NAME
    Session::Token - Secure, efficient, simple random session token
    generation

SYNOPSIS
  Simple 128-bit session token
        my $token = Session::Token->new->get;
        ## 74da9DABOqgoipxqQDdygw

  Keep generator around
        my $generator = Session::Token->new;

        my $token = $generator->get;
        ## bu4EXqWt5nEeDjTAZcbTKY

        my $token2 = $generator->get;
        ## 4Vez56Zc7el5Ggx4PoXCNL

  Custom entropy in bits
        my $token = Session::Token->new(entropy => 256)->get;
        ## WdLiluxxZVkPUHsoqnfcQ1YpARuj9Z7or3COA4HNNAv

  Custom alphabet and length
        my $token = Session::Token->new(alphabet => 'ACGT', length => 100_000_000)->get;
        ## AGTACTTAGCAATCAGCTGGTTCATGGTTGCCCCCATAG...

DESCRIPTION
    This module provides a secure, efficient, and simple interface for
    creating session tokens, password reset codes, temporary passwords,
    random identifiers, and anything else you can think of.

    When a Session::Token object is created, 1024 bytes will be read from
    "/dev/urandom" (Linux, Solaris, most BSDs), "/dev/arandom" (some older
    BSDs), or with Crypt::Random::Source::Strong::Win32 (Windows). These
    bytes will be used to seed the <ISAAC-32> pseudo random number
    generator.

    Once a generator is created, you can repeatedly call the "get" method on
    the generator object and it will return new tokens.

    IMPORTANT: If your application calls "fork", make sure that any
    generators are re-created in one of the processes after the fork since
    forking will duplicate the generator state and otherwise both parent and
    child processes will go on to produce identical tokens.

    ISAAC is a cryptographically secure PRNG that improves on the well known
    RC4 algorithm in some important areas. For instance, it doesn't have
    short cycles like RC4 does. A theoretical shortest possible cycle in
    ISAAC is "2**40", although no cycles this short have ever been found
    (and probably don't exist at all). On average, ISAAC cycles are a
    ridiculous "2**8295".

    After the generator context is created, no system calls are used to
    generate tokens. This is one way that Session::Token helps with
    efficiency. This is only important for certain use cases (generally not
    web sessions).

    In a server application the most important reason you should use the
    "keep generator around" mode instead of creating Session::Token objects
    every time you need a token is that in this mode generating a new token
    cannot fail due to a full descriptor table. Creating new generators for
    every token can fail for this reason. Programs that re-use the generator
    are also more efficient and are less likely to cause problems in
    "chroot" environments.

    Aside: Some crappy (usually C) programs that assume opening
    "/dev/urandom" will always succeed can return session tokens based only
    on the contents of nulled or uninitialised memory! Unix really ought to
    provide a system call for random data.

CUSTOM ALPHABETS
    Being able to choose exactly which characters appear in your token is
    sometimes useful. This set of characters is called the *alphabet*. The
    default alphabet size is 62 characters: uppercase latin letters,
    lowercase latin letters, and digits ("a-zA-Z0-9").

    For some purposes, base-62 is a sweet spot. It is much more compact than
    hexadecimal encoding which helps with efficiency because session tokens
    are usually transfered over the network many times during a session
    (often uncompressed in HTTP headers).

    Also, base-62 tokens don't use "wacky" characters like base-64 encodings
    do. These characters sometimes cause encoding/escaping problems (ie when
    embedded in URLs) and are annoying because often you can't select tokens
    by double-clicking on them.

    Although the default is base-62, there are all kinds of reasons you
    might like to use another alphabet. One example is if your users are
    reading tokens from a print-out or SMS or whatever, you may choose to
    omit characters like "o", "O", and 0 that can easily be confused.

    To set a custom alphabet, just pass in either a string or an array of
    characters to the "alphabet" parameter of the constructor:

        Session::Token->new(alphabet => '01')->get;
        Session::Token->new(alphabet => ['0', '1'])->get; # same thing
        Session::Token->new(alphabet => ['a'..'z'])->get; # character range

ENTROPY
    There are two ways to specify the length of tokens. The first is
    directly in terms of characters:

        print Session::Token->new(length => 5)->get;
        ## -> wpLH4

    The second way is to specify their minimum entropy in terms of bits:

        print Session::Token->new(entropy => 24)->get;
        ## -> Fo5SX

    In the above example, the resulting token is guaranteed to have at least
    24 bits of entropy. Given the default base-62 alphabet, we can compute
    the exact entropy of a 5 character token as follows:

        $ perl -E 'say 5 * log(62)/log(2)'
        29.7709815519344

    So these tokens have about 29.8 bits of entropy. Note that if we removed
    one character from this token, it would bring it below our desired 24
    bits of entropy:

        $ perl -E 'say 4 * log(62)/log(2)'
        23.8167852415475

    The default minimum entropy is 128 bits. Default tokens are 22
    characters long and therefore have about 131 bits of entropy:

        $ perl -E 'say 22 * log(62)/log(2)'
        130.992318828511

    An interesting observation is that 128-bit base-64 tokens also require
    22 characters and these tokens contain only 1 more bit of entropy.

    Another Session::Token design criterion is that all tokens should be the
    same length. The default token length is 22 characters and the tokens
    are always exactly 22 characters (no more, no less). This is nice
    because it makes writing matching regular expressions easier, simplifies
    storage (you never have to store length), and causes various log files
    and things to line up neatly on your screen. Instead of tokens that are
    exactly "N" characters, some libraries that use arbitrary precision
    arithmetic end up creating tokens of *at most* "N" characters.

    In summary, the default token length of exactly 22 characters is a
    consequence of other decisions: base-62 representation, 128 bit minimum
    token entropy, and consistent token length.

MOD BIAS
    Many token generation libraries, especially ones that implement custom
    alphabets, make the mistake of generating a random value, computing its
    modulus over the size of an alphabet, and then using this modulus to
    index into the alphabet to determine an output character.

    Why is this bad? Consider the alphabet "abc". The ideal output
    probability distribution for each character in the token is:

        P(a) = 1/3
        P(b) = 1/3
        P(c) = 1/3

    Assume we have a uniform random number source that generates values in
    the set "[0,1,2,3]" (most PRNGs provide sequences of bits, in other
    words power-of-2 size sets). If we use the naïve modulus algorithm
    described above, 0 maps to "a", 1 maps to "b", 2 maps to "c", and 3
    *also* maps to "a". Instead of the even distribution above, we have the
    following biased distribution:

        P(a) = 2/4 = 1/2
        P(b) = 1/4
        P(c) = 1/4

    Session::Token eliminates this bias in the above case by only using 0,
    1, and 2, and throwing away all 3s (also see the "t/no-mod-bias.t"
    test).

    Of course throwing away a portion of random data is slightly
    inefficient. In the worst case scenario of an alphabet with 129
    characters, for each output byte this module consumes on average 1.9845
    bytes from the random number generator. This inefficiency isn't a
    problem because ISAAC is extremely fast.

    Note that if your application issues biased tokens then some tokens are
    more likely than other tokens which provides a starting point for token
    guessing. If the tokens are unbiased then all tokens are equally likely
    and there is no starting point.

INTRODUCING BIAS
    If your alphabet contains the same character two or more times, this
    character will be more biased than any characters that only occur once.
    You should be very careful that your alphabets don't overlap if you are
    trying to create random session tokens.

    However, if you wish to introduce bias this library doesn't try to stop
    you. (Maybe it should issue a warning?)

        Session::Token->new(alphabet => '0000001', length => 5000)->get; # don't do this
        ## -> 0000000000010000000110000000000000000000000100...

    Due to a limitation discussed below, alphabets larger than 256 aren't
    currently supported so your bias can't get very granular.

    Aside: If you have a constant-biased output stream like the above
    example produces then you can re-construct an un-biased bit sequence
    with the von neumann algorithm. This works by comparing pairs of bits.
    If the pair consists of identical bits, it is discarded. Otherwise the
    order of the different bits is used to determine an output bit, ie 00
    and 11 are discarded but 01 and 10 are mapped to output bits of 0 and 1
    respectively. This only works if the bias in each bit is constant (like
    all characters in a Session::Token are).

ALPHABET SIZE LIMITATION
    Due to a limitation in this module's code, alphabets can't be larger
    than 256 characters. Everywhere the above manual says "characters" it
    actually means bytes. This isn't a Unicode limitation per se, just the
    maximum size of the alphabet. Remember you can easily map bytes to
    characters with tr.

        use utf8; 
        $z = Session::Token->new(alphabet => '01', length => 10)->get;
        $z =~ tr/01/-λ/;
        ## -> λλ--λλλλ-λ

    However, if you wanted to natively support high code points, there is no
    point in hard-coding a limitation on the size of Unicode or some
    arbitrary machine word. Instead, arbitrary precision "characters" should
    be supported with bigint. Here's an example of kinda doing that in lisp:
    <isaac.lisp>.

    This module is not designed to be the ultimate random number generator
    and at this time I think changing the design as described above would
    interfere with its goal of being secure, efficient, and simple.

SEEDING
    This module is designed to always seed itself from "/dev/urandom" or
    "/dev/arandom". You should never need to seed it yourself.

    However if you know what you're doing, you can pass in a custom seed as
    a 1024 byte long string. For example, here is how to create a "null
    seeded" generator:

        my $gen = Session::Token(seed => "\x00" x 1024);

    This is done in the test-suite, but obviously don't do this in regular
    applications because the generated tokens will always be the same.

    One valid reason for seeding is if you have some reason to believe that
    there isn't enough entropy in your kernel's randomness pool and
    therefore you don't trust "/dev/urandom". In this case you should
    acquire your own seed data from somewhere trustworthy (maybe
    "/dev/random" or a previously stored trusted seed).

BUGS
    It might be a good idea if this library could detect forks and re-seed
    in the child process.

    There is currently no way to extract the seed from a Session::Token
    object. Note when implementing this: The saved seed must either store
    the current state of the ISAAC round as well as the 1024 byte "randsl"
    array or else do some kind of minimum fast forwarding in order to
    protect against a partially duplicated keystream bug.

SEE ALSO
    <The Session::Token github repo>

    There are lots of different modules for generating random data.

    Like this module, perl's "rand()" function implements a PRNG in
    user-space seeded from "/dev/urandom". However, perl "rand()" is seeded
    with a mere 4 bytes from "/dev/urandom" and the perldoc doesn't seem to
    specify a PRNG algorithm, so I prefer not to use "rand()" for session
    tokens.

    Data::Token is the first thing I saw when I looked around on CPAN. It
    has an inflexible and unspecified (?) alphabet. It tries to get its
    source of unpredictability from UUIDs and then hashes these UUIDs with
    SHA1. I think this is bad design because some standard UUID formats
    aren't designed to be unpredictable at all. Knowing a target's MAC
    address and the rough time the token was issued may help you predict a
    reduced area of token-space to concentrate guessing attacks upon. I
    don't know if Data::Token uses these types of UUIDs or the potentially
    secure "version 4" UUIDs, but because this wasn't addressed in the
    documentation and because of an apparent misapplication of hash
    functions (if you really had a good random UUID type, there would be no
    need to hash), I don't feel good about using this module.

    There are several decent random number generators like
    Math::Random::Secure, Crypt::URandom &c, but they usually don't
    implement alphabets and some of them require you open "/dev/urandom" for
    every chunk of random bytes. Note that Math::Random::Secure does prevent
    mod bias in its random integers though.

    String::Random is a cool module with a neat regexp-like language for
    specifying random tokens which is more flexible than alphabets. However,
    inspecting the code indicates that it uses perl's "rand()". Also, the
    lack of performance, bias, and security discussion in the docs made me
    decide to not use this otherwise very interesting module.

    String::Urandom has alphabets, but it uses the flawed mod algorithm
    described above and opens "/dev/urandom" on every token. The docs say
    "this module was intended to be used as a pseudorandom string generator
    for less secure applications where response timing may be an issue."
    What the... ?

    Data::Random is also a pretty nice looking library but it seems to use
    "rand()" and the docs don't discuss security.

AUTHOR
    Doug Hoyte, "<doug@hcsw.org>"

COPYRIGHT & LICENSE
    Copyright 2012 Doug Hoyte.

    This module is licensed under the same terms as perl itself.

    ISAAC code:

        By Bob Jenkins.  My random number generator, ISAAC.  Public Domain

