macutil(1) Mail Avenger 0.8.4 macutil(1)NAME
macutil, sendmac - Message Authentication Code utility
SYNOPSISmacutil--gen [options]
macutil--sender [template] [--from name] [options]
macutil--check [options] code
macutil [options] --sendmail [sendmail-options]
sendmac [sendmail-options]
DESCRIPTIONmacutil generates and checks the validity of codes that can be embedded
in temporary email addresses. The codes are calculated using a secret
passphrase stored in a file. Thus, someone who does not know the
passphrase cannot easily generate a valid code. Each code has a
configurable expiration time after which it becomes invalid.
To use macutil, you must create a file containing a passphrase. The
default location of this file is $HOME/.avenger/.macpass, though the
location can be overridden with the MACUTIL_PASSFILE environment
variable or --passfile= command-line option. The file should contain a
passphrase followed by a newline. The maximum allowed length of the
passphrase is 64 characters. Do not use your Unix login password or
any password you have used for a sensitive application, as macutil's
password will be stored in cleartext and thus be relatively easy to
compromise.
Running macutil--gen generates a new code and writes it to standard
output.
Running macutil--check code checks the validity of code. If the code
is valid and has not expired, macutil exits with status 0. If the code
is invalid or has expired, macutil prints a message to standard error
and exits with a non-zero exit code.
The following options affect macutil's behavior:
--gen (-g)
Generates a code, as described above.
--sender template (-s template)
This option is like --gen, but outputs a complete email address,
instead of just a code. The address is formatted based on
template. template should contain an email address with a "*"
character. The "*" will be replaced by a code. For example, if
template is "myname+bounces+*", running "macutil --sender" might
output:
myname+bounces+zjkifk8kuvsy7rubu7vqadmwnn
Don't forget to quote the "*" character when invoking macutil from
a shell.
--from name (-f name)
This option, in conjunction with --sender, produces output more
suitable for the "From:" field in an email message header. For
example, if name is set to "Mail Avenger", running "macutil
--sender 'myname+tmp+*host' --from 'Mail Avenger'" might output:
Mail Avenger <myname+tmp+zjkifk8kuvsy7rubu7vqadmwnn@host>
Note that if the MACUTIL_SENDER environment variable has been set,
this will be used as a default vaule for the --sender option if you
invoke macutil--from and don't specify a --sender.
--fromexp phrase
In conjunction with the --from option, this option includes an
expiration time for the address in a comment. For example,
supplying a phrase of "address expires" would result in output like
this:
Mail Avenger (address expires 07 Dec 2004)
<myname+tmp+zjkifk8kuvsy7rubu7vqadmwnn@host>
--check (-c)
Checks a code, as described above. Exits 0 on success; exits non-
zero with a message to standard error if the code is invalid.
--passfile=file (-p file)
Specify the passphrase file to use.
Note that if file contains multiple passphrases, one per line,
--gen always uses the first passphrase in the file. --check,
however, will try all passphrases until one succeeds, and only
output failure if they all fail. In this way, you can change your
passphrase, but keep accepting the old one for a time by leaving it
as the second line of the file.
--expire=date
Specify the expiration date for the code. date can be an absolute
number of seconds since midnight, Jan 1, 1970, GMT. Alternatively
(and perhaps more usefully), it can be expressed relative to the
current time, as:
+numh
+numD
+numW
to specify num hours, days, or weeks in the future. The full range
of suffixes allowed is s, m, h, D, W, M, and Y, which designate
seconds, minutes, hours, days, weeks, months, and years,
respectively. The default expiration time is 21 days ("+21D").
--aux=string
Permutes the algorithm using string. You must specify the same
--aux argument when both generating and checking codes. This
allows you to re-use the same password for different sets of codes.
For example, you might require tokens generated with "macutil --gen
--aux=list1" to be embedded in recipient addresses for one mailing
list, and "macutil --gen --aux=list2" to be embedded in recipient
addresses for another. Someone who has an address that is valid
for one list will still not be able to send to the other.
--date=date
Run as if the current time were date. As with --expire, date can
be an absolute number or can be relative to the current time. Use
- instead of + to specify a time in the past (e.g., -numh or
-numD).
--sendmail
This option must be the last sendmac option. It tells macutil to
run sendmail with the remaining arguments you have specified, but
to insert the options -f address at the beginning of the argument
list, where address is generated as with the --sender option. You
must specify an address template, either through explicit use of
the --sender option, or by setting the MACUTIL_SENDER environment
variable.
For example, if MACUTIL_SENDER is "myname+bounces+*", running
"macutil --sendmail friend@domain.com" might run the command:
sendmail -f \
myname+bounces+zjkifk8kuvsy7rubu7vqadmwnn \
friend@domain.com
Note that if invoke the macutil program as "sendmac" (or as any
other name you link it to beginning with the four letters "send"),
it will automatically behave as though there were an extra first
argument of --sendmail. (In this case, you cannot specify any
sendmac options, but you can still control sendmac's behavior
through the environment variables listed below.)
ENVIRONMENT
MACUTIL_EXPIRE
Sets the expiration time if not explicitly overwritten by the
--expire flag. If MACUTIL_EXPIRE is not set, macutil uses a
default value of "+21D" (21 days).
MACUTIL_FROMEXP
If this option is set to phrase, then the output of "sendmac
--from" will always behave as though an extra --fromexp phrase
argument had been supplied.
MACUTIL_PASSFILE
Specifies a passphrase file other than the default of
$HOME/.avenger/.macpass.
MACUTIL_SENDER
Specifies a template sender address to use as a default value of
--sender with the --sendmail and --from options. See the
descriptions of the --sendmail and --from options above for more
information.
MACUTIL_SENDMAIL
Specifies the path to sendmail for the --sendmail option. The
default is just sendmail.
FILES
$HOME/.avenger/.macpass
SEE ALSOavenger(1)
The Mail Avenger home page: <http://www.mailavenger.org/>.
BUGSmacutil is designed to provide casual security against people trying to
guess a valid temporary email address. Don't use it where stronger
authentication is required. In particular, for any given passphrase, a
random code will be valid (at least on some date) with probability 1 in
2^64. While these are tough odds to beat, cryptographers generally
prefer a margin of safety closer to 1 in 2^128 for high-security
applications (though that would require longer codes).
Someone who sees a valid code can mount an off-line dictionary attack
against your passphrase. In other words, while it is hard recover your
passphrase outright, given a valid code, it is is easy to verify
whether a particular guess of your passphrase is correct. By guessing
every word in the dictionary, an attacker can recover weak passphrases.
Technically, the cryptographic operation performed on the keys is
encryption, not a message authentication code (or MAC). Hence, one
could argue the utility is misnamed.
AUTHOR
David Mazieres
Mail Avenger 0.8.4 2013-07-13 macutil(1)
