(Objective-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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#import <CkoBinData.h>
#import <CkoStringBuilder.h>

// 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.

CkoBinData *bd = [[CkoBinData alloc] init];

BOOL success = [bd LoadFile: @"qa_data/p7m/a.p7m"];
if (success == NO) {
    NSLog(@"%@",@"Failed to load the file.");
    return;
}

CkoStringBuilder *sb = [[CkoStringBuilder alloc] init];
[sb Append: [bd GetEncodedChunk: [NSNumber numberWithInt: 0] numBytes: [NSNumber numberWithInt: 2] encoding: @"hex"]];

BOOL caseSensitive = YES;
if ([sb ContentsEqual: @"3080" caseSensitive: caseSensitive] || [sb ContentsEqual: @"3081" caseSensitive: caseSensitive] || [sb ContentsEqual: @"3082" caseSensitive: caseSensitive] || [sb ContentsEqual: @"3083" caseSensitive: caseSensitive]) {
    NSLog(@"%@",@"This is a .p7m or .p7s file.");
}
else {
    NSLog(@"%@",@"This is not a .p7m or .p7s file.");
}