Cipher decryption issue : javax.crypto.IllegalBlockSizeException: last block incomplete in decryption

Question

I have to encryption and decryption file in android , and the encrypted file (Image ,video ) can decrypted from other device as well.

I encryption and decryption in same device its working fine but when i am switch the device user the encrypted file to decryption its show me error in doFinal()

javax.crypto.IllegalBlockSizeException: last block incomplete in decryption

is any way to encrypt file from one device and access to all other device like password encryption in android.and using that password key to other device we access the file info.

// code

private boolean encrypt() {
        try {
            String path = Environment.getExternalStorageDirectory() + File.separator + "Download/tools.png";
            String pathe = Environment.getExternalStorageDirectory() + File.separator + "Download/tools.png";
            byte[] fileData = FileUtils.readFile(path);
            byte[] encodedBytes = EncryptDecryptUtils.encode(EncryptDecryptUtils.getInstance(this).getSecretKey(), fileData);
            FileUtils.saveFile(encodedBytes, pathe);
            return true;
        } catch (Exception e) {
           Toast.makeText(this, "File Encryption failed.\nException: " + e.getMessage(), Toast.LENGTH_SHORT).show();

        }
        return false;
    }

    /**
     * Decrypt and return the decoded bytes
     *
     * @return
     */
    private byte[] decrypt() {

        try {
            String pathe = Environment.getExternalStorageDirectory() + File.separator + "Download/tools.png";
            byte[] fileData = FileUtils.readFile(pathe);
            byte[] decryptedBytes = EncryptDecryptUtils.decode(EncryptDecryptUtils.getInstance(this).getSecretKey(), fileData);
            return decryptedBytes;
        } catch (Exception e) {
         Toast.makeText(this, "File Decryption failed.\nException: " + e.getMessage(), Toast.LENGTH_SHORT).show();

          }
        return null;
    }

Below is the code of EncryptDecryptUtils class

public static byte[] encode(SecretKey yourKey, byte[] fileData)
            throws Exception {
        byte[] data = yourKey.getEncoded();
        SecretKeySpec skeySpec = new SecretKeySpec(data, 0, data.length, EncoDecode.KEY_SPEC_ALGORITHM);
        Cipher cipher = Cipher.getInstance(EncoDecode.CIPHER_ALGORITHM, EncoDecode.PROVIDER);
        cipher.init(Cipher.ENCRYPT_MODE, skeySpec, new IvParameterSpec(new byte[cipher.getBlockSize()]));
        return cipher.doFinal(fileData);
    }
public static byte[] decode(SecretKey yourKey, byte[] fileData)
        throws Exception {
    byte[] decrypted;
    Cipher cipher = Cipher.getInstance(EncoDecode.CIPHER_ALGORITHM, EncoDecode.PROVIDER);
    cipher.init(Cipher.DECRYPT_MODE, yourKey, new IvParameterSpec(new byte[cipher.getBlockSize()]));
    Log.d("value ", "decode() returned: " + cipher.toString());
    decrypted = cipher.doFinal(fileData);
    Log.d("", "decode() returned: " + decrypted.length);
    return decrypted;
}



public SecretKey getSecretKey() {
        String encodedKey = "8qkWUsFfdY8yy5lIad4rjw==";
        byte[] decodedKey = Base64.decode(encodedKey, Base64.NO_WRAP);
        SecretKey originalKey = new SecretKeySpec(decodedKey, 0, decodedKey.length, EncoDecode.KEY_SPEC_ALGORITHM);
        return originalKey;
    }

try to convert Base64.encodeBase64String(encrypted); byte[] encrypted = cipher.doFinal(value.getBytes()); end decode before decrypt. — kelalaka, Oct 24 '18 at 12:15
i have use this code : String encodedKey = Base64.encodeToString(secretKey.getEncoded(), Base64.NO_WRAP); prefUtils.saveSecretKey(encodedKey); and get value in getSecretKey String encodedKey = prefUtils.getSecretKey(); in place of String encodedKey = "8qkWUsFfdY8yy5lIad4rjw=="; but not wokring — Ravi Jat, Oct 24 '18 at 12:35
[this](https://stackoverflow.com/questions/15554296/simple-java-aes-encrypt-decrypt-example) might help you — kelalaka, Oct 24 '18 at 12:39
Different devices can use different defaults. Don't rely on any system defaults, instead explicitly say what you are using for everything. Generally that will mean using the version of the function with more parameters, so all of the parameters are specified, rather than assumed. For example, do not use `String.getBytes()`, instead use `String.getBytes(UTF-8)`, or whatever, so the character encoding is explicitly specified. — rossum, Oct 24 '18 at 12:53

Ebbe M. Pedersen · Answer 1 · 2018-10-24T13:25:56.213

You are geting an IllegalBlockSizeException on your decrypt, as the data you provide to the decrypt ain't a multiplum of the encryption ciphers block size. An encrypt in block mode would not produce data with illegal block size, so you can with high certainty conclude that your data have been corrupted in some way from when it left the encrypt, to it entered the decrypt.

You can easily verify this by printing the size of the output from return cipher.doFinal(fileData); in your encode(..) methode and the size of fileData in your decode(..) just prior to your decrypted = cipher.doFinal(fileData); They should be the same, and also be a multiplum of the block size of the cipher.

Finding why the data ain't the same, is then just a matter of debugging until you find the culprit.

Cipher decryption issue : javax.crypto.IllegalBlockSizeException: last block incomplete in decryption

1 Answers1