HASHCASH(1)hashcashHASHCASH(1)NAMEhashcash - 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 parameteriz‐
able amount of work to compute for the sender. The recipient can ver‐
ify received stamps efficiently. This package also includes a sha1
implementation which is equivalent in functionality to the sha1sum pro‐
gram available on some systems.
The hashcash proof-of-work function is to compute partial hash preim‐
ages (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 hash‐
cash 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 Pro‐
tection". 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 recipi‐
ent (eg with email.) The suggested value of 28 days should be safe for
normal email delivery delays. The choice is a trade-off between data‐
base 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 sepa‐
rately using just the -p now option to request just the purge opera‐
tion. 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 mini‐
mum 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 insuffi‐
cient 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 = min‐
utes, h = hours, d = days, M = months, y = Y = years, and s = sec‐
onds).
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 resolu‐
tion of time created in the stamp. Without the -e option, the
default resolution is days (time format: YYMMDD). Alternate for‐
mats 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 sec‐
onds.
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 combi‐
nation with -m to specify an implicit time field width under the
description of the -e flag. Valid widths are 6,10 or 12 chars cor‐
responding 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 hash‐
cash 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 for‐
mat. 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 alter‐
nate 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 subdo‐
mains. 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 char‐
acter 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
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 preim‐
age (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 mix‐
master 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 num‐
ber 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 dou‐
ble 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 preim‐
age 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 recipi‐
ent 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 cre‐
ation, 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 veri‐
fied (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 cre‐
ated 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, illustrat‐
ing 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 subse‐
quently 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 for‐
mat.
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 sequen‐
tially).
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 mal‐
formed, 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>
SEE ALSOsha1sum(1), sha1-hashcash(1), sha1(1), http://www.hashcash.org/
1.22 2006-04-07 HASHCASH(1)
