chore: Update comment to be correct (#282)

The sequence number starts at 1.
0 is not valid.

Author: seebees

modules/serialize/src/serialize_factory.ts

@@ -43,7 +43,10 @@ export function serializeFactory (fromUtf8: (input: any) => Uint8Array) {
   }
 
   function frameIv (ivLength: IvLength, sequenceNumber: number) {
-    /* Precondition: sequenceNumber must conform to the specification. i.e. 0 - (2^32 - 1) */
+    /* Precondition: sequenceNumber must conform to the specification. i.e. 1 - (2^32 - 1)
+     * The sequence number starts at 1
+     * https://github.com/awslabs/aws-encryption-sdk-specification/blob/master/data-format/message-body.md#sequence-number
+     */
     needs(sequenceNumber > 0 && SequenceIdentifier.SEQUENCE_NUMBER_END >= sequenceNumber, 'sequenceNumber out of bounds')
 
     const buff = new Uint8Array(ivLength)

modules/serialize/test/serialize_factory.test.ts

@@ -30,7 +30,7 @@ describe('serializeFactory:frameIv', () => {
     expect(test).to.deep.equal(fixtures.basicFrameIV())
   })
 
-  it('Precondition: sequenceNumber must conform to the specification. i.e. 0 - (2^32 - 1)', () => {
+  it('Precondition: sequenceNumber must conform to the specification. i.e. 1 - (2^32 - 1)', () => {
     const fromUtf8 = () => { throw new Error('not used') }
     const { frameIv } = serializeFactory(fromUtf8)
     expect(() => frameIv(12, 0)).to.throw()