--- CBOR-XS/README 2013/10/26 23:02:55 1.4 +++ CBOR-XS/README 2020/12/01 01:50:01 1.20 @@ -10,24 +10,48 @@ # OO-interface $coder = CBOR::XS->new; - #TODO + $binary_cbor_data = $coder->encode ($perl_value); + $perl_value = $coder->decode ($binary_cbor_data); -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). + # 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 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. + 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. @@ -55,7 +79,21 @@ 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]}); + 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 @@ -79,7 +117,7 @@ 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 + See "SECURITY CONSIDERATIONS", below, for more info on why this is useful. $cbor = $cbor->max_size ([$maximum_string_size]) @@ -93,9 +131,259 @@ 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 + 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 + (), 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 + (), 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. @@ -112,11 +400,71 @@ 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. + 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 @@ -133,7 +481,7 @@ support, 64 bit integers will be truncated or otherwise corrupted. byte strings - Byte strings will become octet strings in Perl (the byte values + 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 @@ -147,23 +495,21 @@ Perl array or hash, respectively. The keys of the map will be stringified during this process. - true, false - These CBOR values become "CBOR::XS::true" and "CBOR::XS::false", + 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. You can check whether a scalar is a CBOR boolean by - using the "CBOR::XS::is_bool" function. + numbers 1 and 0 (for true and false) or to throw an exception on + access (for error). See the Types::Serialiser manpage for details. - null, undefined - CBOR null and undefined values becomes "undef" in Perl (in the - future, Undefined may raise an exception or something else). - - tags - Tagged items consists of a numeric tag and another CBOR value. The - tag 55799 is ignored (this tag implements the magic header). - - All other tags are currently converted into a CBOR::XS::Tagged - object, which is simply a blessed array reference consistsing of the - numeric tag value followed by the (decoded) BOR value. + 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 @@ -171,13 +517,14 @@ 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. + 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. + 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. @@ -186,27 +533,33 @@ 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. + Other unblessed references will be represented using the indirection + tag extension (tag value 22098, + ). 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. + value will be encoded as appropriate for the value. You must use + "CBOR::XS::tag" to create such objects. - CBOR::XS::true, CBOR::XS::false - These special values become CBOR true and CBOR false values, - respectively. You can also use "\1" and "\0" directly if you want. - - blessed objects - Other blessed objects currently need to have a "TO_CBOR" method. It - will be called on every object that is being serialised, and must - return something that can be encoded in CBOR. + 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 - TODO Simple Perl scalars (any scalar that is not a reference) are - the most difficult objects to encode: CBOR::XS will encode undefined + 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: @@ -216,7 +569,7 @@ encode_cbor [-3.0e17] # yields [-3e+17] my $value = 5; encode_cbor [$value] # yields [5] - # used as string, so dump as string + # used as string, so dump as string (either byte or text) print $value; encode_cbor [$value] # yields ["5"] @@ -230,6 +583,22 @@ $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 @@ -247,18 +616,343 @@ than IEEE double to represent numerical values are supported, but might suffer loss of precision. - MAGIC HEADER + 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_bool $value + Converts a Perl boolean (which can be any kind of scalar) into a + CBOR boolean. Strictly the same, but shorter to write, than: + + $value ? Types::Serialiser::true : Types::Serialiser::false + + 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- $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. + CBOR string without changing its 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 + 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. - CBOR and JSON +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 + . + + 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, ) + 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, ) + 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, + ) + These tags are automatically decoded when encountered. They are only + encoded, however, when "pack_strings" is enabled. + + 22098 (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). @@ -272,38 +966,140 @@ CBOR intact. SECURITY CONSIDERATIONS - When you are using CBOR in a protocol, talking to untrusted potentially - hostile creatures requires relatively few measures. + 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. - 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. + 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 @@ -322,6 +1118,15 @@ 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 @@ -342,6 +1147,9 @@ 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/