(C) How to Tell if a File is .p7m or .p7s

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?

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#include <C_CkBinData.h>
#include <C_CkStringBuilder.h>

void ChilkatSample(void)
    {
    HCkBinData bd;
    BOOL success;
    HCkStringBuilder sb;
    BOOL caseSensitive;

    // 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 = CkBinData_Create();

    success = CkBinData_LoadFile(bd,"qa_data/p7m/a.p7m");
    if (success == FALSE) {
        printf("Failed to load the file.\n");
        CkBinData_Dispose(bd);
        return;
    }

    sb = CkStringBuilder_Create();
    CkStringBuilder_Append(sb,CkBinData_getEncodedChunk(bd,0,2,"hex"));

    caseSensitive = TRUE;
    if (CkStringBuilder_ContentsEqual(sb,"3080",caseSensitive) || CkStringBuilder_ContentsEqual(sb,"3081",caseSensitive) || CkStringBuilder_ContentsEqual(sb,"3082",caseSensitive) || CkStringBuilder_ContentsEqual(sb,"3083",caseSensitive)) {
        printf("This is a .p7m or .p7s file.\n");
    }
    else {
        printf("This is not a .p7m or .p7s file.\n");
    }



    CkBinData_Dispose(bd);
    CkStringBuilder_Dispose(sb);

    }