NAME

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

VERSION

version v1.0.0

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 digital fingerprints (a.k.a. signatures, not to be confiused with public key 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 digital fingerprint 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. (YAML, Data::Dumper or Sereal::Encoder should be acceptable alternatives, provided that you enable canonical encoding, and in the case of Sereal, explicitly specify a protocol version.)

By default, it uses JSON::MaybeXS. The choice for using JSON is based on the following considerations:

• JSON is a simple, text-based format. The output is not likely to change between module versions.

• Consistent encoding of integers are strings.

• Classes can be extended with hooks for JSON serialization.

• Speed is not a factor.

However, see "LIMITATIONS" below.

LIMITATIONS

Encoding

The default JSON serializer will use UTF-8 encoding by default, which generally removes an issue with Storable when identical strings have different encodings.

Numeric values may be inconsistently represented if they are not numified, e.g. "123" vs 123 may not produce the same JSON. (This too is an issue with Storable.)

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, or in some cases different encodings of the same scalar, 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.

SOURCE

The development version is on github at https://github.com/robrwo/Object-Signature-Portable and may be cloned from git://github.com/robrwo/Object-Signature-Portable.git

BUGS

Please report any bugs or feature requests on the bugtracker website https://github.com/robrwo/Object-Signature-Portable/issues

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.

AUTHOR

Robert Rothenberg <rrwo@cpan.org>

Acknowledgements

Thanks to various people at YAPC::EU 2014 for suggestions about Sereal::Encoder.

COPYRIGHT AND LICENSE

This software is Copyright (c) 2013-2014, 2019-2020 by Robert Rothenberg.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)