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LibCrypto: Fix bug in big int subtraction

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A regression test was added to the suite.

This commit also generally simplifies the subtraction method.
This commit is contained in:
Itamar 2020-04-09 16:29:09 +03:00 committed by Andreas Kling
parent 0d2777752e
commit 709c691f38
2 changed files with 19 additions and 16 deletions
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26
Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp
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@ -79,25 +79,19 @@ UnsignedBigInteger UnsignedBigInteger::sub(const UnsignedBigInteger& other) cons
}
u8 borrow = 0;
for (size_t i = 0; i < other.length(); ++i) {
// This assertion should not fail, because we verified that *this>other at the beginning of the function
ASSERT(!(borrow == 1 && m_words[i] == 0));
if (m_words[i] - borrow < other.m_words[i]) {
u64 after_borrow = static_cast<u64>(m_words[i] - borrow) + (UINT32_MAX + 1);
result.m_words.append(static_cast<u32>(after_borrow - static_cast<u64>(other.m_words[i])));
borrow = 1;
} else {
result.m_words.append(m_words[i] - borrow - other.m_words[i]);
borrow = 0;
for (size_t i = 0; i < length(); ++i) {
u32 other_word = (i < other.length()) ? other.m_words[i] : 0;
i64 temp = static_cast<i64>(m_words[i]) - static_cast<i64>(other_word) - static_cast<i64>(borrow);
// If temp < 0, we had an underflow
borrow = (temp >= 0) ? 0 : 1;
if (temp < 0) {
temp += (UINT32_MAX + 1);
}
result.m_words.append(temp);
}
for (size_t i = other.length(); i < length(); ++i) {
ASSERT(!(borrow == 1 && m_words[i] == 0));
result.m_words.append(m_words[i] - borrow);
borrow = 0;
}
// This assertion should not fail, because we verified that *this>=other at the beginning of the function
ASSERT(borrow == 0);
return result;
}

9
Userland/test-crypto.cpp
View file

@ -915,6 +915,15 @@ void bigint_subtraction()
// this test only verifies that we don't crash on an assertion
PASS;
}
{
I_TEST((BigInteger | Subtraction Regerssion 1));
auto num = Crypto::UnsignedBigInteger { 1 }.shift_left(256);
if (num.sub(1).words() == Vector<u32> { 4294967295, 4294967295, 4294967295, 4294967295, 4294967295, 4294967295, 4294967295, 4294967295, 0 }) {
PASS;
} else {
FAIL(Incorrect Result);
}
}
}
void bigint_multiplication()