Java

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?

Chilkat2 Java

import com.chilkatsoft.chilkat2.BinData;
import com.chilkatsoft.chilkat2.ChilkatException;
import com.chilkatsoft.chilkat2.StringBuilder;

public class ChilkatExample {

    public static void main(String[] args) {
        // 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.

        BinData bd = new BinData();

        try {
            bd.loadFile("qa_data/p7m/a.p7m");
        } catch (ChilkatException e) {
            System.out.println("Failed to load the file.");
            return;
        }

        StringBuilder sb = new StringBuilder();
        sb.append(bd.getEncodedChunk(0, 2, "hex"));

        boolean caseSensitive = true;
        if (sb.contentsEqual("3080", caseSensitive) || (sb.contentsEqual("3081", caseSensitive) || (sb.contentsEqual("3082", caseSensitive) || sb.contentsEqual("3083", caseSensitive)))) {
            System.out.println("This is a .p7m or .p7s file.");
        } else {
            System.out.println("This is not a .p7m or .p7s file.");
        }
    }
}