1 files changed, 100 insertions, 0 deletions
diff --git a/nss/fuzz/mpi_helper.cc b/nss/fuzz/mpi_helper.cc
new file mode 100644
index 0000000..65cf4b9
--- /dev/null
+++ b/nss/fuzz/mpi_helper.cc
@@ -0,0 +1,100 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Helper functions for MPI fuzzing targets. */
+
+#include "mpi_helper.h"
+#include <cstdlib>
+#include <random>
+
+char *to_char(const uint8_t *x) {
+  return reinterpret_cast<char *>(const_cast<unsigned char *>(x));
+}
+
+// Check that the two numbers are equal.
+void check_equal(BIGNUM *b, mp_int *m, size_t max_size) {
+  char *bnBc = BN_bn2hex(b);
+  char mpiMc[max_size];
+  mp_tohex(m, mpiMc);
+  std::string bnA(bnBc);
+  std::string mpiA(mpiMc);
+  OPENSSL_free(bnBc);
+  // We have to strip leading zeros from bignums, ignoring the sign.
+  if (bnA.at(0) != '-') {
+    bnA.erase(0, std::min(bnA.find_first_not_of('0'), bnA.size() - 1));
+  } else if (bnA.at(1) == '0') {
+    bnA.erase(1, std::min(bnA.find_first_not_of('0', 1) - 1, bnA.size() - 1));
+  }
+
+  if (mpiA != bnA) {
+    std::cout << "openssl: " << std::hex << bnA << std::endl;
+    std::cout << "nss:     " << std::hex << mpiA << std::endl;
+  }
+
+  assert(mpiA == bnA);
+}
+
+// Parse data into two numbers for MPI and OpenSSL Bignum.
+void parse_input(const uint8_t *data, size_t size, BIGNUM *A, BIGNUM *B,
+                 mp_int *a, mp_int *b) {
+  // Note that b might overlap a.
+  size_t len = (size_t)size / 2;
+  assert(mp_read_raw(a, to_char(data), len) == MP_OKAY);
+  assert(mp_read_raw(b, to_char(data) + len, len) == MP_OKAY);
+  // Force a positive sign.
+  // TODO: add tests for negatives.
+  MP_SIGN(a) = MP_ZPOS;
+  MP_SIGN(b) = MP_ZPOS;
+
+  // Skip the first byte as it's interpreted as sign by NSS.
+  assert(BN_bin2bn(data + 1, len - 1, A) != nullptr);
+  assert(BN_bin2bn(data + len + 1, len - 1, B) != nullptr);
+
+  check_equal(A, a, 2 * size + 1);
+  check_equal(B, b, 2 * size + 1);
+}
+
+// Parse data into a number for MPI and OpenSSL Bignum.
+void parse_input(const uint8_t *data, size_t size, BIGNUM *A, mp_int *a) {
+  assert(mp_read_raw(a, to_char(data), size) == MP_OKAY);
+
+  // Force a positive sign.
+  // TODO: add tests for negatives.
+  MP_SIGN(a) = MP_ZPOS;
+
+  // Skip the first byte as it's interpreted as sign by NSS.
+  assert(BN_bin2bn(data + 1, size - 1, A) != nullptr);
+
+  check_equal(A, a, 4 * size + 1);
+}
+
+// Take a chunk in the middle of data and use it as modulus.
+std::tuple<BIGNUM *, mp_int> get_modulus(const uint8_t *data, size_t size,
+                                         BN_CTX *ctx) {
+  BIGNUM *r1 = BN_CTX_get(ctx);
+  mp_int r2;
+  assert(mp_init(&r2) == MP_OKAY);
+
+  size_t len = static_cast<size_t>(size / 4);
+  if (len != 0) {
+    assert(mp_read_raw(&r2, to_char(data + len), len) == MP_OKAY);
+    MP_SIGN(&r2) = MP_ZPOS;
+
+    assert(BN_bin2bn(data + len + 1, len - 1, r1) != nullptr);
+    check_equal(r1, &r2, 2 * len + 1);
+  }
+
+  // If we happen to get 0 for the modulus, take a random number.
+  if (mp_cmp_z(&r2) == 0 || len == 0) {
+    mp_zero(&r2);
+    BN_zero(r1);
+    std::mt19937 rng(data[0]);
+    std::uniform_int_distribution<mp_digit> dist(1, MP_DIGIT_MAX);
+    mp_digit x = dist(rng);
+    mp_add_d(&r2, x, &r2);
+    BN_add_word(r1, x);
+  }
+
+  return std::make_tuple(r1, r2);
+}