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;
}
}
public EncryptedMessage Encrypt(string messageText, int recieverId)
{
if (messageText == null)
{
throw new ArgumentException("Message text cannot be null");
}
IContactModel receiverContact = _storageService.GetContacts().FirstOrDefault(c => c.Id == recieverId);
if (receiverContact == null)
{
throw new ArgumentException("Contact with id of receiverId does not exist");
}
EncryptedMessage encMsg = new EncryptedMessage();
AesCipher aes = new AesCipher();
// set initiazlization vector
encMsg.Iv = FormatConverter.BytesToString64(aes.IV);
// enccrypt message text symmetrically
byte[] messageBytes = FormatConverter.StringToBytes(messageText);
byte[] encryptedMessageBytes = aes.Encrypt(messageBytes);
encMsg.Body = FormatConverter.BytesToString64(encryptedMessageBytes);
// encrypt symmetric key with receivers public key
string receiverPublicKey = receiverContact.PublicKey;
RsaCipher rsa = new RsaCipher(receiverPublicKey);
byte[] encryptedSymmetricKeyBytes = rsa.Encrypt(aes.Key);
encMsg.SymmetricKey = FormatConverter.BytesToString64(encryptedSymmetricKeyBytes);
// create digital signature of message text (using senders private key)
string senderKeyPair = _storageService.GetUser().KeyPair;
rsa = new RsaCipher(senderKeyPair);
byte[] digitalSignatureBytes = rsa.CreateDigitalSignature(messageBytes);
encMsg.DigitalSignature = FormatConverter.BytesToString64(digitalSignatureBytes);
return encMsg;
}