How to resolve the algorithm Base64 decode data step by step in the FreeBASIC programming language

Published on 12 May 2024 09:40 PM
#Freebasic

How to resolve the algorithm Base64 decode data step by step in the FreeBASIC programming language

Table of Contents

Problem Statement

See Base64 encode data. Now write a program that takes the output of the Base64 encode data task as input and regenerate the original file. When working on the VBA implementation I found several 'solutions' on the net, including one from the software maker himself, that showed output with incorrect padding. Obviously with incorrect padding in the output you can not decode correctly to the original file again.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Base64 decode data step by step in the FreeBASIC programming language

Source code in the freebasic programming language

Dim Shared As String B64
B64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ" & _
"abcdefghijklmnopqrstuvwxyz" & _
"0123456789+/"

Function MIMEDecode(s As String ) As Integer
    If Len(s) Then
        MIMEdecode = Instr(B64,s) - 1
    Else
        MIMEdecode = -1
    End If
End Function

Function Decode64(s As String) As String
    Dim As Integer w1, w2, w3, w4
    Dim As String  mD
    For n As Integer = 1 To Len(s) Step 4
        w1 = MIMEdecode(Mid(s,n+0,1))
        w2 = MIMEdecode(Mid(s,n+1,1))
        w3 = MIMEdecode(Mid(s,n+2,1))
        w4 = MIMEdecode(Mid(s,n+3,1))
        If w2 >-1 Then mD+= Chr(((w1* 4 + Int(w2/16)) And 255))
        If w3 >-1 Then mD+= Chr(((w2*16 + Int(w3/ 4)) And 255))
        If w4 >-1 Then mD+= Chr(((w3*64 + w4        ) And 255))
    Next n
    Return mD
End Function

Dim As String msg64 = "VG8gZXJyIGlzIGh1bWFuLCBidXQgdG8gcmVhbGx5IGZvdWwgdGhpbmdzIHVw" & _
"IHlvdSBuZWVkIGEgY29tcHV0ZXIuCiAgICAtLSBQYXVsIFIuIEVocmxpY2g="
Print msg64
Print: Print(Decode64(msg64))
Sleep

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