1Crypt::Random::Seed(3)User Contributed Perl DocumentationCrypt::Random::Seed(3)
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6 Crypt::Random::Seed - Simple method to get strong randomness
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9 Version 0.03
10
12 use Crypt::Random::Seed;
13 my $source = new Crypt::Random::Seed;
15 die "No strong sources exist" unless defined $source;
16 my $seed_string = $source->random_bytes(4);
17 my @seed_values = $source->random_values(4);
18 # Only non-blocking sources
20 my $nonblocking_source = Crypt::Random::Seed->new( NonBlocking=>1 );
21 # Blacklist sources (never choose the listed sources)
23 my $nowin32_source = Crypt::Random::Seed->new( Never=>['Win32'] );
24 # Whitelist sources (only choose from these sources)
26 my $devr_source = Crypt::Random::Seed->new( Only=>['TESHA2'] );
27 # Supply a custom source.
29 my $user_src = Crypt::Random::Seed->new( Source=>sub { myfunc(shift) } );
30 # Or supply a list of [name, sub, is_blocking, is_strong]
31 $user_src = Crypt::Random::Seed->new(
32 Source=>['MyRandomFunction',sub {myfunc(shift)},0,1] );
33 # Given a source there are a few things we can do:
35 say "My randomness source is ", $source->name();
36 say "I am a blocking source" if $source->is_blocking();
37 say "I am a strong randomness source" if $source->is_strong()
38 say "Four 8-bit numbers:",
39 join(",", map { ord $source->random_bytes(1) } 1..4);'
40 say "Four 32-bit numbers:", join(",", $source->random_values(4));
41
43 A simple mechanism to get strong randomness. The main purpose of this
44 module is to provide a simple way to generate a seed for a PRNG such as
45 Math::Random::ISAAC, for use in cryptographic key generation, or as the
46 seed for an upstream module such as Bytes::Random::Secure. Flags for
47 requiring non-blocking sources are allowed, as well as a very simple
48 method for plugging in a source.
49 The randomness sources used are, in order:
51 User supplied.
53 If the constructor is called with a Source defined, then it is
54 used. It is not checked vs. other flags (NonBlocking, Never,
55 Only).
56 Win32 Crypto API.
58 This will use "CryptGenRandom" on Windows 2000 and "RtlGenRand" on
59 Windows XP and newer. According to MSDN, these are well-seeded
60 CSPRNGs (FIPS 186-2 or AES-CTR), so will be non-blocking.
61 EGD / PRNGD.
63 This looks for sockets that speak the EGD
64 <http://egd.sourceforge.net/> protocol, including PRNGD
65 <http://prngd.sourceforge.net/>. These are userspace entropy
66 daemons that are commonly used by OpenSSL, OpenSSH, and GnuGP. The
67 locations searched are "/var/run/egd-pool", "/dev/egd-pool",
68 "/etc/egd-pool", and "/etc/entropy". EGD is blocking, while PRNGD
69 is non-blocking (like the Win32 API, it is really a seeded CSPRNG).
70 However there is no way to tell them apart, so we treat it as
71 blocking. If your O/S supports /dev/random, consider HAVEGED
72 <http://www.issihosts.com/haveged/> as an alternative (a system
73 daemon that refills /dev/random as needed).
74 /dev/random.
76 The strong source of randomness on most UNIX-like systems. Cygwin
77 uses this, though it maps to the Win32 API. On almost all systems
78 this is a blocking source of randomness -- if it runs out of
79 estimated entropy, it will hang until more has come into the
80 system. If this is an issue, which it often is on embedded
81 devices, running a tool such as HAVEGED
82 <http://www.issihosts.com/haveged/> will help immensely.
83 /dev/urandom.
85 A nonblocking source of randomness that we label as weak, since it
86 will continue providing output even if the actual entropy has been
87 exhausted.
88 TESHA2.
90 Crypt::Random::TESHA2 is a Perl module that generates random bytes
91 from an entropy pool fed with timer/scheduler variations.
92 Measurements and tests are performed on installation to determine
93 whether the source is considered strong or weak. This is entirely
94 in portable userspace, which is good for ease of use, but really
95 requires user verification that it is working as expected if we
96 expect it to be strong. The concept is similar to
97 Math::TrulyRandom though updated to something closer to what
98 TrueRand 2.1 does vs. the obsolete version 1 that Math::TrulyRandom
99 implements. It is very slow and has wide speed variability across
100 platforms : I've seen numbers ranging from 40 to 150,000 bits per
101 second.
102 A source can also be supplied in the constructor. Each of these
104 sources will have its debatable points about perceived strength. E.g.
105 Why is /dev/urandom considered weak while Win32 is strong? Can any
106 userspace method such as TrueRand or TESHA2 be considered strong?
107 SOURCE TABLE
109 This table summarizes the default sources:
110 +------------------+-------------+------------+--------------------+
112 | SOURCE | STRENGTH | BLOCKING | NOTE |
113 |------------------+-------------+------------+--------------------|
114 | RtlGenRandom | Strong(1) | No | Default WinXP+ |
115 |------------------+-------------+------------+--------------------|
116 | CryptGenRandom | Strong(1) | No | Default Win2000 |
117 |------------------+-------------+------------+--------------------|
118 | EGD | Strong | Yes(2) | also PRNGD, etc. |
119 |------------------+-------------+------------+--------------------|
120 | /dev/random | Strong | Yes | Typical UNIX |
121 |------------------+-------------+------------+--------------------|
122 | /dev/urandom | Weak | No | Typical UNIX NB |
123 |------------------+-------------+------------+--------------------|
124 | TESHA2-strong | Strong | No | |
125 |------------------+-------------+------------+--------------------|
126 | TESHA2-weak | Weak | No | |
127 +------------------+-------------+------------+--------------------+
128 The alias 'Win32' can be used in whitelist and blacklist and will match
130 both the Win32 sources "RtlGenRandom" and "CryptGenRandom". The alias
131 'TESHA2' may be similarly used and matches both the weak and strong
132 sources.
133 1) Both CryptGenRandom and RtlGenRandom are considered strong by this
135 package, even though both are seeded CSPRNGs so should be the equal of
136 /dev/urandom in this respect. The CryptGenRandom function used in
137 Windows 2000 has some known issues so should be considered weaker.
138 2) EGD is blocking, PRNGD is not. We cannot tell the two apart. There are
140 other software products that use the same protocol, and each will act
141 differently. E.g. EGD mixes in system entropy on every request, while
142 PRNGD mixes on a time schedule.
143 STRENGTH
145 In theory, a strong generator will provide true entropy. Even if a
146 third party knew a previous result and the entire state of the
147 generator at any time up to when their value was returned, they could
148 still not effectively predict the result of the next returned value.
149 This implies the generator must either be blocking to wait for entropy
150 (e.g. /dev/random) or go through some possibly time-consuming process
151 to gather it (TESHA2, EGD, the HAVEGE daemon refilling /dev/random).
152 Note: strong in this context means practically strong, as most
153 computers don't have a true hardware entropy generator. The goal is to
154 make all the attackers ill-gotten knowledge give them no better
155 solution than if they did not have the information.
156 Creating a satisfactory strength measurement is problematic. The Win32
158 Crypto API is considered "strong" by most customers and every other
159 Perl module, however it is a well seeded CSPRNG according to the MSDN
160 docs, so is not a strong source based on the definition in the previous
161 paragraph. Similarly, almost all sources consider /dev/urandom to be
162 weak, as once it runs out of entropy it returns a deterministic
163 function based on its state (albeit one that cannot be run either
164 direction from a returned result if the internal state is not known).
165 Because of this confusion, I have removed the "Weak" configuration
167 option that was present in version 0.01. It will now be ignored. You
168 should be able to use a combination of whitelist, blacklist, and the
169 source's "is_strong" return value to decide if this meets your needs.
170 On Win32, you really only have a choice of Win32 and TESHA2. The
171 former is going to be what most people want, and can be chosen even
172 with non-blocking set. On most UNIX systems, "/dev/random" will be
173 chosen for blocking and "/dev/urandom" for non-blocking, which is what
174 should be done in most cases.
175 BLOCKING
177 EGD and /dev/random are blocking sources. This means that if they run
178 out of estimated entropy, they will pause until they've collected more.
179 This means your program also pauses. On typical workstations this may
180 be a few seconds or even minutes. On an isolated network server this
181 may cause a delay of hours or days. EGD is proactive about gathering
182 more entropy as fast as it can. Running a tool such as the HAVEGE
183 daemon or timer_entropyd can make /dev/random act like a non-blocking
184 source, as the entropy daemon will wake up and refill the pool almost
185 instantly.
186 Win32, PRNGD, and /dev/urandom are fast nonblocking sources. When they
188 run out of entropy, they use a CSPRNG to keep supplying data at high
189 speed. However this means that there is no additional entropy being
190 supplied.
191 TESHA2 is nonblocking, but can be very slow. /dev/random can be faster
193 if run on a machine with lots of activity. On an isolated server,
194 TESHA2 may be much faster. Also note that the blocking sources such as
195 EGD and /dev/random both try to maintain reasonably large entropy
196 pools, so small requests can be supplied without blocking.
197 IN PRACTICE
199 Use the default to get the best source known. If you know more about
200 the sources available, you can use a whitelist, blacklist, or a custom
201 source. In general, to get the best source (typically Win32 or
202 /dev/random):
203 my $source = Crypt::Random::Seed->new();
205 To get a good non-blocking source (Win32 or /dev/urandom):
207 my $source = Crypt::Random::Seed->new(NonBlocking => 1);
209
211 new
212 The constructor with no arguments will find the first available source
213 in its fixed list and return an object that performs the defined
214 methods. If no sources could be found (quite unusual) then the
215 returned value will be undef.
216 Optional parameters are passed in as a hash and may be mixed.
218 NonBlocking => boolean
220 Only non-blocking sources will be allowed. In practice this means EGD
222 and /dev/random will not be chosen (except on FreeBSD where it is non-
223 blocking).
224 Only => [list of strings]
226 Takes an array reference containing one or more string source names.
228 No source whose name does not match one of these strings will be
229 chosen. The string 'Win32' will match either of the Win32 sources, and
230 'TESHA2' will match both the strong and weak versions.
231 Never => [list of strings]
233 Takes an array reference containing one or more string source names.
235 No source whose name matches one of these strings will be chosen. The
236 string 'Win32' will match either of the Win32 sources, and 'TESHA2'
237 will match both the strong and weak versions.
238 Source => sub { ... }
240 Uses the given anonymous subroutine as the generator. The subroutine
242 will be given an integer (the argument to "random_bytes") and should
243 return random data in a string of the given length. For the purposes
244 of the other object methods, the returned object will have the name
245 'User', and be considered non-blocking and non-strong.
246 Source => ['name', sub { ... }, is_blocking, is_strong]
248 Similar to the simpler source routine, but also allows the other source
250 parameters to be defined. The name may not be one of the standard
251 names listed in the "name" section.
252 random_bytes($n)
254 Takes an integer and returns a string of that size filled with random
255 data. Returns an empty string if the argument is not defined or is not
256 more than zero.
257 random_values($n)
259 Takes an integer and returns an array of that many random 32-bit
260 values. Returns an empty array if the argument is not defined or is
261 not more than zero.
262 name
264 Returns the text name of the random source. This will be one of:
265 "User" for user defined, "CryptGenRandom" for Windows 2000 Crypto API,
266 "RtlGenRand" for Windows XP and newer Crypto API, "EGD" for a known
267 socket speaking the EGD protocol, "/dev/random" for the UNIX-like
268 strong randomness source, "/dev/urandom" for the UNIX-like non-blocking
269 randomness source, "TESHA2-strong" for the userspace entropy method
270 when considered strong, "TESHA2-weak" for the userspace entropy method
271 when considered weak. Other methods may be supported in the future.
272 User supplied sources may be named anything other than one of the
273 defined names.
274 is_strong
276 Returns 1 or 0 indicating whether the source is considered a strong
277 source of randomness. See the "STRENGTH" section for more discussion
278 of what this means, and the source table for what we think of each
279 source.
280 is_blocking
282 Returns 1 or 0 indicating whether the source can block on read. Be
283 aware that even if a source doesn't block, it may be extremely slow.
284
286 Dana Jacobsen <dana@acm.org>
287
289 To the best of my knowledge, Max Kanat-Alexander was the original
290 author of the Perl code that uses the Win32 API. I used his code as a
291 reference.
292 David Oswald gave me a lot of help with API discussions and code
294 reviews.
295
297 The first question one may ask is "Why yet another module of this
298 type?" None of the modules on CPAN quite fit my needs, hence this.
299 Some alternatives:
300 Crypt::Random::Source
302 A comprehensive system using multiple plugins. It has a nice API, but
303 uses Any::Moose which means you're loading up Moose or Mouse just to
304 read a few bytes from /dev/random. It also has a very long dependency
305 chain, with on the order of 40 modules being installed as prerequisites
306 (depending of course on whether you use any of them on other projects).
307 Lastly, it requires at least Perl 5.8, which may or may not matter to
308 you. But it matters to some other module builders who end up with the
309 restriction in their modules.
310 Crypt::URandom
312 A great little module that is almost what I was looking for.
313 Crypt::Random::Seed will act the same if given the constructor:
314 my $source = Crypt::Random::Seed->new(
316 NonBlocking => 1,
317 Only => [qw(/dev/random /dev/urandom Win32)]
318 );
319 croak "No randomness source available" unless defined $source;
320 Or you can leave out the "Only" and have TESHA2 as a backup.
322 Crypt::Random
324 Requires Math::Pari which makes it unacceptable in some environments.
325 Has more features (numbers in arbitrary bigint intervals or bit sizes).
326 Crypt::Random::Seed is taking a simpler approach, just handling
327 returning octets and letting upstream modules handle the rest.
328 Data::Entropy
330 An interesting module that contains a source encapsulation (defaults to
331 system rand, but has many plugins), a good CSPRNG (AES in counter
332 mode), and the Data::Entropy::Algorithms module with many ways to get
333 bits, ints, bigints, floats, bigfloats, shuffles, and so forth. From
334 my perspective, the algorithms module is the highlight, with a lot of
335 interesting code.
336 Upstream modules
338 Some modules that could use this module to help them:
339 Bytes::Random::Secure, Math::Random::ISAAC, Math::Random::Secure, and
340 Math::Random::MT to name a few.
341
343 Copyright 2013 by Dana Jacobsen <dana@acm.org>
344 This program is free software; you can redistribute it and/or modify it
346 under the same terms as Perl itself.
347 The software is provided "AS IS", without warranty of any kind, express
349 or implied, including but not limited to the warranties of
350 merchantability, fitness for a particular purpose and noninfringement.
351 In no event shall the authors or copyright holders be liable for any
352 claim, damages or other liability, whether in an action of contract,
353 tort or otherwise, arising from, out of or in connection with the
354 software or the use or other dealings in the software.
355perl v5.34.0 2021-07-22 Crypt::Random::Seed(3)