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Document boolean constraint requirement for gates. #87

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13 changes: 13 additions & 0 deletions circuits/gates.circom
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,14 @@
*/
pragma circom 2.0.0;

/*
Boolean logic gates.

Inputs generally must be constrained to 0 or 1, using Num2Bits or a*(a-1) === 0 etc.
Most languages extend boolean logic to non boolean types, but using non-booleans here will lead to subtle soundness bugs.
*/

// Requires binary inputs: [2,2] -> -4, should be 0
template XOR() {
signal input a;
signal input b;
Expand All @@ -26,6 +34,7 @@ template XOR() {
out <== a + b - 2*a*b;
}

// Applies to non-binary numbers: gives non-zero output iff both inputs are non-zero because a and b are coprime with p
template AND() {
signal input a;
signal input b;
Expand All @@ -34,6 +43,7 @@ template AND() {
out <== a*b;
}

// Requires binary inputs: [2,2] -> 0, should be non-zero
template OR() {
signal input a;
signal input b;
Expand All @@ -42,13 +52,15 @@ template OR() {
out <== a + b - a*b;
}

// Requires binary inputs: 2 -> -1, should be 0
template NOT() {
signal input in;
signal output out;

out <== 1 + in - 2*in;
}

// Requires binary inputs: [2,1] -> -1, should be 0
template NAND() {
signal input a;
signal input b;
Expand All @@ -57,6 +69,7 @@ template NAND() {
out <== 1 - a*b;
}

// Requires binary inputs: [2,1] -> 0, should be non-zero
template NOR() {
signal input a;
signal input b;
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