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 collision 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 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
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 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 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 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.
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 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 = 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 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 seconds.
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
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
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 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 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 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 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 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 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 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 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 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 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 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.
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-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:
- [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 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
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>
SEE ALSO¶
sha1sum(1),
sha1(1),
http://www.hashcash.org/