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+
+Password Hashing
+========================================
+
+Storing passwords for user authentication purposes in plaintext is the
+simplest but least secure method; when an attacker compromises the
+database in which the passwords are stored, they immediately gain
+access to all of them. Often passwords are reused among multiple
+services or machines, meaning once a password to a single service is
+known an attacker has a substantial head start on attacking other
+machines.
+
+The general approach is to store, instead of the password, the output
+of a one way function of the password. Upon receiving an
+authentication request, the authenticator can recompute the one way
+function and compare the value just computed with the one that was
+stored. If they match, then the authentication request succeeds. But
+when an attacker gains access to the database, they only have the
+output of the one way function, not the original password.
+
+Common hash functions such as SHA-256 are one way, but used alone they
+have problems for this purpose. What an attacker can do, upon gaining
+access to such a stored password database, is hash common dictionary
+words and other possible passwords, storing them in a list. Then he
+can search through his list; if a stored hash and an entry in his list
+match, then he has found the password. Even worse, this can happen
+*offline*: an attacker can begin hashing common passwords days,
+months, or years before ever gaining access to the database. In
+addition, if two users choose the same password, the one way function
+output will be the same for both of them, which will be visible upon
+inspection of the database.
+
+There are two solutions to these problems: salting and
+iteration. Salting refers to including, along with the password, a
+randomly chosen value which perturbs the one way function. Salting can
+reduce the effectivness of offline dictionary generation (because for
+each potential password, an attacker would have to compute the one way
+function output for all possible salts - with a large enough salt,
+this can make the problem quite difficult). It also prevents the same
+password from producing the same output, as long as the salts do not
+collide. With a large salt (say 80 to 128 bits) this will be quite
+unlikely. Iteration refers to the general technique of forcing
+multiple one way function evaluations when computing the output, to
+slow down the operation. For instance if hashing a single password
+requires running SHA-256 100,000 times instead of just once, that will
+slow down user authentication by a factor of 100,000, but user
+authentication happens quite rarely, and usually there are more
+expensive operations that need to occur anyway (network and database
+I/O, etc). On the other hand, an attacker who is attempting to break a
+database full of stolen password hashes will be seriously
+inconvenienced by a factor of 100,000 slowdown; they will be able to
+only test at a rate of .0001% of what they would without iterations
+(or, equivalently, will require 100,000 times as many zombie botnet
+hosts).
+
+Botan provides two techniques for password hashing, bcrypt and
+passhash9.
+
+.. _bcrypt:
+
+Bcrypt Password Hashing
+----------------------------------------
+
+Bcrypt is a password hashing scheme originally designed for use in
+OpenBSD, but numerous other implementations exist. It is made
+available by including ``bcrypt.h``. Bcrypt provides outputs that
+look like this::
+
+  "$2a$12$7KIYdyv8Bp32WAvc.7YvI.wvRlyVn0HP/EhPmmOyMQA4YKxINO0p2"
+
+.. cpp:function:: std::string generate_bcrypt(const std::string& password, \
+   RandomNumberGenerator& rng, u16bit work_factor = 10)
+
+   Takes the password to hash, a rng, and a work factor. Higher values
+   increase the amount of time the algorithm runs, increasing the cost
+   of cracking attempts. The resulting hash is returned as a string.
+
+.. cpp:function:: bool check_bcrypt(const std::string& password, \
+   const std::string& hash)
+
+   Takes a password and a bcrypt output and returns true if the
+   password is the same as the one that was used to generate the
+   bcrypt hash.
+
+Here is an example of using bcrypt:
+
+.. literalinclude:: examples/bcrypt.cpp
+
+.. _passhash9:
+
+Passhash9
+----------------------------------------
+
+Botan also provides a password hashing technique called passhash9, in
+``passhash9.h``, which is based on PBKDF2. Its outputs look like::
+
+  "$9$AAAKxwMGNPSdPkOKJS07Xutm3+1Cr3ytmbnkjO6LjHzCMcMQXvcT"
+
+.. cpp:function:: std::string generate_passhash9(const std::string& password, \
+   RandomNumberGenerator& rng, u16bit work_factor = 10, byte alg_id = 0)
+
+   Functions much like ``generate_bcrypt``. The last parameter,
+   ``alg_id``, specifies which PRF to use. Currently defined values
+   are
+
+   ======= ==============
+   Value   PRF algorithm
+   ======= ==============
+   0       HMAC(SHA-1)
+   1       HMAC(SHA-256)
+   2       CMAC(Blowfish)
+   ======= ==============
+
+.. cpp:function:: bool check_passhash9(const std::string& password, \
+   const std::string& hash)
+
+   Functions much like ``check_bcrypt``