When changing the password for a user on my system, why does the size of /etc/shadow never get bigger or smaller?

Question

When changing the password for a user on my Ubuntu system, I've noticed that no matter the length of the password that I choose the /etc/shadow file will always stay the same size and never increase/decrease.

How is this possible?

Why should it? It doesn't contain passwords. – Pilot6 Feb 28 '19 at 21:46 — Pilot6, Feb 28 '19 at 21:46
If you add another user, it should increase. – Pilot6 Feb 28 '19 at 21:49 — Pilot6, Feb 28 '19 at 21:49

score 3 · Accepted Answer · answered Feb 28 '19 at 21:46

3

Because it's saved as a hash and hashes of the same algorithm are the same length no matter what.

In Ubuntu I think the default algorithm for user passwords is sha512, which, as the name implies is always 512bit = 86byte long.

See:

answered Feb 28 '19 at 21:46

pLumo

26,947

Thank you - and for your help everybody else. :) – elliott94 Mar 02 '19 at 12:00

score 2 · Answer 2 · answered Feb 28 '19 at 21:47

The password is converted to a hash and that is a one way process.

So when you enter a password it will be converted to a hash and the hash will be compared with the hash in /etc/shadow and if it is a match you have entered the correct password.

Ref.: https://www.aychedee.com/2012/03/14/etc_shadow-password-hash-formats/

When changing the password for a user on my system, why does the size of /etc/shadow never get bigger or smaller?

2 Answers2