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