{"id":33,"date":"2011-08-16T14:00:10","date_gmt":"2011-08-16T14:00:10","guid":{"rendered":"http:\/\/www.vidarholen.net\/contents\/blog\/?p=33"},"modified":"2011-07-28T08:32:39","modified_gmt":"2011-07-28T08:32:39","slug":"implementation-of-sha512-crypt-vs-md5-crypt","status":"publish","type":"post","link":"https:\/\/www.vidarholen.net\/contents\/blog\/?p=33","title":{"rendered":"Implementation of SHA512-crypt vs MD5-crypt"},"content":{"rendered":"

If you have a new installation, you’re probably using SHA512-based passwords instead of the older MD5-based passwords described in detail in the previous post<\/a>, which I’ll assume you’ve read. sha512-crypt is very similar to md5-crypt, but with some interesting differences.<\/p>\n

Since the implementation of sha512 is really less interesting than the comparison with md5-crypt, I’ll describe it by striking out the relevant parts of the md5-crypt description and writing in what sha512-crypt does instead.<\/p>\n

Like md5-crypt, it can be divided into three phases. Initialization, loop, and finalization. <\/p>\n

    \n
  1. Generate a simple md5<\/strike> sha512<\/i> hash based on the salt and password<\/li>\n
  2. Loop 1000<\/strike> 5000<\/i> times, calculating a new sha512 hash based on the previous hash concatenated with alternatingly the hash of the<\/i> password and the salt. Additionally, sha512-crypt allows you to specify a custom number of rounds, from 1000 to 999999999<\/i><\/li>\n
  3. Use a special base64 encoding on the final hash to create the password hash string<\/li>\n<\/ol>\n

     <\/p>\n

    The main differences are the higher number of rounds, which can be user selected for better (or worse) security, the use of the hashed password and salt in each round, rather than the unhashed ones, and a few tweaks of the initialization step.<\/p>\n

     <\/p>\n

    Here’s the real sha512-crypt initialization.<\/p>\n

      \n
    1. Let “password” be the user’s ascii password, “salt” the ascii salt (truncated to 8<\/strike> 16<\/i> chars) , and “magic” the string “$1$”<\/strike><\/li>\n
    2. Start by computing the Alternate sum, sha512(password + salt + password)<\/code><\/li>\n
    3. Compute the Intermediate0<\/sub> sum by hashing the concatenation of the following strings:\n
        \n
      1. Password<\/li>\n
      2. Magic<\/strike><\/li>\n
      3. Salt<\/li>\n
      4. length(password) bytes of the Alternate sum, repeated as necessary<\/li>\n
      5. For each bit in length(password), from low to high and stopping after the most significant set bit\n