These are notes from my research that led to the publication of the password hashers article. This article is more technical than the previous ones and compares the various cryptographic primitives and algorithms used in the various software I have reviewed. The criteria for inclusion on this list is fairly vague: I mostly included a password hasher if it was significantly different from the previous implementations in some way, and I have included all the major ones I could find as well.
Nic Wolff claims to be the first to have written such a program, all the way back in 2003. Back then the hashing algorithm was MD5, although Wolff has now updated the algorithm to use SHA-1 and still maintains his webpage for public use. Another ancient but unrelated implementation, is the Standford University Applied Cryptography's pwdhash software. That implementation was published in 2004 and unfortunately, that implementation was not updated and still uses MD5 as an hashing algorithm, but at least it uses HMAC to generate tokens, which makes the use of rainbow tables impractical. Those implementations are the simplest password hashers: the inputs are simply the site URL and a password. So the algorithms are, basically, for Wolff's:
token = base64(SHA1(password + domain))
And for Standford's PwdHash:
token = base64(HMAC(MD5, password, domain)))
Another unrelated implementation that is still around is supergenpass is a bookmarklet that was created around 2007, originally using MD5 as well but now supports SHA512 now although still limited to 24 characters like MD5 (which needlessly limits the entropy of the resulting password) and still defaults MD5 with not enough rounds (10, when key derivation recommendations are more generally around 10 000, so that it's slower to bruteforce).
Note that Chris Zarate, the supergenpass author, actually credits Nic Wolff as the inspiration for his implementation. Supergenpass is still in active development and is available for the browser (as a bookmarklet) or mobile (as an webpage). Supergenpass allows you to modify the password length, but also add an extra profile secret which adds to the password and generates a personalized identicon presumably to prevent phishing but it also introduces the interesting protection, the profile-specific secret only found later in Password Hasher Plus. So the Supergenpass algorithm looks something like this:
token = base64(SHA512(password + profileSecret + ":" + domain, rounds))
Another popular implementation is the Wijjo Password Hasher, created around 2006. It was probably the first shipped as a browser extension which greatly improved the security of the product as users didn't have to continually download the software on the fly. Wijjo's algorithm also improved on the above algorithms, as it uses HMAC-SHA1 instead of plain SHA-1 or HMAC-MD5, which makes it harder to recover the plaintext. Password Hasher allows you to set different password policies (use digits, punctuation, mixed case, special characters and password length) and saves the site names it uses for future reference. It also happens that the Wijjo Password Hasher, in turn, took its inspiration on different project, hashapass.com, created in 2006 and also based on HMAC-SHA-1. Indeed, hashapass "can easily be generated on almost any modern Unix-like system using the following command line pattern":
echo -n parameter \ | openssl dgst -sha1 -binary -hmac password \ | openssl enc -base64 \ | cut -c 1-8
So the algorithm here is obviously:
token = base64(HMAC(SHA1, password, domain + ":" + counter)))[:8]
... although in the case of Password Hasher, there is a special routine that takes the token and inserts random characters in locations determined by the sum of the values of the characters in the token.
token = base64(HMAC(SHA1, password, base64(HMAC(SHA1, profileSecret, domain + ":" + counter))))
Honestly, that seems rather strange, but it's what I read from the source code, which is available only after decompressing the extension nowadays. I would have expected the simplest version:
token = base64(HMAC(SHA1, HMAC(SHA1, profileSecret, password), domain + ":" + counter))
The idea here would be "hide" the master password from bruteforce attacks as soon as possible... But maybe this is all equivalent.
Regardless, Password Hasher Plus then takes the token and applies the same special character insertion routine as the Password Hasher.
Last year, Guillaume Vincent a french self-described "humanist and scuba diving fan" released the lesspass extension for Chrome, Firefox and Android. Lesspass introduces several interesting features. It is probably the first to include a commandline version. It also uses a more robust key derivation algorithm (PBKDF2) and takes into account the username on the site, allowing multi account support. The original release (version 1) used only 8192 rounds which is now considered too low. In the bug report it was interesting to note that LessPass couldn't do the usual practice of running the key derivation for 1 second to determine the number of rounds needed as the results need to be deterministic.
At first glance, the LessPass source code seems clear and easy to read which is always a good sign, but of course, the devil is in the details. One key feature that is missing from Password Hasher Plus is the profile-specific seed, although it should be impossible, for a hostile web page to steal keystrokes from a browser extension, as far as I know.
The algorithm then gets a little more interesting:
entropy = PBKDF2(SHA256, masterPassword, domain + username + counter, rounds, length) where rounds=10000 length=32
entropy is then used to pick characters to match the chosen profile.
There is also a lesspass-specific character picking routing that is also not base64, and different from the original Password Hasher algorithm.
A review of password hashers would hardly be complete without mentioning the Master Password and its elaborate algorithm. While the applications surrounding the project are not as refined (there is no web browser plugin and the web interface can't be easily turned into a bookmarklet), the algorithm has been well developed. Of all the password managers reviewed here, Master Password uses one of the strongest key derivation algorithms out there, scrypt:
key = scrypt( password, salt, cost, size, parallelization, length ) where salt = "com.lyndir.masterpassword" + len(username) + name cost = 32768 size = 8 parallelization = 2 length = 64 entropy = hmac-sha256(key, "com.lyndir.masterpassword" + len(domain) + domain + counter )
Master Password the uses one of 6 sets of "templates" specially
crafted to be "easy for a user to read from a screen and type using a
keyboard or smartphone" and "compatible with most site's password
policies", our "transferable" criteria defined
in the first passwords article. For example, the default template
mixes vowels, consonants, numbers and symbols, but carefully avoiding
possibly visibly similar characters like
(although it does mix
l, oddly enough).
The main strength of Master Password seems to be the clear definition of its algorithm (although Hashpass.com does give out OpenSSL commandline examples...), which led to its reuse in another application called freepass. The Master Password app also doubles as a stateful password manager...
I have also considered including easypasswords, which uses PBKDF2-HMAC-SHA1, in my list of recommendations. I discovered only recently that the author wrote a detailed review of many more password hashers and scores them according to their relative strength. In the end, I ended up covering more LessPass since the design is very similar and LessPass does seem a bit more usable. Covering LessPass also allowed me to show the contrast and issues regarding the algorithm changes, for example.
It is also interesting to note that the EasyPasswords author has criticized the Master Password algorithm quite severely:
[...] scrypt isn’t being applied correctly. The initial scrypt hash calculation only depends on the username and master password. The resulting key is combined with the site name via SHA-256 hashing then. This means that a website only needs to break the SHA-256 hashing and deduce the intermediate key — as long as the username doesn’t change this key can be used to generate passwords for other websites. This makes breaking scrypt unnecessary[...]
During a discussion with the Master Password author, he outlined that "there is nothing "easy" about brute-force deriving a 64-byte key through a SHA-256 algorithm." SHA-256 is used in the last stage because it is "extremely fast". scrypt is used as a key derivation algorithm to generate a large secret and is "intentionnally slow": "we don't want it to be easy to reverse the master password from a site password". "But it' unnecessary for the second phase because the input to the second phase is so large. A master password is tiny, there are only a few thousand or million possibilities to try. A master key is 8^64, the search space is huge. Reversing that doesn't need to be made slower. And it's nice for the password generation to be fast after the key has been prepared in-memory so we can display site passwords easily on a mobile app instead of having to lock the UI a few seconds for every password."
Finally, I considered covering Blum's Mental Hash (also covered here and elsewhere). This consists of an algorithm that can basically be ran by the human brain directly. It's not for the faint of heart, however: if I understand it correctly, it will require remembering a password that is basically a string of 26 digits, plus compute modulo arithmetics on the outputs. Needless to say, most people don't do modulo arithmetics every day...