Alice and Bob and PGP Keys

Last week Alice and Bob showed how to cryptographically sign messages so that the recipient can be sure that the message came from the purported sender and hasn’t been forged by a third party. They can only do that if they can securely retrieve the senders public key – which means they need to retrieve it from the actual sender, rather than an impostor, and be sure it’s not tampered with en route. How does this work in practice?
If I want to send someone an encrypted email, or I want to verify that a signed email I received from them is valid, I need a copy of their public key (almost certainly their PGP key, in practice). Perhaps I retrieve it from their website, or from a copy they’ve sent me in the past, or even from a public keyserver. Depending on how I retrieved the key, and how confident I need to be about the key ownership, I might want to double check that the key belongs to who I think it does. I can check that using the fingerprint of the key.
A key fingerprint looks like this:

pub 4096R/856DCBBC 2014-09-22 [expires: 2024-09-19] Key fingerprint = 7393 3400 738B ABFD 3C7A 4F31 B3E7 BD58 856D CBBC
uid Steve Atkins <steve@wordtothewise.com>

I can call my correspondent on the ‘phone to check that the fingerprint of the key I have matches their key, or check the fingerprint against somewhere they’ve published it, maybe a public blog post, or on their business card:
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Another option is to use a service like keybase.io, which allow people to connect their keys to their online or social media presence, and provides some tools to make using PGP less user-hostile.
If this all seems a bit ad hoc to you, you’re right. The original approach PGP used was a web-of-trust, where you’d nominate people whose public keys you had, and you knew, as trusted or partially trusted. If someone was introduced to you, and their public key was vouched for by some configurable number of people you trusted then you’d believe that the key belonged to them. Combine that with key-signing-parties (which aren’t as exciting as they sound, just a bunch of people getting together to vouch for each others PGP keys), and you end up with a decentralized approach that sounds wonderful to an introverted crypto-nerd, and works well enough within small groups, but isn’t attractive to enough people to be able to take advantage of the network effect to break out beyond those small cliques. Web-of-trust requires far too much work and advance planning to be convenient for casual use of encrypted email other than amongst small groups of crypto-nerd friends, yet isn’t really secure enough to rely on where it’s important.
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Cryptography and Email

A decade or so ago it was fairly rare for cryptography and email technology to intersect – there was S/MIME (which I’ve seen described as having “more implementations than users”) and PGP, which was mostly known for adding inscrutable blocks of text to mail and for some interesting political fallout, but not much else.
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That’s changing, though. Authentication and privacy have been the focus of much of the development around email for the past few years, and cryptography, specifically public-key cryptography, is the tool of choice.
DKIM uses public-key cryptography to let the author (or their ESP, or anyone else) attach their identity to the message in a way that’s almost impossible to forge. That lets the recipient make informed decisions about whether to deliver the email or not.
DKIM relies on DNS to distribute it’s public keys, so if you can interfere with DNS, you can compromise DKIM. More than that, if you can compromise DNS you can break many security processes – interfering with DNS is an early part of many attacks. DNSSEC (Domain Name System Security Extensions) lets you be more confident that the results you get back from a DNS query are valid. It’s all based on public-key cryptography. It’s taken a long time to deploy, but is gaining steam.
TLS has escaped from the web, and is used in several places in email. For end users it protects their email (and their passwords) as they send mail via their smarthost or fetch it from their IMAP server. More recently, though, it’s begun to be used “opportunistically” to protect mail as it travels between servers – more than half of the mail gmail sees is protected in transit. Again, public-key cryptography. Perhaps you don’t care about the privacy of the mail you’re sending, but the recipient ISP may. Google already give better search ranking for web pages served over TLS – I wouldn’t be surprised if they started to give preferential treatment to email delivered via TLS.
The IETF is beginning to discuss end-to-end encryption of mail, to protect mail against interception and traffic analysis. I’m not sure exactly where it’s going to end up, but I’m sure the end product will be cobbled together using, yes, public-key cryptography. There are existing approaches that work, such as S/MIME and PGP, but they’re fairly user-hostile. Attempts to package them in a more user-friendly manner have mostly failed so far, sometimes spectacularly. (Hushmail sacrificed end-to-end security for user convenience, while Lavabit had similar problems and poor legal advice).
Not directly email-related, but after the flurry of ESP client account breaches a lot of people got very interested in two-factor authentication for their users. TOTP (Time-Based One-Time Passwords) – as implemented by SecureID and Google Authenticator, amongst many others – is the most commonly used method. It’s based on public-key cryptography. (And it’s reasonably easy to integrate into services you offer).
Lots of the other internet infrastructure you’re relying on (BGP, syn cookies, VPNs, IPsec, https, anything where the manual mentions “certificate” or “key” …) rely on cryptography to work reliably. Knowing a little about how cryptography works can help you understand all of this infrastructure and avoid problems with it. If you’re already a cryptography ninja none of this will be a surprise – but if you’re not, I’m going to try and explain some of the concepts tomorrow.

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