diff --git a/modules/encrypt-browser/src/encrypt.ts b/modules/encrypt-browser/src/encrypt.ts
index 97d5783b8..33ef913e7 100644
--- a/modules/encrypt-browser/src/encrypt.ts
+++ b/modules/encrypt-browser/src/encrypt.ts
@@ -34,6 +34,7 @@ import {
   ContentType,
   serializeSignatureInfo,
   FRAME_LENGTH,
+  MESSAGE_ID_LENGTH,
   raw2der
 } from '@aws-crypto/serialize'
 import { fromUtf8 } from '@aws-sdk/util-utf8-browser'
@@ -81,9 +82,7 @@ export async function encrypt (
   const { material, context } = await cmm.getEncryptionMaterials(encryptionRequest)
   const { kdfGetSubtleEncrypt, subtleSign, dispose } = await getEncryptHelper(material)
 
-  // Why is this here?
-  const idLength = 16
-  const messageId = await backend.randomValues(idLength)
+  const messageId = await backend.randomValues(MESSAGE_ID_LENGTH)
 
   const { id, ivLength } = material.suite
 
diff --git a/modules/encrypt-node/src/encrypt_stream.ts b/modules/encrypt-node/src/encrypt_stream.ts
index 9e261b269..0d126d896 100644
--- a/modules/encrypt-node/src/encrypt_stream.ts
+++ b/modules/encrypt-node/src/encrypt_stream.ts
@@ -25,7 +25,8 @@ import { randomBytes } from 'crypto'
 import {
   MessageHeader, // eslint-disable-line no-unused-vars
   serializeFactory, kdfInfo, ContentType, SerializationVersion, ObjectType,
-  FRAME_LENGTH
+  FRAME_LENGTH,
+  MESSAGE_ID_LENGTH
 } from '@aws-crypto/serialize'
 
 // @ts-ignore
@@ -92,7 +93,7 @@ export function encryptStream (
 export function getEncryptionInfo (material : NodeEncryptionMaterial, frameLength: number, context: EncryptionContext) {
   const { kdfGetCipher } = getEncryptHelper(material)
 
-  const messageId = randomBytes(16)
+  const messageId = randomBytes(MESSAGE_ID_LENGTH)
   const { id, ivLength } = material.suite
   const messageHeader: MessageHeader = Object.freeze({
     version: SerializationVersion.V1,
diff --git a/modules/serialize/src/identifiers.ts b/modules/serialize/src/identifiers.ts
index 945e54b9e..a92e28318 100644
--- a/modules/serialize/src/identifiers.ts
+++ b/modules/serialize/src/identifiers.ts
@@ -78,3 +78,5 @@ Object.freeze(Maximum)
 
 // Default frame length when using framing
 export const FRAME_LENGTH = 4096
+// Message ID length as defined in specification
+export const MESSAGE_ID_LENGTH = 16