Read magic numbers

Question

Hello all

I can't figure out how to make it done, to read 64 bytes of the file and check it against a list of magic numbers.

The key is that each signature/magic number has different length. but I wanna read the first 64 bytes only once.

    Public Function DetectMagicNo(ByVal InputFile As String) As String
        Using MyFileStream As New FileStream(InputFile, FileMode.Open, FileAccess.Read)
            Dim MyBytes(63) As Byte
            Dim OGG As Byte() = {&H4F, &H67, &H67, &H53}
            Dim PSD As Byte() = {&H38, &H42, &H50, &H53}
            Dim MP3 As Byte() = {&HFF, &HFB, &HFF, &HF3, &HFF, &HF2}
            '''full list
            MyFileStream.Read(MyBytes, 0, 64)
            Select Case True
                Case MyBytes.SequenceEqual(OGG)
                    DetectMagicNo = "OGG"
                Case MyBytes.SequenceEqual(PSD)
                    DetectMagicNo = "PSD"
                Case MyBytes.SequenceEqual(MP3)
                    'full case
            End Select
        End Using
    End Function

Anyone can help please :)

Answer 1

Made a slight change to your Select that should help,

            Select Case True
                Case MyBytes.Take(OGG.Length).SequenceEqual(OGG)
                    DetectMagicNo = "OGG"
                Case MyBytes.Take(PSD.Length).SequenceEqual(PSD)
                    DetectMagicNo = "PSD"
                Case MyBytes.Take(MP3.Length).SequenceEqual(MP3)
                    'full case
            End Select

Answer 2

Hi

Seems you are trying to get an impossible equality. You are comparing a 64 byte sequence with a 4 (or 6 whatever) sequence which can never be equal.

I added a Function that pairs down the 64 bytes for a comparison. I have only tested with one of the file types and works fine so maybe it would work fine for all.

' Form1 with Lavel1
Option Strict On
Option Explicit On
Imports System.IO
Public Class Form1
	Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
		Label1.Text = DetectMagicNo("C:\Users\lesha\Desktop\sample-ogg.ogg")
	End Sub
	Public Function DetectMagicNo(InputFile As String) As String
		Using MyFileStream As New FileStream(InputFile, FileMode.Open, FileAccess.Read)
			Dim MyBytes(63) As Byte
			Dim OGG As Byte() = {&H4F, &H67, &H67, &H53}
			Dim PSD As Byte() = {&H38, &H42, &H50, &H53}
			Dim MP3 As Byte() = {&HFF, &HFB, &HFF, &HF3, &HFF, &HF2}

			MyFileStream.Read(MyBytes, 0, 64)

			Select Case True
				Case Tk(4, MyBytes).SequenceEqual(OGG)
					Return "OGG"
				Case Tk(4, MyBytes).SequenceEqual(PSD)
					Return "PSD"
				Case Tk(6, MyBytes).SequenceEqual(MP3)
					Return "MP3"
			End Select
		End Using
		Return "None"
	End Function
	Function Tk(n As Integer, bytes() As Byte) As Byte()
		Dim b(n - 1) As Byte
		For i As Integer = 0 To n - 1
			b(i) = bytes(i)
		Next
		Return b
	End Function
End Class