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


       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 collision bits:

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

       Get resource name:

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

       Get time remaining until expiry

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


       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
       collisions (the SHA1 hash function is used).  By choosing the number of
       bits of collision, the work required to compute a collision 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 collision size.

       For more detailed discussion of other applications hashcash has been
       used for see


       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

       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 collision size can be
       specified with the -b option.  If no collision 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 collision 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
       collision 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

       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 collisions 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.


       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).


       -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 collision of at least this many
           bits.  When verifying a stamp require that it have a collision 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 =

           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 begining of the nearest day;
           * 2 days < period <= 2 minutes then time format YYMMDDhhmm is used
           rounded down to the begining of the nearest minute;
           * period < 2 minutes then time format YYMMDDhhmmss is used in

           Note the rounding down is based on UTC time, not local time.  This
           can lead to initially suprising 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.

           Note the rounding down is based on UTC time, not local time.  This
           can lead to initially suprising 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.

       -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 collision size would take to compute, if -s given by
           itself, just prints speed of the collision finder.  To print an
           estimate of how long the default number of bits would take use -b

       -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

       -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

           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 collision 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

       -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

       -w  Print number of bits of collision 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 resouce, 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

       -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.)


       Creating stamps

       "hashcash -s"
           Print timing information about how many collisions 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
           collision (but don’t actually compute a collision).

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

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

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

       "hashcash -m foo -b 10"
           Compute 10 bit collision 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 collision) there
           are.  The example is a 24 bit collision, 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 collision 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

       "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
           collision 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

       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 collision 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

       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
           Try verifying an old stamp, the above stamp was created 11 Aug

           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 begining of the specified day in UTC
           time (similarly for alternate units the creation time is rounded
           down to the begining 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 explitly
           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 (collision 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

           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

       "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



       ver = 1
       bits = how many bits of partial-collision 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:

               Note the value can also contain =.  Example extension (not a
               real one):


               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
       collisions with other sender’s stamps
       counter = to find a stamp with the desired number of collision 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


           default double spend database


       "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.


       Written by Adam Back <>


       sha1sum(1), sha1(1),