That is not how encryption works. Every time you encrypt the same plaintext, you MUST get a different output. With GCM this is achieved by using a unique random IV each time – a nonce (number used once).

GCM has its own requirements on IV, namely that an IV shall never be reused, and that is important.

String airSecretKey = "Wk+Uzyyn8991w/2V5OIqiQ==";  // this key is now public so you cannot use it
SecretKey key = new SecretKeySpec(Base64.getDecoder().decode(airSecretKey), "AES");

SecureRandom secureRandom = new SecureRandom();
byte[] iv = new byte[12];
AlgorithmParameters params = new GCMParameterSpec(128, iv);

Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
cipher.init(Cipher.ENCRYPT_MODE, key, params, secureRandom);

cipher.updateAAD(...);  // if used
byte[] encrypted = cipher.doFinal(plaintext);

The Swift version should perform the same operations. Java generates the tag and appends it to the ciphertext. It looks like in Swift you have to handle it manually instead.

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