Provided by: libsession-token-perl_1.503-1build1_amd64 
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 minimum 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 are read from "/dev/urandom" (Linux,
Solaris, most BSDs), "/dev/arandom" (some older BSDs), or
Crypt::Random::Source::Strong::Win32 (Windows). These bytes are used to seed the ISAAC-32
<http://www.burtleburtle.net/bob/rand/isaacafa.html> pseudo random number generator.
Once a generator is created, you can repeatedly call the "get" method on the generator
object and it will return a new token each time.
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 both parent and child processes will go on to produce identical tokens (just like
perl's rand after it is seeded).
After the generator context is created, no system calls are used to generate tokens. This
is one way that Session::Token helps with efficiency. However, this is only important for
certain use cases (generally not web sessions).
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 or initial bias 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
"2**8295".
GENERATORS AND URANDOM
Developers must choose whether a single token generator will be kept around and used to
generate all tokens, or if a new Session::Token object will be created every time a token
is needed. As mentioned above, this module accesses urandom in its constructor for seeding
purposes, but not subsequently while generating tokens.
Generally speaking the generator should be kept around and re-used. Probably the most
important reason for this is that generating a new token from an existing generator cannot
fail due to a full file-descriptor table. Creating a new Session::Token object for every
token can fail because, as described above, the constructor needs to open "/dev/urandom"
and this will not succeed if all allotted descriptors are in use, or if the read is
interrupted by a signal. In these events a perl exception will be thrown.
Programs that re-use a generator are more likely to be portable to "chroot"ed environments
where "/dev/urandom" may not be present. Finally, accessing urandom frequently is
inefficient because it requires making system calls and because (at least on linux)
reading from urandom acquires a system-wide kernel lock.
On the other hand, re-using a generator may be undesirable because servers are typically
started immediately after a system reboot and the kernel's randomness pool might be poorly
seeded at that point. Similarly, when starting a virtual machine a previously used entropy
pool state may be restored. In these cases all subsequently generated tokens will be
derived from a weak/predictable seed. For this reason, you might choose to defer creating
the generator until the first request actually comes in, periodically re-create the
generator object, and/or manually handle seeding in some other way.
Programs that assume opening "/dev/urandom" will always succeed can return session tokens
based only on the contents of nulled or uninitialised memory. This is not the case with
Session::Token since its constructor will always throw an exception if it can't seed
itself. Some modern systems provide system calls with fewer failure modes (ie
`getentropy(2)` on OpenBSD and `getrandom(2)` on linux). Future versions of Session::Token
will likely use these system calls when available.
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 letters, lowercase letters, and digits ("a-zA-Z0-9").
For some purposes, base-62 is a sweet spot. It is more compact than hexadecimal encoding
which helps with efficiency because session tokens are usually transferred 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 for using 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
Constructor args can be a hash-ref too:
Session::Token->new({ alphabet => ['a'..'z'] })->get;
ENTROPY
There are two ways to specify the length of tokens. The most primitive is in terms of
characters:
print Session::Token->new(length => 5)->get;
## -> wpLH4
But the primary 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 contains 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 in base-64 representation, 128-bit minimum tokens also
require 22 characters and that 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). Instead of tokens that are exactly "N" characters, some libraries that use
arbitrary precision arithmetic end up creating tokens of at most "N" characters.
A fixed token length is nice because it makes writing matching regular expressions easier,
simplifies storage (you never have to store length), causes various log files and things
to line up neatly on your screen, and ensures that encrypted tokens won't leak token
entropy due to length (see "VARIABLE LENGTH TOKENS").
In summary, the default token length of exactly 22 characters is a consequence of these
decisions: base-62 representation, 128 bit minimum token entropy, and fixed token length.
MOD BIAS
Some token generation libraries that implement custom alphabets will generate a random
value, compute its modulus over the size of an alphabet, and then use this modulus to
index into the alphabet to determine an output character.
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) and wish to
use the alphabet "abc".
If we use the naïve modulus algorithm described above then 0 maps to "a", 1 maps to "b", 2
maps to "c", and 3 also maps to "a". This results in the following biased distribution for
each character in the token:
P(a) = 2/4 = 1/2
P(b) = 1/4
P(c) = 1/4
Of course in an unbiased distribution, each character would have the same chance:
P(a) = 1/3
P(b) = 1/3
P(c) = 1/3
Bias is undesirable because certain tokens are obvious starting points when token guessing
and certain other tokens are very unlikely. Tokens that are unbiased are equally likely
and therefore there is no obvious starting point with them.
Session::Token provides unbiased tokens regardless of the size of your alphabet (though
see the "INTRODUCING BIAS" section for a mis-use warning). It does this in the same way
that you might simulate producing unbiased random numbers from 1 to 5 given an unbiased
6-sided die: Re-roll every time a 6 comes up.
In the above example, Session::Token eliminates bias by only using values of 0, 1, and 2
(the "t/no-mod-bias.t" test contains some more notes on this topic).
Note that mod bias can be made arbitrarily small by increasing the amount of data consumed
from a random number generator (provided that arbitrary precision modulus is available).
Because this module fundamentally avoids mod bias, it can use each of the 4 bytes from an
ISAAC-32 word for a separate character (excepting "re-rolls").
EFFICIENCY OF RE-ROLLING
Throwing away a portion of random data in order to avoid mod bias is slightly inefficient.
How many bytes from ISAAC do we expect to consume for every character in the token? It
depends on the size of the alphabet.
Session::Token masks off each byte using the smallest power of two greater than or equal
to the alphabet size minus one so the probability that any particular byte can be used is:
P = alphabet_size / next_power_of_two(alphabet_size)
For example, with the default base-62 alphabet "P" is "62/64".
In order to find the average number of bytes consumed for each character, calculate the
expected value "E". There is a probability "P" that the first byte will be used and
therefore only one byte will be consumed, and a probability "1 - P" that "1 + E" bytes
will be consumed:
E = P*1 + (1 - P)*(1 + E)
E = P + 1 + E - P - P*E
0 = 1 - P*E
P*E = 1
E = 1/P
So for the default base-62 alphabet, the average number of bytes consumed for each
character in a token is:
E = 1/(62/64) = 64/62 ≅ 1.0323
Because of the next power of two masking optimisation described above, "E" will always be
less than 2. In the worst case scenario of an alphabet with 129 characters, "E" is roughly
1.9845.
This minor inefficiency isn't an issue because the ISAAC implementation used is quite fast
and this module is very thrifty in how it uses ISAAC's output.
INTRODUCING BIAS
If your alphabet contains the same character two or more times, this character will be
more biased than a character that only occurs once. You should be careful that your
alphabets don't repeat in this way 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 print 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. If you like, you can map
tokens onto new alphabets as long as they aren't more than 256 characters long. Here is
how to generate a 128-bit minimum entropy token using the lowercase greek alphabet (note
that both forms of lowercase sigma are included which may not be desirable):
use utf8;
my $token = Session::Token->new(alphabet => [map {chr} 0..25])->get;
$token = join '', map {chr} map {ord($_) + ord('α')} split //, $token;
# ρφνδαπξδββφδοςλχτμγσψδψζειετ
Here's an interesting way to generate a uniform random integer between 0 to 999 inclusive:
0 + Session::Token->new(alphabet => ['0'..'9'], length => 3)->get
If you wanted to natively support high code points, there is no point in hard-coding a
limitation on the size of Unicode or even the (higher) limitation of perl characters.
Instead, arbitrary precision "characters" should be supported with bigint. Here's an
example of something similar in lisp: isaac.lisp <http://hcsw.org/downloads/isaac.lisp>.
This module is not however 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.
TOKEN TEMPLATES
String::Random has a method called "randpattern" where you provide a pattern that serves
as a template when creating the token. You define the meaning of 1 or more template
characters and each one that occurs in the pattern is replaced by a random character from
a corresponding alphabet.
Andrew Beverley requested this feature for Session::Token and I suggested approximately
the following:
use Session::Token;
sub token_template {
my (%m) = @_;
%m = map { $_ => Session::Token->new(alphabet => $m{$_}, length => 1) } keys %m;
return sub {
my $v = shift;
$v =~ s/(.)/exists $m{$1} ? $m{$1}->get : $1/eg;
return $v;
};
}
In order to use "token_template" you should pass it key-vaue pairs of the different token
characters and the alphabets they represent. It will return a sub that should be passed
the template pattern and it will return the resulting random tokens.
For example, here is how to create UUID version 4
<https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_.28random.29>
tokens:
sub uuid_v4_generator {
my $t = token_template(
x => [ 0..9, 'a'..'f' ],
y => [ 8, 9, 'a', 'b' ],
);
return sub {
return $t->('xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx');
}
}
"uuid_v4_generator" returns a generator function that will return tokens of the following
form:
1b782499-9913-4726-a80a-25e7b2221a7c
90f85a64-d826-43bf-98e7-94ba87406bfb
b8b73175-3cce-4861-b43b-3dec5ed5d641
3afb64ab-6de3-4647-bbff-eb94dfa7d4b0
447d2001-2aec-4d32-9910-8c289ae34c48
Note that characters in the pattern which don't have template characters defined ("-" and
4 in the above example) are passed through to the output token.
SEEDING
This module is designed to always seed itself from your kernel's secure random number
source. 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 to compare against Jenkins' reference ISAAC output, but
obviously don't do this in regular applications because the generated tokens will be the
same every time your program is run.
One valid reason for manually 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).
VARIABLE LENGTH TOKENS
As mentioned above, all tokens produced by a Session::Token generator are the same length.
If you prefer tokens of variable length, it is possible to post-process the tokens in
order to achieve this so long as you keep some things in mind.
If you randomly truncate tokens created by Session::Token, be careful not to introduce
bias. For example, if you choose the length of the token as a uniformly distributed random
length between 8 and 10, then the output will be biased towards shorter token sizes.
Length 8 tokens should appear less frequently than length 9 or 10 tokens because there are
fewer of them.
Another approach is to eliminate leading characters of a given value in the same way as
leading 0s are commonly eliminated from numeric representations. Although this approach
doesn't introduce bias, the tokens 1 and 01 are not distinct so it does not increase token
entropy given a fixed maximum token length which is the main reason for preferring
variable length tokens. The ideal variable length algorithm would generate both 1 and 01
tokens (with identical frequency of course).
Implementing unbiased, variable-length tokens would complicate the Session::Token
implementation especially since you should still be able to specify minimum entropy
variable-length tokens. Minimum entropy is the primary input to Session::Token, not token
length. This is the reason that the default token length of 22 isn't hard-coded anywhere
in the Session::Token source code (but 128 is).
The final reason that Session::Token discourages variable length tokens is that they can
leak token information through a side-channel. This could occur when a message is
encrypted but the length of the original message can be inferred from the encrypted
ciphertext.
BUGS
Should check for biased alphabets and print warnings.
Would be cool if it could detect forks and warn or re-seed in the child process (without
incurring "getpid" overhead).
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 output-stream bug.
Doesn't work on perl 5.6 and below due to the use of ":raw" (thanks CPAN testers). It
could probably use "binmode" instead, but meh.
On windows we use Crypt::Random::Source::Strong::Win32 which has a big dependency tree. We
should instead use a slimmer module like Crypt::Random::Seed.
COMMAND-LINE APP
There is a command-line application called App::Session::Token which is a convenience
wrapper around Session::Token. You can generate session tokens by running the
"session-token" binary:
$ echo "Your password is `session-token`"
Your password is 8Yom6z4AeB1RXxCGzklJFt
It supports all the options of this module via command line parameters, and multiple
session tokens can be generated with the "--num" (aka "-n") switch. For example:
$ session-token --alphabet ABC --entropy 32 --num 5
BACAACABCCCCAACBBBCAB
BCBACACBBCACCBABABCBA
ABBBCBABBACBBBCBBBCCA
AACCBBBCCAAACBABACABC
CCABCABBCCCAACAAACCAA
SEE ALSO
The Session::Token github repo <https://github.com/hoytech/Session-Token>
App::Session::Token
Presentation for Toronto Perl Mongers
<https://www.youtube.com/watch?v=c2KZBTtrmZE?start=3705>
There are lots of different modules for generating random data. If the characterisations
of any of them below are inaccurate or out-of-date, please file a github issue and I will
correct them.
Like this module, perl's "rand()" function implements a user-space PRNG seeded from
"/dev/urandom". However, perl's "rand()" is not secure. Perl doesn't specify a PRNG
algorithm at all. On linux, whatever it is is seeded with a mere 4 bytes from
"/dev/urandom".
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 SHA-1. I think this is bad design because some standard UUID
formats aren't designed to be unpredictable at all. This is acknowledged in RFC 4122
section 6: "Do not assume that UUIDs are hard to guess; they should not be used as
security capabilities (identifiers whose mere possession grants access)." With certain
UUIDs, knowing a target's MAC address or 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 an unpredictable UUID, 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 and
Crypt::URandom but they usually don't implement alphabets and some of them require you
open or read from "/dev/urandom" for every chunk of random bytes. Note that
Math::Random::Secure does prevent mod bias in its random integers and could be used to
implement unbiased alphabets (slowly).
String::Random has a neat regexp-like language for specifying random tokens which is more
flexible than alphabets. However, it uses perl's "rand()" and its documentation fails to
discuss performance, bias, or security. See the "TOKEN TEMPLATES" section for a similar
feature.
String::Urandom has alphabets, but it uses the flawed mod algorithm described above and
opens "/dev/urandom" for every token.
There are other modules like Data::Random, App::Genpass, String::MkPasswd,
Crypt::RandPasswd, Crypt::GeneratePassword, and Data::SimplePassword but they use insecure
PRNGs such as "rand()" or mersenne twister, don't adequately deal with bias, and/or don't
let you specify generic alphabets.
Bytes::Random::Secure has alphabets (aka "bags"), uses ISAAC, and avoids mod bias using
the re-roll algorithm. It is much slower than Session::Token (even when using
Math::Random::ISAAC::XS) but does support alphabets larger than 256 and might work in
environments without XS.
Neil Bowers has conducted a 3rd party review <http://neilb.org/reviews/passwords.html> of
various token/password generation modules including Session::Token.
Leo Zovic has created a Common Lisp implementation of session-token
<https://github.com/Inaimathi/session-token>.
AUTHOR
Doug Hoyte, "<doug@hcsw.org>"
COPYRIGHT & LICENSE
Copyright 2012-2016 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