diff --git a/fuzz/CMakeLists.txt b/fuzz/CMakeLists.txt
index 5eff3d9..eddb38c 100644
--- a/fuzz/CMakeLists.txt
+++ b/fuzz/CMakeLists.txt
@@ -3,6 +3,24 @@
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-missing-prototypes")
 
 add_executable(
+  bn_mod_exp
+
+  bn_mod_exp.cc
+)
+
+target_link_libraries(bn_mod_exp Fuzzer)
+target_link_libraries(bn_mod_exp crypto)
+
+add_executable(
+  bn_div
+
+  bn_div.cc
+)
+
+target_link_libraries(bn_div Fuzzer)
+target_link_libraries(bn_div crypto)
+
+add_executable(
   privkey
 
   privkey.cc
diff --git a/fuzz/bn_div.cc b/fuzz/bn_div.cc
new file mode 100644
index 0000000..8994826
--- /dev/null
+++ b/fuzz/bn_div.cc
@@ -0,0 +1,69 @@
+/* Copyright (c) 2017, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include <openssl/bn.h>
+#include <openssl/bytestring.h>
+#include <openssl/span.h>
+
+#define CHECK(expr)                 \
+  do {                              \
+    if (!(expr)) {                  \
+      printf("%s failed\n", #expr); \
+      abort();                      \
+    }                               \
+  } while (false)
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *buf, size_t len) {
+  CBS cbs, child0, child1;
+  uint8_t sign0, sign1;
+  CBS_init(&cbs, buf, len);
+  if (!CBS_get_u8_length_prefixed(&cbs, &child0) ||
+      !CBS_get_u8(&child0, &sign0) ||
+      CBS_len(&child0) == 0 ||
+      !CBS_get_u8_length_prefixed(&cbs, &child1) ||
+      !CBS_get_u8(&child1, &sign1) ||
+      CBS_len(&child1) == 0) {
+    return 0;
+  }
+
+  bssl::UniquePtr<BIGNUM> numerator(
+      BN_bin2bn(CBS_data(&child0), CBS_len(&child0), nullptr));
+  BN_set_negative(numerator.get(), sign0 % 2);
+  bssl::UniquePtr<BIGNUM> divisor(
+      BN_bin2bn(CBS_data(&child1), CBS_len(&child1), nullptr));
+  BN_set_negative(divisor.get(), sign1 % 2);
+
+  if (BN_is_zero(divisor.get())) {
+    return 0;
+  }
+
+  bssl::UniquePtr<BN_CTX> ctx(BN_CTX_new());
+  bssl::UniquePtr<BIGNUM> result(BN_new());
+  bssl::UniquePtr<BIGNUM> remainder(BN_new());
+  CHECK(ctx);
+  CHECK(result);
+  CHECK(remainder);
+
+
+  CHECK(BN_div(result.get(), remainder.get(), numerator.get(), divisor.get(),
+               ctx.get()));
+  CHECK(BN_ucmp(remainder.get(), divisor.get()) < 0);
+
+  // Check that result*divisor+remainder = numerator.
+  CHECK(BN_mul(result.get(), result.get(), divisor.get(), ctx.get()));
+  CHECK(BN_add(result.get(), result.get(), remainder.get()));
+  CHECK(BN_cmp(result.get(), numerator.get()) == 0);
+
+  return 0;
+}
diff --git a/fuzz/bn_mod_exp.cc b/fuzz/bn_mod_exp.cc
new file mode 100644
index 0000000..e38236d
--- /dev/null
+++ b/fuzz/bn_mod_exp.cc
@@ -0,0 +1,115 @@
+/* Copyright (c) 2017, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include <openssl/bn.h>
+#include <openssl/bytestring.h>
+#include <openssl/mem.h>
+
+#define CHECK(expr)                 \
+  do {                              \
+    if (!(expr)) {                  \
+      printf("%s failed\n", #expr); \
+      abort();                      \
+    }                               \
+  } while (false)
+
+// Basic implementation of mod_exp using square and multiple method.
+int mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+            BN_CTX *ctx) {
+  if (BN_is_one(m)) {
+    BN_zero(r);
+    return 1;
+  }
+
+  bssl::UniquePtr<BIGNUM> exp(BN_dup(p));
+  bssl::UniquePtr<BIGNUM> base(BN_new());
+  if (!exp || !base) {
+    return 0;
+  }
+  if (!BN_one(r) || !BN_nnmod(base.get(), a, m, ctx)) {
+    return 0;
+  }
+
+  while (!BN_is_zero(exp.get())) {
+    if (BN_is_odd(exp.get())) {
+      if (!BN_mul(r, r, base.get(), ctx) || !BN_nnmod(r, r, m, ctx)) {
+        return 0;
+      }
+    }
+    if (!BN_rshift1(exp.get(), exp.get()) ||
+        !BN_mul(base.get(), base.get(), base.get(), ctx) ||
+        !BN_nnmod(base.get(), base.get(), m, ctx)) {
+      return 0;
+    }
+  }
+
+  return 1;
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *buf, size_t len) {
+  CBS cbs, child0, child1, child2;
+  uint8_t sign;
+  CBS_init(&cbs, buf, len);
+  if (!CBS_get_u8_length_prefixed(&cbs, &child0) ||
+      !CBS_get_u8(&child0, &sign) ||
+      CBS_len(&child0) == 0 ||
+      !CBS_get_u8_length_prefixed(&cbs, &child1) ||
+      CBS_len(&child1) == 0 ||
+      !CBS_get_u8_length_prefixed(&cbs, &child2) ||
+      CBS_len(&child2) == 0) {
+    return 0;
+  }
+  bssl::UniquePtr<BIGNUM> base(
+      BN_bin2bn(CBS_data(&child0), CBS_len(&child0), nullptr));
+  BN_set_negative(base.get(), sign % 2);
+  bssl::UniquePtr<BIGNUM> power(
+      BN_bin2bn(CBS_data(&child1), CBS_len(&child1), nullptr));
+  bssl::UniquePtr<BIGNUM> modulus(
+      BN_bin2bn(CBS_data(&child2), CBS_len(&child2), nullptr));
+
+  if (BN_is_zero(modulus.get())) {
+    return 0;
+  }
+
+  bssl::UniquePtr<BN_CTX> ctx(BN_CTX_new());
+  bssl::UniquePtr<BN_MONT_CTX> mont(BN_MONT_CTX_new());
+  bssl::UniquePtr<BIGNUM> result(BN_new());
+  bssl::UniquePtr<BIGNUM> expected(BN_new());
+  CHECK(ctx);
+  CHECK(mont);
+  CHECK(result);
+  CHECK(expected);
+
+  CHECK(mod_exp(expected.get(), base.get(), power.get(), modulus.get(),
+                ctx.get()));
+  CHECK(BN_mod_exp(result.get(), base.get(), power.get(), modulus.get(),
+                   ctx.get()));
+  CHECK(BN_cmp(result.get(), expected.get()) == 0);
+
+  if (BN_is_odd(modulus.get())) {
+    CHECK(BN_MONT_CTX_set(mont.get(), modulus.get(), ctx.get()));
+    CHECK(BN_mod_exp_mont(result.get(), base.get(), power.get(), modulus.get(),
+                          ctx.get(), mont.get()));
+    CHECK(BN_cmp(result.get(), expected.get()) == 0);
+    CHECK(BN_mod_exp_mont_consttime(result.get(), base.get(), power.get(),
+                                    modulus.get(), ctx.get(), mont.get()));
+    CHECK(BN_cmp(result.get(), expected.get()) == 0);
+  }
+
+  uint8_t *data = (uint8_t *)OPENSSL_malloc(BN_num_bytes(result.get()));
+  BN_bn2bin(result.get(), data);
+  OPENSSL_free(data);
+
+  return 0;
+}