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?

Java

import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String argv[])
  {
    boolean 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 = new CkBinData();

    success = bd.LoadFile("qa_data/p7m/a.p7m");
    if (success == false) {
        System.out.println("Failed to load the file.");
        return;
        }

    CkStringBuilder sb = new CkStringBuilder();
    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.");
        }
  }
}