NAME

Object::Signature::Portable - generate portable signatures of objects

SYNOPSIS

    use Object::Signature::Portable;

    my $sig = signature( $object ); # MD5 hex of object signature

    my $sig = signature(
      digest => 'SHA1',             # SHA-1 digest
      format => 'b64udigest',       # as URL-safe base-64
      data   => $object,
    );

DESCRIPTION

This module provides a simple function for generating portable cryptographic signatures of Perl data structures.

The object is serialized into a canonical JSON structure, and then hashed using the MD5 algorithm.

Any two machines running different versions of Perl on different architectures should produce identical signatures.

Note that this module is useful in cases where the consistency of signatures between machine is more important than the speed of signature generation.

However, the serialization method, hash algorithm and signature format can be customized, as needed.

EXPORTS

signature

  my $sig = signature( $data );

  my $sig = signature(
    data       => $data,
    digest     => 'MD5',         # default
    format     => 'hexdigest',   # default
    serializer => sub { ... },
  );

Generate a cryptographic signature of the $data.

The following options are supported:

digest

The cryptographic digest algorithm, as supported by Crypt::Digest.

format

The Crypt::Digest formatting method for the signature, which can be one of:

digest

The raw bytes of the digest.

hexdigest

The digest as a string of hexidecimal numbers.

b64digest

The digest as a MIME base-64 string.

b64udigest

The digest as a URL-friendly base-64 string.

prefix

If set to a true value, the digest is prefixed by the name of the digest algorithm.

This is useful when you may want to change the digest algorithm used by an application in the future, but do not want to regenerate signatures for existing objects in a data store.

serializer

The serialization method, which is a subroutine that takes the data as a single argument, and returns the serialized data to be hashed.

It is recommended that you use a serializer that produces canonical (normalized) output, and preferably one that produces consistent output across all of the platforms that you are using.

By default, it uses JSON::MaybeXS. See "LIMITATIONS" below.

LIMITATIONS

Signatures for Arbitrary Objects

By default, this module uses JSON::MaybeXS to serialize Perl objects.

This requires the objects to have a TO_JSON method in order to be serialized. Unfortunately, this is not suitable for many objects (particularly those generated by modules that are not under your control, e.g. many CPAN modules) without monkey-patching or subclassesing them.

One solution is to use a different serializer that can handle the object.

Alternatively, you can write a wrapper function that uses a module such as Object::Serializer to translate an object into a hash reference that can then be passed to the signature function, e.g.

    package Foo;

    use parent 'Object::Serializer';

    use Object::Signature::Portable ();

    sub signature {
        my $self = shift;
        return Object::Signature::Portable::signature(
          data => $self->serialize
        );
    }

Note that Object::Serializer allows you to define custom serialization strategies for various reference types.

Portability

The portability of signatures across different versions of JSON::MaybeXS is, of course, dependent upon whether those versions will produce consistent output.

If you are concerned about this, then write our own serializer, or avoid upgrading JSON::MaybeXS until you are sure that the it will produce consistent signatures.

Security

This module is intended for generating signatures of Perl data structures, as a simple means of determining whether two structures are different.

For that purpose, the MD5 algorithm is probably good enough. However, if you are hashing that in part comes from untrusted sources, or the consequences of two different data structures having the same signature are significant, then you should consider using a different algorithm.

This module is not intended for hashing passwords.

SEE ALSO

Similar Modules

Object::Signature

This uses Storable to serialise objects and generate a MD5 hexidecimal string as a signature.

This has the drawback that machines with different architectures, different versions of Perl, or different versions Storable may not produce the same signature for the same data. (This does not mean that Storable is unable to de-serialize data produced by different versions; it only means that the serialized data is not identical across different versions.)

Object::Signature does not allow for customizing the hash algorithm or signature format.

Object::Signature::Portable module can replicate the signatures generated by Object::Signature, using the following:

  use Storable 2.11;

  my $sig = signature(
    data       => $data,
    serializer => sub {
      local $Storable::canonical = 1;
      return Storable::nfreeze($_[0]);
    },
  );

As noted above, using Storable will not produce portable signatures.

AUTHOR

Robert Rothenberg <rrwo@cpan.org> (on behalf of Foxtons, Ltd.)

LICENSE AND COPYRIGHT

Copyright 2013-2014 Robert Rothenberg.

This program is free software; you can redistribute it and/or modify it under the terms of the the Artistic License (2.0). You may obtain a copy of the full license at:

http://www.perlfoundation.org/artistic_license_2_0

Any use, modification, and distribution of the Standard or Modified Versions is governed by this Artistic License. By using, modifying or distributing the Package, you accept this license. Do not use, modify, or distribute the Package, if you do not accept this license.

If your Modified Version has been derived from a Modified Version made by someone other than you, you are nevertheless required to ensure that your Modified Version complies with the requirements of this license.

This license does not grant you the right to use any trademark, service mark, tradename, or logo of the Copyright Holder.

This license includes the non-exclusive, worldwide, free-of-charge patent license to make, have made, use, offer to sell, sell, import and otherwise transfer the Package with respect to any patent claims licensable by the Copyright Holder that are necessarily infringed by the Package. If you institute patent litigation (including a cross-claim or counterclaim) against any party alleging that the Package constitutes direct or contributory patent infringement, then this Artistic License to you shall terminate on the date that such litigation is filed.

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