fix(stdlib): zero-pad bufferFromFields when declared length exceeds payload

yarn-project/stdlib/src/abi/buffer.test.ts

@@ -1,3 +1,5 @@
+import { Fr } from '@aztec/foundation/curves/bn254';
+
 import { bufferAsFields, bufferFromFields } from './buffer.js';
 
 describe('buffer', () => {
@@ -11,4 +13,44 @@ describe('buffer', () => {
     const buffer = Buffer.from('1234567890abcdef'.repeat(10), 'hex');
     expect(() => bufferAsFields(buffer, 3)).toThrow(/exceeds maximum size/);
   });
+
+  it('pads with zeros when declared length exceeds payload', () => {
+    // Create a small buffer, encode it, then truncate the field array before decoding.
+    const buffer = Buffer.from('aabbccdd', 'hex'); // 4 bytes
+    const fields = bufferAsFields(buffer, 10);
+    // Declared length is 4 bytes, stored in fields[0]. Payload fields follow.
+    // Artificially inflate the declared length to 62 bytes (2 full fields).
+
+    const inflatedFields = [new Fr(62), ...fields.slice(1)];
+    const result = bufferFromFields(inflatedFields);
+    // Result should be exactly 62 bytes: original 4 bytes followed by 58 zero bytes.
+    expect(result.length).toBe(62);
+    expect(result.subarray(0, 4).toString('hex')).toEqual('aabbccdd');
+    expect(result.subarray(4).every(b => b === 0)).toBe(true);
+  });
+
+  it('pads with zeros when payload fields are truncated', () => {
+    // Simulate the blob reconstruction scenario: declared length says 93 bytes (3 fields),
+    // but only 1 payload field is present.
+
+    const payloadField = Fr.fromBuffer(
+      Buffer.from('00' + 'ab'.repeat(31), 'hex'), // 31 bytes of 0xab
+    );
+    // Declared length = 93 bytes (would need 3 fields), but only 1 field in payload.
+    const fields = [new Fr(93), payloadField];
+    const result = bufferFromFields(fields);
+    expect(result.length).toBe(93);
+    // First 31 bytes come from the single payload field.
+    expect(result.subarray(0, 31).every(b => b === 0xab)).toBe(true);
+    // Remaining 62 bytes are zero-padded.
+    expect(result.subarray(31).every(b => b === 0)).toBe(true);
+  });
+
+  it('returns exact buffer when payload matches declared length', () => {
+    const buffer = Buffer.from('ff'.repeat(31), 'hex'); // exactly 1 field of payload
+    const fields = bufferAsFields(buffer, 5);
+    const result = bufferFromFields(fields);
+    expect(result.length).toBe(31);
+    expect(result.toString('hex')).toEqual(buffer.toString('hex'));
+  });
 });

yarn-project/stdlib/src/abi/buffer.ts

@@ -26,11 +26,32 @@ export function bufferAsFields(input: Buffer, targetLength: number): Fr[] {
 }
 
 /**
- * Recovers a buffer from an array of fields.
- * @param fields - An output from bufferAsFields.
- * @returns The recovered buffer.
+ * Recovers a buffer from an array of fields previously encoded with bufferAsFields.
+ *
+ * The first field encodes the byte length of the original buffer. The remaining fields
+ * each carry 31 bytes of payload (the leading byte of each 32-byte field element is skipped).
+ *
+ * If the declared byte length exceeds the bytes available from the payload fields, the result
+ * is zero-padded to the full declared length. This is important for correctness when the field
+ * array has been truncated (e.g. contract class logs reconstructed from blobs using a short
+ * emittedLength): without padding, the resulting buffer would be shorter than declared, causing
+ * bytecode commitment computations to diverge from what the circuit produced.
+ *
+ * @param fields - An output from bufferAsFields: [byteLength, ...payloadFields].
+ * @returns A buffer of exactly `byteLength` bytes.
  */
 export function bufferFromFields(fields: Fr[]): Buffer {
   const [length, ...payload] = fields;
-  return Buffer.concat(payload.map(f => f.toBuffer().subarray(1))).subarray(0, length.toNumber());
+  const byteLength = length.toNumber();
+  const raw = Buffer.concat(payload.map(f => f.toBuffer().subarray(1)));
+  if (raw.length >= byteLength) {
+    return raw.subarray(0, byteLength);
+  }
+  // Pad with zeros if the declared length exceeds the available payload bytes.
+  // This ensures the returned buffer always matches the declared length, so that
+  // downstream bytecode commitment computations are consistent even when the
+  // source field array was truncated (e.g. reconstructed from blob with a short emittedLength).
+  const result = Buffer.alloc(byteLength);
+  raw.copy(result);
+  return result;
 }