Zig

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?

Zig

const std = @import("std");
const chilkat = @import("chilkat");

pub fn main(init: std.process.Init) !void {
    const alloc = init.arena.allocator();

    // 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.

    const bd = try chilkat.BinData.init();
    defer bd.deinit();

    bd.loadFile("qa_data/p7m/a.p7m") catch {
        std.debug.print("Failed to load the file.\n", .{});
        return;
    };

    const sb = try chilkat.StringBuilder.init();
    defer sb.deinit();
    sb.append(try bd.getEncodedChunk(alloc, 0, 2, "hex")) catch {};

    const case_sensitive = true;
    if (sb.contentsEqual("3080", case_sensitive) or (sb.contentsEqual("3081", case_sensitive) or (sb.contentsEqual("3082", case_sensitive) or sb.contentsEqual("3083", case_sensitive)))) {
        std.debug.print("This is a .p7m or .p7s file.\n", .{});
    } else {
        std.debug.print("This is not a .p7m or .p7s file.\n", .{});
    }
}