man CBOR::XS (3): 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

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 an (almost) superset of the JSON data model, i.e. when you can represent something useful in JSON, you should be able to represent it in CBOR.

In short, CBOR is a faster and quite 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 and speed is less important you might want to compare both formats first).

To give you a general idea about speed, with texts in the megabyte range, "CBOR::XS" usually encodes roughly twice as fast as Storable or JSON::XS and decodes about 15%-30% faster than those. The shorter the data, the worse Storable performs in comparison.

Regarding compactness, "CBOR::XS"-encoded data structures are usually about 20% smaller than the same data encoded as (compact) JSON or Storable.

In addition to the core CBOR data format, this module implements a number of extensions, to support cyclic and shared data structures (see "allow_sharing" and "allow_cycles"), string deduplication (see "pack_strings") and scalar references (always enabled).

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:

   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 = $cbor->allow_unknown ([$enable])

$enabled = $cbor->get_allow_unknown

If $enable is true (or missing), then "encode" will not throw an exception when it encounters values it cannot represent in CBOR (for example, filehandles) but instead will encode a CBOR "error" value.

If $enable is false (the default), then "encode" will throw an exception when it encounters anything it cannot encode as CBOR.

This option does not affect "decode" in any way, and it is recommended to leave it off unless you know your communications partner.

$cbor = $cbor->allow_sharing ([$enable])

$enabled = $cbor->get_allow_sharing

If $enable is true (or missing), then "encode" will not double-encode values that have been referenced before (e.g. when the same object, such as an array, is referenced multiple times), but instead will emit a reference to the earlier value.

This means that such values will only be encoded once, and will not result in a deep cloning of the value on decode, in decoders supporting the value sharing extension. This also makes it possible to encode cyclic data structures (which need "allow_cycles" to ne enabled to be decoded by this module).

It is recommended to leave it off unless you know your communication partner supports the value sharing extensions to CBOR (<http://cbor.schmorp.de/value-sharing>), as without decoder support, the resulting data structure might be unusable.

Detecting shared values incurs a runtime overhead when values are encoded that have a reference counter large than one, and might unnecessarily increase the encoded size, as potentially shared values are encode as shareable whether or not they are actually shared.

At the moment, only targets of references can be shared (e.g. scalars, arrays or hashes pointed to by a reference). Weirder constructs, such as an array with multiple ``copies'' of the same string, which are hard but not impossible to create in Perl, are not supported (this is the same as with Storable).

If $enable is false (the default), then "encode" will encode shared data structures repeatedly, unsharing them in the process. Cyclic data structures cannot be encoded in this mode.

This option does not affect "decode" in any way - shared values and references will always be decoded properly if present.

$cbor = $cbor->allow_cycles ([$enable])

$enabled = $cbor->get_allow_cycles

If $enable is true (or missing), then "decode" will happily decode self-referential (cyclic) data structures. By default these will not be decoded, as they need manual cleanup to avoid memory leaks, so code that isn't prepared for this will not leak memory.

If $enable is false (the default), then "decode" will throw an error when it encounters a self-referential/cyclic data structure.

FUTURE DIRECTION: the motivation behind this option is to avoid real cycles - future versions of this module might chose to decode cyclic data structures using weak references when this option is off, instead of throwing an error.

This option does not affect "encode" in any way - shared values and references will always be encoded properly if present.

$cbor = $cbor->pack_strings ([$enable])

$enabled = $cbor->get_pack_strings

If $enable is true (or missing), then "encode" will try not to encode the same string twice, but will instead encode a reference to the string instead. Depending on your data format, this can save a lot of space, but also results in a very large runtime overhead (expect encoding times to be 2-4 times as high as without).

It is recommended to leave it off unless you know your communications partner supports the stringref extension to CBOR (<http://cbor.schmorp.de/stringref>), as without decoder support, the resulting data structure might not be usable.

If $enable is false (the default), then "encode" will encode strings the standard CBOR way.

This option does not affect "decode" in any way - string references will always be decoded properly if present.

$cbor = $cbor->text_keys ([$enable])

$enabled = $cbor->get_text_keys

If $enabled is true (or missing), then "encode" will encode all perl hash keys as CBOR text strings/UTF-8 string, upgrading them as needed.

If $enable is false (the default), then "encode" will encode hash keys normally - upgraded perl strings (strings internally encoded as UTF-8) as CBOR text strings, and downgraded perl strings as CBOR byte strings.

This option does not affect "decode" in any way.

This option is useful for interoperability with CBOR decoders that don't treat byte strings as a form of text. It is especially useful as Perl gives very little control over hash keys.

Enabling this option can be slow, as all downgraded hash keys that are encoded need to be scanned and converted to UTF-8.

$cbor = $cbor->text_strings ([$enable])

$enabled = $cbor->get_text_strings

This option works similar to "text_keys", above, but works on all strings (including hash keys), so "text_keys" has no further effect after enabling "text_strings".

If $enabled is true (or missing), then "encode" will encode all perl strings as CBOR text strings/UTF-8 strings, upgrading them as needed.

If $enable is false (the default), then "encode" will encode strings normally (but see "text_keys") - upgraded perl strings (strings internally encoded as UTF-8) as CBOR text strings, and downgraded perl strings as CBOR byte strings.

This option does not affect "decode" in any way.

This option has similar advantages and disadvantages as "text_keys". In addition, this option effectively removes the ability to encode byte strings, which might break some "FREEZE" and "TO_CBOR" methods that rely on this, such as bignum encoding, so this option is mainly useful for very simple data.

$cbor = $cbor->validate_utf8 ([$enable])

$enabled = $cbor->get_validate_utf8

If $enable is true (or missing), then "decode" will validate that elements (text strings) containing UTF-8 data in fact contain valid UTF-8 data (instead of blindly accepting it). This validation obviously takes extra time during decoding.

The concept of ``valid UTF-8'' used is perl's concept, which is a superset of the official UTF-8.

If $enable is false (the default), then "decode" will blindly accept UTF-8 data, marking them as valid UTF-8 in the resulting data structure regardless of whether that's true or not.

Perl isn't too happy about corrupted UTF-8 in strings, but should generally not crash or do similarly evil things. Extensions might be not so forgiving, so it's recommended to turn on this setting if you receive untrusted CBOR.

This option does not affect "encode" in any way - strings that are supposedly valid UTF-8 will simply be dumped into the resulting CBOR string without checking whether that is, in fact, true or not.

$cbor = $cbor->filter ([$cb->($tag, $value)])

$cb_or_undef = $cbor->get_filter

Sets or replaces the tagged value decoding filter (when $cb is specified) or clears the filter (if no argument or "undef" is provided).

The filter callback is called only during decoding, when a non-enforced tagged value has been decoded (see ``TAG HANDLING AND EXTENSIONS'' for a list of enforced tags). For specific tags, it's often better to provide a default converter using the %CBOR::XS::FILTER hash (see below).

The first argument is the numerical tag, the second is the (decoded) value that has been tagged.

The filter function should return either exactly one value, which will replace the tagged value in the decoded data structure, or no values, which will result in default handling, which currently means the decoder creates a "CBOR::XS::Tagged" object to hold the tag and the value.

When the filter is cleared (the default state), the default filter function, "CBOR::XS::default_filter", is used. This function simply looks up the tag in the %CBOR::XS::FILTER hash. If an entry exists it must be a code reference that is called with tag and value, and is responsible for decoding the value. If no entry exists, it returns no values.

Example: decode all tags not handled internally into "CBOR::XS::Tagged" objects, with no other special handling (useful when working with potentially ``unsafe'' CBOR data).

   CBOR::XS->new->filter (sub { })->decode ($cbor_data);

Example: provide a global filter for tag 1347375694, converting the value into some string form.

   $CBOR::XS::FILTER{1347375694} = sub {
      my ($tag, $value);
      "tag 1347375694 value $value"
   };

$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)

INCREMENTAL PARSING

In some cases, there is the need for incremental parsing of JSON texts. While this module always has to keep both CBOR text and resulting Perl data structure in memory at one time, it does allow you to parse a CBOR stream incrementally, using a similar to using ``decode_prefix'' to see if a full CBOR object is available, but is much more efficient.

It basically works by parsing as much of a CBOR string as possible - if the CBOR data is not complete yet, the pasrer will remember where it was, to be able to restart when more data has been accumulated. Once enough data is available to either decode a complete CBOR value or raise an error, a real decode will be attempted.

A typical use case would be a network protocol that consists of sending and receiving CBOR-encoded messages. The solution that works with CBOR and about anything else is by prepending a length to every CBOR value, so the receiver knows how many octets to read. More compact (and slightly slower) would be to just send CBOR values back-to-back, as "CBOR::XS" knows where a CBOR value ends, and doesn't need an explicit length.

The following methods help with this:

@decoded = $cbor->incr_parse ($buffer)

This method attempts to decode exactly one CBOR value from the beginning of the given $buffer. The value is removed from the $buffer on success. When $buffer doesn't contain a complete value yet, it returns nothing. Finally, when the $buffer doesn't start with something that could ever be a valid CBOR value, it raises an exception, just as "decode" would. In the latter case the decoder state is undefined and must be reset before being able to parse further.

This method modifies the $buffer in place. When no CBOR value can be decoded, the decoder stores the current string offset. On the next call, continues decoding at the place where it stopped before. For this to make sense, the $buffer must begin with the same octets as on previous unsuccessful calls.

You can call this method in scalar context, in which case it either returns a decoded value or "undef". This makes it impossible to distinguish between CBOR null values (which decode to "undef") and an unsuccessful decode, which is often acceptable.

@decoded = $cbor->incr_parse_multiple ($buffer)

Same as "incr_parse", but attempts to decode as many CBOR values as possible in one go, instead of at most one. Calls to "incr_parse" and "incr_parse_multiple" can be interleaved.

$cbor->incr_reset

Resets the incremental decoder. This throws away any saved state, so that subsequent calls to "incr_parse" or "incr_parse_multiple" start to parse a new CBOR value from the beginning of the $buffer again.

This method can be caled at any time, but it must be called if you want to change your $buffer or there was a decoding error and you want to reuse the $cbor object for future incremental parsings.

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.
tagged values: Tagged items consists of a numeric tag and another CBOR value.
See ``TAG HANDLING AND EXTENSIONS'' and the description of "->filter" for details on which tags are handled how.
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 typeless language. That means this module 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. This order can be different each time a hash is encoded.

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 will be represented using the indirection tag extension (tag value 22098, <http://cbor.schmorp.de/indirection>). CBOR decoders are guaranteed to be able to decode these values somehow, by either ``doing the right thing'', decoding into a generic tagged object, simply ignoring the tag, or something else.

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 must 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 ``TAG HANDLING AND EXTENSIONS'' for specific classes handled by this module, and ``OBJECT SERIALISATION'' for generic object serialisation.

simple scalars

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 (either byte or text)
   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 whether a string is encoded as byte or text string by using "utf8::upgrade" and "utf8::downgrade" (if "text_strings" is disabled):

  utf8::upgrade $x;   # encode $x as text string
  utf8::downgrade $x; # encode $x as byte string

Perl doesn't define what operations up- and downgrade strings, so if the difference between byte and text is important, you should up- or downgrade your string as late as possible before encoding. You can also force the use of CBOR text strings by using "text_keys" or "text_strings".

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 implements both a CBOR-specific and the generic Types::Serialier object serialisation protocol. The following subsections explain both methods.

ENCODING

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 cannot 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.

These methods MUST NOT change the data structure that is being serialised. Failure to comply to this can result in memory corruption - and worse.

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

DECODING

Objects encoded via "TO_CBOR" cannot (normally) 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 $uri; # make sure it will be encoded as UTF-8 string
     CBOR::XS::tag 32, "$_[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 its meaning.

This string is available as $CBOR::XS::MAGIC. This module does not prepend this string to the CBOR data it generates, but it will ignore 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];

TAG HANDLING AND EXTENSIONS

This section describes how this module handles specific tagged values and extensions. If a tag is not mentioned here and no additional filters are provided for it, then the default handling applies (creating a CBOR::XS::Tagged object on decoding, and only encoding the tag when explicitly requested).

Tags not handled specifically 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.

Future versions of this module reserve the right to special case additional tags (such as base64url).

ENFORCED TAGS

These tags are always handled when decoding, and their handling cannot be overridden by the user.

26 (perl-object, <http://cbor.schmorp.de/perl-object>)

These tags are automatically created (and decoded) for serialisable objects using the "FREEZE/THAW" methods (the Types::Serialier object serialisation protocol). See ``OBJECT SERIALISATION'' for details.

28, 29 (shareable, sharedref, <http://cbor.schmorp.de/value-sharing>)

These tags are automatically decoded when encountered (and they do not result in a cyclic data structure, see "allow_cycles"), resulting in shared values in the decoded object. They are only encoded, however, when "allow_sharing" is enabled.

Not all shared values can be successfully decoded: values that reference themselves will currently decode as "undef" (this is not the same as a reference pointing to itself, which will be represented as a value that contains an indirect reference to itself - these will be decoded properly).

Note that considerably more shared value data structures can be decoded than will be encoded - currently, only values pointed to by references will be shared, others will not. While non-reference shared values can be generated in Perl with some effort, they were considered too unimportant to be supported in the encoder. The decoder, however, will decode these values as shared values.

256, 25 (stringref-namespace, stringref, <http://cbor.schmorp.de/stringref>)

These tags are automatically decoded when encountered. They are only encoded, however, when "pack_strings" is enabled.

22098 (indirection, <http://cbor.schmorp.de/indirection>)

This tag is automatically generated when a reference are encountered (with the exception of hash and array references). It is converted to a reference when decoding.

55799 (self-describe CBOR, RFC 7049)

This value is not generated on encoding (unless explicitly requested by the user), and is simply ignored when decoding.

NON-ENFORCED TAGS

These tags have default filters provided when decoding. Their handling can be overridden by changing the %CBOR::XS::FILTER entry for the tag, or by providing a custom "filter" callback when decoding.

When they result in decoding into a specific Perl class, the module usually provides a corresponding "TO_CBOR" method as well.

When any of these need to load additional modules that are not part of the perl core distribution (e.g. URI), it is (currently) up to the user to provide these modules. The decoding usually fails with an exception if the required module cannot be loaded.

0, 1 (date/time string, seconds since the epoch)

These tags are decoded into Time::Piece objects. The corresponding "Time::Piece::TO_CBOR" method always encodes into tag 1 values currently.

The Time::Piece API is generally surprisingly bad, and fractional seconds are only accidentally kept intact, so watch out. On the plus side, the module comes with perl since 5.10, which has to count for something.

2, 3 (positive/negative bignum)

These tags are decoded into Math::BigInt objects. The corresponding "Math::BigInt::TO_CBOR" method encodes ``small'' bigints into normal CBOR integers, and others into positive/negative CBOR bignums.

4, 5, 264, 265 (decimal fraction/bigfloat)

Both decimal fractions and bigfloats are decoded into Math::BigFloat objects. The corresponding "Math::BigFloat::TO_CBOR" method always encodes into a decimal fraction (either tag 4 or 264).

NaN and infinities are not encoded properly, as they cannot be represented in CBOR.

See ``BIGNUM SECURITY CONSIDERATIONS'' for more info.

30 (rational numbers)

These tags are decoded into Math::BigRat objects. The corresponding "Math::BigRat::TO_CBOR" method encodes rational numbers with denominator 1 via their numerator only, i.e., they become normal integers or "bignums".

See ``BIGNUM SECURITY CONSIDERATIONS'' for more info.

21, 22, 23 (expected later JSON conversion)

CBOR::XS is not a CBOR-to-JSON converter, and will simply ignore these tags.

32 (URI)

These objects decode into URI objects. The corresponding "URI::TO_CBOR" method again results in a CBOR URI value.

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.

BIGNUM SECURITY CONSIDERATIONS

CBOR::XS provides a "TO_CBOR" method for both Math::BigInt and Math::BigFloat that tries to encode the number in the simplest possible way, that is, either a CBOR integer, a CBOR bigint/decimal fraction (tag 4) or an arbitrary-exponent decimal fraction (tag 264). Rational numbers (Math::BigRat, tag 30) can also contain bignums as members.

CBOR::XS will also understand base-2 bigfloat or arbitrary-exponent bigfloats (tags 5 and 265), but it will never generate these on its own.

Using the built-in Math::BigInt::Calc support, encoding and decoding decimal fractions is generally fast. Decoding bigints can be slow for very big numbers (tens of thousands of digits, something that could potentially be caught by limiting the size of CBOR texts), and decoding bigfloats or arbitrary-exponent bigfloats can be extremely slow (minutes, decades) for large exponents (roughly 40 bit and longer).

Additionally, Math::BigInt can take advantage of other bignum libraries, such as Math::GMP, which cannot handle big floats with large exponents, and might simply abort or crash your program, due to their code quality.

This can be a concern if you want to parse untrusted CBOR. If it is, you might want to disable decoding of tag 2 (bigint) and 3 (negative bigint) types. You should also disable types 5 and 265, as these can be slow even without bigints.

Disabling bigints will also partially or fully disable types that rely on them, e.g. rational numbers that use bignums.

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.

LIMITATIONS ON PERLS WITHOUT 64-BIT INTEGER SUPPORT

On perls that were built without 64 bit integer support (these are rare nowadays, even on 32 bit architectures, as all major Perl distributions are built with 64 bit integer support), support for any kind of 64 bit integer in CBOR is very limited - most likely, these 64 bit values will be truncated, corrupted, or otherwise not decoded correctly. This also includes string, array and map sizes that are stored as 64 bit integers.

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 <[email protected]>
 http://home.schmorp.de/