

Password Schemes
================


Password scheme means the format in which the password is stored in >>password databases<<. The most commonly used password schemes are: 

 * *PLAIN*: Password is in plaintext. 
 * *CRYPT*: Traditional DES-crypted password in '/etc/passwd' (e.g. "pass" = 'vpvKh.SaNbR6s') 

 * Dovecot uses libc's 'crypt()' function, which means that CRYPT usually is able to also decrypt MD5-CRYPT and possibly also other password schemes. 

 * *MD5-CRYPT*: MD5 based salted password hash nowadays commonly used in '/etc/shadow'. (e.g. "pass" = '$1$ozdpg0V0$0fb643pVsPtHVPX8mCZYW/') 

 * *MD5*: Alias for MD5-CRYPT. Dovecot versions earlier than v1.0.rc16 need to use this instead of MD5-CRYPT. This name is deprecated because MD5-CRYPT isn't an actual MD5 hash. 

 * *PLAIN-MD5*: An actual MD5 hash of the password. (e.g. "pass" = '1a1dc91c907325c69271ddf0c944bc72') 
Password databases have a default password scheme: 

 * >>SQL<<: See 'default_pass_scheme' setting in 'dovecot-sql.conf' 
 * >>LDAP<<: See 'default_pass_scheme' setting in 'dovecot-ldap.conf' 
 * >>PasswdFile<<, >>Passwd<<, >>Shadow<<, >>VPopMail<<: CRYPT is used by default and can't be changed currently. 
 * >>PAM<<, >>BSDAuth<<, >>CheckPassword<<: Dovecot never even sees the password with these databases, so Dovecot has nothing to do with what password scheme is used. 
The password scheme can be overridden for each password by prefixing it with {SCHEME}, for example: '{PLAIN}pass'. 
Dovecot contains a 'dovecotpw' utility which can be used to easily generate passwords for a wanted scheme. 


What scheme to use?
===================


With most installations it doesn't really matter what scheme you're using. If you already have users with existing passwords, it's easiest to just keep using the same scheme. Otherwise just pick something strong enough, for example SSHA. 
The main idea behind storing passwords in non-plaintext scheme is that if an attacker gets access to your server, he can't easily just get all users' passwords and start using them. With stronger schemes it takes more time to crack the passwords. 


Non-plaintext authentication mechanisms
=======================================


See >>Authentication/Mechanisms<< for explanation of auth mechanisms. Most installations use only plaintext mechanisms, so you can skip this section unless you know you want to use them. 
The problem with non-plaintext auth mechanisms is that the password must be stored either in plaintext, or using a mechanism-specific scheme that's incompatible with all other non-plaintext mechanisms. For example if you're going to use CRAM-MD5 authentication, the password needs to be stored in either PLAIN or CRAM-MD5 scheme. If you want to allow both CRAM-MD5 and DIGEST-MD5, the password must be stored in plaintext. 
In future it's possible that Dovecot could support multiple passwords in different schemes for a single user. 


Supported schemes
=================


*PLAIN*, *CRYPT* and *MD5-CRYPT* schemes were explained above. 
Non-plaintext mechanism specific schemes: 

 * *LANMAN*: DES-based encryption. Used sometimes with NTLM mechanism. 
 * *NTLM*: MD4 sum of the password stored in hex. Used with NTLM mechanism. 
 * *RPA*: Used with RPA mechanism. 
 * *CRAM-MD5*: Used with CRAM-MD5 mechanism (v1.0.rc16 and later, for older use HMAC-MD5) 

 * *HMAC-MD5*: Deprecated name for CRAM-MD5. The password isn't really in a standard HMAC-MD5 format. 

 * *DIGEST-MD5*: Used with >>DIGEST-MD5 mechanism<<. The username is included in the hash, so it's not possible to use the hash for different usernames. 
MD5 based schemes: 

 * *PLAIN-MD5*: MD5 sum of the password stored in hex. 
 * *LDAP-MD5*: MD5 sum of the password stored in base64. 
 * *SMD5*: Salted MD5 sum of the password stored in base64. 
SHA based schemes: 

 * *SHA*: SHA1 sum of the password stored in base64. 
 * *SSHA*: Salted SHA1 sum of the password stored in base64. 
 * *SHA256*: SHA256 sum of the password stored in base64. (v1.1 and later).  
For some schemes (e.g. PLAIN-MD5, SHA) Dovecot is able to detect if the password hash is base64 or hex encoded, so both can be used. 'dovecotpw' anyway generates the passwords using the encoding mentioned above. 
(This file was created from the wiki on 2007-12-11 04:42)
