pk, unsigned char *sk) {
  size_t i;
  indcpa_keypair(pk, sk);
  for (i = 0; i < KYBER_INDCPA_PUBLICKEYBYTES; i++) {
    sk[i + KYBER_INDCPA_SECRETKEYBYTES] = pk[i];
  }
  sha3_256(sk + KYBER_SECRETKEYBYTES - 2 * KYBER_SYMBYTES, pk,
           KYBER_PUBLICKEYBYTES);
  randombytes(sk + KYBER_SECRETKEYBYTES - KYBER_SYMBYTES,
              KYBER_SYMBYTES); /* Value z for pseudo-random output on reject */
  return 0;
}

/*************************************************
 * Name:        crypto_kem_enc
 *
 * Description: Generates cipher text and shared
 *              secret for given public key
 *
 * Arguments:   - unsigned char *ct:       pointer to output cipher text (an
 *already allocated array of CRYPTO_CIPHERTEXTBYTES bytes)
 *              - unsigned char *ss:       pointer to output shared secret (an
 *already allocated array of CRYPTO_BYTES bytes)
 *              - const unsigned char *pk: pointer to input public key (an
 *already allocated array of CRYPTO_PUBLICKEYBYTES bytes)
 *
 * Returns 0 (success)
 **************************************************/
int crypto_kem_enc(unsigned char *ct, unsigned char *ss,
                   const unsigned char *pk) {
  unsigned char kr[2 * KYBER_SYMBYTES]; /* Will contain key, coins */
  unsigned char buf[2 * KYBER_SYMBYTES];

  randombytes(buf, KYBER_SYMBYTES);
  sha3_256(buf, buf, KYBER_SYMBYTES); /* Don't release system RNG output */

  sha3_256(buf + KYBER_SYMBYTES, pk,
           KYBER_PUBLICKEYBYTES); /* Multitarget countermeasure for coins +
                                     contributory KEM */
  sha3_512(kr, buf, 2 * KYBER_SYMBYTES);

  indcpa_enc(ct, buf, pk,
             kr + KYBER_SYMBYTES); /* coins are in kr+KYBER_SYMBYTES */

  sha3_256(kr + KYBER_SYMBYTES, ct,
           KYBER_CIPHERTEXTBYTES); /* overwrite coins in kr with H(c) */
  sha3_256(ss, kr,
           2 * KYBER_SYMBYTES); /* hash concatenation of pre-k and H(c) to k */
  return 0;
}

/*************************************************
 * Name:        crypto_kem_dec
 *
 * Description: Generates shared secret for given
 *              cipher text and private key
 *
 * Arguments:   - unsigned char *ss:       pointer to output shared secret (an
 *already allocated array of CRYPTO_BYTES bytes)
 *              - const unsigned char *ct: pointer to input cipher text (an
 *already allocated array of CRYPTO_CIPHERTEXTBYTES bytes)
 *              - const unsigned char *sk: pointer to input private key (an
 *already allocated array of CRYPTO_SECRETKEYBYTES bytes)
 *
 * Returns 0.
 *
 * On failure, ss will contain a pseudo-random value.
 **************************************************/
int crypto_kem_dec(unsigned char *ss, const unsigned char *ct,
                   const unsigned char *sk) {
  size_t i;
  int fail;
  unsigned char cmp[KYBER_CIPHERTEXTBYTES];
  unsigned char buf[2 * KYBER_SYMBYTES];
  unsigned char kr[2 * KYBER_SYMBYTES]; /* Will contain key, coins, qrom-hash */
  const unsigned char *pk = sk + KYBER_INDCPA_SECRETKEYBYTES;

  indcpa_dec(buf, ct, sk);

  for (i = 0; i < KYBER_SYMBYTES;
       i++) { /* Multitarget countermeasure for coins + contributory KEM */
    buf[KYBER_SYMBYTES + i] = sk[KYBER_SECRETKEYBYTES - 2 * KYBER_SYMBYTES +
                                 i]; /* Save hash by storing H(pk) in sk */
  }
  sha3_512(kr, buf, 2 * KYBER_SYMBYTES);

  indcpa_enc(cmp, buf, pk,
             kr + KYBER_SYMBYTES); /* coins are in kr+KYBER_SYMBYTES */

  fail = verify(ct, cmp, KYBER_CIPHERTEXTBYTES);

  sha3_256(kr + KYBER_SYMBYTES, ct,
           KYBER_CIPHERTEXTBYTES); /* overwrite coins in kr with H(c)  */

  cmov(kr, sk + KYBER_SECRETKEYBYTES - KYBER_SYMBYTES, KYBER_SYMBYTES,
       fail); /* Overwrite pre-k with z on re-encryption failure */

  sha3_256(ss, kr,
           2 * KYBER_SYMBYTES); /* hash concatenation of pre-k and H(c) to k */

  return 0;
}