Secure defaults for LDAP password (CRYPT/PBKDF2/SHA2/ARGON2) storage

While configuring LDAP you might be asking yourself "what are the different password storage methods/hashes ?".

I did some browsing on the web and found it complictated to figure out what are the password hashing methods to keep your LDAP data secure. That means, you have passwords to access the LDAP database but the database also stores passwords for your users.

You can learn more about password on OWASP's Password Storage Cheat Sheet.

The basics

The manual of slapd.conf says:

  • The must be one of {SSHA}, {SHA}, {SMD5}, {MD5}, {CRYPT}, and {CLEARTEXT}.
  • The default is {SSHA}.

You can also have {ARGON2} in Debian slapd versions after 2.5.4+dfsg-1 thanks to this commit to change the packaging. But you still need to add a line to the slapd.conf file: moduleload argon2.

Let's check each one, starting with the most risky ones:

The format

Since glibc 2.7, the SHA-256 and SHA-512 implementations support a user-supplied number of hashing rounds, defaulting to 5000. If the "$id$" characters in the salt are followed by "rounds=xxx$", where xxx is an integer, then the result has the form

$id$rounds=yyy$salt$hashed

Using slappasswd and the command: slappasswd -h '{CRYPT}' -c '$6$rounds=300000$%.16s' it generates a password in the format:

$6$[cost]$[22 character salt][31 character hash]

For example, with input password abc123xyz, cost 12, and a random salt, the output of bcrypt is the string:

  • SHA-256 $5$
  • SHA-512 $6$
 {CRYPT}$6$rounds=300000$OJaNHOxGKWWtaK$xxxxxxxxxxxxxxxxxx.
 \_____/\/\___________/ \_____________/ \________________/
 Hash   Alg Cost Options     Salt        Hashed password

password-hash {SSHA} {CRYPT} {ARGON2}`
password-crypt-salt-format "$6$rounds=300000$%.16s"

More schemes for slappasswd

Debian bookworm

As I found out on the webpage: slapd-contrib manuals

There is 2 schemes added by the contrib package (slapd-contrib):

  • pbkdf2

  • {PBKDF2} - alias to {PBKDF2-SHA1}

  • {PBKDF2-SHA1} - PBKDF2 using HMAC-SHA-1 as the underlying pseudorandom function

  • {PBKDF2-SHA256} - PBKDF2 using HMAC-SHA-256 as the underlying pseudorandom function

  • {PBKDF2-SHA512} - PBKDF2 using HMAC-SHA-512 as the underlying pseudorandom function

  • sha2

    • {SSHA256} - SHA-256 with salt, giving hash values of 256 bits length
    • {SHA256} - plain SHA-256 giving hash values of 256 bits length
    • {SSHA384} - SHA-384 with salt, giving hash values of 384 bits length
    • {SHA384} - plain SHA-384 giving hash values of 384 bits length
    • {SSHA512} - SHA-512 with salt, giving hash values of 512 bits length
    • {SHA512} - plain SHA-512 giving hash values of 512 bits length

  • smbk5pwd (Samba & Kerberos password sync overlay to slapd) Have fun. I have no way to test it.

    • {K5KEY} - to be used in the userPassword attribute.

  • (Undocumented)

Debian trixie

Released on 08/2025.

slapd-contrib manuals

There is 1 more scheme than in bookworm:

  • argon2
    • {ARGON2} - Argon 2, like {CRYPT} you can use parameters in the salt/format: {ARGON2}$argon2i$v=19$m=4096,t=3,p=1$...

But argon2 got moved from slapd-contrib to slapd in the Debian version 2.5.4+dfsg-1~exp1. So all versions above will find the module in /usr/lib/ldap/argon2.so

And that means that slappasswd wants it this way: slappasswd -o module-load=argon2 -h '{ARGON2}'. And here is the result: {ARGON2}$argon2id$v=19$m=7168,t=5,p=1$oFzolryiDLgxHI07Ut495A$MnuYn7SV7mrtqfq3W/izjmdZpc6S6UOoQNZhBrgbOkM

To change the default parameters you need to use it like this: slappasswd -o module-load="argon2 m=7168" -h '{ARGON2}'.

It supports m=<memory>, p=<parallelism> and t=<iterations> parameters.

The existing list of schemes supported from bookworm remain unchanged.

But one more interesting module is added: ppm (Password Policy Module) - extension of the password policy overlay.

Example

If you have the error: Password generation failed for scheme {PBKDF2}: scheme not recognized You maybe need to install apt install slapd-contrib or/and add -o module-load=pw-pbkdf2 to the command line.

Run slappasswd -o module-load=pw-pbkdf2 -h '{PBKDF2}' And here is the result: {PBKDF2}10000$yRnewNnSbe9LyLCmc7wTdw$YZ.z5OpUKcs8t1lYe3rkPnVS8K4

Note

I am not sure why I was holding publishing this article, normally it is up to date. Please contact me if not.

References

  • https://www.redpill-linpro.com/techblog/2016/08/16/ldap-password-hash.html
  • https://kb.brightcomputing.com/knowledge-base/how-can-i-change-the-password-policy-of-ldap/
  • https://strugglers.net/~andy/mothballed-blog/2010/01/23/openldap-and-md5crypt/
  • https://serverfault.com/a/571989/336084
  • https://github.com/P-H-C/phc-string-format/blob/master/phc-sf-spec.md
  • https://manpages.debian.org/bookworm/libcrypt-dev/crypt.5.en.html
  • https://manpages.debian.org/bookworm/slapd/slapd.conf.5.en.html#password
  • https://www.wolfssl.com/fips-140-3-and-sha-1-retirement/
  • https://www.arsouyes.org/articles/2019/52_Hash_Function_Cryptography/
  • https://passlib.readthedocs.io/en/stable/narr/quickstart.html#recommended-hashes