int crypto_kem_dec()

in src/kem.c [153:252]

• 76 lines of code
• 4 McCabe index (conditional complexity)

int crypto_kem_dec(unsigned char *ss, const unsigned char *ct, const unsigned char *sk)
{ // FrodoKEM's key decapsulation
    uint16_t B[PARAMS_N*PARAMS_NBAR] = {0};
    uint16_t Bp[PARAMS_N*PARAMS_NBAR] = {0};
    uint16_t W[PARAMS_NBAR*PARAMS_NBAR] = {0};                // contains secret data
    uint16_t C[PARAMS_NBAR*PARAMS_NBAR] = {0};
    uint16_t CC[PARAMS_NBAR*PARAMS_NBAR] = {0};
    ALIGN_HEADER(32) uint16_t BBp[PARAMS_N*PARAMS_NBAR] ALIGN_FOOTER(32) = {0};
    ALIGN_HEADER(32) uint16_t Sp[(2*PARAMS_N+PARAMS_NBAR)*PARAMS_NBAR] ALIGN_FOOTER(32) = {0};  // contains secret data
    uint16_t *Ep = (uint16_t *)&Sp[PARAMS_N*PARAMS_NBAR];     // contains secret data
    uint16_t *Epp = (uint16_t *)&Sp[2*PARAMS_N*PARAMS_NBAR];  // contains secret data
    const uint8_t *ct_c1 = &ct[0];
    const uint8_t *ct_c2 = &ct[(PARAMS_LOGQ*PARAMS_N*PARAMS_NBAR)/8];
    const uint8_t *sk_s = &sk[0];
    const uint8_t *sk_pk = &sk[CRYPTO_BYTES];
    const uint16_t *sk_S = (uint16_t *) &sk[CRYPTO_BYTES + CRYPTO_PUBLICKEYBYTES];
    uint16_t S[PARAMS_N * PARAMS_NBAR];                      // contains secret data
    const uint8_t *sk_pkh = &sk[CRYPTO_BYTES + CRYPTO_PUBLICKEYBYTES + 2*PARAMS_N*PARAMS_NBAR];
    const uint8_t *pk_seedA = &sk_pk[0];
    const uint8_t *pk_b = &sk_pk[BYTES_SEED_A];
    uint8_t G2in[BYTES_PKHASH + BYTES_MU];                   // contains secret data via muprime
    uint8_t *pkh = &G2in[0];
    uint8_t *muprime = &G2in[BYTES_PKHASH];                  // contains secret data
    uint8_t G2out[2*CRYPTO_BYTES];                           // contains secret data
    uint8_t *seedSEprime = &G2out[0];                        // contains secret data
    uint8_t *kprime = &G2out[CRYPTO_BYTES];                  // contains secret data
    uint8_t Fin[CRYPTO_CIPHERTEXTBYTES + CRYPTO_BYTES];      // contains secret data via Fin_k
    uint8_t *Fin_ct = &Fin[0];
    uint8_t *Fin_k = &Fin[CRYPTO_CIPHERTEXTBYTES];           // contains secret data
    uint8_t shake_input_seedSEprime[1 + CRYPTO_BYTES];       // contains secret data

#ifdef DO_VALGRIND_CHECK
    VALGRIND_MAKE_MEM_UNDEFINED(sk, CRYPTO_SECRETKEYBYTES);
#endif

    for (size_t i = 0; i < PARAMS_N * PARAMS_NBAR; i++) {
        S[i] = LE_TO_UINT16(sk_S[i]);
    }

    // Compute W = C - Bp*S (mod q), and decode the randomness mu
    frodo_unpack(Bp, PARAMS_N*PARAMS_NBAR, ct_c1, (PARAMS_LOGQ*PARAMS_N*PARAMS_NBAR)/8, PARAMS_LOGQ);
    frodo_unpack(C, PARAMS_NBAR*PARAMS_NBAR, ct_c2, (PARAMS_LOGQ*PARAMS_NBAR*PARAMS_NBAR)/8, PARAMS_LOGQ);
    frodo_mul_bs(W, Bp, S);
    frodo_sub(W, C, W);
    frodo_key_decode((uint16_t*)muprime, W);

    // Generate (seedSE' || k') = G_2(pkh || mu')
    memcpy(pkh, sk_pkh, BYTES_PKHASH);
    shake(G2out, CRYPTO_BYTES + CRYPTO_BYTES, G2in, BYTES_PKHASH + BYTES_MU);

    // Generate Sp and Ep, and compute BBp = Sp*A + Ep. Generate A on-the-fly
    shake_input_seedSEprime[0] = 0x96;
    memcpy(&shake_input_seedSEprime[1], seedSEprime, CRYPTO_BYTES);
    shake((uint8_t*)Sp, (2*PARAMS_N+PARAMS_NBAR)*PARAMS_NBAR*sizeof(uint16_t), shake_input_seedSEprime, 1 + CRYPTO_BYTES);
    for (size_t i = 0; i < (2*PARAMS_N+PARAMS_NBAR)*PARAMS_NBAR; i++) {
        Sp[i] = LE_TO_UINT16(Sp[i]);
    }
    frodo_sample_n(Sp, PARAMS_N*PARAMS_NBAR);
    frodo_sample_n(Ep, PARAMS_N*PARAMS_NBAR);
    frodo_mul_add_sa_plus_e(BBp, Sp, Ep, pk_seedA);

    // Generate Epp, and compute W = Sp*B + Epp
    frodo_sample_n(Epp, PARAMS_NBAR*PARAMS_NBAR);
    frodo_unpack(B, PARAMS_N*PARAMS_NBAR, pk_b, CRYPTO_PUBLICKEYBYTES - BYTES_SEED_A, PARAMS_LOGQ);
    frodo_mul_add_sb_plus_e(W, B, Sp, Epp);

    // Encode mu, and compute CC = W + enc(mu') (mod q)
    frodo_key_encode(CC, (uint16_t*)muprime);
    frodo_add(CC, W, CC);

    // Prepare input to F
    memcpy(Fin_ct, ct, CRYPTO_CIPHERTEXTBYTES);

    // Reducing BBp modulo q
    for (int i = 0; i < PARAMS_N*PARAMS_NBAR; i++) BBp[i] = BBp[i] & ((1 << PARAMS_LOGQ)-1);

    // If (Bp == BBp & C == CC) then ss = F(ct || k'), else ss = F(ct || s)
    // Needs to avoid branching on secret data as per:
    //     Qian Guo, Thomas Johansson, Alexander Nilsson. A key-recovery timing attack on post-quantum 
    //     primitives using the Fujisaki-Okamoto transformation and its application on FrodoKEM. In CRYPTO 2020.
    int8_t selector = ct_verify(Bp, BBp, PARAMS_N*PARAMS_NBAR) | ct_verify(C, CC, PARAMS_NBAR*PARAMS_NBAR);
    // If (selector == 0) then load k' to do ss = F(ct || k'), else if (selector == -1) load s to do ss = F(ct || s)
    ct_select((uint8_t*)Fin_k, (uint8_t*)kprime, (uint8_t*)sk_s, CRYPTO_BYTES, selector);
    shake(ss, CRYPTO_BYTES, Fin, CRYPTO_CIPHERTEXTBYTES + CRYPTO_BYTES);

    // Cleanup:
    clear_bytes((uint8_t *)W, PARAMS_NBAR*PARAMS_NBAR*sizeof(uint16_t));
    clear_bytes((uint8_t *)Sp, PARAMS_N*PARAMS_NBAR*sizeof(uint16_t));
    clear_bytes((uint8_t *)S, PARAMS_N*PARAMS_NBAR*sizeof(uint16_t));
    clear_bytes((uint8_t *)Ep, PARAMS_N*PARAMS_NBAR*sizeof(uint16_t));
    clear_bytes((uint8_t *)Epp, PARAMS_NBAR*PARAMS_NBAR*sizeof(uint16_t));
    clear_bytes(muprime, BYTES_MU);
    clear_bytes(G2out, 2*CRYPTO_BYTES);
    clear_bytes(Fin_k, CRYPTO_BYTES);
    clear_bytes(shake_input_seedSEprime, 1 + CRYPTO_BYTES);
#ifdef DO_VALGRIND_CHECK
    VALGRIND_MAKE_MEM_DEFINED(sk, CRYPTO_SECRETKEYBYTES);
#endif
    return 0;
}