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NAME

macutil, sendmac − Message Authentication Code utility

SYNOPSIS

macutil −−gen [options]

macutil −−sender [template] [−−from name] [options]

macutil −−check [options] code

macutil [options] −−sendmail [sendmail-options]

sendmac [sendmail-options]

macutil 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 [AT] domain.com" might run the command:

    sendmail −f \
        myname+bounces+zjkifk8kuvsy7rubu7vqadmwnn \
        friend [AT] 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

BUGS

macutil 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

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