Password Schemes

Password scheme means the format in which the password is stored in Password databases (passdb). The main reason for choosing a scheme other than PLAIN is to prevent someone with access to the password database (such as a hacker) from stealing users’ passwords and using them to access other services.

What scheme to use?

You should choose the strongest crypt scheme that’s supported by your system. From strongest to weakest:

ARGON2I/ARGON2ID: Argon2 is the winner of password hashing competition held at July 2015. The password will start with $argon2i$ or $argon2id$. You can use -r to tune computational complexity, minimum is 3. ARGON2ID is only available if your libsodium is recent enough. ARGON2 can require quite a hefty amount of virtual memory, so we recommend that you set service auth { vsz_limit = 2G } at least, or more.

BLF-CRYPT: This is the Blowfish crypt (bcrypt) scheme. It is generally considered to be very secure. The encrypted password will start with $2y$ (other generators can generate passwords that have other letters after $2, those should work too.)

Note

v2.2: bcrypt is not available on most Linux distributions). Since v2.3.0 this is provided by dovecot. You can tune the computational cost using -r parameter for doveadm.

SHA512-CRYPT: A strong scheme. The encrypted password will start with $6$

SHA256-CRYPT: A strong scheme. The encrypted password will start with $5$

MD5-CRYPT: A weak but common scheme often used in /etc/shadow. The encrypted password will start with $1$

Note

The above schemes are implemented by the libc’s crypt() function. Using them is especially useful when sharing the same passwords with other software, because most of them support using crypt() to verify the password. However, not all libcs (especially older ones) implement all of the above schemes. See below for other password schemes that are implemented by Dovecot internally (instead of libc).

A few articles about why choosing a good password scheme is important:

• How To Safely Store A Password
• Speed Hashing

It’s not possible to easily switch from one password scheme to another. The only practical way to do this is to wait until user logs in and change the password during the login. This HOWTO shows one way to do this.

Generating encrypted passwords

You can generate passwords for a particular scheme easily with doveadm pw utility. For example:

doveadm pw

New in version v2.3.0.

The scheme defaults to BCRYPT, but you can use -s to override

doveadm pw -s SHA512-CRYPT

To provide password, for scripting purposes, you can use either

doveadm pw -p password

or

printf 'password\npassword\n' | doveadm pw

Default password schemes

Password databases have a default password scheme:

SQL: See default_pass_scheme setting in dovecot-sql.conf.ext

LDAP: See default_pass_scheme setting in dovecot-ldap.conf.ext

PasswdFile: CRYPT is used by default, but can be changed with scheme parameter in passdb args.

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.

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. In addition, the mechanism-specific schemes often offer very little protection. This isn’t a limitation of Dovecot, it’s a requirement for the algorithms to even work.

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.

• 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.
• 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.

New in version v2.2.

• SCRAM-SHA-1: Used with SCRAM-SHA-1 mechanism.

Other supported password schemes

Strong schemes and mechanism-specific schemes are listed above.

• 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 is usually able to recognize MD5-CRYPT and possibly also other password schemes. See all of the *-CRYPT schemes at the top of this page.
• The traditional DES-crypt scheme only uses the first 8 characters of the password, the rest are ignored. Other schemes may have other password length limitations (if they limit the password length at all).

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 (also see below for libc’s SHA* support):

• 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).
• SSHA256: Salted SHA256 sum of the password stored in base64. (v1.2 and later).
• SHA512: SHA512 sum of the password stored in base64. (v2.0 and later).
• SSHA512: Salted SHA512 sum of the password stored in base64. (v2.0 and later).

Other schemes

New in version v2.3.0.

• ARGON2I: ARGON2i password scheme, needs libsodium
• ARGON2ID: ARGON2id password scheme, needs libsodium
• PBKDF2: PKCS5 Password hashing algortihm

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. doveadm pw anyway generates the passwords using the encoding mentioned above.

3rd party password schemes

These plugins are provided by community members, we do not provide support or help with them, please contact the developer(s) directly. Use at your own discretion. Since v2.3.0 ARGON2 is provided by dovecot itself.

• SCRYPT and ARGON2: See https://github.com/LuckyFellow/dovecot-libsodium-plugin/

Encoding

The base64 vs. hex encoding that is mentioned above is simply the default encoding that is used. You can override it for any scheme by adding a “.hex”, “.b64” or “.base64” suffix. For example:

• {SSHA.b64}986H5cS9JcDYQeJd6wKaITMho4M9CrXM contains the password encoded to base64 (just like {SSHA})
• {SSHA.HEX}3f5ca6203f8cdaa44d9160575c1ee1d77abcf59ca5f852d1 contains the password encoded to hex

This can be especially useful with plaintext passwords to encode characters that would otherwise be illegal. For example in passwd-file you couldn’t use a “:” character in the password without encoding it to base64 or hex. For example: {PLAIN}{}:!” is the same as {PLAIN.b64}e1x9OiEiCg==.

You can also specify the encoding with doveadm pw. For example: doveadm pw -s plain.b64

Salting

For the SHA512-CRYPT, SHA256-CRYPT and MD5-CRYPT schemes, the salt is stored before the hash, e.g.: $6$salt$hash. For the BLF-CRYPT scheme, bcrypt stores the salt as part of the hash.

For most of the other salted password schemes (SMD5, SSHA*) the salt is stored after the password hash and its length can vary. When hashing the password, append the salt after the plaintext password, e.g.: SSHA256(pass, salt) = SHA256(pass + salt) + salt.

For example with SSHA256 you know that the hash itself is 32 bytes (256 bits/8 bits per byte). Everything after that 32 bytes is the salt. For example if you have a password:

{SSHA256}SoR/78T5q0UPFng8UCXWQxOUKhzrJZlwfNtllAupAeUT+kQv

After base64 decoding it you’ll see that its length is 36 bytes, so the first 32 bytes are the hash and the following 4 bytes are the salt:

• length: echo SoR/78T5q0UPFng8UCXWQxOUKhzrJZlwfNtllAupAeUT+kQv|base64 -d|wc -c-> 36
• hash: echo SoR/78T5q0UPFng8UCXWQxOUKhzrJZlwfNtllAupAeUT+kQv|base64 -d|dd bs=1 count=32|hexdump -C-> 4a 84 7f ef c4 f9 ab 45 0f 16 78 3c 50 25 d6 43 13 94 2a 1c eb 25 99 70 7c db 65 94 0b a9 01 e5
• salt: echo SoR/78T5q0UPFng8UCXWQxOUKhzrJZlwfNtllAupAeUT+kQv|base64 -d|dd bs=1 skip=32|hexdump -C-> 13 fa 44 2f

Other common hash sizes are:

• MD5: 16 bytes
• SHA: 20 bytes
• SHA256: 32 bytes
• SHA512: 64 bytes

The web management gui VBoxAdm has some code dealing with creation and verification of salted hashes in Perl. However not all password schemes provided by dovecotpw are supported. Have a look at the module VBoxAdm::DovecotPW for more details.