Rust

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?

Rust

// 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.

let bd = chilkat::BinData::new();

if bd.load_file("qa_data/p7m/a.p7m").is_err() {
    println!("Failed to load the file.");
    return;
}

let sb = chilkat::StringBuilder::new();
let _ = sb.append(&bd.get_encoded_chunk(0, 2, "hex").unwrap_or_default());

let case_sensitive = true;
if sb.contents_equal("3080", case_sensitive) || (sb.contents_equal("3081", case_sensitive) || (sb.contents_equal("3082", case_sensitive) || sb.contents_equal("3083", case_sensitive))) {
    println!("This is a .p7m or .p7s file.");
} else {
    println!("This is not a .p7m or .p7s file.");
}