[ViewVC] Diff of: cvs/CBOR-XS/README

Comparing CBOR-XS/README (file contents):
Revision 1.15 by root, Mon Apr 27 20:21:53 2015 UTC vs.
Revision 1.18 by root, Wed Dec 7 14:14:30 2016 UTC

         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
         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
+        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
         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
+        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
         instead will emit a reference to the earlier value.
         This means that such values will only be encoded once, and will not
         result in a deep cloning of the value on decode, in decoders
         supporting the value sharing extension. This also makes it possible
-        to encode cyclic data structures (which need "allow_cycles" to ne
+        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
         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 standard CBOR way.
         This option does not affect "decode" in any way - string references
         will always be decoded properly if present.
+    $cbor = $cbor->text_keys ([$enable])
+    $enabled = $cbor->get_text_keys
+        If $enabled is true (or missing), then "encode" will encode all perl
+        hash keys as CBOR text strings/UTF-8 string, upgrading them as
+        needed.
+        If $enable is false (the default), then "encode" will encode hash
+        keys normally - upgraded perl strings (strings internally encoded as
+        UTF-8) as CBOR text strings, and downgraded perl strings as CBOR
+        byte strings.
+        This option does not affect "decode" in any way.
+        This option is useful for interoperability with CBOR decoders that
+        don't treat byte strings as a form of text. It is especially useful
+        as Perl gives very little control over hash keys.
+        Enabling this option can be slow, as all downgraded hash keys that
+        are encoded need to be scanned and converted to UTF-8.
+    $cbor = $cbor->text_strings ([$enable])
+    $enabled = $cbor->get_text_strings
+        This option works similar to "text_keys", above, but works on all
+        strings (including hash keys), so "text_keys" has no further effect
+        after enabling "text_strings".
+        If $enabled is true (or missing), then "encode" will encode all perl
+        strings as CBOR text strings/UTF-8 strings, upgrading them as
+        needed.
+        If $enable is false (the default), then "encode" will encode strings
+        normally (but see "text_keys") - upgraded perl strings (strings
+        internally encoded as UTF-8) as CBOR text strings, and downgraded
+        perl strings as CBOR byte strings.
+        This option does not affect "decode" in any way.
+        This option has similar advantages and disadvantages as "text_keys".
+        In addition, this option effectively removes the ability to encode
+        byte strings, which might break some "FREEZE" and "TO_CBOR" methods
+        that rely on this, such as bignum encoding, so this option is mainly
+        useful for very simple data.
     $cbor = $cbor->validate_utf8 ([$enable])
     $enabled = $cbor->get_validate_utf8
         If $enable is true (or missing), then "decode" will validate that
         elements (text strings) containing UTF-8 data in fact contain valid
         UTF-8 data (instead of blindly accepting it). This validation
         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 thats true or not.
+        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.
         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.
+        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).
            $CBOR::XS::FILTER{1347375694} = sub {
               my ($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.
     $cbor->incr_reset
         Resets the incremental decoder. This throws away any saved state, so
         that subsequent calls to "incr_parse" or "incr_parse_multiple" start
         to parse a new CBOR value from the beginning of the $buffer again.
-        This method can be caled at any time, but it *must* be called if you
+        This method can be called at any time, but it *must* be called if
-        want to change your $buffer or there was a decoding error and you
+        you want to change your $buffer or there was a decoding error and
-        want to reuse the $cbor object for future incremental parsings.
+        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
     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
-        hahs is encoded.
+        hash is encoded.
         Currently, tied hashes will use the indefinite-length format, while
         normal hashes will use the fixed-length format.
     array references
            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 ie encoded as byte or text string by
+        You can force whether a string is encoded as byte or text string by
-        using "utf8::upgrade" and "utf8::downgrade"):
+        using "utf8::upgrade" and "utf8::downgrade" (if "text_strings" is
+        disabled):
           utf8::upgrade $x;   # encode $x as text string
           utf8::downgrade $x; # encode $x as byte string
         Perl doesn't define what operations up- and downgrade strings, so if
         the difference between byte and text is important, you should up- or
-        downgrade your string as late as possible before encoding.
+        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
            $x += 0;     # numify it, ensuring it will be dumped as a number
           "$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"
     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 overriden by the user.
+    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, L <http://cbor.schmorp.de/value-sharing>)
+    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.
         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, L
+    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 refernces). It is converted to
+        (with the exception of hash and array references). It is converted
-        a reference when decoding.
+        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 overriden by changing the %CBOR::XS::FILTER entry for the tag, or
+    can be overridden by changing the %CBOR::XS::FILTER entry for the tag,
-    by providing a custom "filter" callback when decoding.
+    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
     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 (decimal fraction/bigfloat)
+    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.
+        encodes into a decimal fraction (either tag 4 or 264).
-        CBOR cannot represent bigfloats with *very* large exponents -
-        conversion of such big float objects is undefined.
-        Also, NaN and infinities are not encoded properly.
+        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.
     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
+    Tl;dr... if you want to decode or encode CBOR from untrusted sources,
-    hostile creatures requires relatively few measures.
+    you should start with a coder object created via "new_safe":
+       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 of all, your CBOR decoder should be secure, that is, should not
+        First and foremost, your CBOR decoder should be secure, that is,
-    have any buffer overflows. Obviously, this module should ensure that and
+        should not have any buffer overflows or similar bugs that could
+        potentially be exploited. Obviously, this module should ensure that
-    I am trying hard on making that true, but you never know.
+        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".
+    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", 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
-    Second, you need to avoid resource-starving attacks. That means you
+        You need to avoid resource-starving attacks. That means you should
-    should limit the size of CBOR data you accept, or make sure then when
+        limit the size of CBOR data you accept, or make sure then when your
-    your resources run out, that's just fine (e.g. by using a separate
+        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
+        process that can crash safely). The size of a CBOR string in octets
-    usually a good indication of the size of the resources required to
+        is usually a good indication of the size of the resources required
-    decode it into a Perl structure. While CBOR::XS can check the size of
+        to decode it into a Perl structure. While CBOR::XS can check the
-    the CBOR text, it might be too late when you already have it in memory,
+        size of the CBOR text (using "max_size"), it might be too late when
-    so you might want to check the size before you accept the string.
+        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
-    Third, CBOR::XS recurses using the C stack when decoding objects and
+        CBOR::XS recurses using the C stack when decoding objects and
-    arrays. The C stack is a limited resource: for instance, on my amd64
+        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
+        machine with 8MB of stack size I can decode around 180k nested
-    but only 14k nested CBOR objects (due to perl itself recursing deeply on
+        arrays but only 14k nested CBOR objects (due to perl itself
-    croak to free the temporary). If that is exceeded, the program crashes.
+        recursing deeply on croak to free the temporary). If that is
-    To be conservative, the default nesting limit is set to 512. If your
+        exceeded, the program crashes. To be conservative, the default
-    process has a smaller stack, you should adjust this setting accordingly
+        nesting limit is set to 512. If your process has a smaller stack,
-    with the "max_depth" method.
+        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 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
+        Something else could bomb you, too, that I forgot to think of. In
-    case, you get to keep the pieces. I am always open for hints, though...
+        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
+BIGNUM SECURITY CONSIDERATIONS
-    structures in its error messages, so when you serialise sensitive
+    CBOR::XS provides a "TO_CBOR" method for both Math::BigInt and
-    information you might want to make sure that exceptions thrown by
+    Math::BigFloat that tries to encode the number in the simplest possible
-    CBOR::XS will not end up in front of untrusted eyes.
+    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.

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Comparing CBOR-XS/README (file contents): Revision 1.15 by root, Mon Apr 27 20:21:53 2015 UTC vs. Revision 1.18 by root, Wed Dec 7 14:14:30 2016 UTC

Diff Legend

Comparing CBOR-XS/README (file contents):
Revision 1.15 by root, Mon Apr 27 20:21:53 2015 UTC vs.
Revision 1.18 by root, Wed Dec 7 14:14:30 2016 UTC