Token Mechanism

[TheAspens]

This pull request updates the method for creating random strings (tokens) to use cryptographically secure functions and it adds a token table and functions for interacting with the table. This will be used by both issue #2447 and issue #2451

[TheAspens]

The design of this was documented under "Token Generation" and "Token Usage" here: https://boinc.berkeley.edu/trac/wiki/RightToErasure#TokenGeneration (which is the full design document for issue #2447 )

[davidpanderson]

PHP provides a function for making crypto-secure random strings:
http://php.net/manual/en/function.openssl-random-pseudo-bytes.php

[TheAspens]

This code uses http://php.net/manual/en/function.random-bytes.php which is also a PHP function and is recommended as superior to openssl-random-psuedo-bytes.php.

The inclusion of https://github.com/paragonie/random_compat is recommended on the php.net website with this comment under the description of random-bytes again see http://php.net/manual/en/function.random-bytes.php

Note: Although this function was added to PHP in PHP 7.0, a » userland implementation is available for PHP 5.2 to 5.6, inclusive.

There are lots of recommendations that direct people to use random-bytes.php as preferred over openssl-random-psuedo-bytes.php

• https://www.reddit.com/r/PHPhelp/comments/6umjwl/random_bytes_vs_openssl_random_pseudo_bytes_which/

• https://stackoverflow.com/questions/38716613/generate-a-single-use-token-in-php-random-bytes-or-openssl-random-pseudo-bytes

• https://paragonie.com/blog/2015/07/how-safely-generate-random-strings-and-integers-in-php

[TheAspens]

@davidpanderson - do you have any other objections or can you go ahead and merge this?

[davidpanderson]

Looks good; sorry for the delay.

Small note: at some point I started using "double" for unix timestamps because of 2038.
But it doesn't matter.

[JuhaSointusalo]

Sorry for being late. You allow token.expire_time to be null. Was that intentional?

[TheAspens]

@JuhaSointusalo - Yes - I wanted to allow for tokens that do not expire.

[JuhaSointusalo]

@TheAspens Ok, thanks for explaining. I'm not sure if looking up valid non-expiring tokens works but I suppose that can wait until the more urgent stuff is done.