Difference between revisions of "SHA password hashes"

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[[Category:Security (English)]]
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[[Category:Security]]
==Why Should You Use SHA-2?==
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{{note|1= With {{pkg|shadow}} 4.1.4.3-3 ''sha512'' is the default for new passwords (see [https://bugs.archlinux.org/task/13591#comment85993 bug 13591] and corresponding [https://projects.archlinux.org/svntogit/packages.git/commit/trunk?h=packages/shadow&id=98001501a8306ef5a0df55d1cffc048851894940 commit]).}}
In Linux distributions login passwords are commonly hashed and stored in the /etc/shadow file using the [[Wikipedia:MD5|MD5 algorithm]]. The security of the MD5 hash function has been severely compromised by [[Wikipedia:MD5#Collision_vulnerabilities|collision vulnerabilities]]. This does not mean MD5 is insecure for password hashing but in the interest of decreasing vulnerabilities a more secure and robust algorithm that has no known weaknesses (i.e. SHA) is recommended.
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The following tutorial uses the ''sha512'' hash function, which has been  recommended by the NSA for Red Hat Enterprise Linux 5. Alternatively, [[Wikipedia:SHA-2|SHA-2]] consists of three additional hash functions with digests that are 224, 256 or 384 bits.
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If your current password was created with {{pkg|shadow}} version prior to 4.1.4.3-3 (2011-11-26) you are using MD5. To start using a SHA-512 hash you just need to change your password with ''passwd''.
{{Warning|The very minimal terminal manager ''fgetty'' doesn't support sha512 password hashing. Enabling sha512 with ''fgetty'' will cause you to be locked out. The default tty manager ''agetty'' and minimal tty manager ''mingetty'' both support sha512.}}
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==Editing the Necessary Files==
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==Benefits of SHA-2 over MD5==
{{note|You must have root privileges to edit the files within this section.}}
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In Linux distributions login passwords are commonly hashed and stored in the {{ic|/etc/shadow}} file using the [[Wikipedia:MD5|MD5 algorithm]]. The security of the MD5 hash function has been severely compromised by [[Wikipedia:MD5#Collision_vulnerabilities|collision vulnerabilities]]. This does not mean MD5 is insecure for password hashing but in the interest of decreasing vulnerabilities a more secure and robust algorithm that has no known weaknesses (e.g. SHA-512) is recommended.
===Editing /etc/pam.d/passwd===
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A default {{Filename|/etc/pam.d/passwd}} should look like the following:
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#%PAM-1.0
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#password required pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 retry=3
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#password required pam_unix.so md5 shadow use_authtok
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password required pam_unix.so md5 shadow nullok
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Open {{Filename|/etc/pam.d/passwd}} with a text editor and replace <tt>md5</tt> with <tt>sha512</tt> on the uncommented line. At the end of of the uncommented line add the <tt>rounds</tt> option.
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The ''rounds=N'' option helps to improve [[Wikipedia:Key Strengthening|key strengthening]]. The number of rounds has a larger impact on security than the selection of a hash function. For example, ''rounds=65536'' means that an attacker has to compute 65536 hashes for each password he tests against the hash in your /etc/shadow. Therefore the attacker will be delayed by a factor of 65536. This also means that your computer must compute 65536 hashes every time you login, but even on slow computers that takes less than 1 second.
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The following tutorial uses the SHA-512 hash function, which has been recommended by the United States' National Security Agency (NSA) for Red Hat Enterprise Linux 5. Alternatively, [[Wikipedia:SHA-2|SHA-2]] consists of four additional hash functions with digests that are 224, 256, 384, and 512 bits.
  
{{note|For a more detailed explanation of the {{Filename|/etc/pam.d/passwd}} password options check the [http://linux.die.net/man/8/pam_unix pam man page].}}
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==Increasing Security==
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{{note|You must have root privileges to edit this file.}}
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The {{ic|1=rounds=N}} option helps to improve [[Wikipedia:Key stretching|key strengthening]]. The number of rounds has a larger impact on security than the selection of a hash function. For example, {{ic|1=rounds=65536}} means that an attacker has to compute 65536 hashes for each password he tests against the hash in your {{ic|/etc/shadow}}. Therefore the attacker will be delayed by a factor of 65536. This also means that your computer must compute 65536 hashes every time you log in, but even on slow computers that takes less than 1 second. If you do not use the {{ic|rounds}} option, then glibc will '''default''' to '''5000''' rounds for SHA-512. Additionally, the default value for the {{ic|rounds}} option can be found in {{ic|sha512-crypt.c}}.
  
After applying the above changes your {{Filename|/etc/pam.d/passwd}} file should look like this:
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Open {{ic|/etc/pam.d/passwd}} with a text editor and add the {{ic|rounds}} option at the end of of the uncommented line. After applying this change the line should look like this:
#%PAM-1.0
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  password required pam_unix.so sha512 shadow nullok '''rounds=65536'''
#password required pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 retry=3
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#password required pam_unix.so md5 shadow use_authtok
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  password required pam_unix.so '''sha512''' shadow nullok '''rounds=65536'''
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===Editing /etc/default/passwd===
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{{note|For a more detailed explanation of the {{ic|/etc/pam.d/passwd}} password options check the [http://linux.die.net/man/8/pam_unix PAM man page].}}
Your default {{Filename|/etc/default/passwd}} file should look like this:
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# This file contains some information for
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# the passwd (1) command and other tools
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# creating or modifying passwords.
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# Define default crypt hash
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# CRYPT={des,md5,blowfish}
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CRYPT=des
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# Use another crypt hash for group passwowrds.
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# This is used by gpasswd, fallback is the CRYPT entry.
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# GROUP_CRYPT=des
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# We can override the default for a special service
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# by appending the service name (FILES, YP, NISPLUS, LDAP)
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# for local files, use a more secure hash. We
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# don't need to be portable here:
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CRYPT_FILES=blowfish
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# sometimes we need to specify special options for
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# a hash (variable is prepended by the name of the
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# crypt hash).
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BLOWFISH_CRYPT_FILES=5
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# For NIS, we should always use DES:
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CRYPT_YP=des
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On line 7 of the above example file, change
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CRYPT=des
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to
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CRYPT=sha512
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{{Note|It's unclear whether this is still necessary with the {{Filename|/etc/shadow}} mechanism.}}
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==Re-Hash the Passwords==
 
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Even though you have changed the encryption settings, your passwords are not automatically re-hashed. To fix this, you must reset all user passwords so that they can be re-hashed.
===Editing /etc/login.defs===
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According to ''passwd'''s man page, this file has to be edited when the /etc/shadow mechanism is used for storing passwords. Add the following line to {{Filename|/etc/login.defs}}:
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ENCRYPT_METHOD=SHA512
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==Final Steps==
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Even though you have changed the encryption, your passwords are not automatically rehashed. To fix this, you must reset all user passwords so that they can be rehashed.
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As root issue the following the command,
 
As root issue the following the command,
 
  # passwd <username>
 
  # passwd <username>
where <tt><username></tt> is the name of the user whose password you are changing. Then re-enter their current password, and it will be rehashed using the SHA-2 function!
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where {{ic|<username>}} is the name of the user whose password you are changing. Then re-enter their current password, and it will be re-hashed using the SHA-2 function.
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To verify that your passwords have been re-hashed, check the {{ic|/etc/shadow}} file as root. Passwords hashed with SHA-256 should begin with a {{ic|$5}} and passwords hashed with SHA-512 will begin with {{ic|$6}}.
  
To verify that your passwords have been rehashed, check the {{Filename|/etc/shadow}} file as root. Passwords hashed with ''sha256'' should begin with a '''$5''' and passwords hashed with ''sha512'' will begin with '''$6'''.
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==Known Problems==
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===fgetty===
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Arch Linux is using SHA-512 password hashing by default (since 2011-11-26). The very minimal terminal manager ''fgetty'' does not support SHA-512 password hashing by default. Enabling SHA-512 with the default ''fgetty'' will cause you to be locked out.
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A patched version of ''fgetty'' is in the [[Arch User Repository|AUR]] named {{AUR|fgetty-pam}} which adds SHA-512 support.

Revision as of 00:34, 6 December 2012

Note: With shadow 4.1.4.3-3 sha512 is the default for new passwords (see bug 13591 and corresponding commit).

If your current password was created with shadow version prior to 4.1.4.3-3 (2011-11-26) you are using MD5. To start using a SHA-512 hash you just need to change your password with passwd.

Benefits of SHA-2 over MD5

In Linux distributions login passwords are commonly hashed and stored in the /etc/shadow file using the MD5 algorithm. The security of the MD5 hash function has been severely compromised by collision vulnerabilities. This does not mean MD5 is insecure for password hashing but in the interest of decreasing vulnerabilities a more secure and robust algorithm that has no known weaknesses (e.g. SHA-512) is recommended.

The following tutorial uses the SHA-512 hash function, which has been recommended by the United States' National Security Agency (NSA) for Red Hat Enterprise Linux 5. Alternatively, SHA-2 consists of four additional hash functions with digests that are 224, 256, 384, and 512 bits.

Increasing Security

Note: You must have root privileges to edit this file.

The rounds=N option helps to improve key strengthening. The number of rounds has a larger impact on security than the selection of a hash function. For example, rounds=65536 means that an attacker has to compute 65536 hashes for each password he tests against the hash in your /etc/shadow. Therefore the attacker will be delayed by a factor of 65536. This also means that your computer must compute 65536 hashes every time you log in, but even on slow computers that takes less than 1 second. If you do not use the rounds option, then glibc will default to 5000 rounds for SHA-512. Additionally, the default value for the rounds option can be found in sha512-crypt.c.

Open /etc/pam.d/passwd with a text editor and add the rounds option at the end of of the uncommented line. After applying this change the line should look like this:

password	required	pam_unix.so sha512 shadow nullok rounds=65536
Note: For a more detailed explanation of the /etc/pam.d/passwd password options check the PAM man page.

Re-Hash the Passwords

Even though you have changed the encryption settings, your passwords are not automatically re-hashed. To fix this, you must reset all user passwords so that they can be re-hashed.

As root issue the following the command,

# passwd <username>

where <username> is the name of the user whose password you are changing. Then re-enter their current password, and it will be re-hashed using the SHA-2 function.

To verify that your passwords have been re-hashed, check the /etc/shadow file as root. Passwords hashed with SHA-256 should begin with a $5 and passwords hashed with SHA-512 will begin with $6.

Known Problems

fgetty

Arch Linux is using SHA-512 password hashing by default (since 2011-11-26). The very minimal terminal manager fgetty does not support SHA-512 password hashing by default. Enabling SHA-512 with the default fgetty will cause you to be locked out. A patched version of fgetty is in the AUR named fgetty-pamAUR which adds SHA-512 support.