docs: update spec

SPEC.md

@@ -7,7 +7,7 @@ _eddy_ uses **ChaCha20** stream cipher together with **BLAKE2b** hash function i
 
 After initializing a ChaCha20 instance with the key from the previous step and a **random 12-byte nonce**, a **BLAKE2b key** is generated by XORing a 64-byte zero-block using this instance - in Go, such a block is created with `make([]byte, 64)`.
 
-The file MAC tag is calculated from the ciphertext, not plaintext.
+The file MAC tag is calculated from the ciphertext, not plaintext. This is called "Encrypt-then-MAC" (EtM).
 
 ### File format
 An encrypted file has the following structure. No delimiters or flags. This provides plausible deniability - the file is generally indistinguishable from, say, a compressed file or a randomly generated one.