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Fix handling of large RHS in mp_add_integer_into.

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While looking over the code for other reasons, I happened to notice
that the internal function mp_add_masked_integer_into was using a
totally wrong condition to check whether it was about to do an
out-of-range right shift: it was comparing a shift count measured in
bits against BIGNUM_INT_BYTES.

The resulting bug hasn't shown up in the code so far, which I assume
is just because no caller is passing any RHS to mp_add_integer_into
bigger than about 1 or 2. And it doesn't show up in the test suite
because I hadn't tested those functions. Now I am testing them, and
the newly added test fails when built for 16-bit BignumInt if you back
out the actual fix in this commit.

(cherry picked from commit 921118dbea)
This commit is contained in:
Simon Tatham 2020-02-19 19:08:51 +00:00
parent 5b45c32ab7
commit 45e4fbf207
2 changed files with 19 additions and 2 deletions
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5
mpint.c
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@ -758,7 +758,7 @@ static BignumCarry mp_add_masked_integer_into(
for (size_t i = 0; i < rw; i++) {
BignumInt aword = mp_word(a, i);
size_t shift = i * BIGNUM_INT_BITS;
BignumInt bword = shift < BIGNUM_INT_BYTES ? b >> shift : 0;
BignumInt bword = shift < CHAR_BIT*sizeof(b) ? b >> shift : 0;
BignumInt out;
bword = (bword ^ b_xor) & b_and;
BignumADC(out, carry, aword, bword, carry);
@ -799,7 +799,8 @@ static void mp_add_integer_into_shifted_by_words(
* shift n down. If it's 0, we add zero bits into r, and
* leave n alone. */
BignumInt bword = n & -(BignumInt)indicator;
uintmax_t new_n = (BIGNUM_INT_BITS < 64 ? n >> BIGNUM_INT_BITS : 0);
uintmax_t new_n = (BIGNUM_INT_BYTES < sizeof(n) ?
n >> BIGNUM_INT_BITS : 0);
n ^= (n ^ new_n) & -(uintmax_t)indicator;
BignumInt aword = mp_word(a, i);

16
test/cryptsuite.py
View File

@ -335,6 +335,22 @@ class mpint(MyTestBase):
bm = mp_copy(bi)
self.assertEqual(int(mp_mul(am, bm)), ai * bi)
def testAddInteger(self):
initial = mp_copy(4444444444444444444444444)
x = mp_new(mp_max_bits(initial) + 64)
# mp_{add,sub}_integer_into should be able to cope with any
# uintmax_t. Test a number that requires more than 32 bits.
mp_add_integer_into(x, initial, 123123123123123)
self.assertEqual(int(x), 4444444444567567567567567)
mp_sub_integer_into(x, initial, 123123123123123)
self.assertEqual(int(x), 4444444444321321321321321)
# mp_mul_integer_into only takes a uint16_t integer input
mp_mul_integer_into(x, initial, 10001)
self.assertEqual(int(x), 44448888888888888888888884444)
def testDivision(self):
divisors = [1, 2, 3, 2**16+1, 2**32-1, 2**32+1, 2**128-159,
141421356237309504880168872420969807856967187537694807]