PowerShell

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?

PowerShell

Add-Type -Path "C:\chilkat\ChilkatDotNet47-x64\ChilkatDotNet47.dll"

$success = $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.

$bd = New-Object Chilkat.BinData

$success = $bd.LoadFile("qa_data/p7m/a.p7m")
if ($success -eq $false) {
    $("Failed to load the file.")
    exit
}

$sb = New-Object Chilkat.StringBuilder
$sb.Append($bd.GetEncodedChunk(0,2,"hex"))

$caseSensitive = $true
if ($sb.ContentsEqual("3080",$caseSensitive) -or $sb.ContentsEqual("3081",$caseSensitive) -or $sb.ContentsEqual("3082",$caseSensitive) -or $sb.ContentsEqual("3083",$caseSensitive)) {
    $("This is a .p7m or .p7s file.")
}
else {
    $("This is not a .p7m or .p7s file.")
}