Ubuntu Manpage: hashcash - hashcash anti-spam / denial of service counter-measure tool

NAME

       hashcash - hashcash anti-spam / denial of service counter-measure tool

SYNOPSIS

       Mint (create) stamps:

       hashcash -m [ [ options ] [ resource ... ] ... ]

       Check stamps:

       hashcash -c [ [ options ] -r resource ... ] [ stamp ... ]

       Speed estimates:

       hashcash -s [ options ] [ -b bits ]

       Purge database:

       hashcash -p now [ -j resource ] [ -k ] [ -t time ] [ -u ]

       Count partial preimage bits:

       hashcash -w [ options ] [ stamp ... ]

       Get resource name:

       hashcash -n [ options ] [ stamp ... ]

       Get time remaining until expiry

       hashcash -l [ options ] -e period [ stamp ... ]

DESCRIPTION

       Hashcash  is  a  denial-of-service  counter measure tool.  Its main current use is to help hashcash users
       avoid losing email due to content based and blacklist based anti-spam systems.

       The hashcash tool allows you to create hashcash stamps to attach  to  emails  you  send,  and  to  verify
       hashcash  stamps  attached  to  emails  you  receive.   Email  senders  attach  hashcash  stamps with the
       X-Hashcash: header.  Vendors and authors of anti-spam tools are  encouraged  to  exempt  mail  sent  with
       hashcash from their blacklists and content based filtering rules.

       A  hashcash stamp constitutes a proof-of-work which takes a parameterizable amount of work to compute for
       the sender.  The recipient can verify received stamps efficiently.

       The hashcash proof-of-work function is to compute partial hash  preimages  (the  SHA1  hash  function  is
       used).   By choosing the number of bits of the preimage that are equal to 0, the work required to compute
       a partial preimage can be made arbitrarily expensive -- from fractions of a second to minutes  or  hours.
       The verification is efficient requiring the same small cost whatever the preimage size.

       For   more   detailed   discussion   of   other   applications   hashcash   has   been   used   for   see
       http://www.hashcash.org/

USAGE NOTES

In this man page a resource name is the name of the service or address the stamp is created for. In the
case of email, the resource name is the recipient's email address in the form user@domain.com.

Minting stamps

The -m flag must be given to mint a stamp.

The resource name (recipient's email address) to mint the stamp against can be passed as an argument, or
if omitted is read from stdin. If stdin is a tty the user is prompted, if stdin is a pipe the resource
name is just silently read. The desired preimage size can be specified with the -b option. If no
preimage size is specified, the default is 20 bits. See also the -b default option.

Checking stamps

The -c flag must be given to check a stamps expiry. The stamp to check can be given as an argument to
"hashcash". If no stamp is given the stamp is read from stdin. If stdin is a tty the user will be
prompted, if stdin is a pipe the stamp is just silently read. A resource name (the recipient's email
address) can be given with the -r option. If a resource name is given the resource name is compared to
the resource name in the stamp, if they do not match, the stamp is rejected.

Note: if no resource name is given the stamp is anyway checked to see if it is otherwise valid, but it
could be minted for a different resource, which would allow stamps to be reused across different
resources, so hashcash will return unchecked exit code on exit.

Stamps are by default considered to be valid for 28 days. The validity period can be changed using the
-e flag.

If the stamp has expired or has a date in the future the stamp is rejected and the program exits
immediately.

If a required preimage size is given with the -b flag, the stamps value is computed and compared, if the
stamp has insufficent value it is rejected, and the program exits immediately. If the -b flag is not
given, the stamp is checked to see if it is otherwise valid, but hashcash will return unchecked exit code
on exit.

If the stamp is double spent the stamp is rejected. Double spending protection is discussed in more
detail below in "Double Spending Protection". If double spending protection is not enabled, the stamp
could be double spent, so hashcash will return unchecked exit code (exit code 2) on exit.

The -w flag can be used to request that the number of bits of the preimage are counted and displayed. The
-n flag can be used to request that the resource name in the stamp is parsed out and displayed. The -l
flag can be used to request the number of seconds until expiry of the stamp is output.

The program will only return exit codes valid or invalid if the -c flag is used, the -b flag is used, -d,
-r resource are used. These are the minimum set of options necessary to fully check the validty of a
stamp. If these criteria are not met, the program will return exit code unchecked (exit code 2) on exit.
(See also the -y flag.)

Double Spending Protection

If the -d flag is used when checking stamps, a database of spent stamps is kept.

By default stamps expire after 28 days, without expiry the database would grow indefinately. You can
specify an alternate expiry period with the -e flag. The recommended (and default) expiry period for
email is 28 days. After the expiry period amount of time, the stamp is anyway considered expired and may
be purged from the database to save space. (See "Purging Periodically vs on Next Access" for how to
purge stamps.)

For efficiency reasons a stamp is verified before it is checked in the database; if it is otherwise
invalid no database activity will occur.

Note: The decision about how long the stamp should be considered valid is up to the verifier. If it is
too short it is possible for some applications that the stamp will expire before arriving at the
recipient (eg with email.) The suggested value of 28 days should be safe for normal email delivery
delays. The choice is a trade-off between database size and risk of expiry prior to arrival, and depends
on the application.

Note: Different stamps in the same database can have different validity periods, so for example stamps
for different resources with different validity periods can be stored in the same database, or the
recipient may change the validity period for future stamps without affecting the validity of old stamps.

Purging Periodically vs on Next Access

To purge old stamps periodically while checking stamps use the -p period option to purge no sooner than
the given time period since the last purge. Purging can be used with the -k option to purge unexpired
stamps also, and with the -j resource flag to purge only stamps for the given resource.

There are circumstances where it may be inconvenient to purge stamps on the next access, for example if
there is a large double spend database which takes some time to purge, and the response time of the
hashcash checker is important. To avoid this problem, purging can be done separately using just the -p
now option to request just the purge operation. On unix for example you could call "hashcash -p now" in
a cron job once per day, or on demand when disk was running low.

Speed Estimates

The -s flag requests measurement of how many preimage can be tested per second. No stamp is minted, or
verified.

If the -b flag is used with this option, instead an estimate of how many seconds it would take to mint a
stamp of the given size in bits is computed. To find out how much time it will take to mint a default
sized stamp use -s -b default.

Notes

All informational output is printed on stderr. Minted stamps, and results of stamp verification and
timing are printed on stdout. The quiet flag -q suppresses all informational output. The -v flag
requests more informational output. The requested output, which is the only information that is output
in quiet mode (when -q is specified) is printed on standard output. If stdout is a pipe, or when quiet
mode is in effect the output is printed without description (ie just bits, just seconds, just resource).

OPTIONS

-c Check the expiry information of stamps given as an argument or on stdin. (Use with -b, -d and -r
resource to fully check stamps).

-m Mint stamps with the resources given as arguments or on stdin.

-b bits
When minting a stamp, create a preimage of at least this many bits. When verifying a stamp require
that it have a preimage of at minimum this many bits, otherwise reject it. If omitted the default is
used.

When checking stamps, require that the stamps have this many bits.

The default number of bits can be specified with -b default. Bits relative to the default can also
be specified with -b +n for n bits more than the default and -b -n for n bits less than the default.

-b default, -b +0 and -b -0 are all equivalent.

When doing the speed test -s, can to measure speed of default token with -s -b default.

-r resource
When minting stamps, the resource name (recipient's email address) to mint the stamp against can be
given either with -r resource or as an argument to "hashcash".

When checking stamps, the resource name (your own email address) is given with the -r option. If the
resource name is given it is checked against the resource name in the stamp, and if they do not match
the stamp is rejected. Note if the resource name is not given, stamps for other resources would be
accepted, and therefore hashcash returns exit code unchecked (exit code 2) on exit.

-o When verifying stamps multiple resources can be given. By default the resources are just checked one
by one until a matching valid resource is found. However when you use wildcards or regular
expressions (see -E), it is useful to be able to specify that one resource overrides another. For
example this: -b15 -r adam@dev.null -o -b10 *@dev.null states that mail to address adam@dev.null
requires 15 bits, but mail to *@dev.null requires only 10 bits. If we omitted the -o override
relationship between the two resources, a stamp of 10 bits would be accepted for address
adam@dev.null because while it would be rejected as having insufficient bits under the first rule, it
would be accepted under the 2nd rule. The -o option allows you avoid this problem.

-e time
Expiry period for spent stamps. While checking stamps (using the -c flag), if the stamp was minted
more than the specified amount of time ago, it is considered expired. If this option is not used, by
default stamps expire after 28 days. The expiry period is given in seconds by default (an argument
of 0 means forever). A single character suffix can be used to specify alternate units (m = minutes,
h = hours, d = days, M = months, y = Y = years, and s = seconds).

If used with the -d option, the spent stamp and its expiry period is recorded in the database. See
the -p option for description of how to purge stamps from the database.

While minting stamps, the -e flag can have an effect on the resolution of time created in the stamp.
Without the -e option, the default resolution is days (time format: YYMMDD). Alternate formats based
on range of expiry period are as follows:

While minting you can also given an explicit time width with the -z option instead. (-z overrides -e
if both are given. If neither are given the default is 6 chars (time format: YYMMDD)).

The rules for automatically determining appropriate time width from -e if no -z option is given are:

* period >= 2 years then time format YY is used rounded down to the nearest year start;
* 2 years < period <= 2 months then time format YYMM is used rounded down to the nearest month start;
* 2 months < period <= 2 days then time format YYMMDD is used rounded down to the beginning of the
nearest day;
* 2 days < period <= 2 minutes then time format YYMMDDhhmm is used rounded down to the beginning of
the nearest minute;
* period < 2 minutes then time format YYMMDDhhmmss is used in seconds.

Note the rounding down is based on UTC time, not local time. This can lead to initially surprising
results when rounding down to eg days in time zones other than GMT (UTC = GMT). It may be clearer to
understand if you use the -u option.

-z width
The -z option is for use during minting and allows user choice of width of time width field. See
also the -e option given in combination with -m to specify an implicit time field width under the
description of the -e flag. Valid widths are 6,10 or 12 chars corresponding respectively to: YYMMDD,
YYMMDDhhmm, and YYMMDDhhmmss rounded down to the nearest day, or minute respectively.

-g period
The -g option is for use when checking hashcash stamps with the -c option and specifies a grace
period for clock skew, ie if a hashcash stamp arrives with a date in the future or in the past it
will not be rejected as having a futuristic date (or as being expired) unless it is more futuristic
(or has been expired for longer) than this period. The default is 2 days, which means as long as the
sending system's clock is no more than 2 days ahead (or 2 days behind) of the receiving system's
clock, the hashcash stamp will still be accepted.

The default units for grace period are seconds. A single character suffix can be used to specify
alternate units (m = minutes, h = hours, d = days, M = months, y = Y = years, and s = seconds).

-d Store stamps in a double spend database. If stamp has been seen before it will be rejected even if
it is otherwise valid. The default database file is database.sdb in the current directory. Only
otherwise valid stamps will be stored in the database. Only fully validated stamps will be stored in
the database, unless the -y option is given.

-f dbname
Use dbname instead of default filename for double spend database.

-p period
Purges the database of expired stamps if the given time period has passed since the last time it was
purged. As a convenience -p now is equivalent to -p 0 both of which mean purge now, regardless of
when the database was last purged.

If used in combination with -j resource only the stamps minted for the given resource are purged.

If used in combination with -k all stamps even un-expired stamps are purged. Can be used in
combination with -t time to expire as if the current time were the given time.

-k Use with option -p to request all stamps are purged rather than just expired ones.

-j resource
Use with option -p to request that just stamps matching the given resource name are to be purged,
rather than the default which is to purge all expired stamps. If the resource name is the empty
string, all stamps are matched (this is equivalent to omitting the -j option).

Note the -E, -M and -S type of match flags also apply to resources given with the -j resource flag.

-s Print timing information only, and don't proceed to create a stamp. If combined with -b bits flag
print estimate of how long the requested preimage size would take to compute, if -s given by itself,
just prints speed of the preimage finder. To print an estimate of how long the default number of
bits would take use -b default.

-h Print short usage information.

-v Print more verbose informational output about the stamp minting or verification. (If -v is the only
argument, prints the tool version number.)

-V Prints tool version number.

-q Batch mode. Prints no information other than output. This option overrides the -v option.

-X When minting, prints the hashcash email X-header 'X-Hashcash: ' before the stamp. Without this
option just the bare stamp is printed.

When checking, after scanning stamps given as arguments, scans stdin for lines starting with the
string 'X-Hashcash:', and uses the rest of the matching line as the stamp. Only the lines up to and
ending at the first blank line are scanned (see also -i flag which can be used to override this). A
blank line is the separator used to separate the headers from the body of a mail message or USENET
article. This is meant to make it convenient to pipe a mail message or USENET article to hashcash on
stdin.

-x extension
An extension string composed of name value sets. The extension format is described below in the
section on the hashcash stamp format. This allows users to define their own stamp extensions which
are hashed into the stamp, verified by recipients that support them, and ignored by recipients that
don't support them. Note the extension hook mechanism has not yet been implemented. This will come
in a subsequent release.

-i When checking and using the -X flag, ignore the blank line boundary between headers and body of the
message, and check for stamps in the body too if one is not found in the headers.

-t time
Pretend the current time is the time given for purposes of minting stamps, verifying stamps and
purging old stamps from the database. Time is given in a format based on UTCTIME format
YYMMDD[hhmm[ss]].

Time is expressed in local time by default. Use with -u flag to give time in UTC (GMT).

You can also give time relative to the current time by prefixing the argument with + or -. The
default units for relative time are seconds. A single character suffix can be used to specify
alternate units (m = minutes, h = hours, d = days, M = months, y = Y = years, and s = seconds).

Note: when time is expressed in local time, if there is daylight savings in your timezone, there are
one or two ambiguous hours per year at the time of change from daylight savings time to normal time.

-u Input and output absolute times in UTC (GMT) instead of local time.

-a period
Add (or subtract if number is negative) a random value from the current time before minting the
stamp. This hides the time the stamp was created, which may be useful for anonymous users. Note
adding (rather than subtracting) a random time may be risky if the stamp takes less than the added
time to arrive as the recipient will reject stamps with time stamps in the future.

-n Print resource name parsed from stamp being verified. Returns exit code unchecked on exit.

-l Print number of seconds left before stamp expires. Returns exit code unchecked on exit.

Note: the calculation includes the grace period, so can be up to 2 times grace period longer than you
might otherwise expect (clock fast but system has to presume it could be slow). If you want to
exclude the grace period add -g0 to set grace period to 0 for the calculation.

-w Print number of bits of preimage of stamp. Returns exit code unchecked on exit.

-y Returns success if the stamp is valid even if it is not fully checked. Use with -c where not all of
-d, -r are specified to get success exit code on valid but partially checked stamp. Similarly can
use with -n, -l, -w with same effect.

-M When checking stamps, allow wildcard * matching in the resource name to make it simpler to specify
multiple email addresses and to allow matching catch-all addresses and addresses including
subdomains. This is the default. See also -S, -E and -C

-S When checking stamps use simple text compare to compare resource names to those in stamps. See also
-M, -E and -C.

-E When checking stamps use regular expressions to specify resource names to make it simpler to specify
multiple email addresses, catch-all addresses, classes of extension addresses and addresses including
subdomains. Note regular expression syntax is POSIX style: special characters do not need to be
quoted to have their special meaning; but they do have to be quoted with \ to that character in the
searched string. The regular expression automatically has ^ added at the beginning and $ added at
the end, if they are not specified. The special characters ^ matches the beginning of the resource,
and $ matches the end of resource.

(Note even if compiled with BSD regular expressions, POSIX style syntax is used; also note BSD
regular expressions do not support ranges {}.)

-C By default resources are canonicalized to lower case on minting and on checking. The -C flag
overrides this so that resources are treated as case sensitive on checking, and not canonizalized on
minting.

-P Print progress info (number of iterations, expected iterations, percentage done, best stamp size
found so far).

-O core
Select hashcash core with that number. Currently 0-9 are valid cores. Not all cores work on all
architectures. Eg some are x86 specific assembler, others PPC specific assembler. If a core is not
valid hashcash returns failure and explains what happened.

-Z n
Compress the stamp. This is a time vs space trade off. Larger stamps are faster, but arguably
slightly ugly. For fastest stamps (the default) use -Z 0; for partly compressed stamps use -Z 1; for
very compressed, but somewhat slow stamps use -Z 2. (Note: due to a late discovered bug, -Z2 is the
same as -Z1 for now until I can fix that.)

EXAMPLES

Creating stamps

"hashcash -s"
Print timing information about how many preimages the machine can try per second.

"hashcash -sv"
More accurate but quite slow benchmarking of different processor specific minting cores.

"hashcash -s -b default"
Print how long it would take the machine to compute a default sized preimage (but don't actually
compute a preimage).

"hashcash -s -b 32"
Print how long it would take the machine to compute a 32 bit preimage (but don't actually compute a
preimage).

"hashcash -m"
Mint a stamp. Will prompt for resource name and mint with default value (number of preimage bits).

"hashcash -m foo"
Compute preimage on resource foo. Will mint with default value (number of preimage bits).

"hashcash -m foo -b 10"
Compute 10 bit preimage on resource foo.

"hashcash -a -3d"
Subtract a random time of between 0 days and 3 days from the stamp's creation time. This is the same
fuzz factor used by mixmaster to reduce risk of timing-correlations.

Examining Stamps

"hashcash -w 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Report the value of the stamp (how many bits of preimage) there are. The example is a 24 bit
preimage, which takes on average 25 seconds to create on a 3Ghz P4.

"hashcash -mq -b 10 foo ⎪ hashcash -w"
Create a stamp in batch mode, pass to hashcash on stdin to verify, have it print how many bits there
were.

"hashcash -n 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Report the resource name from the stamp. The resource name in the example is foo.

"hashcash -l -e 30y 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Report how long until the stamp expires if it expires in 30 years from its creation date. (Note
dates too far into the future run into the 2038 end of Epoch, which is the unix time analog of the
y2k bug).

Verifying Stamps

"hashcash -c 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Check if the stamp is valid. Note as we are not checking the stamp in a double spend database, and
did not specify a resource name or required number of bits of preimage and hashcash will consider the
stamp not fully checked, and it will report it as valid but not fully unchecked, or as invalid if
there is any problem with the stamp.

"hashcash -c -b24 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Check that the value of the stamp is greater or equal to 24 bits. This example has 24 bit value. If
you increase the requested number of bits or replace the stamp with one with less than 24 bit
preimage the stamp will be rejected.

"hashcash -c -b24 -r foo 1:24:040806:foo::511801694b4cd6b0:1e7297a"
As above check if the stamp has sufficient value, but in addition check that the resource name given
matches the resource name in the stamp.

Double Spending Prevention

The examples given in "Verifying Stamps" can be modified to keep a double spend database so that the same
stamp will not be accepted twice. Note a stamp will only be checked in and added to the database if it
is otherwise valid and fully checked (a required number of bits of preimage has been specified and a
resource has been specified).

"hashcash -cd -b 10 -r foo 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Check the stamp and add to double spent database if it's valid (has correct resource name and
sufficient value).

"hashcash -cd -b 10 -r foo 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Try to double spend the stamp. It will be rejected as double spent.

Stamp Expiry

To prevent the double spend database growing indefinately, the recipient can request that stamps be no
older than a specified period. After expiry old stamps can dropped from the double spend database as
they will no longer be needed -- expired stamps can be rejected based purely on their old date, so the
space taken by expired stamps in the double spend database can be saved without risk of accepting an
expired though otherwise valid stamp.

The third field of the stamp is the UTC time since 1st January 1970. The default time format is YYMMDD,
time rounded down to the nearest day. The default validity period is 28 days.

You can provide an alternative validity period with the -e option.

"hashcash -cd -b 10 -e 2d -r foo 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Try verifying an old stamp, the above stamp was created 11 Aug 2002.

We gave option -e 2d so the stamps expiry date is 2 days after creation, which is now in the past.

Note: if the creation time is expressed in the stamp in days, the precise creation date is the
beginning of the specified day in UTC time (similarly for alternate units the creation time is
rounded down to the beginning of the unit it is expressed in). For units in days, for example, this
may mean depending on your time zone that the stamp appears to be considered invalid in under the
specified expiry period in days relative to your relative view of what day it is, as the calculation
is based on current time in UTC, and the creation time of the stamp is expressed in UTC time.

"hashcash -cd -b 10 -r foo 1:24:040806:foo::511801694b4cd6b0:1e7297a"
Test whether the stamp is otherwise valid, apart from having expired. Omitting the -e tells hashcash
that the stamp will never expire. An expiry period of forever can also be given explicitly like
this: -e 0, where an expiry period of 0 means forever.

Purging old stamps

If the -c, -d options are used together, each time a stamp is checked, if it is valid and all of the
mandatory aspects of the stamp are verified (preimage bits check, resource name check) then the stamp and
its expiry period is written to the database file. The default expiry period if an expiry period is not
given explicitly with the -e option is 28 days (ie stamps expire after 4 weeks).

First mint and then add a stamp:

"hashcash -m -b 10 foo -e 1m > stamp"
Note: we specified an expiry on minting in this example, to ensure that the stamp creation time is
given in high enough resolution in the stamp that the stamp will not be considered expired at time of
creation. (Recall the default resolution is in days, a stamp created with a creation time rounded
down to the beginging of the day is unlikely to be considered valid 1 minute later unless you mint it
at midnight UTC time.)

"hashcash -cd -e 1m -b 10 -r foo < stamp"
The stamp expires in 1 minute. Wait 1 minute and then explicitly request that expired stamps be
purged:

"hashcash -p now"
Then try resubmitting the same stamp:

"hashcash -cd -e 1m -b 10 -r foo < stamp"
and the stamp will be rejected anyway as it has expired, illustrating why it was not necessary to
keep this stamp in the database.

With the default database (the sdb format) the database contents are human readable, so you can view
their contents by cating them to the terminal:

"cat hashcash.sdb"
to see that the stamp really is added and then after puring subsequently purged due to expiry.

Purging old stamps on Demand

As a convenience you can purge at the same time as checking stamps by using the -p option with the -c
option.

"hashcash -m -b 10 foo > stamp"
"hashcash -cd -p now -e 1 -b 10 -r foo < stamp"
It may be inefficient to purge stamps on every use as the entire database has to be scanned for
expired stamps. By giving a time period to the -p option, you can tell "hashcash" to purge no more
frequently than that time period since the previous purge.

For example:

"hashcash -cd -p 1d -e 1 -b 10 -r foo < stamp"
tells "hashcash" to purge any expired stamps no more than once per day.

"hashcash -p 1M -j foo"
tells "hashcash" to purge only expired stamps matching resource foo once per month.

"hashcash -p now -k"
tells "hashcash" to purge all stamps (expired and unexpired) now.

stamp format (version 1)

       The current stamp format is version 1.  This tool can verify hashcash version 0 stamps also, but  version
       0  stamps  are  no  longer  created as they are being phased out in favor of the more extensible v1 stamp
       format.

       ver:bits:date:resource:[ext]:rand:counter

       where

       ver = 1
       bits = how many bits of partial-preimage the stamp is claimed to have
       date = YYMMDD[hhmm[ss]]
       resource = resource string (eg IP address, email address)
       ext = extension -- ignored in the current version
           Format of extension:

           [name1[=val1[,val2...]];[name2[=val1[,val2...]]...]]
               Note the value can also contain =.  Example extension (not a real one):

                       name1=2,3;name2;name3=var1=2,var2=3,2,val

               Which would be extension name1 has values 2 and 3; extension name2 has no values; extension name3
               has 3 values "var1=2", "var2=3", "2" and "val".  The hashcash extension may interpret the  values
               as it sees fit eg "var1=2" could be the value of an option to the extension name3.

       rand = string of random characters from alphabet a-zA-Z0-9+/= to avoid preimage with other sender's
       stamps
       counter = to find a stamp with the desired number of preimage bits need to try lots of different strings
       this counter is incremented on each try. The Counter is also composed of characters from the alphabet
       a-zA-Z0-9+/=.  (Note an implementation is not required to count sequentially).

FILES

       hashcash.sdb
           default double spend database

EXIT STATUS

       "hashcash" returns success (exit code 0) after successfully minting a stamp, after fully checking a stamp
       and finding it valid, and after a timing test.

       If  when  checking  a  stamp  it  is  found  to be invalid (due to being malformed, being expired, having
       insufficient value, having a date in the future, or being double spent), "hashcash" returns failure (exit
       code 1).

       If insufficient options are given to fully check  a  stamp,  if  the  stamp  is  otherwise  valid  return
       unchecked (exit code 2).  If the -y flag is given and hashcash would normally return unchecked, exit code
       success is returned instead.

       If  any exception occurs (file read failure for database checking or corrupted database contents) an exit
       status of 3 is returned.

AUTHOR

       Written by Adam Back <adam@cypherspace.org>