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
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).
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.
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).
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 = new_safe CBOR::XS
Create a new, safe/secure CBOR::XS object. This is similar to "new",
but configures the coder object to be safe to use with untrusted
data. Currently, this is equivalent to:
my $cbor = CBOR::XS
->new
->forbid_objects
->filter (\&CBOR::XS::safe_filter)
->max_size (1e8);
But is more future proof (it is better to crash because of a change
than to be exploited in other ways).
$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 be
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 encoded 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 = $cbor->forbid_objects ([$enable])
$enabled = $cbor->get_forbid_objects
Disables the use of the object serialiser protocol.
If $enable is true (or missing), then "encode" will will throw an
exception when it encounters perl objects that would be encoded
using the perl-object tag (26). When "decode" encounters such tags,
it will fall back to the general filter/tagged logic as if this were
an unknown tag (by default resulting in a "CBOR::XC::Tagged"
object).
If $enable is false (the default), then "encode" will use the
Types::Serialiser object serialisation protocol to serialise objects
into perl-object tags, and "decode" will do the same to decode such
tags.
See "SECURITY CONSIDERATIONS", below, for more info on why
forbidding this protocol can be useful.
$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
automatically encode byte strings, which might break some "FREEZE"
and "TO_CBOR" methods that rely on this.
A workaround is to use explicit type casts, which are unaffected by
this option.
$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. "CBOR::XS" provides a number of default filter
functions already, the the %CBOR::XS::FILTER hash can be freely
extended with more.
"CBOR::XS" additionally provides an alternative filter function that
is supposed to be safe to use with untrusted data (which the default
filter might not), called "CBOR::XS::safe_filter", which works the
same as the "default_filter" but uses the %CBOR::XS::SAFE_FILTER
variable instead. It is prepopulated with the tag decoding functions
that are deemed safe (basically the same as %CBOR::XS::FILTER
without all the bignum tags), and can be extended by user code as
wlel, although, obviously, one should be very careful about adding
decoding functions here, since the expectation is that they are safe
to use on untrusted data, after all.
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 $value"
};
Example: provide your own filter function that looks up tags in your
own hash:
my %my_filter = (
998347484 => sub {
my ($tag, $value);
"tag 998347484 value $value"
};
);
my $coder = CBOR::XS->new->filter (sub {
&{ $my_filter{$_[0]} or return }
});
Example: use the safe filter function (see "SECURITY CONSIDERATIONS"
for more considerations on security).
CBOR::XS->new->filter (\&CBOR::XS::safe_filter)->decode ($cbor_data);
$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 strings are self-delimited, so it is
possible to concatenate CBOR strings without any delimiters or size
fields and recover their data.
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 called 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
More options are available, see "TYPE CASTS", below, and the
"text_keys" and "text_strings" options.
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.
TYPE CASTS
EXPERIMENTAL: As an experimental extension, "CBOR::XS" allows you to
force specific cbor types to be used when encoding. That allows you to
encode types not normally accessible (e.g. half floats) as well as force
string types even when "text_strings" is in effect.
Type forcing is done by calling a special "cast" function which keeps a
copy of the value and returns a new value that can be handed over to any
CBOR encoder function.
The following casts are currently available (all of which are unary
operators):
CBOR::XS::as_int $value
Forces the value to be encoded as some form of (basic, not bignum)
integer type.
CBOR::XS::as_text $value
Forces the value to be encoded as (UTF-8) text values.
CBOR::XS::as_bytes $value
Forces the value to be encoded as a (binary) string value.
CBOR::XS::as_float16 $value
Forces half-float (IEEE 754 binary16) encoding of the given value.
CBOR::XS::as_float32 $value
Forces single-float (IEEE 754 binary32) encoding of the given value.
CBOR::XS::as_float64 $value
Forces double-float (IEEE 754 binary64) encoding of the given value.
=item, CBOR::XS::as_cbor $cbor_text
Bot a type cast per-se, this type cast forces the argument to eb
encoded as-is. This can be used to embed pre-encoded CBOR data.
Note that no checking on the validity of the $cbor_text is done -
it's the callers responsibility to correctly encode values.
Example: encode a perl string as binary even though "text_strings" is in
effect.
CBOR::XS->new->text_strings->encode ([4, "text", CBOR::XS::bytes "bytevalue"]);
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
Tl;dr... if you want to decode or encode CBOR from untrusted sources,
you should start with a coder object created via "new_safe" (which
implements the mitigations explained below):
my $coder = CBOR::XS->new_safe;
my $data = $coder->decode ($cbor_text);
my $cbor = $coder->encode ($data);
Longer version: When you are using CBOR in a protocol, talking to
untrusted potentially hostile creatures requires some thought:
Security of the CBOR decoder itself
First and foremost, your CBOR decoder should be secure, that is,
should not have any buffer overflows or similar bugs that could
potentially be exploited. Obviously, this module should ensure that
and I am trying hard on making that true, but you never know.
CBOR::XS can invoke almost arbitrary callbacks during decoding
CBOR::XS supports object serialisation - decoding CBOR can cause
calls to *any* "THAW" method in *any* package that exists in your
process (that is, CBOR::XS will not try to load modules, but any
existing "THAW" method or function can be called, so they all have
to be secure).
Less obviously, it will also invoke "TO_CBOR" and "FREEZE" methods -
even if all your "THAW" methods are secure, encoding data structures
from untrusted sources can invoke those and trigger bugs in those.
So, if you are not sure about the security of all the modules you
have loaded (you shouldn't), you should disable this part using
"forbid_objects" or using "new_safe".
CBOR can be extended with tags that call library code
CBOR can be extended with tags, and "CBOR::XS" has a registry of
conversion functions for many existing tags that can be extended via
third-party modules (see the "filter" method).
If you don't trust these, you should configure the "safe" filter
function, "CBOR::XS::safe_filter" ("new_safe" does this), which by
default only includes conversion functions that are considered
"safe" by the author (but again, they can be extended by third party
modules).
Depending on your level of paranoia, you can use the "safe" filter:
$cbor->filter (\&CBOR::XS::safe_filter);
... your own filter...
$cbor->filter (sub { ... do your stuffs here ... });
... or even no filter at all, disabling all tag decoding:
$cbor->filter (sub { });
This is never a problem for encoding, as the tag mechanism only
exists in CBOR texts.
Resource-starving attacks: object memory usage
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 (using "max_size" - done by "new_safe"), 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.
As for encoding, it is possible to construct data structures that
are relatively small but result in large CBOR texts (for example by
having an array full of references to the same big data structure,
which will all be deep-cloned during encoding by default). This is
rarely an actual issue (and the worst case is still just running out
of memory), but you can reduce this risk by using "allow_sharing".
Resource-starving attacks: stack overflows
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.
Resource-starving attacks: CPU en-/decoding complexity
CBOR::XS will use the Math::BigInt, Math::BigFloat and Math::BigRat
libraries to represent encode/decode bignums. These can be very slow
(as in, centuries of CPU time) and can even crash your program (and
are generally not very trustworthy). See the next section on bignum
security for details.
Data breaches: leaking information in error messages
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.
Something else...
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...
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
value in CBOR is very limited - most likely, these 64 bit values will be
truncated, corrupted, or otherwise not decoded correctly. This also
includes string, float, 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.
SEE ALSO
The JSON and JSON::XS modules that do similar, but human-readable,
serialisation.
The Types::Serialiser module provides the data model for true, false and
error values.
AUTHOR
Marc Lehmann <
[email protected]>
http://home.schmorp.de/
