NAME

CBOR::XS - Concise Binary Object Representation (CBOR, RFC7049)

SYNOPSIS

use CBOR::XS;

$binary_cbor_data = encode_cbor $perl_value;
$perl_value       = decode_cbor $binary_cbor_data;

# OO-interface

$coder = CBOR::XS->new;
$binary_cbor_data = $coder->encode ($perl_value);
$perl_value       = $coder->decode ($binary_cbor_data);

# prefix decoding

my $many_cbor_strings = ...;
while (length $many_cbor_strings) {
   my ($data, $length) = $cbor->decode_prefix ($many_cbor_strings);
   # data was decoded
   substr $many_cbor_strings, 0, $length, ""; # remove decoded cbor string
}

DESCRIPTION

WARNING! This module is very new, and not very well tested (that's up to you to do). Furthermore, details of the implementation might change freely before version 1.0. And lastly, the object serialisation protocol depends on a pending IANA assignment, and until that assignment is official, this implementation is not interoperable with other implementations (even future versions of this module) until the assignment is done.

You are still invited to try out CBOR, and this module.

This module converts Perl data structures to the Concise Binary Object Representation (CBOR) and vice versa. CBOR is a fast binary serialisation format that aims to use a superset of the JSON data model, i.e. when you can represent something in JSON, you should be able to represent it in CBOR.

In short, CBOR is a faster and very compact binary alternative to JSON, with the added ability of supporting serialisation of Perl objects. (JSON often compresses better than CBOR though, so if you plan to compress the data later you might want to compare both formats first).

The primary goal of this module is to be correct and the secondary goal is to be fast. To reach the latter goal it was written in C.

See MAPPING, below, on how CBOR::XS maps perl values to CBOR values and vice versa.

FUNCTIONAL INTERFACE

The following convenience methods are provided by this module. They are exported by default:

$cbor_data = encode_cbor $perl_scalar: Converts the given Perl data structure to CBOR representation. Croaks on error.
$perl_scalar = decode_cbor $cbor_data: The opposite of encode_cbor: expects a valid CBOR string to parse, returning the resulting perl scalar. Croaks on error.

OBJECT-ORIENTED INTERFACE

The object oriented interface lets you configure your own encoding or decoding style, within the limits of supported formats.

$cbor = new CBOR::XS

Creates a new CBOR::XS object that can be used to de/encode CBOR strings. All boolean flags described below are by default disabled.

The mutators for flags all return the CBOR object again and thus calls can be chained:

#TODO my $cbor = CBOR::XS->new->encode ({a => [1,2]});

$cbor = $cbor->max_depth ([$maximum_nesting_depth])

$max_depth = $cbor->get_max_depth

Sets the maximum nesting level (default 512) accepted while encoding or decoding. If a higher nesting level is detected in CBOR data or a Perl data structure, then the encoder and decoder will stop and croak at that point.

Nesting level is defined by number of hash- or arrayrefs that the encoder needs to traverse to reach a given point or the number of { or [ characters without their matching closing parenthesis crossed to reach a given character in a string.

Setting the maximum depth to one disallows any nesting, so that ensures that the object is only a single hash/object or array.

If no argument is given, the highest possible setting will be used, which is rarely useful.

Note that nesting is implemented by recursion in C. The default value has been chosen to be as large as typical operating systems allow without crashing.

See SECURITY CONSIDERATIONS, below, for more info on why this is useful.

$cbor = $cbor->max_size ([$maximum_string_size])

$max_size = $cbor->get_max_size

Set the maximum length a CBOR string may have (in bytes) where decoding is being attempted. The default is 0, meaning no limit. When decode is called on a string that is longer then this many bytes, it will not attempt to decode the string but throw an exception. This setting has no effect on encode (yet).

If no argument is given, the limit check will be deactivated (same as when 0 is specified).

See SECURITY CONSIDERATIONS, below, for more info on why this is useful.

$cbor_data = $cbor->encode ($perl_scalar)

Converts the given Perl data structure (a scalar value) to its CBOR representation.

$perl_scalar = $cbor->decode ($cbor_data)

The opposite of encode: expects CBOR data and tries to parse it, returning the resulting simple scalar or reference. Croaks on error.

($perl_scalar, $octets) = $cbor->decode_prefix ($cbor_data)

This works like the decode method, but instead of raising an exception when there is trailing garbage after the CBOR string, it will silently stop parsing there and return the number of characters consumed so far.

This is useful if your CBOR texts are not delimited by an outer protocol and you need to know where the first CBOR string ends amd the next one starts.

CBOR::XS->new->decode_prefix ("......")
=> ("...", 3)

MAPPING

This section describes how CBOR::XS maps Perl values to CBOR values and vice versa. These mappings are designed to "do the right thing" in most circumstances automatically, preserving round-tripping characteristics (what you put in comes out as something equivalent).

For the more enlightened: note that in the following descriptions, lowercase perl refers to the Perl interpreter, while uppercase Perl refers to the abstract Perl language itself.

CBOR -> PERL

integers

CBOR integers become (numeric) perl scalars. On perls without 64 bit support, 64 bit integers will be truncated or otherwise corrupted.

byte strings

Byte strings will become octet strings in Perl (the byte values 0..255 will simply become characters of the same value in Perl).

UTF-8 strings

UTF-8 strings in CBOR will be decoded, i.e. the UTF-8 octets will be decoded into proper Unicode code points. At the moment, the validity of the UTF-8 octets will not be validated - corrupt input will result in corrupted Perl strings.

arrays, maps

CBOR arrays and CBOR maps will be converted into references to a Perl array or hash, respectively. The keys of the map will be stringified during this process.

null

CBOR null becomes undef in Perl.

true, false, undefined

These CBOR values become Types:Serialiser::true, Types:Serialiser::false and Types::Serialiser::error, respectively. They are overloaded to act almost exactly like the numbers 1 and 0 (for true and false) or to throw an exception on access (for error). See the Types::Serialiser manpage for details.

CBOR tag 256 (perl object)

The tag value 256 (TODO: pending iana registration) will be used to deserialise a Perl object serialised with FREEZE. See "OBJECT SERIALISATION", below, for details.

CBOR tag 55799 (magic header)

The tag 55799 is ignored (this tag implements the magic header).

other CBOR tags

Tagged items consists of a numeric tag and another CBOR value. Tags not handled internally are currently converted into a CBOR::XS::Tagged object, which is simply a blessed array reference consisting of the numeric tag value followed by the (decoded) CBOR value.

In the future, support for user-supplied conversions might get added.

anything else

Anything else (e.g. unsupported simple values) will raise a decoding error.

PERL -> CBOR

The mapping from Perl to CBOR is slightly more difficult, as Perl is a truly typeless language, so we can only guess which CBOR type is meant by a Perl value.

hash references

Perl hash references become CBOR maps. As there is no inherent ordering in hash keys (or CBOR maps), they will usually be encoded in a pseudo-random order.

Currently, tied hashes will use the indefinite-length format, while normal hashes will use the fixed-length format.

array references

Perl array references become fixed-length CBOR arrays.

other references

Other unblessed references are generally not allowed and will cause an exception to be thrown, except for references to the integers 0 and 1, which get turned into false and true in CBOR.

CBOR::XS::Tagged objects

Objects of this type must be arrays consisting of a single [tag, value] pair. The (numerical) tag will be encoded as a CBOR tag, the value will be encoded as appropriate for the value. You cna use CBOR::XS::tag to create such objects.

Types::Serialiser::true, Types::Serialiser::false, Types::Serialiser::error

These special values become CBOR true, CBOR false and CBOR undefined values, respectively. You can also use \1, \0 and \undef directly if you want.

other blessed objects

Other blessed objects are serialised via TO_CBOR or FREEZE. See "OBJECT SERIALISATION", below, for details.

simple scalars

TODO Simple Perl scalars (any scalar that is not a reference) are the most difficult objects to encode: CBOR::XS will encode undefined scalars as CBOR null values, scalars that have last been used in a string context before encoding as CBOR strings, and anything else as number value:

# dump as number
encode_cbor [2]                      # yields [2]
encode_cbor [-3.0e17]                # yields [-3e+17]
my $value = 5; encode_cbor [$value]  # yields [5]

# used as string, so dump as string
print $value;
encode_cbor [$value]                 # yields ["5"]

# undef becomes null
encode_cbor [undef]                  # yields [null]

You can force the type to be a CBOR string by stringifying it:

my $x = 3.1; # some variable containing a number
"$x";        # stringified
$x .= "";    # another, more awkward way to stringify
print $x;    # perl does it for you, too, quite often

You can force the type to be a CBOR number by numifying it:

my $x = "3"; # some variable containing a string
$x += 0;     # numify it, ensuring it will be dumped as a number
$x *= 1;     # same thing, the choice is yours.

You can not currently force the type in other, less obscure, ways. Tell me if you need this capability (but don't forget to explain why it's needed :).

Perl values that seem to be integers generally use the shortest possible representation. Floating-point values will use either the IEEE single format if possible without loss of precision, otherwise the IEEE double format will be used. Perls that use formats other than IEEE double to represent numerical values are supported, but might suffer loss of precision.

OBJECT SERIALISATION

This module knows two way to serialise a Perl object: The CBOR-specific way, and the generic way.

Whenever the encoder encounters a Perl object that it cnanot serialise directly (most of them), it will first look up the TO_CBOR method on it.

If it has a TO_CBOR method, it will call it with the object as only argument, and expects exactly one return value, which it will then substitute and encode it in the place of the object.

Otherwise, it will look up the FREEZE method. If it exists, it will call it with the object as first argument, and the constant string CBOR as the second argument, to distinguish it from other serialisers.

The FREEZE method can return any number of values (i.e. zero or more). These will be encoded as CBOR perl object, together with the classname.

If an object supports neither TO_CBOR nor FREEZE, encoding will fail with an error.

Objects encoded via TO_CBOR cannot be automatically decoded, but objects encoded via FREEZE can be decoded using the following protocol:

When an encoded CBOR perl object is encountered by the decoder, it will look up the THAW method, by using the stored classname, and will fail if the method cannot be found.

After the lookup it will call the THAW method with the stored classname as first argument, the constant string CBOR as second argument, and all values returned by FREEZE as remaining arguments.

EXAMPLES

Here is an example TO_CBOR method:

sub My::Object::TO_CBOR {
   my ($obj) = @_;

   ["this is a serialised My::Object object", $obj->{id}]
}

When a My::Object is encoded to CBOR, it will instead encode a simple array with two members: a string, and the "object id". Decoding this CBOR string will yield a normal perl array reference in place of the object.

A more useful and practical example would be a serialisation method for the URI module. CBOR has a custom tag value for URIs, namely 32:

sub URI::TO_CBOR {
   my ($self) = @_;
   my $uri = "$self"; # stringify uri
   utf8::upgrade $uri; # make sure it will be encoded as UTF-8 string
   CBOR::XS::tagged 32, "$_[0]"
}

This will encode URIs as a UTF-8 string with tag 32, which indicates an URI.

Decoding such an URI will not (currently) give you an URI object, but instead a CBOR::XS::Tagged object with tag number 32 and the string - exactly what was returned by TO_CBOR.

To serialise an object so it can automatically be deserialised, you need to use FREEZE and THAW. To take the URI module as example, this would be a possible implementation:

sub URI::FREEZE {
   my ($self, $serialiser) = @_;
   "$self" # encode url string
}

sub URI::THAW {
   my ($class, $serialiser, $uri) = @_;

   $class->new ($uri)
}

Unlike TO_CBOR, multiple values can be returned by FREEZE. For example, a FREEZE method that returns "type", "id" and "variant" values would cause an invocation of THAW with 5 arguments:

sub My::Object::FREEZE {
   my ($self, $serialiser) = @_;

   ($self->{type}, $self->{id}, $self->{variant})
}

sub My::Object::THAW {
   my ($class, $serialiser, $type, $id, $variant) = @_;

   $class-<new (type => $type, id => $id, variant => $variant)
}

MAGIC HEADER

There is no way to distinguish CBOR from other formats programmatically. To make it easier to distinguish CBOR from other formats, the CBOR specification has a special "magic string" that can be prepended to any CBOR string without changing it's meaning.

This string is available as $CBOR::XS::MAGIC. This module does not prepend this string tot he CBOR data it generates, but it will ignroe it if present, so users can prepend this string as a "file type" indicator as required.

THE CBOR::XS::Tagged CLASS

CBOR has the concept of tagged values - any CBOR value can be tagged with a numeric 64 bit number, which are centrally administered.

CBOR::XS handles a few tags internally when en- or decoding. You can also create tags yourself by encoding CBOR::XS::Tagged objects, and the decoder will create CBOR::XS::Tagged objects itself when it hits an unknown tag.

These objects are simply blessed array references - the first member of the array being the numerical tag, the second being the value.

You can interact with CBOR::XS::Tagged objects in the following ways:

$tagged = CBOR::XS::tag $tag, $value: This function(!) creates a new CBOR::XS::Tagged object using the given $tag (0..2**64-1) to tag the given $value (which can be any Perl value that can be encoded in CBOR, including serialisable Perl objects and CBOR::XS::Tagged objects).
$tagged->[0]
$tagged->[0] = $new_tag
$tag = $tagged->tag
$new_tag = $tagged->tag ($new_tag): Access/mutate the tag.
$tagged->[1]
$tagged->[1] = $new_value
$value = $tagged->value
$new_value = $tagged->value ($new_value): Access/mutate the tagged value.

EXAMPLES

Here are some examples of CBOR::XS::Tagged uses to tag objects.

You can look up CBOR tag value and emanings in the IANA registry at http://www.iana.org/assignments/cbor-tags/cbor-tags.xhtml.

Prepend a magic header ($CBOR::XS::MAGIC):

my $cbor = encode_cbor CBOR::XS::tag 55799, $value;
# same as:
my $cbor = $CBOR::XS::MAGIC . encode_cbor $value;

Serialise some URIs and a regex in an array:

my $cbor = encode_cbor [
   (CBOR::XS::tag 32, "http://www.nethype.de/"),
   (CBOR::XS::tag 32, "http://software.schmorp.de/"),
   (CBOR::XS::tag 35, "^[Pp][Ee][Rr][lL]\$"),
];

Wrap CBOR data in CBOR:

my $cbor_cbor = encode_cbor
   CBOR::XS::tag 24,
      encode_cbor [1, 2, 3];

CBOR and JSON

CBOR is supposed to implement a superset of the JSON data model, and is, with some coercion, able to represent all JSON texts (something that other "binary JSON" formats such as BSON generally do not support).

CBOR implements some extra hints and support for JSON interoperability, and the spec offers further guidance for conversion between CBOR and JSON. None of this is currently implemented in CBOR, and the guidelines in the spec do not result in correct round-tripping of data. If JSON interoperability is improved in the future, then the goal will be to ensure that decoded JSON data will round-trip encoding and decoding to CBOR intact.

SECURITY CONSIDERATIONS

When you are using CBOR in a protocol, talking to untrusted potentially hostile creatures requires relatively few measures.

First of all, your CBOR decoder should be secure, that is, should not have any buffer overflows. Obviously, this module should ensure that and I am trying hard on making that true, but you never know.

Second, you need to avoid resource-starving attacks. That means you should limit the size of CBOR data you accept, or make sure then when your resources run out, that's just fine (e.g. by using a separate process that can crash safely). The size of a CBOR string in octets is usually a good indication of the size of the resources required to decode it into a Perl structure. While CBOR::XS can check the size of the CBOR text, it might be too late when you already have it in memory, so you might want to check the size before you accept the string.

Third, CBOR::XS recurses using the C stack when decoding objects and arrays. The C stack is a limited resource: for instance, on my amd64 machine with 8MB of stack size I can decode around 180k nested arrays but only 14k nested CBOR objects (due to perl itself recursing deeply on croak to free the temporary). If that is exceeded, the program crashes. To be conservative, the default nesting limit is set to 512. If your process has a smaller stack, you should adjust this setting accordingly with the max_depth method.

Something else could bomb you, too, that I forgot to think of. In that case, you get to keep the pieces. I am always open for hints, though...

Also keep in mind that CBOR::XS might leak contents of your Perl data structures in its error messages, so when you serialise sensitive information you might want to make sure that exceptions thrown by CBOR::XS will not end up in front of untrusted eyes.

CBOR IMPLEMENTATION NOTES

This section contains some random implementation notes. They do not describe guaranteed behaviour, but merely behaviour as-is implemented right now.

64 bit integers are only properly decoded when Perl was built with 64 bit support.

Strings and arrays are encoded with a definite length. Hashes as well, unless they are tied (or otherwise magical).

Only the double data type is supported for NV data types - when Perl uses long double to represent floating point values, they might not be encoded properly. Half precision types are accepted, but not encoded.

Strict mode and canonical mode are not implemented.

THREADS

This module is not guaranteed to be thread safe and there are no plans to change this until Perl gets thread support (as opposed to the horribly slow so-called "threads" which are simply slow and bloated process simulations - use fork, it's much faster, cheaper, better).

(It might actually work, but you have been warned).

BUGS

While the goal of this module is to be correct, that unfortunately does not mean it's bug-free, only that I think its design is bug-free. If you keep reporting bugs they will be fixed swiftly, though.

Please refrain from using rt.cpan.org or any other bug reporting service. I put the contact address into my modules for a reason.

AUTHOR

Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/

To install CBOR::XS, copy and paste the appropriate command in to your terminal.

cpanm

cpanm CBOR::XS

CPAN shell

perl -MCPAN -e shell
install CBOR::XS

For more information on module installation, please visit the detailed CPAN module installation guide.

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NAME

SYNOPSIS

DESCRIPTION

FUNCTIONAL INTERFACE

OBJECT-ORIENTED INTERFACE

MAPPING

CBOR -> PERL

PERL -> CBOR

OBJECT SERIALISATION

EXAMPLES

MAGIC HEADER

THE CBOR::XS::Tagged CLASS

EXAMPLES

CBOR and JSON

SECURITY CONSIDERATIONS

CBOR IMPLEMENTATION NOTES

THREADS

BUGS

SEE ALSO

AUTHOR

Module Install Instructions