Difference between revisions of "SHA password hashes"

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Open {{ic|/etc/pam.d/passwd}} with a text editor and replace {{ic|md5}} with {{ic|sha512}} on the uncommented line. At the end of of the uncommented line, add the {{ic|rounds}} option.
 
Open {{ic|/etc/pam.d/passwd}} with a text editor and replace {{ic|md5}} with {{ic|sha512}} on the uncommented line. At the end of of the uncommented line, add the {{ic|rounds}} option.
  
The {{ic|1=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, {{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}}.
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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}}.
  
 
{{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].}}
 
{{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].}}

Revision as of 15:46, 24 January 2012

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Reason: please use the first argument of the template to provide a brief explanation. (Discuss in Talk:SHA password hashes#)
Note: With shadow 4.1.4.3-3 sha512 is the default for new passwords (see bug 13591 and corresponding commit).

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.

Support

Warning: 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.

Arch Linux's default tty manager agetty and the minimal tty manager mingetty both support SHA-512. Additionally, a patched version of fgetty is in the AUR named fgetty-pamAUR which adds SHA-512 support.

Editing the Necessary Files

Note: You must have root privileges to edit the files within this section.

Editing /etc/pam.d/passwd

A default /etc/pam.d/passwd should look like the following:

#%PAM-1.0
#password	required	pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 retry=3
#password	required	pam_unix.so md5 shadow use_authtok
password	required	pam_unix.so md5 shadow nullok

Open /etc/pam.d/passwd with a text editor and replace md5 with sha512 on the uncommented line. At the end of of the uncommented line, add the rounds option.

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.

Note: For a more detailed explanation of the /etc/pam.d/passwd password options check the PAM man page.

After applying the above changes your /etc/pam.d/passwd file should look like this:

#%PAM-1.0
#password	required	pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 retry=3
#password	required	pam_unix.so md5 shadow use_authtok
password	required	pam_unix.so sha512 shadow nullok rounds=65536

Editing /etc/default/passwd

Your default /etc/default/passwd file should look like this:

# This file contains some information for
# the passwd (1) command and other tools 
# creating or modifying passwords.

# Define default crypt hash
# CRYPT={des,md5,blowfish}
CRYPT=des

# Use another crypt hash for group passwowrds.
# This is used by gpasswd, fallback is the CRYPT entry.
# GROUP_CRYPT=des


# We can override the default for a special service
# by appending the service name (FILES, YP, NISPLUS, LDAP)

# for local files, use a more secure hash. We
# do not need to be portable here:
CRYPT_FILES=blowfish
# sometimes we need to specify special options for
# a hash (variable is prepended by the name of the
# crypt hash).
BLOWFISH_CRYPT_FILES=5

# For NIS, we should always use DES:
CRYPT_YP=des

On line 7 of the above example file, change

CRYPT=des

to

CRYPT=sha512
Note: It is unclear whether this is still necessary with the /etc/shadow mechanism.
Note: It is not necessary to edit /etc/login.defs. For more see here.

Final Steps

Even though you have changed the encryption, 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.