What Should Be The Data Type For The Hashed Value Of A Password Encrypted Using Pbkdf2?

I am trying to learn to use PBKDF2 hash functions for storing passwords in the database. I have a rough draft of the procedure that I'll be using to generate the hashed function. B

Solution 1:

The first link, as always, is Thomas Pornin's canonical answer to How to securely hash passwords.

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Storage in the database

• The hash can be stored in BINARY format for the least transformations and smallest number of bytes; see below for sizes.

  • Alternately, store it in a CHAR after converting to hex, which costs a transformation and double the bytes of the BINARY size

  • Alternatively, store it in a CHAR after converting to Base64, which costs a transformation and 4/3rds the number of bytes of BINARY size plus padding

  • i.e. PBKDF2-HMAC-SHA-512 where all 64 bytes of output are used would be

    • BINARY(64) as binary

    • CHAR(128) as hex

    • CHAR(88) as Base64

• The number of iterations should be stored in an INT, so it can be trivially increased later

• The salt, which must be a per-user, cryptographically random value, can be stored in a BINARY format for the smallest number of bytes, and should be at least 12, and preferably 16-24 bytes long.

  • i.e. for a 16 byte binary salt

    • BINARY(16) as binary

    • CHAR(32) as hex

    • CHAR(24) as Base64

• Optionally a password hash algorithm version as a small INT type

  • i.e. 1 for PBKDF2-HMAC-SHA-512, and then later if you change to BCrypt, 2 for BCrypt, etc.

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Normal PBKDF2 considerations

• Consider using PBKDF2-HMAC-SHA-512, as SHA-512 in particular has 64-bit operations that reduce the advantage most GPU based attackers have over you as of early 2016.

• Use a high (hundreds of thousands or high tens of thousands) of iterations.

• Don't ask for a larger number of PBKDF2 output bytes than the native hash function supports

  • SHA-512 <= 64 bytes

  • SHA-384 <= 48 bytes

  • SHA-256 <= 32 bytes

  • SHA-224 <= 28 bytes

  • MD5 <= 20 bytes