diff options
| -rw-r--r-- | src/lib/math/bigint/bigint.cpp | 21 | ||||
| -rw-r--r-- | src/tests/test_bigint.cpp | 29 |
2 files changed, 40 insertions, 10 deletions
diff --git a/src/lib/math/bigint/bigint.cpp b/src/lib/math/bigint/bigint.cpp index b996f0fa2..36f1d2883 100644 --- a/src/lib/math/bigint/bigint.cpp +++ b/src/lib/math/bigint/bigint.cpp @@ -163,18 +163,19 @@ void BigInt::encode_words(word out[], size_t size) const */ uint32_t BigInt::get_substring(size_t offset, size_t length) const { - if(length > 32) - throw Invalid_Argument("BigInt::get_substring: Substring size " + std::to_string(length) + " too big"); + if(length == 0 || length > 32) + throw Invalid_Argument("BigInt::get_substring invalid substring length"); - uint64_t piece = 0; - for(size_t i = 0; i != 8; ++i) - { - const uint8_t part = byte_at((offset / 8) + (7-i)); - piece = (piece << 8) | part; - } - - const uint64_t mask = (static_cast<uint64_t>(1) << length) - 1; + const size_t byte_offset = offset / 8; const size_t shift = (offset % 8); + const uint32_t mask = 0xFFFFFFFF >> (32 - length); + + const uint8_t b0 = byte_at(byte_offset); + const uint8_t b1 = byte_at(byte_offset + 1); + const uint8_t b2 = byte_at(byte_offset + 2); + const uint8_t b3 = byte_at(byte_offset + 3); + const uint8_t b4 = byte_at(byte_offset + 4); + const uint64_t piece = make_uint64(0, 0, 0, b4, b3, b2, b1, b0); return static_cast<uint32_t>((piece >> shift) & mask); } diff --git a/src/tests/test_bigint.cpp b/src/tests/test_bigint.cpp index ad0af03f4..33c617239 100644 --- a/src/tests/test_bigint.cpp +++ b/src/tests/test_bigint.cpp @@ -36,6 +36,7 @@ class BigInt_Unit_Tests final : public Test results.push_back(test_random_prime()); results.push_back(test_encode()); results.push_back(test_bigint_io()); + results.push_back(test_get_substring()); return results; } @@ -206,6 +207,34 @@ class BigInt_Unit_Tests final : public Test return result; } + Test::Result test_get_substring() + { + const size_t trials = 1000; + + Test::Result result("BigInt get_substring"); + + const Botan::BigInt r(Test::rng(), 250); + + for(size_t s = 1; s <= 32; ++s) + { + for(size_t trial = 0; trial != trials; ++trial) + { + const size_t offset = Test::rng().next_byte(); + + const uint32_t val = r.get_substring(offset, s); + + Botan::BigInt t = r >> offset; + t.mask_bits(s); + + const uint32_t cmp = t.to_u32bit(); + + result.test_eq("Same value", size_t(val), size_t(cmp)); + } + } + + return result; + } + Test::Result test_bigint_io() { Test::Result result("BigInt IO operators"); |