leah blogs: October 2019

09oct2019 · Ken Thompson's Unix password

Somewhere around 2014 I found an /etc/passwd file in some dumps of the BSD 3 source tree, containing passwords of all the old timers such as Dennis Ritchie, Ken Thompson, Brian W. Kernighan, Steve Bourne and Bill Joy.

Since the DES-based crypt(3) algorithm used for these hashes is well known to be weak (and limited to at most 8 characters), I thought it would be an easy target to just crack these passwords for fun.

Well known tools for this are john and hashcat.

Quickly, I had cracked a fair deal of these passwords, many of which were very weak. (Curiously, bwk used /.,/.,, which is easy to type on a QWERTY keyboard.)

However, kens password eluded my cracking endeavor. Even an exhaustive search over all lower-case letters and digits took several days (back in 2014) and yielded no result. Since the algorithm was developed by Ken Thompson and Robert Morris, I wondered what’s up there. I also realized, that, compared to other password hashing schemes (such as NTLM), crypt(3) turns out to be quite a bit slower to crack (and perhaps was also less optimized).

Did he really use uppercase letters or even special chars? (A 7-bit exhaustive search would still take over 2 years on a modern GPU.)

The topic came up again earlier this month on The Unix Heritage Society mailing list, and I shared my results and frustration of not being able to break kens password.

Finally, today this secret was resolved by Nigel Williams:

From: Nigel Williams <nw@retrocomputingtasmania.com>
Subject: Re: [TUHS] Recovered /etc/passwd files

ken is done:


took 4+ days on an AMD Radeon Vega64 running hashcat at about 930MH/s
during that time (those familiar know the hash-rate fluctuates and
slows down towards the end).

This is a chess move in descriptive notation, and the beginning of many common openings. It fits very well to Ken Thompson’s background in computer chess.

I’m very happy that this mystery has been solved now and I’m pleased of the answer.

[Update 16:29: fix comment on chess.]

NP: Mel Stone—By Now

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