I have generated on my android application a pair of RSA Keys.
I receive from a web service - an AES Key, encrypted with my RSA public key - a String encoded with the AES key.
So I must do the following: - decrypt the AES Key - decrypt the string with the obtained AES Key.
To generate the RSA Keys I did:
keyGen = KeyPairGenerator.getInstance("RSA");
keyGen.initialize(size);
keypair = keyGen.genKeyPair();
privateKey = keypair.getPrivate();
publicKey = keypair.getPublic();
On RSA decrypt I use :
public static byte[] decryptRSA( PrivateKey key, byte[] text) throws Exception
{
byte[] dectyptedText = null;
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.DECRYPT_MODE, key);
dectyptedText = cipher.doFinal(text);
return dectyptedText;
}
On AES decrypt I use:
public static byte[] decryptAES(byte[] key, byte[] text) throws Exception {
SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");
Cipher cipher = Cipher.getInstance("AES/ECB/PKCS1Padding");
cipher.init(Cipher.DECRYPT_MODE, skeySpec);
byte[] decrypted = cipher.doFinal(text);
return decrypted;
}
So, in my code, to obtain the decrypted AES Key I do
byte[] decryptedAESKey = sm.decryptRSA(key, Base64.decode(ReceivedBase64EncryptedAESKey));
byte[] decryptedString = sm.decryptAES(decryptedAESKey, Base64.decode(ReceivedEncryptedAESString));
On the end I get a null for decryptedString. What am I doing wrong ?
Well, the thing is that the key decrypted was 8 byte long and I had to make it 16 byte to be AES 128 bits compatible
So, I made a method to convert it back
private static byte[] GetKey(byte[] suggestedKey)
{
byte[] kRaw = suggestedKey;
ArrayList<Byte> kList = new ArrayList<Byte>();
for (int i = 0; i < 128; i += 8)
{
kList.add(kRaw[(i / 8) % kRaw.length]);
}
byte[] byteArray = new byte[kList.size()];
for(int i = 0; i<kList.size(); i++){
byteArray[i] = kList.get(i);
}
return byteArray;
}
And the rewritten decrypt method:
public static byte[] decryptAES(byte[] key, byte[] text) throws Exception {
SecretKeySpec skeySpec = new SecretKeySpec(GetKey(key), "AES");
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS7Padding","BC");
byte [] iv = new byte[cipher.getBlockSize()];
for(int i=0;i<iv.length;i++)iv[i] = 0;
IvParameterSpec ivSpec = new IvParameterSpec(iv);
cipher.init(Cipher.DECRYPT_MODE, skeySpec, ivSpec);
byte[] decrypted = cipher.doFinal(text);
return decrypted;
}
I'm not sure what language or libraries you are using (looks like Java?), but some really general things to try:
ReceivedEncryptedAESString
and the output of the Base64.decode
to check they look alright. cipher
after the construction step and the init step.
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