C++

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?

C++

#include <CkBinData.h>
#include <CkStringBuilder.h>

void ChilkatSample(void)
    {
    bool 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.

    CkBinData bd;

    success = bd.LoadFile("qa_data/p7m/a.p7m");
    if (success == false) {
        std::cout << "Failed to load the file." << "\r\n";
        return;
    }

    CkStringBuilder sb;
    sb.Append(bd.getEncodedChunk(0,2,"hex"));

    bool caseSensitive = true;
    if (sb.ContentsEqual("3080",caseSensitive) || sb.ContentsEqual("3081",caseSensitive) || sb.ContentsEqual("3082",caseSensitive) || sb.ContentsEqual("3083",caseSensitive)) {
        std::cout << "This is a .p7m or .p7s file." << "\r\n";
    }
    else {
        std::cout << "This is not a .p7m or .p7s file." << "\r\n";
    }
    }