--- CBOR-XS/README 2013/10/25 23:09:45 1.1 +++ CBOR-XS/README 2013/10/27 22:48:12 1.5 @@ -0,0 +1,469 @@ +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 IS A PRE-ALPHA RELEASE! IT WILL CRASH, CORRUPT YOUR DATA + AND EAT YOUR CHILDREN! (Actually, apart from being untested and a bit + feature-limited, it might already be useful). + + 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. + + This makes it a faster and more compact binary alternative to JSON, with + the added ability of supporting serialising of perl objects. + + 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. + + 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- $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. + +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. + +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 + http://home.schmorp.de/ +