Lua

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?

Lua

local chilkat = require("chilkat")

-- 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.

local bd = chilkat.BinData.new()

local success = bd:load_file("qa_data/p7m/a.p7m")
if not success then
    print("Failed to load the file.")
    return
end

local sb = chilkat.StringBuilder.new()
sb:append(bd:get_encoded_chunk(0, 2, "hex"))

local caseSensitive = true
if sb:contents_equal("3080", caseSensitive) or (sb:contents_equal("3081", caseSensitive) or (sb:contents_equal("3082", caseSensitive) or sb:contents_equal("3083", caseSensitive))) then
    print("This is a .p7m or .p7s file.")
else
    print("This is not a .p7m or .p7s file.")
end