B4X

How to Tell if a File is .p7m or .p7s

See more Misc Examples

This example explains how to detect if a file is a .p7m or .p7s. It provides as solution to the following problem:

Sometimes people submit P7S/P7M with wrong extensions. Can I check if file is P7S or P7M with Chilkat?

Also see What is a P7M or P7S File?

B4X

Dim success As Boolean = False

'  A .p7m or .p7s file is binary and contains PKCS7.
'  PKCS7 is ASN.1, and always begins with a SEQUENCE tag (a byte equal to 0x30)
'  followed by an encoded length.
'  If the file is larger than 128 bytes (and it SHOULD be), the encoded length will begin with either 0x80, 0x81, 0x82, or 0x83.
'  It can also begin with 0x84, but only for very large files (where the length is too large for 3 bytes).
'  See How ASN.1 Encodes Lengths

'  Therefore, if we have a file that might be .p7m or .p7s, we can check by
'  examining the 1st 2 bytes of the file.

Dim bd As ChilkatBinData
bd.Initialize

success = bd.LoadFile("qa_data/p7m/a.p7m")
If success = False Then
    Log("Failed to load the file.")
    Return
End If


Dim sb As ChilkatStringBuilder
sb.Initialize
sb.Append(bd.GetEncodedChunk(0, 2, "hex"))

Dim caseSensitive As Boolean = True
If sb.ContentsEqual("3080", caseSensitive) Or sb.ContentsEqual("3081", caseSensitive) Or sb.ContentsEqual("3082", caseSensitive) Or sb.ContentsEqual("3083", caseSensitive) Then
    Log("This is a .p7m or .p7s file.")
Else
    Log("This is not a .p7m or .p7s file.")
End If