On Ubuntu/Debian mkpasswd
is part of the package whois and implemented in mkpasswd.c
which as actually just a sophisticated wrapper around the crypt()
function in glibc declared in unistd.h
. crypt() takes two arguments password and salt. Password is "test" in this case, salt is prepended by "$6$" for the SHA-512 hash (see SHA-crypt) so "$6$Zem197T4" is passed to crypt().
Maybe you noticed the -R
option of mkpasswd
which determines the number of rounds. In the document you'll find a default of 5000 rounds. This is the first hint why the result would never be equal to the simple concatenation of salt and password, it's not hashed only once. Actually if you pass -R 5000
you get the same result. In this case "$6$rounds=5000$Zem197T4" is passed to crypt() and the implementation in glibc (which is the libc of Debian/Ubuntu) extracts the method and number of rounds from this.
What happens inside crypt() is more complicated than just computing a single hash and the result is base64 encoded in the end. That's why the result you showed contains all kinds of characters after the last '$' and not only [0-9a-f] as in the typical hex string of a SHA-512 hash. The algorithm is described in detail in the already mentioned SHA-Crypt document.
I think this is not feasible.
As @Eir Nym says, you fairly obviously can't translate, for example,
an MD5-hash of a password into a SHA-256 hash, without knowing the
password.
However you would think that you'd be able to translate a SHA-256
hash (according to crypt(3)) into a SHA-256 hash (according to
OpenLDAP): surely it's just a matter of identifying the salt and hash
in one format, and reordering them into the format the other expects.
No? What, it's not?!
In OpenLDAP, the format of the userPassword
attribute, though not
quite documented, is relatively straightforward: it's the fixed-length
hash of the password followed by the salt (see eg
contrib/slapd-modules/passwd/sha2
in the OpenLDAP
sources). This is also evident in notes such as this OpenLDAP FAQ-o-matic article which shows how to generate a {SSHA}
hash from password + salt.
For systems which use crypt(3), however, you'll see ‘password hashes’
of the form
$id$salt$hash
where $5$
indicates SHA-256, $6$
indicates SHA-512, and so on. The
hash
element of that is the base64 encoded form of some
function of key, salt, and (in some variants) number of encryption
rounds, but the actual function is... not documented.
- The Wikipedia page on crypt(3)
mentions that ‘Over time various algorithms have been introduced.’
- It points to the ‘Password Hashing
Competition’
specification (PHC), which narrows down the gross format of the ‘hash’
string, but not as much as you'd hope. It doesn't even
conclusively identify what base64 variant the string uses (it
appears to, but acknowledges that characters
[.-]
can appear in
a base64 string without saying what they are).
- The passlib
library
acknowledges some of the mess (‘All of the above is guesswork
based on examination of existing hashes and OS implementations’)
and recommends the PHC format.
- A linuxquestions.org
question
indicates that the relevant base64 encoding is peculiar.
So we can look at the glibc sources
for crypt. There, we discover (in crypt_util.c
) that the base64
alphabet is indeed odd, using "./0-9A-Za-b"
rather than the more
usual (and PHC-specified) "A-Za-z0-9+/"
(with occasional variants in
the last two characters). Not helpful, but we could cope if this were
the only difference.
However, the sha256-crypt.c
function in that library (for example)
takes the key and salt and does something very odd and long-winded
with them to produce the hash
value. I wouldn't presume to venture an
opinion on whether what it's doing is sensible, but it's certainly not
straightforward.
What that means is that although the crypt(3) $5$
and OpenLDAP ‘SHA-256’
hashes are on the face of it using the same cryptographic primitive,
they are actually using it in sufficiently different ways that they
amount to different hash functions so that, as with the point at the
top, you can't translate one to the other without knowing the
password. A similar point can be made about $6$
/SHA-512 passwords.
This is very annoying.
Parenthetical note: As it happens, it is possible to convert the $1$
/SMD5 ‘hashes’ between crypt(3) format and OpenLDAP:
$1$salt$hash
corresponds to
'{SMD5}' + hash + salt
where +
is simple string concatenation. But we probably shouldn't be using
MD5 hashes any more, so this doesn't much help.
Note also that you can port your crypt(3) hashes to OpenLDAP unchanged, using
{CRYPT}$1$salt$hash
if your libc is compatible (which it typically will be, if OpenLDAP is running on Linux), but OpenLDAP won't generate new passwords this way, so this is a one-way street.
Best Answer
Setting a password with
passwd
orchpasswd
generates a random salt, so users who happen to have the same password would not have identical hashes. In order to have identical hashes this way, you'd have to have a misconfigured system that somehow doesn't save entropy between reboots, and systems that are so completely identical as to repeat the random seed (that can happen with VMs, especially when resuming from snapshots), and for all the passwords to be generated after reading exactly the same number of bytes from/dev/urandom
. This is highly unlikely.Thus, if you see identical hashes, it means that the hashes were copied. The passwords were deliberately made the same, rather than by coincidentally setting the same password multiple times. Either the administrator directly edited the password database and copying the hashes, or they used something like
chpasswd -e
to supply hashes.The only bad consequence of repeating a password hash is that it makes it apparent that the accounts have the same password. With distinct salts, as is normally the case, it would be impossible to tell that two accounts have the same password except by guessing the password.