What Does ICryptoTransform.TransformFinalBlock Actually Do?

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C#

VB

.NET

I am trying to figure out some code a coworker wrote, but the documentation for TransformFinalBlock does not really explain anything. It says it "transforms the specified region of the specified byte array" and returns "the computed transform", but it doesn't explain what kind of transform is performed. Here is the code my coworker wrote:

Public Shared Function Encrypt(ByVal str As String, ByVal strKey As String) As String
    cryptDES3.Key = cryptMD5Hash.ComputeHash(ASCIIEncoding.ASCII.GetBytes(strKey))
    cryptDES3.Mode = CipherMode.ECB
    Dim desdencrypt As ICryptoTransform = cryptDES3.CreateEncryptor()
    Dim buff() As Byte = ASCIIEncoding.ASCII.GetBytes(str)
    Return (Convert.ToBase64String(desdencrypt.TransformFinalBlock(buff, 0, buff.Length)))
End Function

I'm thinking this is not the correct way to perform encryption (I'm using a technique that makes use of CryptoStream), but I can't be sure the above code is incorrect. Mabye this is just a clever way to avoid a stream when you're working with byte arrays anyway. Not sure. I thought I'd save myself a few hours in case any of you know more about encryption than I do.