Why do I get a BadPaddingException from RSA decryption?

Question

I used RSA algorithm to encrypt and decrypt. When I encrypt a string, it's working properly. When I decrypt, I get an error. Below, I post my code.

public final String modulusString ="..............";
public final String publicExponentString = "AQAB";

/* Encryption */
byte[] modulebytes = Base64.decode(modulusString);
byte[] exponentbytes = Base64.decode(publicExponentString);
BigInteger module = new BigInteger(1,modulebytes);
BigInteger publicexponent = new BigInteger(1,exponentbytes);
RSAPublicKeySpec rsaPubKey = new RSAPublicKeySpec(module, publicexponent);
KeyFactory fact = KeyFactory.getInstance("RSA");
PublicKey pubKey = fact.generatePublic(rsaPubKey);

Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pubKey);

byte[] plainBytes = EncryptionValue.getBytes("UTF-8");
byte[] cipherData = cipher.doFinal( plainBytes );
String encryptedString = Base64.encode(cipherData);

return encryptedString;

/* Decryption */
byte[] modulebytes = Base64.decode(modulusString);
byte[] exponentbytes = Base64.decode(publicExponentString);

BigInteger modulus = new BigInteger(1, modulebytes );
BigInteger exponent = new BigInteger(1, exponentbytes);

RSAPrivateKeySpec rsaPrivKey = new RSAPrivateKeySpec(modulus, exponent);
KeyFactory fact = KeyFactory.getInstance("RSA");
PrivateKey privKey = fact.generatePrivate(rsaPrivKey);

Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.DECRYPT_MODE, privKey);

byte[] base64String = Base64.decode(DecryptionValue);
byte[] plainBytes = new String(base64String).getBytes("UTF-8");
plainBytes = cipher.update(plainBytes);
byte[] values = cipher.doFinal(plainBytes);

return new String(values, "UTF-8");

Exception in thread "main" javax.crypto.BadPaddingException: Decryption error
  at sun.security.rsa.RSAPadding.unpadV15(RSAPadding.java:380)
  at sun.security.rsa.RSAPadding.unpad(RSAPadding.java:291)
  at com.sun.crypto.provider.RSACipher.doFinal(RSACipher.java:363)
  at com.sun.crypto.provider.RSACipher.engineDoFinal(RSACipher.java:389)
  at javax.crypto.Cipher.doFinal(Cipher.java:2121)
  at cryptocodefinal.CryptoCodeFinal.DecryptionValue(CryptoCodeFinal.java:79)
  at cryptocodefinal.CryptoCodeFinal.main(CryptoCodeFinal.java:148)

Answer 1

You appear to be decrypting with the public key. That won't work. You need to need to decrypt with the private exponent that goes with the public exponent used for encryption.

You can't decrypt without the private key. That is the point of asymmetric cryptography.