public bool Decrypt(EncryptedMessage encryptedMessage, int senderId, out string messageText)
{
if (encryptedMessage == null)
{
throw new ArgumentException("Encrypted message cannot be null");
}
if (encryptedMessage.Body == null || encryptedMessage.DigitalSignature == null ||
encryptedMessage.SymmetricKey == null || encryptedMessage.Iv == null)
{
throw new ArgumentException("Not all encrypted message fields are initialized");
}
IContactModel senderContact = _storageService.GetContacts().FirstOrDefault(c => c.Id == senderId);
if (senderContact == null)
{
throw new ArgumentException("Contact with id of senderId does not exist");
}
string receiverKeyPair = _storageService.GetUser().KeyPair;
string senderPublicKey = senderContact.PublicKey;
try
{
// decrypt symmetric key with receivers private key
RsaCipher rsa = new RsaCipher(receiverKeyPair);
byte[] encryptedSymmetricKeyBytes = FormatConverter.String64ToBytes(encryptedMessage.SymmetricKey);
byte[] decryptedSymmetricKeyBytes = rsa.Decrypt(encryptedSymmetricKeyBytes);
// decrypt message text with jsut decrypted symmetric key
byte[] ivBytes = FormatConverter.String64ToBytes(encryptedMessage.Iv);
AesCipher aes = new AesCipher(decryptedSymmetricKeyBytes, ivBytes);
byte[] encryptedMessageBytes = FormatConverter.String64ToBytes(encryptedMessage.Body);
byte[] decryptedMessageBytes = aes.Decrypt(encryptedMessageBytes);
// set message text out parameter
messageText = FormatConverter.BytesToString(decryptedMessageBytes);
// verify digital signature
rsa = new RsaCipher(senderPublicKey);
byte[] digitalSignatureBytes = FormatConverter.String64ToBytes(encryptedMessage.DigitalSignature);
bool signatureOk = rsa.VerifyDigitalSignature(decryptedMessageBytes, digitalSignatureBytes);
return(signatureOk);
}
catch (Exception ex)
{
messageText = null;
return(false);
}
}