Cannot decrypt RSA encrypted text : due to : input too large for RSA cipher

Question

Hi,
I am in a fix trying to decrypt this RSA encrypted String ... plzz help
I have the encrypted text as a String.
This is what I do to decrypt it using the Private key
- Determine the block size of the Cipher object
- Get the array of bytes from the String
- Find out how many block sized partitions I have in the array
- Encrypt the exact block sized partitions using update() method
- Ok, now its easy to find out how many bytes remain (using % operator)
- If the remaining bytes is 0 then simply call the 'doFinal()'
i.e. the one which returns an array of bytes and takes no args
- If the remaining bytes is not zero then call the
'doFinal(byte [] input, int offset, in inputLen)' method for the
bytes which actually remained
However, this doesnt work. This is making me go really crazy.
Can anyone point out whats wrong ? Plzz
Here is the (childish) code
Cipher rsaDecipher = null;
//The initialization stuff for rsaDecipher
//The rsaDecipher Cipher is using 256 bit keys
//I havent specified anything regarding padding
//And, I am using BouncyCastle
String encryptedString;
// read in the string from the network
// this string is encrypted using an RSA public key generated earlier
// I have to decrypt this string using the corresponding Private key
byte [] input = encryptedString.getBytes();
int blockSize = rsaDecipher.getBlockSize();
int outputSize = rsaDecipher.getOutputSize(blockSize);
byte [] output = new byte[outputSize];
int numBlockSizedPartitions = input.length / blockSize;
int numRemainingBytes = input.length % blockSize;
boolean hasRemainingBytes = false;
if (numRemainingBytes > 0)
  hasRemainingBytes = true;
int offset = 0;
int inputLen = blockSize;
StringBuffer buf = new StringBuffer();
for (int i = 0; i < numBlockSizedPartitions; i++) {
  output = rsaDecipher.update(input, offset, blockSize);
  offset += blockSize;
  buf.append(new String(output));
if (hasRemainingBytes) {
  //This is excatly where I get the "input too large for RSA cipher"
  //Which is suffixed with ArrayIndexOutofBounds
  output = rsaDecipher.doFinal(input,offset,numRemainingBytes);
} else {
  output = rsaDecipher.doFinal();
buf.append(new String(output));
//After having reached till here, will it be wrong if I assumed that I
//have the properly decrypted string ???

Answer 1

Hi,
I am in a fix trying to decrypt this RSA encrypted
String ... plzz helpYou're already broken at this point.
Repeat after me: ciphertext CANNOT be safely represented as a String. Strings have internal structure - if you hand ciphertext to the new String(byte[]) constructor, it will eat your ciphertext and leave you with garbage. Said garbage will fail to decrypt in a variety of puzzling fashions.
If you want to transmit ciphertext as a String, you need to use something like Base64 to encode the raw bytes. Then, on the receiving side, you must Base64-DEcode back into bytes, and then decrypt the resulting byte[].
Second - using RSA as a general-purpose cipher is a bad idea. Don't do that. It's slow (on the order of 100x slower than the slowest symmetric cipher). It has a HUGE block size (governed by the keysize). And it's subject to attack if used as a stream-cipher (IIRC - I can no longer find the reference for that, so take it with a grain of salt...) Standard practice is to use RSA only to encrypt a generated key for some symmetric algorithm (like, say, AES), and use that key as a session-key.
At any rate - the code you posted is broken before you get to this line:byte [] input = encryptedString.getBytes();Go back to the encrypting and and make it stop treating your ciphertext as a String.
Grant