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Lyd/underflow constraints #494

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8df4208
fix(circuits): underflow constraints
lydiagarms Jun 24, 2026
62d98ca
chore(tests): add test contract for underflows
lydiagarms Jul 2, 2026
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1 change: 1 addition & 0 deletions circuits/common/splitCommitments.zok
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,7 @@ def main(

// prepare secret state 'newCommitments' for commitments
field newCommitment_0_value_field = value;
assert(oldCommitment_0_value >= value);
field newCommitment_1_value_field = oldCommitment_0_value - value;

// preimage check - newCommitment_commitment
Expand Down
25 changes: 11 additions & 14 deletions src/boilerplate/circuit/zokrates/raw/BoilerplateGenerator.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -335,29 +335,20 @@ class BoilerplateGenerator {
];
},

postStatements({ name: x, isWhole, isNullified, newCommitmentValue, structProperties, structPropertiesTypes, typeName }): string[] {
postStatements({ name: x, isWhole, isNullified, structProperties, structPropertiesTypes, typeName }): string[] {
// if (!isWhole && !newCommitmentValue) throw new Error('PATH');
let y = isWhole ? x : x.slice(0, -2);
const lines: string[] = [];
if (!isWhole && isNullified) {
// decrement
const i = parseInt(x.slice(-1), 10);
const x0 = x.slice(0, -1) + `${i-2}`;
const x1 = x.slice(0, -1) + `${i-1}`;
if (!structProperties) {
lines.push(
`assert(${y} >0);
// TODO: assert no under/overflows

field ${x}_newCommitment_value_field = ${y};`
`field ${x}_newCommitment_value_field = ${y};`
);
} else {
// TODO types for each structProperty
lines.push(
`${structProperties.map(p => newCommitmentValue[p] === '0' ? '' : `assert(${y}.${p} > 0);`).join('\n')}
// TODO: assert no under/overflows

${typeName} ${x}_newCommitment_value = ${typeName} { ${structProperties.map(p => ` ${p}: ${y}.${p}`)} };`
`${typeName} ${x}_newCommitment_value = ${typeName} { ${structProperties.map(p => ` ${p}: ${y}.${p}`)} };`
);
}
} else {
Expand Down Expand Up @@ -551,9 +542,15 @@ class BoilerplateGenerator {
statements({ name: x, subtrahend, newCommitmentValue, structProperties, memberName}): string[] {
if (subtrahend.decrementType === '-='){
if (structProperties) {
return [`${x}.${memberName} = ${x}.${memberName} - (${newCommitmentValue});`]
return [
`assert(${x}.${memberName} >= (${newCommitmentValue}));
${x}.${memberName} = ${x}.${memberName} - (${newCommitmentValue});`,
];
}
return [`${x} = ${x} - (${newCommitmentValue});`];
return [
`assert(${x} >= (${newCommitmentValue}));
${x} = ${x} - (${newCommitmentValue});`,
];
} else if (subtrahend.decrementType === '='){
if (structProperties) {
return [`${x}.${memberName} = ${newCommitmentValue};`]
Expand Down
184 changes: 166 additions & 18 deletions src/codeGenerators/circuit/zokrates/toCircuit.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,92 @@ const keepOneTrailingSemicolon = (code: string) => {
return code.endsWith('}') ? code : code.replace(/;+$/, '') + ';';
};

// Traverses a node to check for any guards that are necessary to prevent underflows
const underflowGuardConditions = (
node: any,
codeGeneratorState: any,
seen = new Set<any>(),
): string[] => {
if (!node) return [];
if (Array.isArray(node)) return node.flatMap(child => underflowGuardConditions(child, codeGeneratorState, seen));
if (typeof node !== 'object') return [];
if (seen.has(node)) return [];
seen.add(node);

switch (node.nodeType) {
// If the node is a binary operation with operator "-", we return a guard to prevent underflows, analogously to solidity,
// otherwise we traverse the child nodes
case 'BinaryOperation': {
const subExpressionConditions = [
...underflowGuardConditions(node.leftExpression, codeGeneratorState, seen),
...underflowGuardConditions(node.rightExpression, codeGeneratorState, seen),
];
if (node.operator !== '-') return subExpressionConditions;
return [
...subExpressionConditions,
`${codeGenerator(node.leftExpression, codeGeneratorState)} >= ${codeGenerator(node.rightExpression, codeGeneratorState)}`,
];
}

// We only need to prevent underflows in if statements if the branch of the if statement is taken
case 'Conditional': {
const condition = removeTrailingSemicolon(codeGenerator(node.condition, codeGeneratorState));
return [
...underflowGuardConditions(node.condition, codeGeneratorState, seen),
...underflowGuardConditions(node.trueExpression, codeGeneratorState, seen).map(guard => `!(${condition}) || (${guard})`),
...underflowGuardConditions(node.falseExpression, codeGeneratorState, seen).map(guard => `(${condition}) || (${guard})`),
];
}

case 'UnaryOperation': {
const subExpressionConditions = underflowGuardConditions(
node.subExpression,
codeGeneratorState,
seen,
);
if (node.operator !== '--') return subExpressionConditions;
return [
...subExpressionConditions,
`${codeGenerator(node.subExpression, codeGeneratorState)} >= 1`,
];
}

default:
return Object.values(node).flatMap(child => underflowGuardConditions(child, codeGeneratorState, seen));
}
};

const underflowGuardAssertions = (node: any, codeGeneratorState: any) => {
return underflowGuardConditions(node, codeGeneratorState)
.map(guard => ` assert(${guard});`)
.join('\n');
};

// Adds a guard to the statement to prevent underflows
const withUnderflowGuardAssertions = (node: any, statement: string, codeGeneratorState: any) => {
const guards = underflowGuardAssertions(node, codeGeneratorState);
return guards ? `${guards}\n${statement}` : statement;
};

const assignmentStatement = (node: any, codeGeneratorState: any) =>
`${codeGenerator(node.leftHandSide, codeGeneratorState)} ${node.operator} ${codeGenerator(node.rightHandSide, codeGeneratorState)};`;

// Return an underflow guard that only is enforced if a condition is satisfied
const conditionalUnderflowGuardAssertions = (
node: any,
condition: string,
activeWhenTrue: boolean,
codeGeneratorState: any,
) => {
return underflowGuardConditions(node, codeGeneratorState)
.map(guard =>
activeWhenTrue
? `\n assert(!(${condition}) || (${guard}));`
: `\n assert((${condition}) || (${guard}));`,
)
.join('');
};

function poseidonLibraryChooser(fileObj: string) {
if (!fileObj.includes('poseidon')) return fileObj;
let poseidonFieldCount = 0;
Expand Down Expand Up @@ -241,12 +327,12 @@ codeGeneratorState.wrapperFunctions.set(functionName, wrapperFunction);
if(node.initialValue?.nodeType === 'InternalFunctionCall'){
if(!declarations) return ;
if(node.initialValue?.expression?.nodeType === 'BinaryOperation')
return `${declarations} = ${codeGenerator(node.initialValue.expression, codeGeneratorState)};`;
return `${declarations} = ${node.initialValue.name};`;
return withUnderflowGuardAssertions(node.initialValue, `${declarations} = ${codeGenerator(node.initialValue.expression, codeGeneratorState)};`, codeGeneratorState);
return withUnderflowGuardAssertions(node.initialValue, `${declarations} = ${node.initialValue.name};`, codeGeneratorState);
}
const initialValue = codeGenerator(node.initialValue, codeGeneratorState);

return `${declarations} = ${initialValue};`;
return withUnderflowGuardAssertions(node.initialValue, `${declarations} = ${initialValue};`, codeGeneratorState);
}

case 'ElementaryTypeName':
Expand All @@ -262,6 +348,21 @@ codeGeneratorState.wrapperFunctions.set(functionName, wrapperFunction);

case 'ExpressionStatement': {
if (node.isVarDec) {
if (node.expression?.nodeType === 'Assignment') {
const declarationType =
node.expression?.leftHandSide?.typeName === 'bool'
? 'bool'
: 'field';
return withUnderflowGuardAssertions(
node.expression,
`
${declarationType} mut ${assignmentStatement(
node.expression,
codeGeneratorState,
)}`,
codeGeneratorState,
);
}
if (node.expression?.leftHandSide?.typeName === 'bool'){
return `
bool mut ${codeGenerator(node.expression, codeGeneratorState)}`;
Expand Down Expand Up @@ -295,13 +396,21 @@ codeGeneratorState.wrapperFunctions.set(functionName, wrapperFunction);
return ` ` ;

case 'Assignment':
return `${codeGenerator(node.leftHandSide, codeGeneratorState)} ${node.operator} ${codeGenerator(node.rightHandSide, codeGeneratorState)};`;
return withUnderflowGuardAssertions(
node,
assignmentStatement(node, codeGeneratorState),
codeGeneratorState,
);

case 'UnaryOperation':
if (node.subExpression?.typeName?.name === 'bool' && node.operator === '!'){
return `${node.operator}${node.subExpression.name};`;
}
return `${codeGenerator(node.initialValue, codeGeneratorState)} = ${codeGenerator(node.subExpression, codeGeneratorState)} ${node.operator[0]} 1;`;
return withUnderflowGuardAssertions(
node,
`${codeGenerator(node.initialValue, codeGeneratorState)} = ${codeGenerator(node.subExpression, codeGeneratorState)} ${node.operator[0]} 1;`,
codeGeneratorState,
);

case 'BinaryOperation':
if (node.operator === '/') {
Expand Down Expand Up @@ -361,6 +470,8 @@ codeGeneratorState.wrapperFunctions.set(functionName, wrapperFunction);
node.condition.rightExpression.name = node.condition.rightExpression.name.replace('_temp','');
if(node.condition.leftExpression.nodeType == 'Identifier')
node.condition.leftExpression.name = node.condition.leftExpression.name.replace('_temp','');
const conditionUnderflowGuards = underflowGuardAssertions(node.condition, codeGeneratorState);
if (conditionUnderflowGuards) initialStatements += `\n${conditionUnderflowGuards}`;
initialStatements+= `
assert(!(${codeGenerator(node.condition, codeGeneratorState)}));`;
return initialStatements;
Expand All @@ -374,11 +485,17 @@ codeGeneratorState.wrapperFunctions.set(functionName, wrapperFunction);
${varDec} ${codeGenerator(elt, codeGeneratorState)}_temp = ${codeGenerator(elt, codeGeneratorState)};`;
}
});
const condition = removeTrailingSemicolon(codeGenerator(node.condition, codeGeneratorState));
// Check for underflow in the condition of the if statement
const conditionUnderflowGuards = underflowGuardAssertions(node.condition, codeGeneratorState);
if (conditionUnderflowGuards) initialStatements += `\n${conditionUnderflowGuards}`;
for (let i =0; i<node.trueBody.length; i++) {
// We may have a statement that is not within the If statement but included due to the ordering (e.g. b_1 =b)
if (node.trueBody[i].outsideIf) {
trueStatements += `${codeGenerator(node.trueBody[i], codeGeneratorState)}`;
} else {
// we need the underflow body only in the case that the true body is executed
trueStatements += conditionalUnderflowGuardAssertions(node.trueBody[i], condition, true, codeGeneratorState);
if (node.trueBody[i].expression.nodeType === 'UnaryOperation'){
trueStatements+= `
${codeGenerator(node.trueBody[i].expression.subExpression, codeGeneratorState)} = if (${removeTrailingSemicolon(codeGenerator(node.condition, codeGeneratorState))}) { ${removeTrailingSemicolon(codeGenerator(node.trueBody[i].expression.subExpression, codeGeneratorState))} ${node.trueBody[i].expression.operator[0]} 1 } else { ${removeTrailingSemicolon(codeGenerator(node.trueBody[i].expression.subExpression, codeGeneratorState))} };`
Expand All @@ -392,6 +509,8 @@ codeGeneratorState.wrapperFunctions.set(functionName, wrapperFunction);
if (node.falseBody[j].outsideIf) {
falseStatements += `${codeGenerator(node.falseBody[j], codeGeneratorState)}`;
} else {
// we need the underflow body only in the case that the false body is executed
falseStatements += conditionalUnderflowGuardAssertions(node.falseBody[j], condition, false, codeGeneratorState);
if (node.falseBody[j].expression.nodeType === 'UnaryOperation'){
falseStatements+= `
${codeGenerator(node.falseBody[j].expression.subExpression, codeGeneratorState)} = if (${removeTrailingSemicolon(codeGenerator(node.condition, codeGeneratorState))}) { ${removeTrailingSemicolon(codeGenerator(node.falseBody[j].expression.subExpression, codeGeneratorState))} } else { ${codeGenerator(node.falseBody[j].expression.subExpression, codeGeneratorState)} ${node.falseBody[j].expression.operator[0]} 1 };`;
Expand All @@ -408,14 +527,22 @@ codeGeneratorState.wrapperFunctions.set(functionName, wrapperFunction);

case 'ForStatement':
switch (node.initializationExpression.nodeType) {
case 'ExpressionStatement':
return `for u32 ${codeGenerator(node.condition.leftExpression, codeGeneratorState)} in ${codeGenerator(node.initializationExpression.expression.rightHandSide, codeGeneratorState)}..${node.condition.rightExpression.value} {
case 'ExpressionStatement': {
const initialValue = node.initializationExpression.expression.rightHandSide;
// Check for any underflow guards necessary in the initialization, because they are
// not covered elsewhere
return withUnderflowGuardAssertions(initialValue, `for u32 ${codeGenerator(node.condition.leftExpression, codeGeneratorState)} in ${codeGenerator(initialValue, codeGeneratorState)}..${node.condition.rightExpression.value} {
${keepOneTrailingSemicolon(codeGenerator(node.body, codeGeneratorState))}
}`;
case 'VariableDeclarationStatement':
return `for u32 ${codeGenerator(node.condition.leftExpression, codeGeneratorState)} in ${codeGenerator(node.initializationExpression.initialValue, codeGeneratorState)}..${node.condition.rightExpression.value} {
}`, codeGeneratorState);
}
case 'VariableDeclarationStatement': {
const initialValue = node.initializationExpression.initialValue;
// Check for any underflow guards necessary in the initialization, because they are
// not covered elsewhere
return withUnderflowGuardAssertions(initialValue, `for u32 ${codeGenerator(node.condition.leftExpression, codeGeneratorState)} in ${codeGenerator(initialValue, codeGeneratorState)}..${node.condition.rightExpression.value} {
${keepOneTrailingSemicolon(codeGenerator(node.body, codeGeneratorState))}
}`;
}`, codeGeneratorState);
}
default:
break;
}
Expand All @@ -433,20 +560,41 @@ codeGeneratorState.wrapperFunctions.set(functionName, wrapperFunction);
case 'Assert':
// only happens if we have a single bool identifier which is a struct property
// these get converted to fields so we need to assert == 1 rather than true
if (node.arguments[0].isStruct && node.arguments[0].nodeType === "MemberAccess") return `
assert(${node.arguments.flatMap(codeGenerator)} == 1);`;
return `
assert(${node.arguments.flatMap(codeGenerator)});`;
if (node.arguments[0].isStruct && node.arguments[0].nodeType === "MemberAccess") {
return withUnderflowGuardAssertions(
node,
`
assert(${node.arguments.flatMap(codeGenerator)} == 1);`,
codeGeneratorState,
);
}
return withUnderflowGuardAssertions(
node,
`
assert(${node.arguments.flatMap(codeGenerator)});`,
codeGeneratorState,
);

case 'Boilerplate':
return Circuitbp.generateBoilerplate(node);

case 'BoilerplateStatement': {
let newComValue = '';
if (node.bpType === 'incrementation') newComValue = codeGenerator(node.addend, codeGeneratorState);
if (node.bpType === 'decrementation') newComValue = codeGenerator(node.subtrahend, codeGeneratorState);
let guardNode;
if (node.bpType === 'incrementation') {
guardNode = node.addend;
newComValue = codeGenerator(node.addend, codeGeneratorState);
}
if (node.bpType === 'decrementation') {
guardNode = node.subtrahend;
newComValue = codeGenerator(node.subtrahend, codeGeneratorState);
}
node.newCommitmentValue = newComValue;
return Circuitbp.generateBoilerplate(node);
// Check for underflows in the incrementation/ decrementation,
// e.g. for a += b -c, check b >= c
return guardNode
? withUnderflowGuardAssertions(guardNode, Circuitbp.generateBoilerplate(node), codeGeneratorState)
: Circuitbp.generateBoilerplate(node);
}

// And if we haven't recognized the node, we'll throw an error.
Expand Down
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