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LibCrypto: Fix a bug in big int addition

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There was a bug when dealing with a carry when the addition
result for the current word was UINT32_MAX.

This commit also adds a regression test for the bug.
This commit is contained in:
Itamar 2020-04-08 16:00:02 +03:00 • committed by Andreas Kling
parent e0cf40518c
commit 2843dce498
2 changed files with 9 additions and 1 deletions
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2
Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp
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@ -39,7 +39,7 @@ UnsignedBigInteger UnsignedBigInteger::add(const UnsignedBigInteger& other)
u32 word_addition_result = shorter->m_words[i] + longer->m_words[i]; u32 word_addition_result = shorter->m_words[i] + longer->m_words[i];
u8 carry_out = 0; u8 carry_out = 0;
// if there was a carry, the result will be smaller than any of the operands // if there was a carry, the result will be smaller than any of the operands
if (word_addition_result < shorter->m_words[i]) { if (word_addition_result + carry < shorter->m_words[i]) {
carry_out = 1; carry_out = 1;
} }
if (carry) { if (carry) {

8
Userland/test-crypto.cpp
View file

@ -843,6 +843,14 @@ void bigint_addition_edgecases()
FAIL(Incorrect Result); FAIL(Incorrect Result);
} }
} }
{
I_TEST((BigInteger | Borrow with zero));
Crypto::UnsignedBigInteger num1({ UINT32_MAX - 3, UINT32_MAX });
Crypto::UnsignedBigInteger num2({ UINT32_MAX - 2, 0 });
if (num1.add(num2).words() == Vector<u32> { 4294967289, 0, 1 }) {
PASS;
} else {
FAIL(Incorrect Result);
} }
void bigint_subtraction() void bigint_subtraction()