Question

I am not sure which algorithm crypt() uses when hashing. I looked on the PHP manual, but it just says that it uses whatever is available. But how do I know which one it uses, and if it does use one, how to tell it which one to use? I am using MAMP currently as my development environment, but I figure there must be a way to find out with a statement in PHP.

Answer 1

You specify the algorithm as part of the salt string. For example, starting with $2a$ gives you a Blowfish cypher. If the machine does not support the algorithm you are trying to use, you won’t get a meaningful result. You can attempt to find out in advance which algorithms are supported by checking some of the predefined constants, such as CRYPT_BLOWFISH, although I have noticed that the constants CRYPT_SHA256 and CRYPT_SHA512 are not always defined, at least on PHP 5.2. Starting with PHP 5.3, PHP has its own implementations of the algorithms, so it does not matter what the system has available at PHP compile time like it does in PHP 5.2 and earlier. The Suhosin patch for PHP 5.2 supposedly adds at least Blowfish, but its implementation does not seem to be compatible with the one used in PHP 5.3.

The PHP docs for the crypt() function do provide some information on how to use the salt string to specify which algorithm to use:

• CRYPT_STD_DES – Standard DES-based hash with a two character salt
  from the alphabet "./0-9A-Za-z". Using invalid characters in the salt
  will cause crypt() to fail.
• CRYPT_EXT_DES – Extended DES-based
  hash. The "salt" is a 9-character string consisting of an underscore
  followed by 4 bytes of iteration count and 4 bytes of salt. These are
  encoded as printable characters, 6 bits per character, least
  significant character first. The values 0 to 63 are encoded as
  "./0-9A-Za-z". Using invalid characters in the salt will cause crypt()
  to fail.
• CRYPT_MD5 – MD5 hashing with a twelve character salt
  starting with $1$
• CRYPT_BLOWFISH – Blowfish hashing with a salt as
  follows: "$2a$", a two digit cost parameter, "$", and 22 digits from
  the alphabet "./0-9A-Za-z". Using characters outside of this range in
  the salt will cause crypt() to return a zero-length string. The two
  digit cost parameter is the base-2 logarithm of the iteration count
  for the underlying Blowfish-based hashing algorithmeter and must be in
  range 04-31, values outside this range will cause crypt() to fail.
• CRYPT_SHA256 – SHA-256 hash with a sixteen character salt prefixed
  with $5$. If the salt string starts with ’rounds=$’, the numeric
  value of N is used to indicate how many times the hashing loop should
  be executed, much like the cost parameter on Blowfish. The default
  number of rounds is 5000, there is a minimum of 1000 and a maximum of
  999,999,999. Any selection of N outside this range will be truncated
  to the nearest limit.
• CRYPT_SHA512 – SHA-512 hash with a sixteen
  character salt prefixed with $6$. If the salt string starts with
  ’rounds=$’, the numeric value of N is used to indicate how many
  times the hashing loop should be executed, much like the cost
  parameter on Blowfish. The default number of rounds is 5000, there is
  a minimum of 1000 and a maximum of 999,999,999. Any selection of N
  outside this range will be truncated to the nearest limit.

So, to specify that you want the string "password" hashed using Blowfish with 2^10 iterations, you could use

crypt('password', '$2a$10$XA86t7EJ0xD9OYEUbnTulT');

where the string starting with XA86 is the salt.

Finally, if you want more examples or just want something to take care of all this password compatibility business for you, take a look at phpass. It is public domain and works nicely in my experience. It will automatically use the "best" algorithm on the system unless you specify that you want a hash that is compatible with multiple systems, in which case (I think) it uses MD5.