Email security is a rather wide-ranging topic, and one that I've wanted to cover for some time, well before several recent events that have made it come up in the wider public knowledge. There is no way I can hope to cover it in a single post (I think it would outpace even the length of my internationalization discussion), and there are definitely parts for which I am underqualified, as I am by no means an expert in cryptography. Instead, I will be discussing this over the course of several posts of which this is but the first; to ease up on the amount of background explanation, I will assume passing familiarity with cryptographic concepts like public keys, hash functions, as well as knowing what SSL and SSH are (though not necessarily how they work). If you don't have that knowledge, ask Wikipedia.
Before discussing how email security works, it is first necessary to ask what email security actually means. Unfortunately, the layman's interpretation is likely going to differ from the actual precise definition. Security is often treated by laymen as a boolean interpretation: something is either secure or insecure. The most prevalent model of security to people is SSL connections: these allow the establishment of a communication channel whose contents are secret to outside observers while also guaranteeing to the client the authenticity of the server. The server often then gets authenticity of the client via a more normal authentication scheme (i.e., the client sends a username and password). Thus there is, at the end, a channel that has both secrecy and authenticity : channels with both of these are considered secure and channels without these are considered insecure .
In email, the situation becomes more difficult. Whereas an SSL connection is between a client and a server, the architecture of email is such that email providers must be considered as distinct entities from end users. In addition, messages can be sent from one person to multiple parties. Thus secure email is a more complex undertaking than just porting relevant details of SSL. There are two major cryptographic implementations of secure email : S/MIME and PGP. In terms of implementation, they are basically the same , although PGP has an extra mode which wraps general ASCII (known as "ASCII-armor"), which I have been led to believe is less recommended these days. Since I know the S/MIME specifications better, I'll refer specifically to how S/MIME works.
S/MIME defines two main MIME types: multipart/signed, which contains the message text as a subpart followed by data indicating the cryptographic signature, and application/pkcs7-mime, which contains an encrypted MIME part. The important things to note about this delineation are that only the body data is encrypted , that it's theoretically possible to encrypt only part of a message's body, and that the signing and encryption constitute different steps. These factors combine to make for a potentially infuriating UI setup.
How does S/MIME tackle the challenges of encrypting email? First, rather than encrypting using recipients' public keys, the message is encrypted with a symmetric key. This symmetric key is then encrypted with each of the recipients' keys and then attached to the message. Second, by only signing or encrypting the body of the message, the transit headers are kept intact for the mail system to retain its ability to route, process, and deliver the message. The body is supposed to be prepared in the "safest" form before transit to avoid intermediate routers munging the contents. Finally, to actually ascertain what the recipients' public keys are, clients typically passively pull the information from signed emails. LDAP, unsurprisingly, contains an entry for a user's public key certificate, which could be useful in large enterprise deployments. There is also work ongoing right now to publish keys via DNS and DANE.
I mentioned before that S/MIME's use can present some interesting UI design decisions. I ended up actually testing some common email clients on how they handled S/MIME messages: Thunderbird, Apple Mail, Outlook , and Evolution. In my attempts to create a surreptitious signed part to confuse the UI, Outlook decided that the message had no body at all, and Thunderbird decided to ignore all indication of the existence of said part. Apple Mail managed to claim the message was signed in one of these scenarios, and Evolution took the cake by always agreeing that the message was signed . It didn't even bother questioning the signature if the certificate's identity disagreed with the easily-spoofable From address. I was actually surprised by how well people did in my tests—I expected far more confusion among clients, particularly since the will to maintain S/MIME has clearly been relatively low, judging by poor support for "new" features such as triple-wrapping or header protection.
Another fault of S/MIME's design is that it makes the mistaken belief that composing a signing step and an encryption step is equivalent in strength to a simultaneous sign-and-encrypt. Another page describes this in far better detail than I have room to; note that this flaw is fixed via triple-wrapping (which has relatively poor support). This creates yet more UI burden into how to adequately describe in UI all the various minutiae in differing security guarantees. Considering that users already have a hard time even understanding that just because a message says it's from email@example.com doesn't actually mean it's from firstname.lastname@example.org, trying to develop UI that both adequately expresses the security issues and is understandable to end-users is an extreme challenge.
What we have in S/MIME (and PGP) is a system that allows for strong guarantees, if certain conditions are met, yet is also vulnerable to breaches of security if the message handling subsystems are poorly designed. Hopefully this is a sufficient guide to the technical impacts of secure email in the email world. My next post will discuss the most critical component of secure email: the trust model. After that, I will discuss why secure email has seen poor uptake and other relevant concerns on the future of email security.
 This is a bit of a lie: a channel that does secrecy and authentication at different times isn't as secure as one that does them at the same time.
 It is worth noting that authenticity is, in many respects, necessary to achieve secrecy.
 This, too, is a bit of a lie. More on this in a subsequent post.
 I'm very aware that S/MIME and PGP use radically different trust models. Trust models will be covered later.
 S/MIME 3.0 did add a provision stating that if the signed/encrypted part is a message/rfc822 part, the headers of that part should override the outer message's headers. However, I am not aware of a major email client that actually handles these kind of messages gracefully.
 Actually, I tested Windows Live Mail instead of Outlook, but given the presence of an official MIME-to-Microsoft's-internal-message-format document which seems to agree with what Windows Live Mail was doing, I figure their output would be identical.
 On a more careful examination after the fact, it appears that Evolution may have tried to indicate signedness on a part-by-part basis, but the UI was sufficiently confusing that ordinary users are going to be easily confused.