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Increase robustness against UB in secp256k1_scalar_cadd_bit #647

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merged 2 commits into from Oct 28, 2019
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Increase robustness against UB in secp256k1_scalar_cadd_bit #647

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8fe63e5
Increase robustness against UB.
roconnor-blockstream Jul 3, 2019
0d82732
Improve VERIFY_CHECK of overflow in secp256k1_scalar_cadd_bit.
roconnor-blockstream Jul 5, 2019
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5 changes: 4 additions & 1 deletion src/scalar_low_impl.h
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Expand Up @@ -38,8 +38,11 @@ static int secp256k1_scalar_add(secp256k1_scalar *r, const secp256k1_scalar *a,

static void secp256k1_scalar_cadd_bit(secp256k1_scalar *r, unsigned int bit, int flag) {
if (flag && bit < 32)
*r += (1 << bit);
*r += ((uint32_t)1 << bit);
#ifdef VERIFY
VERIFY_CHECK(bit < 32);
/* Verify that adding (1 << bit) will not overflow any in-range scalar *r by overflowing the underlying uint32_t. */
VERIFY_CHECK(((uint32_t)1 << bit) - 1 <= UINT32_MAX - EXHAUSTIVE_TEST_ORDER);
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VERIFY_CHECK(secp256k1_scalar_check_overflow(r) == 0);
#endif
}
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