Using OATH Toolkit with Dropbox

Today there was an announcement that Dropbox supports two-factor authentication. On their page with detailed instructions there is (at the bottom) a link to the man page of the OATH Toolkit command line utility oathtool. OATH Toolkit is available in Ubuntu 12.04 and Debian Wheezy. (Note that the 1.10.4 version in Ubuntu does not support the base32 features.) There is not a lot of details in the documentation on Dropbox’s site on how to use oathtool, but I have experimented a bit with the setup and I’d like to share my findings. I assume you are somewhat familiar with the OATH Toolkit; if not I suggest reading my earlier introduction to OATH Toolkit.

To use OATH Toolkit’s command line utility to generate OTPs that are accepted by Dropbox, here is how you proceed. When you enable two-factor authentication on Dropbox’s site, you must select “Use a mobile app” and on the next screen with the QR code image, click the “enter your secret key manually” link. You will then be presented with a code that looks like this: gr6d 5br7 25s6 vnck v4vl hlao re

Now this code is actually space-delimitted base32 encoded data, without any padding. Since version 1.12.0, oathtool can read base32 encoded keys. However, parsing the raw string fails, so to make it work, you need to remove the spaces and add padding characters. I have yet to see any documentation on the Dropbox implementation, but I assume they always generate 16 binary octets that are base32 encoded into 26 characters like the codes that I have seen. The code is the cryptographic key used for the HMAC-SHA1 computation described in the RFC 6238 that specify OATH TOTP. If you study the base32 encoding you discover that 26 characters needs six pad characters. So converted into proper base32, the string would be gr6d5br725s6vnckv4vlhlaore======. Now generating OTPs are easy, see below.

jas@latte:~$ oathtool --verbose --totp --base32 "gr6d5br725s6vnckv4vlhlaore======"
Hex secret: 347c3e863fd765eab44aaf2ab3ac0e89
Base32 secret: GR6D5BR725S6VNCKV4VLHLAORE======
Digits: 6
Window size: 0
Step size (seconds): 30
Start time: 1970-01-01 00:00:00 UTC (0)
Current time: 2012-08-27 21:22:54 UTC (1346102574)
Counter: 0x2ACA9C5 (44870085)


Dropbox’s implementation is robust in that it requests a valid OTP from me, generated using the secret they just displayed, before proceeding. This verifies that the user was able to import the key correctly, and that the users’ OATH TOTP implementation appears to work. If I type in the OTP generated from oathtool this way, it allowed me to enable two-factor authentication and I agreed. From that point, signing into the Dropbox service will require a OTP. I invoke the tool, using the same arguments as above, and the tool will use the current time to compute a fresh OTP.

Reflecting on how things could work smoother, I suppose oathtool could be more permissive when it performs the base32 decoding so that the user doesn’t have to fix the base32 spacing/padding manually. I’ll consider this for future releases.