AES Encryption and decryption using jks file - aes

Question

I have one small doubt as i am new to AES.
I encrypted a string using one certificate with some password lets say , 'xxx'.
Now i duplicated the certificate by changing the password of it.
When i try to decrypt the encrypted string with the duplicated cert, it says Bad padding exception.Exception in thread "main" javax.crypto.BadPaddingException: Given final block not properly padded
However, when i use the original cert, it decrypts properly.
Could anyone please guide me on it?
public SecretKey retrieveKey(String password, byte[] certFile) throws Exception {
try {
String alias = null;
certPass = password;
char[] pass = certPass.toCharArray();
KeyStore keyStore = KeyStore.getInstance("jceks");
InputStream inputStream = new ByteArrayInputStream(certFile);
keyStore.load(inputStream, pass);
Enumeration enumeration = keyStore.aliases();
while (enumeration.hasMoreElements()) {
alias = (String) enumeration.nextElement();
}
Certificate cert = keyStore.getCertificate(alias);
Key key = cert.getPublicKey();
aesSecretKey = new SecretKeySpec(key.getEncoded(), algorithm);
byte[] encoded = aesSecretKey.getEncoded();
byte[] encryptionKey = Arrays.copyOfRange(encoded, encoded.length - 16, encoded.length);
aesSecretKey = new SecretKeySpec(encryptionKey, algorithm);
} catch (IOException e) {
throw e;
} catch (Exception e) {
throw e;
}
return aesSecretKey;
}

Answer 1

You should use RSA to wrap / unwrap the AES key. The public key is not identical to the AES key, so the following code is certainly incorrect:
Key key = cert.getPublicKey();
aesSecretKey = new SecretKeySpec(key.getEncoded(), algorithm);