fix: BYTES_PER_KEY value

modules/serialize/src/identifiers.ts

@@ -62,8 +62,34 @@ Object.freeze(SequenceIdentifier)
 export enum Maximum {
   // Maximum number of messages which are allowed to be encrypted under a single cached data key
   MESSAGES_PER_KEY = 2 ** 32, // eslint-disable-line no-unused-vars
-  // Maximum number of bytes which are allowed to be encrypted under a single cached data key
-  BYTES_PER_KEY = 2 ** 63 - 1, // eslint-disable-line no-unused-vars
+  /* Maximum number of bytes which are allowed to be encrypted
+   * under a cached single data key across messages.
+   * The _real_ maximum is 2 ** 63 - 1,
+   * However Javascript can only perform safe operations on values
+   * up to Number.MAX_SAFE_INTEGER === 9007199254740991 === 2 ** 53 - 1.
+   * e.g
+   * Number.MAX_SAFE_INTEGER + 1 === Number.MAX_SAFE_INTEGER + 2 => true
+   * Number.MAX_SAFE_INTEGER + 1 > Number.MAX_SAFE_INTEGER + 2 => false
+   * Number.MAX_SAFE_INTEGER + 1 < Number.MAX_SAFE_INTEGER + 2 => false
+   *
+   * This means that after 2 ** 53 - 1 the process of accumulating a byte count
+   * will never yield an accurate comparison and so, never halt.
+   *
+   * The choice here to use 2 ** 53 - 1 instead of Number.MAX_SAFE_INTEGER is deliberate.
+   * This is because in the future Number.MAX_SAFE_INTEGER could be raised to 2 ** 66
+   * or some value larger 2 ** 63.
+   * 
+   * This is *not* the maximum amount of data that can be encrypted under a single data key
+   * or under a single AES operation.
+   * The maximum amount of data that can be safely encrypted under a single AES operation is 2 ** 36 -32.
+   * However the AWS Encryption SDK for Javascript does not support non-framed encryption.
+   * Therefore the largest single AES operation supported
+   * by the AWS Encryption SDK for Javascript is the maximum frame size 2 **32 -1.
+   * The maximum amount of data that can be encrypted by the AWS Encryption SDK
+   * is the maximum number of frames at the maximum frame size.
+   * (number of frames) * (frame size) == (2 ** 32 - 1) * (2 ** 32 -1) ~ 2 ** 64 ~ 1.8e19 bytes.
+   */
+  BYTES_PER_KEY = 2 ** 53 - 1, // eslint-disable-line no-unused-vars
   // Maximum number of frames allowed in one message as defined in specification
   FRAME_COUNT = 2 ** 32 - 1, // eslint-disable-line no-unused-vars
   // Maximum bytes allowed in a single frame as defined in specification