AES encryption (in Java) of different JSON strings always produce same encrypted string as result. Why?

I have a program written in Java which takes JSON string as argument, encrypts it using AES then encodes it using Base64. JSON string is like:

{"a": "b"} or {"a": "n"} or {"a": "k"}  

I.e related object would have one property a. Value part is randomly generated.

Program outputs for above JSON inputs looks like

UBNvKoRoGqk0PTQQL5K4Sw== bKwlToSND3HkceDExEDXSw== u/yKJq1FdoifBM+AnadC3A== 

i.e. they are unique.

Same goes for {"a":"gn"} — random string with length 2. Same for 3 and so on.

But starting from 7 program produces the same encoded string for different inputs. I mean following JSON strings taken as input:

{"a": "pzfovvs"} {"a": "bqwuvck"} 

produces same string as output:


Same goes for length 8 and 9. Starting from 10 results became unique again.

What is the explanation of this strange phenomenon?

(I can post code if needed.)

Ok, here is the code:

import; import; import java.util.Base64; import javax.crypto.Cipher; import javax.crypto.KeyGenerator;  public class JWTEncryptor {  private static String algorithm = "AES"; private static Key key; private static KeyGenerator keyGenerator; private static Cipher cipher;  public static String encrypt(String jwt) throws Exception {     if (key == null || cipher == null) {         setUp();     }     cipher.init(Cipher.ENCRYPT_MODE, key);     return Base64.getEncoder().encodeToString(cipher.doFinal(jwt.getBytes("UTF-8"))); }  private static void setUp() {     try {         cipher = Cipher.getInstance(algorithm);     } catch (Exception e1) {         e1.printStackTrace();     }     if (keyGenerator != null) {         key = keyGenerator.generateKey();         return;     }     try {         keyGenerator = KeyGenerator.getInstance(algorithm);         key = keyGenerator.generateKey();     } catch (NoSuchAlgorithmException e) {         e.printStackTrace();     } }  public static String decrypt(String encryptedJWT) throws Exception {     cipher.init(Cipher.DECRYPT_MODE, key);     return new     String(cipher.doFinal(Base64.getDecoder().decode(encryptedJWT))); }   }