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.
pgp
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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Useful bits of Cryptography – Hashes

More than just PGP

Cryptography is the science of securing communication from adversaries. In the email world it’s most obvious use is tools like PGP or S/MIME that are used to encrypt a message so that it can only be read by the intended recipient, or to sign a message so that the recipient can be sure of who it came from. There are quite a few other aspects of sending email where a little cryptography is useful or essential, though – bounce management, suppression lists, unsubscription handling, DKIM and DMARC, amongst others.

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Make Mail.app work for you

Mark Nottingham (@mnot) posted a good idea to twitter:
 

Highlight e-mails that your MTA receives with TLS. Make sure to include your mail server’s name in the value (here to the left of what’s shown)
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TLS and Encryption

Yesterday I talked about STARTTLS deployment, and how it was a good thing to support to help protect the privacy of your recipients.
STARTTLS is just one aspect of protecting email from eavesdropping; encrypting traffic as the mail is being sent or read and encrypting the message itself using PGP or S/MIME are others. This table shows what approaches protect messages at different stages of the messages life:
[table nl=”~”] Compromise point,SUBMIT~+IMAPS,TLS,PGP /~SMIME
Sender’s computer as mail is sent,,,
Sender’s computer later,,,[icon name=check-square] Sender’s network,[icon name=check-square],,[icon name=check-square] Sender’s ISP,,,[icon name=check-square] Global Internet (passive),,[icon name=check-square],[icon name=check-square] Global Internet (active),,[icon name=question-circle],[icon name=check-square] Global Internet (later),,[icon name=question-circle],[icon name=check-square] 3rd party mail services,,,[icon name=check-square] Recipient’s ISP,,,[icon name=check-square] Recipient’s network,[icon name=check-square],,[icon name=check-square] Recipient’s computer as mail is read,,,
Recipient’s computer later,,,[icon name=check-square] [/table] You can see that if you’re sending really sensitive data, you should be encrypting the entire message with PGP or S/MIME (or not sending the message via email at all). Doing so will protect the content of the mail against pretty most sorts of attack, but is pretty intrusive for the sender and recipient so can’t really be used without prior agreement with the recipient.
The other approaches will make some sorts of passive surveillance much more difficult, though.
Encrypting the connection a user uses to send mail ([rfc 6409]using the SUBMIT protocol[/rfc]) and to read mail ([rfc 2595]using TLS to protect IMAP or POP3[/rfc]) will protect against passive sniffing when the user is on possibly hostile network, such as public wifi or an employers network. That’s an easy place to try and sniff traffic, and if that traffic isn’t protected an attacker can not only read someone’s email, they can steal their credentials and cause all sorts of havoc. All general purpose mail clients and all ISPs support encryption here, so it’s almost universally used.
STARTTLS use with SMTP is all about protecting email traffic when it’s being sent between ISPs – both between the sender’s ISP and the recipient’s and also between any 3rd party mail services (outsourced spam filtering, mailing list providers, vanity domain fowarders, etc.).
I’ve listed three different sorts of attack on that inter-ISP traffic – passive, active and “later”.
A passive attack is where the attacker has the ability to listen to bytes as they go by, but isn’t able to modify or intercept them. While you might think of this as something a nation state would do, via secret agreements with backbone providers or high-tech fiber optic cable taps, there are ways a smaller attacker might be able to compromise an intermediate router and tap that traffic with little risk of detection. Deploying any sort of STARTTLS will protect against this, even if it’s misconfigured, using expired certificates or even just the default setup of a newly deployed mailserver. Facebook describe these weaker forms of STARTTLS as “opportunistic” in their survey – it’s not perfect, but it’s a lot better than nothing.
An active attack is one where the attacker has the ability to intercept and modify traffic between the two ISPs. This seems like it would be harder to do than a passive attack, but it’s often easier, though not as stealthy. Once that’s done, the attacker can pretend to be the recipient ISP and have full access to read, modify or discard messages. To protect against this sort of attack TLS needs to be used not just to encrypt the traffic in-flight, but also to allow the sender to validate that the mailserver they’re talking to really is who they think it is. This is what Facebook describe as “strict” – it requires that the mailserver have a valid certificate, issued by a legitimate certification authority for the domain that the mail is being sent to.
What about “later”? It’s easy to imagine a case where an attacker has been passively monitoring and recording encrypted traffic for a while, and then later they manage to acquire the encryption keys that were used (by, for example, issuing a subpoena to the recipient ISP, or using a compromise to rip them out of your servers memory). With many forms of encryption once you have those private keys it’s possible to decrypt all the traffic you’ve already captured. There are a few algorithms, though, that have what’s known as perfect forward secrecy – knowing the private keys that were used at the time the mail was transferred doesn’t allow you to decrypt them at a later time. If you’re concerned about the privacy of your messages, you should definitely read up on how to set that up.
All of these techniques are a great way to defend against ubiquitous or casual attempts to read your messages, but none of them are proof against a determined attacker. If all else fails, there’s always a wrench attack.
security

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