Implementing RSA in C#

Question

I\'m currently trying to implement a class to handle secure communications between instances of my app using RSACrytoServiceProveider class. First question : is it a good i

Answer 1

I don't know if this snip of code may help you, I've wrote this code to be able to crypt and decrypt with private/public key pairs in diverse crypting algortims and without data to encrypt length issue, infact RSA implementation in .NET suffer whe you try to manage more than 250 (more or less, sorry I don't recall) bytes of data.

I just cut and paste only methods needed, I also cutted xml documentation cause is not in english, if you found this useful let me know, I can post all source. I repeat I do not tested this cut&paste version, but I used the full version of this class that is not that different.

BTW: it's in VB, but if you just need to lurk at it I think it's enough ;)

Namespace Crypto

    Public Class RSACry

        Shared Sub New()
        End Sub

        Public Enum Algorithms
            DES
            TDES
            RC2
            RDAEL
        End Enum

        Public Shared Function Encrypt(ByVal xmlkeystring As String, ByVal typo As Algorithms, ByVal datatoencrypt As String) As String
            Dim rsaer As RSA = Crypto.RSACry.ReadKeyString(xmlkeystring)
            Dim result() As Byte = Crypto.RSACry.EncryptIt(rsaer, typo, datatoencrypt)
            Return System.Convert.ToBase64String(result)
        End Function

        Public Shared Function Decrypt(ByVal xmlkeystring As String, ByVal typo As Algorithms, ByVal datatodecrypt As String) As String
            Dim rsaer As RSA = Crypto.RSACry.ReadKeyString(xmlkeystring)
            Dim result() As Byte = Crypto.RSACry.DecryptIt(rsaer, typo, datatodecrypt)
            Return System.Text.Encoding.UTF8.GetString(result)
        End Function

        Friend Shared Function EncryptIt(ByRef rsaer As RSA, ByVal typo As Algorithms, ByVal datatoencrypt As String) As Byte()
            Dim result() As Byte = Nothing

            Try
                Dim plainbytes() As Byte = System.Text.Encoding.UTF8.GetBytes(datatoencrypt)
                Dim sa As SymmetricAlgorithm = SymmetricAlgorithm.Create(Crypto.RSACry.GetAlgorithmName(typo))
                Dim ct As ICryptoTransform = sa.CreateEncryptor()
                Dim encrypt() As Byte = ct.TransformFinalBlock(plainbytes, 0, plainbytes.Length)
                Dim fmt As RSAPKCS1KeyExchangeFormatter = New RSAPKCS1KeyExchangeFormatter(rsaer)
                Dim keyex() As Byte = fmt.CreateKeyExchange(sa.Key)

                --return the key exchange, the IV (public) and encrypted data 
                result = New Byte(keyex.Length + sa.IV.Length + encrypt.Length) {}
                Buffer.BlockCopy(keyex, 0, result, 0, keyex.Length)
                Buffer.BlockCopy(sa.IV, 0, result, keyex.Length, sa.IV.Length)
                Buffer.BlockCopy(encrypt, 0, result, keyex.Length + sa.IV.Length, encrypt.Length)

            Catch ex As Exception
                Throw New CryptographicException("Unable to crypt: " + ex.Message)
            End Try

            Return result
        End Function

        Friend Shared Function DecryptIt(ByRef rsaer As RSA, ByVal typo As Algorithms, ByVal datatodecrypt As String) As Byte()
            Dim result() As Byte = Nothing

            Try
                Dim encrbytes() As Byte = System.Convert.FromBase64String(datatodecrypt)
                Dim sa As SymmetricAlgorithm = SymmetricAlgorithm.Create(Crypto.RSACry.GetAlgorithmName(typo))
                Dim keyex() As Byte = New Byte((rsaer.KeySize >> 3) - 1) {}
                Buffer.BlockCopy(encrbytes, 0, keyex, 0, keyex.Length)

                Dim def As RSAPKCS1KeyExchangeDeformatter = New RSAPKCS1KeyExchangeDeformatter(rsaer)
                Dim key() As Byte = def.DecryptKeyExchange(keyex)
                Dim iv() As Byte = New Byte((sa.IV.Length - 1)) {}
                Buffer.BlockCopy(encrbytes, keyex.Length, iv, 0, iv.Length)

                Dim ct As ICryptoTransform = sa.CreateDecryptor(key, iv)
                result = ct.TransformFinalBlock(encrbytes, keyex.Length + iv.Length, (encrbytes.Length - 1) - (keyex.Length + iv.Length))
            Catch ex As Exception
                Throw New CryptographicException("Unable to decrypt: " + ex.Message)
            End Try

            Return result
        End Function    

        Friend Shared Function GetAlgorithmName(ByVal typo As Algorithms) As String
            Dim algtype As String = String.Empty

            Select Case typo
                Case Algorithms.DES
                    Return "DES"
                    Exit Select
                Case Algorithms.RC2
                    Return "RC2"
                    Exit Select
                Case Algorithms.RDAEL
                    Return "Rijndael"
                    Exit Select
                Case Algorithms.TDES
                    Return "TripleDES"
                    Exit Select
                Case Else
                    Return "Rijndael"
                    Exit Select
            End Select

            Return algtype
        End Function

        Friend Shared Function ReadKeyString(ByVal xmlkeystring As String) As RSA
            Dim rsaer As RSA = Nothing

            Try
                If (String.IsNullOrEmpty(xmlkeystring)) Then Throw New Exception("Key is not specified")
                rsaer = RSA.Create()
                rsaer.FromXmlString(xmlkeystring)
            Catch ex As Exception
                Throw New CryptographicException("Unable to load key")
            End Try

            Return rsaer
        End Function    

End Namespace