aes: add migration guide for aes API changes #77

Primitive/Symmetric/Cipher/Block/README.md

@@ -1 +1,45 @@
 Symmetric ciphers that work on a block at a time.
+
+## AES Migration Guide
+In [PR #79](https://github.com/GaloisInc/cryptol-specs/pull/79), we simplified the AES modules and in doing so changed the public API used for AES.
+To update to the new module structure, you may need to make changes to your cryptol specs that use AES.
+The new module structure provides concrete instantiations for `AES128`, `AES192`, and `AES256`.
+It also provides a spec `AES_specification` that is generic over the key size, in case you want to implement something that supports multiple key sizes.
+Note that these are only the block cipher, operating over a single block at a time.
+For encryption over longer plaintexts, use a mode of operation.
+
+If you previously used `AES.cry`, you were implicitly using AES256. Update your import line to use `AES256.cry`, and update your encrypt and decrypt functions to change the name and order of arguments.
+Before:
+```haskell
+import Primitive::Symmetric::Cipher::Block::AES
+ct = aesEncrypt(data, key)
+pt = aesDecrypt(ct, key)
+```
+After:
+```haskell
+import Primitive::Symmetric::Cipher::Block::AES256 as AES256
+ct = AES256::encrypt key data
+pt = AES256::decrypt key ct
+```
+
+If you previously used `AES_parameterized.cry`, or if you want to implement something using AES with an arbitrary key length, update your import line to use `AES_specification.cry`, parameterized
+with your own key length parameter. The remaining API is unchanged, although we added a publicly accessible `KeySize` type.
+Before:
+```haskell
+import Primitive::Symmetric::Cipher::Block::AES_parameterized as AES
+
+parameter
+  /** 0: use AES128, 1: use AES192, 2: use AES256 */
+  type Mode : #
+  type constraint (2 >= Mode)
+```
+After:
+```haskell
+parameter
+  // This constraint enforces the standard key sizes of 128, 192, and 256 bits.
+  type KeySize : #
+  type constraint (fin KeySize, KeySize % 64 == 0, KeySize / 64 >= 2, KeySize / 64 <= 4)
+
+import Primitive::Symmetric::Cipher::Block::AES_specification as AES where
+  type KeySize' = KeySize
+```