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How to handle BadPaddingException During AES256 encryption in C# and decryption in Java

I don't know why an error is coming up.
Exception in thread "main" javax.crypto.BadPaddingException: Given final block not properly padded. Such issues can arise if a bad key is used during decryption.
I understand that this error occurs when the incorrect key is used during the decryption. However, if you look at the test results result below, you can see that both C# and Java are the same (Key, IV, Salt is Base64 encoded).
C# Test Result
Java Test Result
It's the same!(Key, IV, Salt)
But the current BadpaddingException error is generated. What could be the problem?
I am attaching my source file.
C# (Encryption)
class AES {
private readonly static string keyStr = "This is Key";
private readonly static string vector = "This is Vector";
public static Rfc2898DeriveBytes MakeKey(string password){
byte[] keyBytes = System.Text.Encoding.UTF8.GetBytes(password);
byte[] saltBytes = SHA512.Create().ComputeHash(keyBytes);
Rfc2898DeriveBytes result = new Rfc2898DeriveBytes(keyBytes, saltBytes, 65536);
return result;
}
public static Rfc2898DeriveBytes MakeVector(string vector){
byte[] vectorBytes = System.Text.Encoding.UTF8.GetBytes(vector);
byte[] saltBytes = SHA512.Create().ComputeHash(vectorBytes);
Rfc2898DeriveBytes result = new Rfc2898DeriveBytes(vectorBytes, saltBytes, 65536);
return result;
}
public static void Encrypt(String inputFile, String outputFile) {
using (RijndaelManaged aes = new RijndaelManaged()){
//Create Key and Vector
Rfc2898DeriveBytes key = AES.MakeKey(AES.keyStr);
Rfc2898DeriveBytes vector = AES.MakeVector(AES.vector);
//AES256
aes.BlockSize = 128;
aes.KeySize = 256;
// It is equal in java
// Cipher _Cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING");
aes.Mode = CipherMode.CBC;
aes.Padding = PaddingMode.PKCS7;
aes.Key = key.GetBytes(32); //256bit key
aes.IV = vector.GetBytes(16); //128bit block size
//processing Encrypt
ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);
byte[] encrypted;
using (MemoryStream msEncrypt = new MemoryStream()) {
using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write)) {
byte[] inputBytes = File.ReadAllBytes(inputFile);
csEncrypt.Write(inputBytes, 0, inputBytes.Length);
}
encrypted = msEncrypt.ToArray();
}
string encodedString = Convert.ToBase64String(encrypted);
File.WriteAllText(outputFile, encodedString);
}
}
}
Java (Decryption)
public class AES256File {
private static final String algorithm = "AES";
private static final String blockNPadding = algorithm+"/CBC/PKCS5Padding";
private static final String password = "This is Key";
private static final String IV = "This is Vector";
private static IvParameterSpec ivSpec;
private static Key keySpec;
public static void MakeKey(String password) throws NoSuchAlgorithmException, UnsupportedEncodingException, InvalidKeySpecException{
SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
MessageDigest digest = MessageDigest.getInstance("SHA-512");
byte[] keyBytes = password.getBytes("UTF-8");
// C# : byte[] saltBytes = SHA512.Create().ComputeHash(keyBytes);
byte[] saltBytes = digest.digest(keyBytes);
//256bit
PBEKeySpec pbeKeySpec = new PBEKeySpec(password.toCharArray(), saltBytes, 65536, 256);
Key secretKey = factory.generateSecret(pbeKeySpec);
byte[] key = new byte[32];
System.arraycopy(secretKey.getEncoded(), 0, key, 0, 32);
SecretKeySpec secret = new SecretKeySpec(key, "AES");
setKeySpec(secret);
}
public static void MakeVector(String IV) throws NoSuchAlgorithmException, UnsupportedEncodingException, InvalidKeySpecException{
SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
MessageDigest digest = MessageDigest.getInstance("SHA-512");
byte[] vectorBytes = IV.getBytes("UTF-8");
byte[] saltBytes = digest.digest(vectorBytes);
// 128bit
PBEKeySpec pbeKeySpec = new PBEKeySpec(IV.toCharArray(), saltBytes, 65536, 128);
Key secretIV = factory.generateSecret(pbeKeySpec);
byte[] iv = new byte[16];
System.arraycopy(secretIV.getEncoded(), 0, iv, 0, 16);
IvParameterSpec ivSpec = new IvParameterSpec(iv);
setIvSpec(ivSpec);
}
public void decrypt(File source, File dest) throws Exception {
Cipher c = Cipher.getInstance(blockNPadding);
c.init(Cipher.DECRYPT_MODE, keySpec, ivSpec);
fileProcessing(source, dest, c);
}
public void fileProcessing(File source, File dest, Cipher c) throws Exception{
InputStream input = null;
OutputStream output = null;
try{
input = new BufferedInputStream(new FileInputStream(source));
output = new BufferedOutputStream(new FileOutputStream(dest));
byte[] buffer = new byte[input.available()];
int read = -1;
while((read = input.read(buffer)) != -1){
output.write(c.update(buffer, 0, read));
}
byte[] deryptedBytes = c.doFinal(buffer); // -----------------------> Error!! Showing!
byte[] decodedBytes = Base64.getDecoder().decode(deryptedBytes);
String decodeString = new String(decodedBytes, "UTF-8");
decodedBytes = decodeString.getBytes(StandardCharsets.UTF_8);
output.write(decodedBytes);
}finally{
if(output != null){
try{output.close();}catch(IOException e){}
}
if(input != null){
try{input.close();}catch(IOException e){}
}
}
}
I have verified as below.
Verification Key and IV in C#
//Key Verification
var salt = Convert.ToBase64String(saltBytes);
Console.Write("Salt Result : ");
Console.WriteLine(salt);
var result_test = Convert.ToBase64String(result.GetBytes(32));
Console.Write("Key Test Result: ");
Console.WriteLine(result_test);
//IV Verification (Salt is Using same code)
var result_test = Convert.ToBase64String(result.GetBytes(16));
Console.Write("IV Test Result: ");
Console.WriteLine(result_test);
Verification Key and IV in Java
//Key Verification
/* print Salt */
String base64 = Base64.getEncoder().encodeToString(saltBytes);
System.out.println("Salt Result : " + base64);
/* print Key */
String result_test = Base64.getEncoder().encodeToString(key);
System.out.println("Key Test Result : " + result_test);
/* print generated Key */
System.out.println("Secret Key Result : " + Base64.getEncoder().encodeToString(secret.getEncoded()));
//IV Verification (Salt is Using same code)
/* print IV */
String result_test = Base64.getEncoder().encodeToString(iv);
System.out.println("IV Test Result : " + result_test);
/* print generated IV */
System.out.println("IV Result : " + Base64.getEncoder().encodeToString(ivSpec.getIV()));
Updated
c# .netframework 4.5 / Java8 modified what #Topaco said and confirmed that it worked well.
I want to say thank you very much to #Topaco and #Gusto2, and I'm going to make changes to the parts that have been modified in security, just as #Gusto2 said!

1) In the C# Encrypt-method the plain text is encrypted first and then Base64-encoded. Thus, in the decryption process the data must be Base64-decoded first and then decrypted. Currently this is handled in the wrong order i.e. the data are decrypted first and then decoded. Therefore, in the Java fileProcessing-method replace
while((read = input.read(buffer)) != -1){
output.write(c.update(buffer, 0, read));
}
with
while((read = input.read(buffer)) != -1) {
byte[] bufferEncoded = buffer;
if (read != buffer.length) {
bufferEncoded = Arrays.copyOf(buffer, read);
}
byte[] bufferDecoded = Base64.getDecoder().decode(bufferEncoded);
output.write(c.update(bufferDecoded));
}
2) It's not necessary to pass buffer (or bufferDecoded) to the doFinal-method, since that was already done in the update-method. Thus,
byte[] deryptedBytes = c.doFinal(buffer);
must be replaced with
output.write(c.doFinal());
3) Since the Base64-decoding is already done in 1) in the try-block all lines following the doFinal-statement have to be removed. Overall, this results in
try {
input = new BufferedInputStream(new FileInputStream(source));
output = new BufferedOutputStream(new FileOutputStream(dest));
byte[] buffer = new byte[input.available()];
int read = -1;
while((read = input.read(buffer)) != -1) {
byte[] bufferEncoded = buffer;
if (read != buffer.length) {
bufferEncoded = Arrays.copyOf(buffer, read);
}
byte[] bufferDecoded = Base64.getDecoder().decode(bufferEncoded);
output.write(c.update(bufferDecoded));
}
output.write(c.doFinal());
}
4) The size of the buffer has to be a multiple of 4 in order to ensure a proper Base64-decoding. Thus, it's more reliable to replace
byte[] buffer = new byte[input.available()];
with
byte[] buffer = new byte[4 * (input.available() / 4)];
As long as the data are read in one chunk (which is not guaranteed, see e.g. https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/InputStream.html#available()) there is no problem. However, if the data are read in several chunks it's important to read a multiple of 4 bytes, otherwise the Base64-decoding will fail. That can be easily proved by using a buffer size which isn't a multiple of 4. This point must also be considered if the buffer size is explicitly defined with regard to larger files.

while((read = input.read(buffer)) != -1){
output.write(c.update(buffer, 0, read));
}
byte[] deryptedBytes = c.doFinal(buffer)
you are decrypting the input to a file, then you are using the same cipher instance to decrypt the the last read chunk (again) into a separate array not to the file
quick fix:
while((read = input.read(buffer)) != -1){
output.write(c.update(buffer, 0, read));
}
output.write(c.doFinal()); // write the padded block
if you want to create and print a decrypted String, you need to create a new Cipher instance (or maybe it will be enough to reinitialize the instance, I am not sure) assuming the buffer contains the whole input
c.init(Cipher.DECRYPT_MODE, keySpec, ivSpec);
// assuming the buffer contains the whole input again
byte[] deryptedBytes = c.doFinal(buffer); // decrypting the whole file again
correct approach:
IV is used to securely reuse the same encryption key for multiple encryptions. So if your key is not random, you should generate new random IV for each encryption (and pass the IV along the ciphertext, most often prepended). Otherwise the encryption is not semantically secure and you may create opening for the two pad attack. So deriving IV from the key may not be very secure.
I advice to use any MAC (authentication code) passed along the ciphertext to ensure integrity (e.g. HMAC)
you are still reading all the file input fully into memory, what would not work for REALLY LARGE files. You may initialize the buffer to an arbitrary length (a few MB?) and process the input file as chunked

File Encryption And Decryption In Java, doesn't work decryption

Hi guys I have to do this and I can encrypt file according to the des algorithm but I can not decyrpt again file ,I recieve error messaje like that :
javax.crypto.BadPaddingException Given final block not properly padded
I can not decrypt file I couldnt find why. Can u help me please
Thank you guys.
JAVA CODE :
public class Sifreleme {
public static void encrypt(){
try {
SecretKey key = KeyGenerator.getInstance("DES").generateKey();
FileOutputStream fosKey = new FileOutputStream("..\\KEY");
SecretKeyFactory keyfac = SecretKeyFactory.getInstance("DES");
DESKeySpec keyspec = (DESKeySpec) keyfac.getKeySpec(key, DESKeySpec.class);
fosKey.write(keyspec.getKey());
fosKey.close();
Cipher crypt = Cipher.getInstance("DES");
crypt.init(Cipher.ENCRYPT_MODE, key);
FileInputStream fis = new FileInputStream("C:\\Users\\akif\\Desktop\\zilsesi.mp3");
FileOutputStream fos = new FileOutputStream("C:\\Users\\akif\\Desktop\\sifrelenenzilsesi.mp3");
byte[] arrayBytes = new byte[8];
int bytesReads;
while ((bytesReads = fis.read(arrayBytes)) != -1) {
fos.write(crypt.doFinal(arrayBytes), 0, bytesReads);
}
fis.close();
fos.close();
} catch (Exception e) {
e.printStackTrace();
}
}
public static void decrypt(){
try {
FileInputStream fisKey = new FileInputStream("..\\KEY");
byte[] arrayKey = new byte[fisKey.available()];
fisKey.read(arrayKey);
SecretKey key = new SecretKeySpec(arrayKey, "DES");
Cipher decrypt = Cipher.getInstance("DES");
decrypt.init(Cipher.DECRYPT_MODE, key);
FileInputStream fis = new FileInputStream("C:\\Users\\akif\\Desktop\\sifrelenenzilsesi.mp3");
byte[] encText = new byte[16];
int bytesReads;
while ((bytesReads = fis.read(encText)) != -1) {
fis.read(decrypt.doFinal(encText), 0, bytesReads);
}
fis.close();
System.out.println(new String(encText));
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String []args) throws IOException{
encrypt();
decrypt();
}

Your code here:
while ((bytesReads = fis.read(encText)) != -1) {
fis.read(decrypt.doFinal(encText), 0, bytesReads);
}
Is rather obviously wrong: you need to write the plaintext generated by calling decrypt.doFinal just like you do during encryption. Currently you are overwriting the generated plaintext by the next block(s) of ciphertext because you call read twice in the loop.
Furthermore, depending on your DES Cipher implementation, you forgot about the IV.
A lot of other things are wrong as well, including:
the stream handling using getAvailable();
the use of the 56 bit DES cipher;
the use of ECB mode;
the repeated calls to doFinal (which results in a very large overhead and insecure code);
not using the CipherInputStream and CipherOutputStream (etcetera);
using a string as the key;
forgetting to close your streams when an exception occurs (use the try with resources);
the printStackTracke() exception handling;
the use of static fields as variables.
Using the platform encoding within new String(encText) is only likely wrong.
Note that using the wrong key / ciphertext combination will likely also result in this error.

Decrypting of AES/CBC/PKCS5Padding Error: Given final block not properly padded

I'm getting Given final block not properly padded error while decrypting AES/CBC/PKCS5Padding cipher on large encrypted file.
I think this issue is caused by adding wrong initialization vector in cipher.init() method.
I can't read whole file at runtime, so i need to encrypt fixed-size blocks. At this point I'm creating IV and storing it to .txt file. But in decrypting method I'm using the same IV every decryption cycle. How should I change this?
Encryption:
void encrypt() throws Exception{
char[] password = passwordText.getText().toCharArray();
byte[] salt = new byte[8];
/* Creating and saving salt */
salt = saveSalt(salt);
/* Securing password */
SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
KeySpec spec = new PBEKeySpec(password, salt, 65536, 128);
SecretKey tmp = factory.generateSecret(spec);
SecretKey secret = new SecretKeySpec(tmp.getEncoded(), "AES");
if (choosedFile != null) {
/* Choosing algorithm for decryption */
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
/* Getting plain file */
CipherInputStream fis = new CipherInputStream(new FileInputStream(choosedFile), cipher);
CipherOutputStream fos = new CipherOutputStream(new FileOutputStream(choosedFile+".encrypted"), cipher);
/* Encrypting and Measuring */
long startTime = System.currentTimeMillis();
cipher.init(Cipher.ENCRYPT_MODE, secret);
byte[] rawText = new byte[128];
int count;
while((count = fis.read(rawText)) > 0) {
System.out.println(count);
byte[] encryptedText = cipher.doFinal(rawText);
fos.write(encryptedText, 0, count);
}
long stopTime = System.currentTimeMillis();
fis.close();
fos.close();
/* Creating initialization vector and storing*/
byte[] iVector = cipher.getIV();
saveIVector(iVector);
text.setText(text.getText() + "File was encrypted in " + (stopTime - startTime) + "ms.\n");
}
}
Decryption:
void decrypt() throws Exception {
/* Getting salt */
byte[] salt = getSalt();
/* Getting initialization vector */
byte[] iVector = getIVector();
/* Getting user password */
char[] password = passwordText.getText().toCharArray();
/* Securing password */
SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
KeySpec spec = new PBEKeySpec(password, salt, 65536, 128);
SecretKey tmp = factory.generateSecret(spec);
SecretKey secret = new SecretKeySpec(tmp.getEncoded(), "AES");
if (choosedFile != null) {
/* Choosing algorithm for decryption */
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
/* Getting ciphered file */
CipherInputStream fis = new CipherInputStream(new FileInputStream(choosedFile), cipher);
CipherOutputStream fos = new CipherOutputStream(new FileOutputStream(choosedFile+".decrypted"), cipher);
/* Decrypting and Measuring */
long startTime = System.currentTimeMillis();
cipher.init(Cipher.DECRYPT_MODE, secret, new IvParameterSpec(iVector));
byte[] rawText = new byte[128];
int count;
while((count = fis.read(rawText)) > 0) {
byte[] encryptedText = cipher.doFinal(rawText);
fos.write(encryptedText, 0, count);
}
long stopTime = System.currentTimeMillis();
fis.close();
fos.close();

When using CipherInputStream and CipherOutputStream, the streams handle all the calls to the cipher (that's why you pass the cipher to it on initialization). You just need to initialize it correctly, and stream the data through the stream, and the cipher stream will do the needed calls to update() and doFinal(). Remember to close the steams to trigger the doFinal().
Currently your code passes the data through the cipher several times in an uncontrolled way, and the data is messed up.
Also, you only need a CipherInputStream for decrypt, and a CipherOutputStream for encrypt. In your current code you use both for both encrypt and decrypt.
Encrypt could be something like this (this don't handle the iv ..):
...
cipher.init(Cipher.ENCRYPT_MODE, secret);
InputStream is = new FileInputStream(choosedFile);
OutputStream os = new CipherOutputStream(new FileOutputStream(choosedFile+".encrypted"), cipher);
byte[] buffer = new byte[1024];
int len;
while ((len = is.read(buffer)) != -1) {
os.write(buffer, 0, len);
}
is.close();
os.close();
...

Worried about "until I had to cut large file into fixed-size blocks".
Using "chunk" in place of "block" above because "block"has a specific meaning in block ciphers such as AES.
What are toy doing with the chunks, concatenating them?
With CBC mode, after the first block the the previous encrypted block value is effectively used as the IV for the next block. So when splitting and then concatenating the chunks the value of the last block of the previous chunk is the IV for the next chunk.
See CBC mode.
Or are you doing something completely different?

incomplete RSA decryption with large XML file type - JAVA

I have a problem when decrypting XML type my file my returns incomplete data algorithm and rare symbols.
public File decryptFile(File fileInput, X509Certificate certificate) throws BadPaddingException, Exception {
try (DataInputStream dis = new DataInputStream(new FileInputStream(fileInput))) {
byte[] encryptedKeyBytes = new byte[dis.readInt()];
dis.readFully(encryptedKeyBytes);
PublicKey publicKey = certificate.getPublicKey();
rsaCipher.init(Cipher.DECRYPT_MODE, publicKey);
byte[] rijndaelKeyBytes = rsaCipher.doFinal(encryptedKeyBytes);
SecretKey rijndaelKey = new SecretKeySpec(rijndaelKeyBytes, "Rijndael");
byte[] iv = new byte[16];
dis.read(iv);
IvParameterSpec spec = new IvParameterSpec(iv);
Cipher cipher = Cipher.getInstance("Rijndael/CBC/PKCS5Padding");
cipher.init(Cipher.DECRYPT_MODE, rijndaelKey, spec);
try (CipherInputStream cis = new CipherInputStream(dis, cipher)) {
try (FileOutputStream fos = new FileOutputStream(fileInput.getAbsolutePath() + ".xml")) {
byte[] data = new byte[16];
int theByte;
while ((theByte = cis.read(data)) != -1) {
System.out.print(new String(data));
fos.write(data, 0, theByte);
}
System.out.println("\n\n");
}
}
}
return new File(fileInput.getAbsolutePath() + ".xml");
}
this code returns me the data
</ctaAbonBenef><distPago>00000</distPago><item>00000</item><pagoPoder>N</p�|���[�[W�Z�5��Q�
I think this has to do with UTF-8, but I can not solve.
Now I can also believe that it is the encryption algorithm to use, I leave just in case.
public static void generateFileEncrypt(File fileInput, PrivateKey privateKey, String folderSave) throws Exception {
String fileOutput = folderSave + "\" + fileInput.getName() + ENCRYPTED_FILENAME_SUFFIX;
DataOutputStream output = new DataOutputStream(new FileOutputStream(fileOutput));
Cipher rsaCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
rsaCipher.init(Cipher.ENCRYPT_MODE, privateKey);
KeyGenerator rijndaelKeyGenerator = KeyGenerator.getInstance("Rijndael");
rijndaelKeyGenerator.init(128);
Key rijndaelKey = rijndaelKeyGenerator.generateKey();
byte[] encodedKeyBytes = rsaCipher.doFinal(rijndaelKey.getEncoded());
output.writeInt(encodedKeyBytes.length);
output.write(encodedKeyBytes);
SecureRandom random = new SecureRandom();
byte[] iv = new byte[16];
random.nextBytes(iv);
output.write(iv);
IvParameterSpec spec = new IvParameterSpec(iv);
Cipher symmetricCipher = Cipher.getInstance("Rijndael/CBC/PKCS5Padding");
symmetricCipher.init(Cipher.ENCRYPT_MODE, rijndaelKey, spec);
try (
CipherOutputStream cos = new CipherOutputStream(output, symmetricCipher);
FileInputStream fis = new FileInputStream(fileInput)) {
int theByte;
byte[] data = new byte[16];
while ((theByte = fis.read(data)) != -1) {
System.out.print(new String(data));
cos.write(data, 0, theByte);
}
System.out.println("\n\n");
cos.flush();
}
}
Thanks in advance.

I haven't digested all your code; I stopped when I saw you trying to decrypt with the public key, and encrypting with the private key. That's sort of like a digital signature, but your padding will be all wrong and you should use the Signature class if that is what your really want to do.
The public key is used to encrypt, or to verify a digital signature. Use the private key to decrypt, and see if that resolves your problem.
You are still doing it wrong. Don't call it "encryption" if the key isn't private.
But anyway, I think the printing to stdout looks wrong because you are converting the entire buffer to text. The last block is likely to be padded, so it won't decode to valid text—it's padding; it wasn't part of the input file, and you aren't writing it to the decrypted file, but you are printing it.
Change to encrypt with the public key, decrypt with the private key, and then change your printing to this:
System.out.print(new String(data, 0, theByte));
Even better would be to specify the character set of the data (probably UTF-8, since it's the default for XML).

I think u should do the opposite. encrypt with the public key and decrypt with the private key..

Random access InputStream using AES CTR mode in android

I am unable to find any working example for AES CTR encryption for random access. Can someone guide me how to use the counter in CTR MODE also how to implement jumping to a specific position in the stream?
The default stream implementation (CipherInputStream) doesn't skip the stream and it corrupts the plain text.
I am trying to decrypt encrypted Video file stored on sdcard in Android. An embedded HTTP file server decrypts it on the fly. Everything is working fine until a user performs a seek within the video: the video stops immediately because it receives corrupt video stream.
I am using following code to initialize and encrypt/ decrypt stream (for sake of simplicity i hard coded the keys. it will not be hard coded in production)
ByteBuffer bb = ByteBuffer.allocate(16);
bb.put("1234567891230000".getBytes());
byte[] ivString = bb.array();
// INITIALISATION
String keyString = "1234567812345678";
IvParameterSpec iv = new IvParameterSpec(ivString);
SecretKeySpec keySpec = new SecretKeySpec(keyString.getBytes(), "AES");
// FOR ENCRYPTION
Cipher cipher = Cipher.getInstance("AES/CTR/NoPadding");
cipher.init(Cipher.ENCRYPT_MODE, keySpec, new IvParameterSpec(ivString));
Inputstream encrypted_is = new CipherInputStream(in, cipher);
// FOR DECRYPTION
cipher.init(Cipher.DECRYPT_MODE, keySpec, new IvParameterSpec(ivString));
Inputstream decrypted_is = new CipherInputStream(in, cipher);

You should not implement this using a stream. A stream is used for sequential access to data. Skipping should only be used for jumping short distances forward and mark/reset only for jumping small distances back.
Using a file map is probably the most efficient. For a slightly easier approach but less efficient approach you could use RandomAccessFile instead. You should furthermore use Cipher.getInstance("AES/CTR/NoPadding") using an "IV" that is set to the counter you expect at the location you start within the file.
Sample code for using CTR with offset:
private static final int AES_BLOCK_SIZE = 16;
public static final void jumpToOffset(final Cipher c,
final SecretKey aesKey, final IvParameterSpec iv, final long offset) {
if (!c.getAlgorithm().toUpperCase().startsWith("AES/CTR")) {
throw new IllegalArgumentException(
"Invalid algorithm, only AES/CTR mode supported");
}
if (offset < 0) {
throw new IllegalArgumentException("Invalid offset");
}
final int skip = (int) (offset % AES_BLOCK_SIZE);
final IvParameterSpec calculatedIVForOffset = calculateIVForOffset(iv,
offset - skip);
try {
c.init(Cipher.ENCRYPT_MODE, aesKey, calculatedIVForOffset);
final byte[] skipBuffer = new byte[skip];
c.update(skipBuffer, 0, skip, skipBuffer);
Arrays.fill(skipBuffer, (byte) 0);
} catch (ShortBufferException | InvalidKeyException
| InvalidAlgorithmParameterException e) {
throw new IllegalStateException(e);
}
}
private static IvParameterSpec calculateIVForOffset(final IvParameterSpec iv,
final long blockOffset) {
final BigInteger ivBI = new BigInteger(1, iv.getIV());
final BigInteger ivForOffsetBI = ivBI.add(BigInteger.valueOf(blockOffset
/ AES_BLOCK_SIZE));
final byte[] ivForOffsetBA = ivForOffsetBI.toByteArray();
final IvParameterSpec ivForOffset;
if (ivForOffsetBA.length >= AES_BLOCK_SIZE) {
ivForOffset = new IvParameterSpec(ivForOffsetBA, ivForOffsetBA.length - AES_BLOCK_SIZE,
AES_BLOCK_SIZE);
} else {
final byte[] ivForOffsetBASized = new byte[AES_BLOCK_SIZE];
System.arraycopy(ivForOffsetBA, 0, ivForOffsetBASized, AES_BLOCK_SIZE
- ivForOffsetBA.length, ivForOffsetBA.length);
ivForOffset = new IvParameterSpec(ivForOffsetBASized);
}
return ivForOffset;
}