public byte[] DecryptData(EncryptedPacket encryptedPacket, RSAWithRSAParameterKey rsaParams, DigitalSignatureFuncs digitalSignature)
{
// Decrypt AES Key with RSA
var decryptedSessionKey = rsaParams.DecryptData(encryptedPacket.EncryptedSessionKey);
// Integrity + Signature Check
var hmacToCheck = HMac.ComputeHMACSha256(encryptedPacket.EncryptedData, decryptedSessionKey);
if (!Compare(encryptedPacket.HMAC, hmacToCheck))
{
throw new CryptographicException("HMAC for decryption does not match encrypted package HMAC code received. This means the message has been tampered with.");
}
if (!digitalSignature.VerifySignature(encryptedPacket.HMAC, encryptedPacket.Signature))
{
throw new CryptographicException("Digital Signature of document cannot be verified.");
}
// Decrypt our data with AES using the decryptedSessionKey
return(_aes.Decrypt(encryptedPacket.EncryptedData, decryptedSessionKey, encryptedPacket.IV));
}
private static void TestDigitalSignature()
{
var document = Encoding.UTF8.GetBytes("Document to Sign");
byte[] hashedDocument;
using (var sha256 = SHA256.Create())
{
hashedDocument = sha256.ComputeHash(document);
}
var digitalSignature = new DigitalSignatureFuncs();
digitalSignature.AssignNewKey();
var signature = digitalSignature.SignData(hashedDocument);
var verified = digitalSignature.VerifySignature(hashedDocument, signature);
Console.WriteLine($"Original Text: {Encoding.Default.GetString(document)}");
Console.WriteLine($"Digital Signature: {Convert.ToBase64String(signature)}");
Console.WriteLine(verified ? "The digital signature has been verified." : "The digital signature has NOT been verified.");
}
