I'm getting the error The data to be decrypted exceeds the maximum for this modulus of 256 bytes when decrypting the RSA encrypted string in C#.
What i'm trying to achieve:
- Generate public/private key pairs in C# (RSA)
- Save private key in session/temp storage for use during decryption
- Send/return public key to client/JS for use with encryption
- Encrypt string (max 20 chars) in JS with public key and send to server
- Decrypt encrypted string in server using the private key saved in step 2
What works:
- Generation of public/private key pairs
- Encryption in JS using the library jsencrypt
Code written so far:
C#
///Source: https://stackoverflow.com/questions/17128038/c-sharp-rsa-encryption-decryption-with-transmission
public static void GenerateKeys()
{
//CSP with a new 2048 bit rsa key pair
var csp = new RSACryptoServiceProvider(2048);
//Private key
var privKey = csp.ExportParameters(true);
//Public key
var pubKey = csp.ExportParameters(false);
string privKeyString = string.Empty;
var sw = new System.IO.StringWriter();
var xs = new System.Xml.Serialization.XmlSerializer(typeof(RSAParameters));
xs.Serialize(sw, privKey);
privKeyString = sw.ToString();
//This will give public key in the following format which is required by the JS library
//-----BEGIN PUBLIC KEY-----
//XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
//-----END PUBLIC KEY-----
string publicKeyBase64 = ConvertPublicKeyForJS(pubKey);
//Will be saved in sesssion for later use during decryption
string privateKeyBase64 = Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes(privKeyString));
//Save in session/temp location
//Return publicKeyBase64 to JS
}
/// <summary>
/// Source: https://stackoverflow.com/questions/28406888/c-sharp-rsa-public-key-output-not-correct/28407693#28407693
/// </summary>
/// <param name="publicKey"></param>
/// <returns></returns>
public static string ConvertPublicKeyForJS(RSAParameters publicKey)
{
string output = string.Empty;
using (var stream = new MemoryStream())
{
var writer = new BinaryWriter(stream);
writer.Write((byte)0x30); // SEQUENCE
using (var innerStream = new MemoryStream())
{
var innerWriter = new BinaryWriter(innerStream);
innerWriter.Write((byte)0x30); // SEQUENCE
EncodeLength(innerWriter, 13);
innerWriter.Write((byte)0x06); // OBJECT IDENTIFIER
var rsaEncryptionOid = new byte[] { 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x01 };
EncodeLength(innerWriter, rsaEncryptionOid.Length);
innerWriter.Write(rsaEncryptionOid);
innerWriter.Write((byte)0x05); // NULL
EncodeLength(innerWriter, 0);
innerWriter.Write((byte)0x03); // BIT STRING
using (var bitStringStream = new MemoryStream())
{
var bitStringWriter = new BinaryWriter(bitStringStream);
bitStringWriter.Write((byte)0x00); // # of unused bits
bitStringWriter.Write((byte)0x30); // SEQUENCE
using (var paramsStream = new MemoryStream())
{
var paramsWriter = new BinaryWriter(paramsStream);
EncodeIntegerBigEndian(paramsWriter, publicKey.Modulus); // Modulus
EncodeIntegerBigEndian(paramsWriter, publicKey.Exponent); // Exponent
var paramsLength = (int)paramsStream.Length;
EncodeLength(bitStringWriter, paramsLength);
bitStringWriter.Write(paramsStream.GetBuffer(), 0, paramsLength);
}
var bitStringLength = (int)bitStringStream.Length;
EncodeLength(innerWriter, bitStringLength);
innerWriter.Write(bitStringStream.GetBuffer(), 0, bitStringLength);
}
var length = (int)innerStream.Length;
EncodeLength(writer, length);
writer.Write(innerStream.GetBuffer(), 0, length);
}
var base64 = Convert.ToBase64String(stream.GetBuffer(), 0, (int)stream.Length);
StringBuilder sb = new StringBuilder();
sb.AppendLine("-----BEGIN PUBLIC KEY-----");
sb.AppendLine(base64);
sb.AppendLine("-----END PUBLIC KEY-----");
output = sb.ToString();
}
return output;
}
private static void EncodeLength(BinaryWriter stream, int length)
{
if (length < 0) throw new ArgumentOutOfRangeException("length", "Length must be non-negative");
if (length < 0x80)
{
// Short form
stream.Write((byte)length);
}
else
{
// Long form
var temp = length;
var bytesRequired = 0;
while (temp > 0)
{
temp >>= 8;
bytesRequired++;
}
stream.Write((byte)(bytesRequired | 0x80));
for (var i = bytesRequired - 1; i >= 0; i--)
{
stream.Write((byte)(length >> (8 * i) & 0xff));
}
}
}
private static void EncodeIntegerBigEndian(BinaryWriter stream, byte[] value, bool forceUnsigned = true)
{
stream.Write((byte)0x02); // INTEGER
var prefixZeros = 0;
for (var i = 0; i < value.Length; i++)
{
if (value[i] != 0) break;
prefixZeros++;
}
if (value.Length - prefixZeros == 0)
{
EncodeLength(stream, 1);
stream.Write((byte)0);
}
else
{
if (forceUnsigned && value[prefixZeros] > 0x7f)
{
// Add a prefix zero to force unsigned if the MSB is 1
EncodeLength(stream, value.Length - prefixZeros + 1);
stream.Write((byte)0);
}
else
{
EncodeLength(stream, value.Length - prefixZeros);
}
for (var i = prefixZeros; i < value.Length; i++)
{
stream.Write(value[i]);
}
}
}
public static string DecryptValue(string cypherText)
{
string plainTextData = string.Empty;
string privKeyBase64 = ""; //get value from session/temp storage
string privKeyString = System.Text.Encoding.ASCII.GetString(Convert.FromBase64String(privKeyBase64));
var sr = new System.IO.StringReader(privKeyString);
var xs = new System.Xml.Serialization.XmlSerializer(typeof(RSAParameters));
var privKey = (RSAParameters)xs.Deserialize(sr);
var csp = new RSACryptoServiceProvider();
csp.ImportParameters(privKey);
var bytesCypherText = Convert.FromBase64String(cypherText);
//Problematic line
var bytesPlainTextData = csp.Decrypt(bytesCypherText, false);
plainTextData = System.Text.Encoding.Unicode.GetString(bytesPlainTextData);
return plainTextData;
}
JS
function encrypt(msg, publicKey) {
var jsEncrypt = new JSEncrypt();
jsEncrypt.setPublicKey(publicKey);
var encrypted = jsEncrypt.encrypt(msg);
var base64result = btoa(encrypted);
return base64result;
}
Update/Variation:
If i make the key size 1024 instead of 2048, the error i'm getting is The data to be decrypted exceeds the maximum for this modulus of 128 bytes.. Total bytes of the encrypted string is 172 based on what i can see in the debugger.
Update | Solution:
- Remove this line in the JS
var base64result = btoa(encrypted);. Just return the encrypted value directly. - In C# decrypt method, change this line
var bytesPlainTextData = csp.Decrypt(bytesCypherText, false);tovar bytesPlainTextData = csp.Decrypt(bytesCypherText, RSAEncryptionPadding.Pkcs1);.
Notes:
- Please ignore the incorrect method return types, this is still a POC and a lot of values are being filled manually.
- I'm not an expert in security matters so a lot of the codes have been picked from different sources.
