Recursively Call RSA Encryption To Encrypt Strings Longer Than 128 Bytes

Question

so I am working on an assignment that is meant to teach us the methods of encrypting and decrypting messages.

I have made a simple application that will take in text from one textBox and on a button click reproduce the encrypted bytes into another textBox. Also have another button that when pressed will take the encrypted byte array, and decrypt it.

Everything works, but if my message to be encrypted is larger than 128 bytes than it will throw an error. Makes sense since RSA key size is 128. So I am working on writing some code to handle cases where the byte array exceeds 128.

So far this is what I have come up with:

//Encode the data to encrypt as a byte array
var dataToEncrypt = encoder.GetBytes(txtPlain.Text);

//store dataLength
int dataLength = dataToEncrypt.Length;

//Check if dataLength > 128 
if(dataLength > 128)
{
    //Divide dataLength by 128 to determine how many cycles will be needed
    double numOfCycles = (dataLength / 128);
    //round up to nearest whole number
    cycles = (int) Math.Ceiling(numOfCycles);
}

//for however many cycles
for (int i = 0; i < cycles; i++)
{
    var myByteArray = new byte[128];
    for (int j = 0; j < 128; j++)
    {
       int currentByte = i * 128 + j;
       myByteArray[j] = dataToEncrypt[currentByte];
    }

    var encryptedByteArray = myRSA.Encrypt(myByteArray, false).ToArray();

    var length = encryptedByteArray.Count();
    var item = 0;
    var sb = new StringBuilder();

    //Change each byte in the encrypted byte array to text
    foreach (var x in encryptedByteArray)
    {
        item++;
        sb.Append(x);
        if (item < length) sb.Append(",");
    }
    txtCypher.Text = sb.ToString();   
}

I have tried to calculate how many cycles (encrypting 128 bytes at a time) will be needed, and then repeat this process of encrypting a byte array of 128 bytes and printing out.

However I continue to get an error An unhandled exception of type 'System.Security.Cryptography.CryptographicException' occurred in mscorlib.dll Additional information: Bad Length.

from line: var encryptedByteArray = myRSA.Encrypt(myByteArray, false).ToArray();

It appears that my byte array is exceeding the limit of 128, but I cannot seem to figure out exactly how.

Does the error occur on the first cycle or only subsequent cycles? — user469104, Apr 08 '16 at 19:39
Typically `RSA` is used to encrypt only the key for a symmetric algorithm and not the data itself. — John Alexiou, Apr 08 '16 at 20:03
See http://stackoverflow.com/questions/1496793/rsa-encryption-getting-bad-length — user469104, Apr 08 '16 at 20:03
The limit isn't necessarily 128, it can also be 117 (most likely) or something else depending on the padding used. But do note that RSA really isn't designed for encoding messages - it's more useful for encrypting keys that are in turn used to encrypt messages. — Luaan, Apr 08 '16 at 22:39
If you're using textbook RSA (unpadded), then the duplicate doesn't fit. Are you? Can you show the necessary code? — Artjom B., Apr 08 '16 at 22:39

Recursively Call RSA Encryption To Encrypt Strings Longer Than 128 Bytes

0 Answers0