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".

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.

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.

  $srs = new Mail::SRS(...)
    Construct a new Mail::SRS object and return it. Available parameters
    are:

    Secret
        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
        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
        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.

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

METHODS
  $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.

EXPORTS
    Given :all, this module exports three variables.

    $SRSSEP
        The SRS separator. The choice of 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.

    $SRSTAG
        The SRS0 tag.

    $SRSWRAP
        The SRS1 tag.

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

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.

