Encryption (TripleDES) between Universal App and ASP.Net Web Service - asp.net

Question

I am totally lost. I build a Universal App (VB.net) which is consuming a web service on my (Azure Website Vb.net).
My Universal APP (UA) is sending userdata (Username an EmployeeNR) to a webservice and the Webservice is checking that data.
With TripleDES I encrypted the EmployeeNr and Username in the Universal and send it to the WebService. Now i am struggling how to decrypt the TripleDES on my WebService.
In the Universal App I work with Windows.Security.Cryptography which is not available on the Webservice site (asp.net).
I can only use System.Security.Cryptography.
I can encrypt and decrypt on Windows App site.
I also can encrpyt and decrypt on the Web Service site.
But if I pass an encrypted String form Win App to Web Service. Decryption is not working.
Problem is: On both sides, I use the same STRING, same Key and TripleDES encryption, but the result is different.
MY Encryption side - Windows Universal APP
Function myencrptor(ByVal Plaintext As String, ByVal mykey As String) As String
Dim crypt As SymmetricKeyAlgorithmProvider = SymmetricKeyAlgorithmProvider.OpenAlgorithm(SymmetricAlgorithmNames.TripleDesCbcPkcs7)
Dim PlainTextbuffer As IBuffer = CryptographicBuffer.ConvertStringToBinary(Plaintext, BinaryStringEncoding.Utf8)
Dim mykeybuffer As IBuffer = CryptographicBuffer.ConvertStringToBinary(mykey, BinaryStringEncoding.Utf8)
Dim magickey As CryptographicKey = crypt.CreateSymmetricKey(mykeybuffer)
Dim signed As IBuffer = CryptographicEngine.Encrypt(magickey, PlainTextbuffer, Nothing)
Dim verschlüsselt As String = CryptographicBuffer.EncodeToBase64String(signed)
Return verschlüsselt
End Function
My Server Side
Partial Class regal_appsync_Default
Inherits System.Web.UI.Page
Public NotInheritable Class Simple3Des
Private xTripleDES As New TripleDESCryptoServiceProvider
Sub New(ByVal key As String)
' Initialize the crypto provider.
xTripleDES.Key = TruncateHash(key, xTripleDES.KeySize \ 8)
xTripleDES.IV = TruncateHash("", xTripleDES.BlockSize \ 8)
End Sub
Private Function TruncateHash(ByVal key As String, ByVal length As Integer) As Byte()
Dim sha1 As New SHA1CryptoServiceProvider
' Hash the key.
Dim keyBytes() As Byte = System.Text.Encoding.UTF8.GetBytes(key)
Dim hash() As Byte = sha1.ComputeHash(keyBytes)
' Truncate or pad the hash.
ReDim Preserve hash(length - 1)
Return hash
End Function
Public Function EncryptData(ByVal plaintext As String) As String
' Convert the plaintext string to a byte array.
Dim plaintextBytes() As Byte = System.Text.Encoding.UTF8.GetBytes(plaintext)
' Create the stream.
Dim ms As New System.IO.MemoryStream
' Create the encoder to write to the stream.
Dim encStream As New CryptoStream(ms,
xTripleDES.CreateEncryptor(),
System.Security.Cryptography.CryptoStreamMode.Write)
' Use the crypto stream to write the byte array to the stream.
encStream.Write(plaintextBytes, 0, plaintextBytes.Length)
encStream.FlushFinalBlock()
' Convert the encrypted stream to a printable string.
Return Convert.ToBase64String(ms.ToArray)
End Function
Public Function DecryptData(ByVal encryptedtext As String) As String
' Convert the encrypted text string to a byte array.
Dim encryptedBytes() As Byte = Convert.FromBase64String(encryptedtext)
' Create the stream.
Dim ms As New System.IO.MemoryStream
' Create the decoder to write to the stream.
Dim decStream As New CryptoStream(ms, xTripleDES.CreateDecryptor(), System.Security.Cryptography.CryptoStreamMode.Write)
' Use the crypto stream to write the byte array to the stream.
decStream.Write(encryptedBytes, 0, encryptedBytes.Length)
decStream.FlushFinalBlock()
' Convert the plaintext stream to a string.
Return System.Text.Encoding.UTF8.GetString(ms.ToArray)
End Function
End Class
Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Lbl1.Text = myEncoding(Txt1.Text)
End Sub
Protected Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Lbl1.Text = myDecoding(Lbl1.Text)
End Sub
Function myEncoding(ByVal plainText As String) As String
Dim wrapper As New Simple3Des("012345678901234567891234")
Dim cipherText As String = wrapper.EncryptData(plainText)
Return cipherText
End Function
Function myDecoding(ByVal cipherText As String) As String
' DecryptData throws if the wrong password is used.
Dim wrapper As New Simple3Des("012345678901234567891234")
Try
Dim plainText As String = wrapper.DecryptData(cipherText)
Return plainText
Catch ex As System.Security.Cryptography.CryptographicException
Return "problem"
End Try
End Function
End Class
looking forward to any helpful tips!
best wishes
hendrik