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fix modulo soundness due to overflow #999
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ChihChengLiang
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Dec 27, 2022
| @@ -49,27 +49,31 @@ impl<F: Field> ModGadget<F> { | |||
| 1.expr() - lt.expr() - n_is_zero.expr(), | |||
| ); | |||
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| // Constrain k * n + r no overflow | |||
| cb.add_constraint("overflow == 0 for k * n + r", mul_add_words.overflow()); | |||
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What do you think use require_zero here for clarity?
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| Word::from(2), | ||
| Word::from(3), | ||
| Word::from(0), | ||
| /* magic number (2^256 + 2) / 3, and 2^256 + 2 divisible by 3 */ |
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| /* magic number (2^256 + 2) / 3, and 2^256 + 2 divisible by 3 */ | |
| /* magic number (2^256 + 2) / 3, and 2^256 + 2 is divisible by 3 */ |
| @@ -183,6 +194,20 @@ mod tests { | |||
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| #[test] | |||
| fn test_mod_n_unexpected_rem() { | |||
| // test soundness by manipulating k to make a' = k * n + r and a' >= | |||
| // 2^256 lead to overflow and trigger soundness bug: (a' != a) ^ a' ≡ a | |||
| // (mod 2^256) | |||
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| // (mod 2^256) | |
| // (mod 2^256) | |
| // Here, the attacker tries to convince you of a false statement `2 % 3 = 0` by showing you `2 = ((2^256 + 2) / 3) * 3 + 0`. In the `MulAddWordsGadget`, `k * n + r = a (modulo 2**256)` would have been a valid statement. But the gadget would have the overflow=1. Since we constrain the overflow to be 0 in the ModGadget, the statement would be invalid in the ModGadget. |
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Nov 29, 2023
* fix soundness bug in ecdsa circuit * update parameters * fix: ec_sub_unequal would have panicked for some edge cases (privacy-scaling-explorations#999) * update ecdsa parameters --------- Co-authored-by: Rohit Narurkar <[email protected]>
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Fix issue #996
Changes summary
ModGadgetkso it's able to be unit-test. With k exposing, therefore also move witness generation logic outside.MulModGadget, op implementationktok2, plus addingk1, also passingk1forModGadgetto constrain.MulAddWordsGadgetsaturating_subto avoid underflow panic during carry calculation.