B4J Tutorial [B4x]Padding/depadding PKCS7/5 and other

EDIT: If the message length is equal to the pad length, you have to pad to the next multiple. E.g. 16 bytes would be padded to 32 because one would not know how long the message was because the last byte defines the number of added (=padded) values.

Here's another example to pad/depad data for many use cases (like Encryption).

Explanation on this site: https://asecuritysite.com/encryption/padding

Example how it works:

1. You have a string or bytes with a lenght of 5. Let's say it's the string "Hello"
2. The function you use (e.g. AES) needs the data with a length of a multiple of x (here: 16)
3. You then need to fill the rest of the 16 bytes ("Hello" has 5, so it's 11) with a filler (Pad-Value)
4. 16 bytes is easy bit usually you have much more data to pad
5. The example calculates the next multiple of 16 (you can use any pad-size) and fills it with pad-values
6. The last byte (here the 16th) is filled with the total length of padded bytes (otherwise you wouldn't know how many were added)
7. The pad value (what it uses to fill up) depends on the usage and is regulated (RFC's, etc.)
8. It can be

- any byte value
- random bytes
- the count of the added bytes (here it would be 11 bytes or 0xb for all bytes to fill)

I use it to exchange AES encrypted data with my ESP32 (as far as I know it only supports "no padding" so I had to pad the data on my own).

Padding:

Pad-Mode
1=Random Bytes
2=Value given in PadValue
3=Number of padded bytes is jused as the pad value

Sub Pad (mess() As Byte, PadTo As Int,PadMode As Int,PadValue As Object) As Byte()
    Dim MessLength As Int = mess.Length
    Dim PaddedLength As Int
    PaddedLength = MessLength-MessLength Mod PadTo + PadTo
    Log("Original length: " & MessLength & ", padded: " & PaddedLength)
    Dim PaddedMess(PaddedLength) As Byte
    bc.ArrayCopy(mess,0,PaddedMess,0,mess.Length)
    For i=mess.Length To PaddedLength-2
        Select PadMode
        Case 1    
           PaddedMess(i)=Rnd(0,256)-127
        Case 2
           PaddedMess(i)=PadValue
        Case 3
           PaddedMess(i)=PaddedLength-MessLength
        End Select 
    Next 
    PaddedMess(PaddedLength-1)=PaddedLength-MessLength
    Return PaddedMess

End Sub

Un-Pad:

Sub Unpad (PaddedMess() As Byte) As Byte()
    Dim OrigMess(PaddedMess.Length-PaddedMess(PaddedMess.Length-1)) As Byte
    bc.ArrayCopy(PaddedMess,0,OrigMess,0,OrigMess.Length)
    Return OrigMess
End Sub

Call:

Dim padded() As Byte=Pad("Hello".GetBytes("UTF8"),16,2,15)
Dim unpadded() As Byte= Unpad(padded)
Log(BytesToString(unpadded,0,unpadded.Length,"UTF8"))