如何使用Mimekit检查MIME消息是否已加密或签名？

Question

I want to JUST check if a MIME message is encrypted or signed using Mimekit.

I don't want to decrypt it or verify its signature, I just want to know if it is encrypted or signed.

I expected to find this as functions: IsSigned() & IsEncrypted() in the MimeMessage Class but these functions don't exist!

Thank you.

Answer 1

Those methods don't exist because a MimeMessage itself cannot be signed or encrypted, only the Body (or a subset of the body) of the message can be signed or encrypted.

A very simple solution is to do this:

var pkcs7 = message.Body as ApplicationPkcs7Mime;

bool signed = (message.Body is MultipartSigned) ||
    (pkcs7 != null && pkcs7.SecureMimeType == SecureMimeType.SignedData);

bool encrypted = (message.Body is MultipartEncrypted) ||
    (pkcs7 != null && pkcs7.SecureMimeType == SecureMimeType.EnvelopedData);

Note: MultipartEncrypted is only used with PGP (not S/MIME), so if you only care about S/MIME, you do not need to check for MultipartEncrypted .

Likewise, ApplicationPkcs7Mime is only used for S/MIME, so if you only care about PGP, you do not need to check for ApplicationPkcs7Mime .

Both PGP and S/MIME (can) use MultipartSigned , though, so you'll have to check that in either case.

Since every client I've ever used or received mail from only ever signs and/or encrypts the top-level Body part of the message, the above check is likely all you'll need. However, it is possible for MIME-compliant clients to sign and/or encrypt a subpart of the Body , so you'd have to traverse the MIME tree and check each node if you want to be 100% compliant.