Support a negative base in power_mod

Closes #1168

2 files changed, 28 insertions, 0 deletions

diff --git a/src/tests/data/bn/powmod.vec b/src/tests/data/bn/powmod.vec
index 024a2d36b..2165ed52c 100644
--- a/src/tests/data/bn/powmod.vec
+++ b/src/tests/data/bn/powmod.vec
@@ -216,3 +216,25 @@ Exponent = 0x2000000000000000000000000000000000000000000000000000000000060800000
 Modulus  = 0x40000000000000000000000000000000000000000000000000000000000c100000000000000fffd
 Output = 0x9943fa648c48cb4cd01756bed11e3382aca74d84fb0bf8cf8d56cd4524f80538d4888cbd23b8e2
 
+# Check support for negative base (GH #1168)
+
+Base = -5
+Exponent = 10
+Modulus = 37
+Output = 30
+
+Base = -5
+Exponent = 11
+Modulus = 37
+Output = 35
+
+Base = -0xFFE0000FFF80003F00000000FF
+Exponent = 0x7FF7C00200
+Modulus = 0xFFFFFFC00000000000003FFFFF
+Output = 0xCAEB2FF794C6783C4F1F06E684
+
+Base = -0xF9FFFFF000000FFFFFFFFFFFFFFFC0000000
+Exponent = 0x83FFFF000000000000000003FFFFFFFFFFFF
+Modulus = 0xFFE007FFF9F83FFFFF8F000FFFFFFFFFFFFF
+Output = 7559902530247723476756105022122181875899018
+
diff --git a/src/tests/test_bigint.cpp b/src/tests/test_bigint.cpp
index 0dc07aabf..61be15d15 100644
--- a/src/tests/test_bigint.cpp
+++ b/src/tests/test_bigint.cpp
@@ -492,6 +492,12 @@ class BigInt_Powmod_Test : public Text_Based_Test
 
          result.test_eq("power_mod", Botan::power_mod(base, exponent, modulus), expected);
 
+         /*
+         * Only the basic power_mod interface supports negative base
+         */
+         if(base.is_negative())
+            return result;
+
          Botan::Power_Mod pow_mod1(modulus);
 
          pow_mod1.set_base(base);