AEAD decryption process using AWS KMS & AES/GCM/NoPadding cipher

ae.clj

;; CAVEAT: Ensure your JDK/JRE is configured with Java Cryptography
;; Extension (JCE) Unlimited Strength Jurisdiction Policy Files.
;; Visit http://www.oracle.com/technetwork/java/javase/downloads/index.html
;; and look for "JCE".

;; Given a ciphertext (produced by AES/GCM/NoPadding cipher), a nonce,
;; and a KMS-encrypted content encrypting key (cek), here is the
;; process for producing the plaintext:

(defn b64dec [x] (javax.xml.bind.DatatypeConverter/parseBase64Binary x))

(defn b64enc [x] (javax.xml.bind.DatatypeConverter/printBase64Binary x))

(def kms (com.amazonaws.services.kms.AWSKMSClient.))

(def gcm-param (javax.crypto.spec.GCMParameterSpec. 128 (b64dec "nonceNONCEnonceN")))

(def cek-ciphertext-bytes (b64dec "... elided base64 chars of encrypted CEK ..."))

(def cek-bytes
  (.array
   (.getPlaintext
    (.decrypt kms (doto (com.amazonaws.services.kms.model.DecryptRequest.)
                    (.setCiphertextBlob (java.nio.ByteBuffer/wrap cek-ciphertext-bytes)))))))

(def cek (javax.crypto.spec.SecretKeySpec. cek-bytes "AES"))

(def cipher (javax.crypto.Cipher/getInstance "AES/GCM/NoPadding"))

(.init cipher javax.crypto.Cipher/DECRYPT_MODE cek gcm-param)

;; successful decryption:
(def ciphertext (b64dec "p2D6qwsnyqdUKYB7db8Gzj6WmxxRYrt11B81/06JcO8rG/AK/Tk5SLtkLNh/C6DL5XVP0LgQNOBGV52KJmbzuEzzLu4B+Bdmjs13vLxq8LnNHIH9NCcnxp4DkE+iNVv3N+qEoALmAxXxIi1W73QG4UeHJwWrdvJpm5bu/w/t3fpvoOdKYNCgiZZOEH9XPkNtSnWsPdJZoL6FSHU0qKNQyC/fvTk/o4+lsPGL+eMwLgPMDahIRoZAz5eoQU2gkWLNC+arY76sFjim"))
(.doFinal cipher ciphertext)

;; corrupted ciphertext by changing one character in the base64 repr:
(def ciphertext (b64dec "p2D6qwsnyqdUKYB7db8Gzj6WmxxRYrt11B81/06JcO8rG/AK/Tk5SLtkLNh/C6DL5XVP0LgQNOBGV52KJmbzuEzzLu4B+Bdmjs13vLxq8LnNHIH9NCcnxp4DkE+iNVv3N+qEoALmAxXxIi1W73QG4UeHJwWrdvJpm5bu/w/t3fpvoOdKYNCgiZZOEH9XPkNtSnWsPdJZoL6FSHU0qKNQyC/fvTk/o4+lsPGL+eMwLgPMDahIRoZAz5eoQU2gkWLNC+arY76sFjiM"))
(.doFinal cipher ciphertext)
> AEADBadTagException Tag mismatch!  com.sun.crypto.provider.GaloisCounterMode.decryptFinal (GaloisCounterMode.java:524)