/ 
Mail-SRS-0.19
River stage zero No dependents
/ Mail::SRS

NAME

Mail::SRS - Interface to Sender Rewriting Scheme

SYNOPSIS

use Mail::SRS;
my $srs = new Mail::SRS(
	Secret     => [ .... ],    # scalar or array
	MaxAge     => 49,          # days
	HashLength => 4,           # base64 characters: 4 x 6bits
	HashMin    => 4,           # base64 characters
		);
my $srsaddress = $srs->forward($sender, $alias);
my $sender = $srs->reverse($srsaddress);

DESCRIPTION

The Sender Rewriting Scheme preserves .forward functionality in an SPF-compliant world.

SPF requires the SMTP client IP to match the envelope sender (return-path). When a message is forwarded through an intermediate server, that intermediate server may need to rewrite the return-path to remain SPF compliant. If the message bounces, that intermediate server needs to validate the bounce and forward the bounce to the original sender.

SRS provides a convention for return-path rewriting which allows multiple forwarding servers to compact the return-path. SRS also provides an authentication mechanism to ensure that purported bounces are not arbitrarily forwarded.

SRS is documented at http://spf.pobox.com/srs.html and many points about the scheme are discussed at http://www.anarres.org/projects/srs/

For a better understanding of this code and how it functions, please run the interactive walkthrough in eg/simple.pl in this distribution. To run this from the build directory, type "make teach".

METHODS

$srs = new Mail::SRS(...)

Construct a new Mail::SRS object and return it. Available parameters are:

Secret => $string

A key for the cryptographic algorithms. This may be an array or a single string. A string is promoted into an array of one element.

MaxAge

The maximum number of days for which a timestamp is considered valid. After this time, the timestamp is invalid.

HashLength => $integer

The number of bytes of base64 encoded data to use for the cryptographic hash. More is better, but makes for longer addresses which might exceed the 64 character length suggested by RFC2821. This defaults to 4, which gives 4 x 6 = 24 bits of cryptographic information, which means that a spammer will have to make 2^24 attempts to guarantee forging an SRS address.

HashMin => $integer

The shortest hash which we will allow to pass authentication. Since we allow any valid prefix of the full SHA1 HMAC to pass authentication, a spammer might just suggest a hash of length 0. We require at least HashMin characters, which must all be correct. Naturally, this must be no greater than HashLength and will default to HashLength unless otherwise specified.

Separator => $character

Some subclasses require other parameters. See their documentation for details.

$srsaddress = $srs->forward($sender, $alias)

Map a sender address into a new sender and a cryptographic cookie. Returns an SRS address to use as the new sender.

There are alternative subclasses, some of which will return SRS compliant addresses, some will simply return non-SRS but valid RFC821 addresses. See the interactive walkthrough for more information on this ("make teach").

$sender = $srs->reverse($srsaddress)

Reverse the mapping to get back the original address. Validates all cryptographic and timestamp information. Returns the original sender address.

$srs->compile($sendhost, $senduser)

This method, designed to be overridden by subclasses, takes as parameters the original host and user and must compile a new username for the SRS transformed address. It is expected that this new username will be joined on $SRSSEP, and will contain a hash generated from $self->hash_create(...), and possibly a timestamp generated by $self->timestamp_create().

$srs->parse($srsuser)

This method, designed to be overridden by subclasses, takes an SRS-transformed username as an argument, and must reverse the transformation produced by compile(). It is required to verify any hash and timestamp in the parsed data, using $self->hash_verify($hash, ...) and $self->timestamp_check($timestamp).

$srs->timestamp_create([$time])

Return a two character timestamp representing 'today', or $time if given. $time is a Unix timestamp (seconds since the aeon).

$srs->timestamp_check($timestamp)

Return 1 if a timestamp is valid, undef otherwise. There are 4096 possible timestamps, used in a cycle. At any time, $srs->{MaxAge} timestamps in this cycle are valid, the last one being today. A timestamp from the future is not valid, neither is a timestamp from too far into the past. Of course if you go far enough into the future, the cycle wraps around, and there are valid timestamps again, but the likelihood of a random timestamp being valid is 4096/$srs->{MaxAge}, which is usually quite small: 1 in 132 by default.

$srs->time_check($time)

Similar to $srs->timestamp_check($timestamp), but takes a Unix time, and checks that an alias created at that Unix time is still valid. This is designed for use by subclasses with storage backends.

$srs->hash_create(@data)

Returns a cryptographic hash of all data in @data. Any piece of data encoded into an address which must remain inviolate should be hashed, so that when the address is reversed, we can check that this data has not been tampered with. You must provide at least one piece of data to this method (otherwise this system is both cryptographically weak and there may be collision problems with sender addresses).

$srs->hash_verify($hash, @data)

Verify that @data has not been tampered with, given the cryptographic hash previously output by $srs->hash_create(); Returns 1 or undef. All known secrets are tried in order to see if the hash was created with an old secret.

$srs->set_secret($new, @old)

Add a new secret to the rewriter. When an address is returned, all secrets are tried to see if the hash can be validated. Don't use "foo".

$srs->get_secret()

Return the list of secrets. These are secret. Don't publish them.

$srs->separator()

Return the initial separator, which follows the SRS tag. This is only used as the initial separator, for the convenience of administrators who wish to make srs0 and srs1 users on their mail servers and require to use + or - as the user delimiter. All other separators in the SRS address must be =.

EXPORTS

Given :all, this module exports the following variables.

$SRSSEP

The SRS separator. The choice of = as internal separator was fairly arbitrary. It cannot be any of the following:

/ +

Used in Base64.

-

Used in domains.

! %

Used in bang paths and source routing.

:

Cannot be used in a Windows NT or Apple filename.

; | *

Shell or regular expression metacharacters are probably to be avoided.

$SRS0TAG

The SRS0 tag.

$SRS1TAG

The SRS1 tag.

$SRSTAG

Deprecated, equal to $SRS0TAG.

$SRSWRAP

Deprecated, equal to $SRS1TAG.

EXTENDING Mail::SRS

Write a subclass. If people mail me asking for callbacks with the hash data from the standard subclasses, I will provide them. Callback hooks have not been provided in this release candidate.

WARNING: MINOR CHANGES since v0.18

$SRSTAG and $SRSWRAP are deprecated.

You must use $SRS0RE and $SRS1RE to detect SRS addresses.

WARNING: MAJOR CHANGES since v0.15

The separator character is now =.
The cryptographic scheme is now HMAC with SHA1.
Only a prefix of the MAC is used.

This API is still a release candidate and should remain relatively stable.

BUGS

Email address parsing for quoted addresses is not yet done properly.

More error checking should be done for invalid SRS addresses.

SEE ALSO

Mail::SRS::Guarded, Mail::SRS::DB, Mail::SRS::Reversable, "make teach", eg/*, http://www.anarres.org/projects/srs/

AUTHOR

Shevek
CPAN ID: SHEVEK
cpan@anarres.org
http://www.anarres.org/projects/

COPYRIGHT

Copyright (c) 2004 Shevek. All rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.