Add fuzzing!
Reviewed-by: Emilia Käsper <emilia@openssl.org>
diff --git a/fuzz/bignum.c b/fuzz/bignum.c
new file mode 100644
index 0000000..28a439e
--- /dev/null
+++ b/fuzz/bignum.c
@@ -0,0 +1,91 @@
+/*
+ * Copyright 2016 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the OpenSSL licenses, (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * https://www.openssl.org/source/license.html
+ * or in the file LICENSE in the source distribution.
+ */
+
+/*
+ * Confirm that a^b mod c agrees when calculated cleverly vs naively, for
+ * random a, b and c.
+ */
+
+#include <stdio.h>
+#include <openssl/bn.h>
+#include "fuzzer.h"
+
+int LLVMFuzzerTestOneInput(const uint8_t *buf, size_t len) {
+ int success = 0;
+ static BN_CTX *ctx;
+ static BN_MONT_CTX *mont;
+ static BIGNUM *b1;
+ static BIGNUM *b2;
+ static BIGNUM *b3;
+ static BIGNUM *b4;
+ static BIGNUM *b5;
+
+ if (ctx == NULL) {
+ b1 = BN_new();
+ b2 = BN_new();
+ b3 = BN_new();
+ b4 = BN_new();
+ b5 = BN_new();
+ ctx = BN_CTX_new();
+ mont = BN_MONT_CTX_new();
+ }
+ // Divide the input into three parts, using the values of the first two
+ // bytes to choose lengths, which generate b1, b2 and b3. Use three bits
+ // of the third byte to choose signs for the three numbers.
+ size_t l1 = 0, l2 = 0, l3 = 0;
+ int s1 = 0, s2 = 0, s3 = 0;
+ if (len > 2) {
+ len -= 3;
+ l1 = (buf[0] * len) / 255;
+ ++buf;
+ l2 = (buf[0] * (len - l1)) / 255;
+ ++buf;
+ l3 = len - l1 - l2;
+
+ s1 = buf[0] & 1;
+ s2 = buf[0] & 2;
+ s3 = buf[0] & 4;
+ ++buf;
+ }
+ OPENSSL_assert(BN_bin2bn(buf, l1, b1) == b1);
+ BN_set_negative(b1, s1);
+ OPENSSL_assert(BN_bin2bn(buf + l1, l2, b2) == b2);
+ BN_set_negative(b2, s2);
+ OPENSSL_assert(BN_bin2bn(buf + l1 + l2, l3, b3) == b3);
+ BN_set_negative(b3, s3);
+
+ // mod 0 is undefined
+ if (BN_is_zero(b3)) {
+ success = 1;
+ goto done;
+ }
+
+ OPENSSL_assert(BN_mod_exp(b4, b1, b2, b3, ctx));
+ OPENSSL_assert(BN_mod_exp_simple(b5, b1, b2, b3, ctx));
+
+ success = BN_cmp(b4, b5) == 0;
+ if (!success) {
+ BN_print_fp(stdout, b1);
+ putchar('\n');
+ BN_print_fp(stdout, b2);
+ putchar('\n');
+ BN_print_fp(stdout, b3);
+ putchar('\n');
+ BN_print_fp(stdout, b4);
+ putchar('\n');
+ BN_print_fp(stdout, b5);
+ putchar('\n');
+ }
+
+ done:
+ OPENSSL_assert(success);
+
+ return 0;
+}
