A friend of mine, a software engineer, just asked me this.
Why do you force people to enter that enormous key just to protect their sync data? Passwords are sufficient for banking institutions and payroll facilities, arguably with more important data than your bookmarks. Why not make it optional extra security for those who want it, instead of making everyone pass around a 26 character string to every machine they want to sync from, and risk losing all of their information if it’s lost?
The answer is quite long-winded. Here’s a slightly edited version of my response.
Firstly, we try to make sure that people don’t have to enter it; we’re not blind to the additional complication involved. The sync key is generated for you during setup, so you don’t have to think up another password. It’s stored in Password Manager so you don’t have to remember it. When you set up a new laptop or Android phone you can usually use Easy Setup (the “forefront” UI in Firefox 4), which is much like Bluetooth pairing, so you don’t have to type it. The UI will only continue to hide the Sync key more deeply as we start to introduce better means of credentials exchange (such as QR codes for time-delayed J-PAKE)… in fact, soon enough we’re likely to rename it “Recovery Key”, because that’s what it’s for.
Secondly, that long string is an AES key, with all the joy it brings. We encrypt your data locally because we sync your entire history, bookmarks, and passwords, including access to banks, messages from revolutionary organizations, doctors’ heath data (HIPAA!), and more… and we have over a million active users. A breach without strong local encryption would make the PSN intrusion look like 4chan trolling. We want to ensure that we can’t get your data, either deliberately or under the coercion of the FBI. Being able to recover a user’s data from our servers means we are required to give your browsing history to the FBI if they show up with a warrant. Ever visited thepiratebay?
In essence, we make the same promises as DropBox, but we actually keep them. We really can’t betray your trust, and the sync key is why.
“But why not use the password for encryption?”. I’m glad you asked.
A password is inadequate for this purpose. We used to allow a user-entered passphrase in place of the sync key, but it had a lot of problems.
For one thing, users didn’t understand why they needed two passwords… and using just one is a terrible idea! Your account password goes over the wire for HTTP auth, and HTTPS is not always a defense — quite apart from the possibility of a compromised HTTP server (an attack vector against which we want to guard), I’ve personally helped out two users whose employers were running SSL MITM proxies, which allows them to snoop HTTPS traffic… including HTTP auth headers. We only detected it because the user’s employer had added their own root certificate to Windows’ cert store, but not Firefox’s, and Firefox threw a certificate error. Your HTTPS traffic is visible to your employer. That’s a terrible thing, but your Sync data is still secure, because we don’t just use your password as an encryption key.
The other issue with passwords is that there just isn’t enough entropy in a user-entered string to support our cryptographic guarantees, even with PBKDF2 as a bootstrap algorithm. Put it this way: is your password twenty-six base36 characters long (a solid 128-bit key), or is it eight to twelve letters with a couple of numbers? I thought so. Most people’s passwords aren’t even that strong.
Payroll facilities… well, they don’t care, and in my experience they typically don’t understand technology too well. Just because ADP, or Sony, or DropBox jump off a bridge doesn’t mean we’re going to throw our users off, too.
Speaking more broadly: it would be really convenient for Firefox Sync to not use encryption. We could let you see your bookmarks in a webpage (a common request), and the client (the code I maintain) would be much simpler! That’s how Chrome approaches this problem… Google wants to see your bookmarks. But that’s not really how Mozilla works; we try to err on the side of safety, freedom, and serving the users’ best interests, rather than opting for the expedient solution. The vast majority of users simply do not have the knowledge to correctly evaluate the decision you’re asking them to make. That’s why users put their bank URL (and credentials, apparently!) into delicious, and put their private keys on the web.
And that’s why we don’t let you upload the contents of your Firefox profile with weak or no encryption.