fix(avm): non-negative uint64 deserialization (#16008)

https://aztecprotocol.slack.com/archives/C04DL2L1UP2/p1753436140955629

Author: fcarreiro

yarn-project/simulator/src/public/avm/avm_memory_types.ts

@@ -81,6 +81,7 @@ function UnsignedIntegerClassFactory(bits: number) {
     public constructor(n: bigint | number) {
       super();
       this.n = BigInt(n);
+      assert(n >= 0n, `${this.constructor.name} cannot handle negative values: ${n}.`);
       assert(n < NewUintClass.mod, `Value ${n} is too large for ${this.constructor.name}.`);
     }
 

yarn-project/simulator/src/public/avm/opcodes/memory.ts

@@ -63,8 +63,8 @@ export class Set extends Instruction {
     private value: bigint | number,
   ) {
     super();
-
-    assert(this.value < Fr.MODULUS, 'Value is larger than Fr.MODULUS');
+    assert(this.value >= 0, `Value ${this.value} is negative`);
+    assert(this.value < Fr.MODULUS, `Value ${this.value} is larger than Fr.MODULUS`);
   }
 
   public async execute(context: AvmContext): Promise<void> {

yarn-project/simulator/src/public/avm/serialization/instruction_serialization.ts

@@ -118,19 +118,38 @@ const OPERAND_SPEC = new Map<OperandType, [number, (offset: number) => OperandNa
   [OperandType.UINT8, [1, Buffer.prototype.readUint8, Buffer.prototype.writeUint8]],
   [OperandType.UINT16, [2, Buffer.prototype.readUint16BE, Buffer.prototype.writeUint16BE]],
   [OperandType.UINT32, [4, Buffer.prototype.readUint32BE, Buffer.prototype.writeUint32BE]],
-  [OperandType.UINT64, [8, Buffer.prototype.readBigInt64BE, Buffer.prototype.writeBigInt64BE]],
-  [OperandType.UINT128, [16, readBigInt128BE, writeBigInt128BE]],
-  [OperandType.FF, [32, readBigInt254BE, writeBigInt254BE]],
+  [OperandType.UINT64, [8, readUint64BE, writeUint64BE]],
+  [OperandType.UINT128, [16, readUint128BE, writeUint128BE]],
+  [OperandType.FF, [32, readUint254BE, writeUint254BE]],
   [OperandType.TAG, [1, Buffer.prototype.readUint8, Buffer.prototype.writeUint8]],
 ]);
 
-function readBigInt254BE(this: Buffer, offset: number): bigint {
-  const totalBytes = 32;
+function readUintBE(buf: Buffer, offset: number, totalBytes: number): bigint {
   let value: bigint = 0n;
   for (let i = 0; i < totalBytes; ++i) {
     value <<= 8n;
-    value |= BigInt(this.readUint8(i + offset));
+    value |= BigInt(buf.readUint8(i + offset));
   }
+  return value;
+}
+
+function writeUintBE(buf: Buffer, value: bigint, totalBytes: number): void {
+  for (let offset = totalBytes - 1; offset >= 0; --offset) {
+    buf.writeUint8(Number(value & 0xffn), offset);
+    value >>= 8n;
+  }
+}
+
+function readUint64BE(this: Buffer, offset: number): bigint {
+  return readUintBE(this, offset, 8);
+}
+
+function writeUint64BE(this: Buffer, value: bigint): void {
+  writeUintBE(this, value, 8);
+}
+
+function readUint254BE(this: Buffer, offset: number): bigint {
+  let value = readUintBE(this, offset, 32);
 
   // In circuit, we only support values up to Fr.MODULUS and any deserialized value
   // would naturally undergo a modulus reduction.
@@ -141,30 +160,16 @@ function readBigInt254BE(this: Buffer, offset: number): bigint {
   return value;
 }
 
-function writeBigInt254BE(this: Buffer, value: bigint): void {
-  const totalBytes = 32;
-  for (let offset = totalBytes - 1; offset >= 0; --offset) {
-    this.writeUint8(Number(value & 0xffn), offset);
-    value >>= 8n;
-  }
+function writeUint254BE(this: Buffer, value: bigint): void {
+  writeUintBE(this, value, 32);
 }
 
-function readBigInt128BE(this: Buffer, offset: number): bigint {
-  const totalBytes = 16;
-  let ret: bigint = 0n;
-  for (let i = 0; i < totalBytes; ++i) {
-    ret <<= 8n;
-    ret |= BigInt(this.readUint8(i + offset));
-  }
-  return ret;
+function readUint128BE(this: Buffer, offset: number): bigint {
+  return readUintBE(this, offset, 16);
 }
 
-function writeBigInt128BE(this: Buffer, value: bigint): void {
-  const totalBytes = 16;
-  for (let offset = totalBytes - 1; offset >= 0; --offset) {
-    this.writeUint8(Number(value & 0xffn), offset);
-    value >>= 8n;
-  }
+function writeUint128BE(this: Buffer, value: bigint): void {
+  writeUintBE(this, value, 16);
 }
 
 /**