2.4.1. Date and Time

Tag value 0 is for date/time strings that follow the standard format described in [RFC3339], as refined by Section 3.3 of [RFC4287].

Tag value 1 is for numerical representation of seconds relative to 1970-01-01T00:00Z in UTC time. (For the non-negative values that POSIX defines, the number of seconds is counted in the same way as for POSIX “seconds since the epoch” [TIME_T].) The tagged item can be a positive or negative integer (major types 0 and 1), or a floating point number (major type 7 with additional information 25, 26 or 27). Note that the number can be negative (time before 1970-01-01T00:00Z) and, if a floating point number, indicate fractional seconds.

2.4.2. Bignums

Bignums are integers that do not fit into the basic integer representations provided by major types 0 and 1. They are encoded as a byte string data item, which is interpreted as an unsigned integer n in network byte order. For tag value 2, the value of the bignum is n. For tag value 3, the value of the bignum is -1 - n. Decoders that understand these tags MUST be able to decode bignums that have leading zeroes.

For example, the number 18446744073709551616 (2**64) is represented as 0b110_00010 (major type 6, tag 2), followed by 0b010_01001 (major type 2, length 9), followed by 0x010000000000000000 (one byte 0x01 and eight bytes 0x00). In hexadecimal:

C2                        -- Tag 2
   29                     -- Byte string of length 9
      010000000000000000  -- Bytes content

2.4.3. Decimal Fractions and Bigfloats

Decimal fractions combine an integer mantissa with a base-10 scaling factor. They are most useful if an application needs the exact representation of a decimal fraction such as 1.1 because there is no exact representation for many decimal fractions in binary floating point.

Bigfloats combine an integer mantissa with a base-2 scaling factor. They are binary floating point values that can exceed the range or the precision of the three IEEE 754 formats supported by CBOR (Section 2.3). Bigfloats may also be used by constrained applications that need some basic binary floating point capability without the need for supporting IEEE 754.

A decimal fraction or a bigfloat is represented as a tagged array that contains exactly two integer numbers: an exponent e and a mantissa m. Decimal fractions (tag 4) use base-10 exponents, the value of a decimal fraction data item is m*(10**e). Bigfloats (tag 5) use base-2 exponents, the value of a bigfloat data item is m*(2**e). The exponent e MUST be represented in an integer of major type 0 or 1, while the mantissa also can be a bignum (Section 2.4.2).

An example of a decimal fraction is that the number 273.15 could be represented as 0b110_00100 (major type of 6 for the tag, additional information of 4 for the type of tag), followed by 0b100_00010 (major type of 4 for the array, additional information of 2 for the length of the array), followed by 0b001_00001 (major type of 1 for the first integer, additional information of 1 for the value of -2), followed by 0b000_11001 (major type of 0 for the second integer, additional information of 25 for a two-byte value), followed by 0b0110101010110011 (27315 in two bytes). In hexadecimal:

C4             -- Tag 4
   82          -- Array of length 2
      21       -- -2
      19 6ab3  -- 27315

An example of a bigfloat is that the number 1.5 could be represented as 0b110_00101 (major type of 6 for the tag, additional information of 5 for the type of tag), followed by 0b100_00010 (major type of 4 for the array, additional information of 2 for the length of the array), followed by 0b001_00000 (major type of 1 for the first integer, additional information of 0 for the value of -1), followed by 0b000_00011 (major type of 0 for the second integer, additional information of 3 for the value of 3). In hexadecimal:

C5             -- Tag 5
   82          -- Array of length 2
      20       -- -1
      03       -- 3

Decimal fractions and bigfloats provide no representation of Infinity, -Infinity, or NaN; if these are needed in place of a decimal fraction or bigfloat, the IEEE 754 half precision representations from Section 2.3 can be used. For constrained applications, where there is a choice between representing a specific number as an integer and as a decimal fraction or bigfloat (such as when the exponent is small and non-negative), there is a quality of implementation expectation that the integer representation is used directly.

2.4.4. Content Hints

The tags in this section are for content hints that might be used by generic CBOR processors.

2.4.4.1. Encoded CBOR data item

Sometimes it is beneficial to carry an embedded CBOR data item that is not meant to be decoded immediately at the time the enclosing data item is being parsed. Tag 24 (CBOR data item) can be used to tag the embedded byte string as a data item encoded in CBOR format.

2.4.4.2. Expected Later Encoding for CBOR to JSON Converters

Tags 21 to 23 indicate that a byte string might require a specific encoding when interoperating with a text-based representation. These tags are useful when an encoder knows that the byte string data it is writing is likely to be later converted to a particular JSON-based usage. That usage specifies that some strings are encoded as Base64, Base64url, and so on. The encoder uses byte strings instead of doing the encoding itself to reduce the message size, to reduce the code size of the encoder, or both. The encoder does not know whether or not the converter will be generic, and therefore wants to say what it believes is the proper way to convert binary strings to JSON.

The data item tagged can be a byte string, or any other data item. In the latter case, the tag applies to all of the byte string data items contained in the data item, except for those contained in a nested expected conversion tagged item.

These three tag types suggest conversions to three of the base data encodings defined in [RFC4648]. For base64url encoding, padding is not used (see section 3.2 of RFC 4648), that is all trailing equals signs (“=”) are removed from the base64url encoded string. Later tags might be defined for other data encodings of RFC 4648, or of other ways to encode binary data in strings.

2.4.4.3. Encoded Text

Some text strings hold data that have formats widely-used on the Internet, and sometimes those formats can be validated and presented to the application in appropriate form by the decoder. There are tags for some of these formats.

• Tag 32 is for URIs, as defined in [RFC3986];
• Tags 33 and 34 are for base64url and base64 encoded text strings, as defined in [RFC4648];
• Tag 35 is for regular expressions in PCRE/JavaScript syntax [ECMA262].
• Tag 36 is for MIME messages (including all headers), as defined in [RFC2045];

Note that tag 33 and 34 differ from 21 and 22 in that the data is transported in base-encoded form for the former and in raw byte string form in the latter case.

2.4.5. Self-describe CBOR

In many applications, it will be clear from the context that CBOR is being employed for encoding a data item. For instance, a specific protocol might specify the use of CBOR, or a media type is indicated that specifies its use. However, there may be applications where such context information is not available, such as when CBOR data is stored in a file and disambiguating metadata is not in use. Here, it may help to have some distinguishing characteristics for the data itself.

Tag 55799 is defined for this purpose. It does not impart any special semantics on the data item that follows, that is, the semantics of a data item tagged with tag 55799 is exactly identical to the semantics of the data item itself.

The serialization of this tag is 0xd9d9f7, which appears not to be in use as a distinguishing mark for frequently used file types. In particular, it is not a valid start of a Unicode text in any Unicode encoding if followed by a valid CBOR data item.

For instance, a decoder might be able to parse both CBOR and JSON. Such a decoder would need to mechanically distinguish the two formats. An easy way for an encoder to help the decoder would be to tag the entire CBOR item with Tag 55799, the serialization of which will never be found at the beginning of a JSON text.

3. Creating CBOR-Based Protocols

Data formats such as CBOR are often used in environments where there is no format negotiation. A specific design goal of CBOR is to not need any included or assumed schema: a decoder can take a CBOR item and decode it with no other knowledge.

Of course, in real-world implementations, the encoder and the decoder will have a shared view of what should be in a CBOR data item. For example, an agreed-to format might be “the item is an array whose first value is a UTF-8 string, the second value is an integer, followed by zero or more floating point numbers” or “a map whose keys are byte strings that has to contain at least one pair whose key is 0xab01”.

This specification puts no restrictions on CBOR-based protocols. An encoder can be capable of encoding as many or as few types of values as is required by the protocol in which it is used; a decoder can be capable of understanding as many or as few types of values as is required by the protocols in which it is used. This lack of restrictions allows CBOR to be used in extremely constrained environments.

This section discusses some considerations in creating CBOR-based protocols. It is advisory only, and explicitly excludes any language from RFC 2119 other than words that could be interpreted as “MAY” in the RFC 2119 sense.

3.1. CBOR in Streaming Applications

In a streaming application, a data stream may be composed of a sequence of CBOR data items concatenated back-to-back. In such an environment, the decoder immediately begins decoding a new data item if data is found after the end of a previous data item.

Not all of the bytes making up a data item may be immediately available to the decoder; some decoders will buffer additional data until a complete data item can be presented to the application. Other decoders can present partial information about a top-level data item to an application, such as the nested data items that could already be decoded, or even parts of a byte string that hasn’t completely arrived yet.

Note that some applications and protocols will not want to use indefinite length encoding. Using indefinite length encoding allows an encoder to not need to marshall all the data for counting, but it requires a decoder to allocate increasing amounts of memory while waiting for the end of the item. This might be fine for some applications but not others.

3.2. Generic Encoders and Decoders

A generic CBOR decoder can decode all well-formed CBOR data and present them to an application. CBOR data are well-formed if the structure of the initial bytes and the byte strings/data items implied by their values is followed and no extraneous data follows (Appendix C).

Even though CBOR attempts to minimize these cases, not all well-formed CBOR data are valid: for example, the format excludes simple values below 32 that are encoded with an extension byte. Also, specific tags may make semantic constraints that may be violated, such as by including a tag in a bignum tag or by following a byte string within a date tag. Finally, the data may be invalid, such as invalid UTF-8 strings or date strings that do not conform to [RFC3339]. There is no requirement that generic encoders and decoders make unnatural choices for their application interface to enable the processing of invalid data. Generic encoders and decoders are expected to forward simple values and tags even if their specific code points had not been registered at the time the encoder/decoder has been written (Section 3.5).

Generic decoders provide ways to present well-formed CBOR values, both valid and invalid, to an application. The diagnostic notation (Section 6) may be used to present well-formed CBOR values to humans.

Generic encoders provide an application interface that allows the application to specify any well-formed value, including simple values and tags unknown to the encoder.

3.3. Syntax Errors

A decoder encountering a CBOR data item that is not well-formed generally can choose to completely fail the decoding (issue an error and/or stop processing altogether), substitute the problematic data and data items using an decoder-specific convention that clearly indicates there has been a problem, or it might take some other action.

3.3.1. Incomplete CBOR data items

The representation of a CBOR data item has a specific length, determined by its initial bytes and by the structure of any data items enclosed in the data items. If less data is available, this can be treated as a syntax error. A decoder may also implement incremental parsing, that is, decode the data item as far as it is available and present the data found so far, (such as in an event-based interface) with the option of continuing the decoding once further data are available.

Examples of incomplete data items include:

• a decoder expecting a certain number of array or map entries instead encounters the end of the data
• a decoder processing what it expects to be the last pair in a map comes to the end of the data
• a decoder has just seen a tag and then encounters the end of the data
• a decoder has seen the beginning of an indefinite length item but encounters the end of the data before it sees the “break” stop code

3.3.2. Malformed Indefinite Length Items

Examples of malformed indefinite length data items include:

• Within an indefinite length byte string or text, a decoder finds an item that is not of the appropriate major type before it finds the “break” stop code
• Within an indefinite length map, a decoder encounters the “break” stop code immediately after reading a key (the value is missing)

Another error is a “break” stop code that is found when there is no immediately enclosing indefinite length item needing to be closed.

3.3.3. Unknown Additional Information Values

At the time this document is written, some additional information values are unassigned and reserved for future versions of this document (see Section 5.2). Since the overall syntax for these additional information values is not yet defined, a decoder that sees an additional information value that it does not understand cannot continue parsing.

3.4. Other Decoding Errors

A CBOR data item may be syntactically well-formed, but present a problem with interpreting the data encoded in it in the CBOR data model. Generally speaking, a decoder that finds a data item with such a problem might issue a warning, might stop processing altogether, might handle the error and make the problematic value available to the application as such, or take some other type of action.

Such problems might include:

Duplicate keys in a map:

Generic decoders (Section 3.2) make data available to applications using the native CBOR data model. That data model includes maps (key-value mappings with unique keys), not multimaps (key-value mappings where multiple entries can have the same key). Thus, a generic decoder that gets a CBOR map item that has duplicate keys will decode to a map with only one instance of that key, or it might stop processing altogether. On the other hand, a “streaming decoder” may not even be able to notice (Section 3.7).

Inadmissible type on the value following a tag:

Tags (Section 2.4) specify what type of data item is supposed to follow the tag; for example, the tags for positive or negative bignums are supposed to be put on byte strings. A decoder that decodes the tagged data item into a native representation (a native big integer in this example) is expected to check the type of the data item being tagged. Even decoders that don’t have such native representations available in their environment may perform the check on those tags known to them and react appropriately.

Invalid UTF-8 string:

A decoder might or might not want to verify that the sequence of bytes in an UTF-8 string (major type 3) is actually valid UTF-8 and react appropriately.

3.5. Handling Unknown Simple Values and Tags

A decoder that comes across a simple value (Section 2.3) that it does not recognize, such as a value that was added to the IANA registry after the decoder was deployed or a value that the decoder chose not to implement, might issue a warning, might stop processing altogether, might handle the error by making the unknown value available to the application as such (as is expected of generic decoders), or take some other type of action.

A decoder that comes across a tag (Section 2.4) that it does not recognize, such as a tag that was added to the IANA registry after the decoder was deployed or a tag that the decoder chose not to implement, might issue a warning, might stop processing altogether, might handle the error and present the unknown tag value together with the contained data item to the application (as is expected of generic decoders), might ignore the tag and simply present the contained data item only to the application, or take some other type of action.

3.6. Numbers

For the purposes of this specification, all number representations for the same numeric value are equivalent. This means that an encoder can encode a floating point value of 0.0 as the integer 0. It, however, also means that an application that expects to find integer values only might find floating point values if the encoder decides these are desirable, such as when the floating point value is more compact than a 64-bit integer.

An application or protocol that uses CBOR might restrict the representations of numbers. For instance, a protocol that only deals with integers might say that floating point numbers may not be used and that decoders of that protocol do not need to be able to handle floating point numbers. Similarly, a protocol or application that uses CBOR might say that decoders need to be able to handle either type of number.

CBOR-based protocols should take into account that different language environments pose different restrictions on the range and precision of numbers that are representable. For example, the JavaScript number system treats all numbers as floating-point, which may result in silent loss of precision in decoding integers with more than 53 significant bits. A protocol that uses numbers should define its expectations on the handling of non-trivial numbers in decoders and receiving applications.

A CBOR-based protocol that includes floating point numbers can restrict which of the three formats (half-precision, single-precision, and double-precision) are to be supported. For an integer-only application, a protocol may want to completely exclude the use of floating point values.

A CBOR-based protocol designed for compactness may want to exclude specific integer encodings that are longer than necessary for the application, such as to save the need to implement 64-bit integers. There is an expectation that encoders will use the most compact integer representation that can represent a given value. However, a compact application should accept values that use a longer-than needed encoding (such as encoding “0” as 0b000_11101 followed by two bytes of 0x00) as long as the application can decode an integer of the given size.

3.7. Specifying Keys for Maps

The encoding and decoding applications need to agree on what types of keys are going to be used in maps. In applications that need to interwork with JSON-based applications, keys probably should be limited to UTF-8 strings only; otherwise, there has to be a specified mapping from the other CBOR types to Unicode characters, and this often leads to implementation errors. In applications where keys are numeric in nature and numeric ordering of keys is important to the application, directly using the numbers for the keys is useful.

If multiple types of keys are to used, consideration should be given to how these types would be represented in the specific programming environments that are to be used. For example, in JavaScript objects, a key of integer 1 cannot be distinguished from a key of string “1”. This means that, if integer keys are used, the simultaneous use of string keys that look like numbers needs to be avoided. Again, this leads to the conclusion that keys should be of a single CBOR type.

Decoders that deliver data items nested within a CBOR data item immediately on decoding them (“streaming decoders”) often do not keep the state that is necessary to ascertain uniqueness of a key in a map. Similarly, an encoder that can start encoding data items before the enclosing data item is completely available (“streaming encoder”) may want to reduce its overhead significantly by relying on its data source to maintain uniqueness.

A CBOR-based protocol should make an intentional decision about what to do when a receiving application does see multiple identical keys in a map. The resulting rule in the protocol should respect the CBOR data model: it cannot prescribe a specific handling of the entries with the identical keys, except that it might have a rule that having identical keys in a map indicates a malformed map and that the decoder has to stop with an error. Duplicate keys are also prohibited by CBOR decoders that are using Section 3.10.

The CBOR data model for maps does not allow ascribing semantics to the order of the key/value pairs in the map representation.
Thus, it would be a very bad practice to define a CBOR-based protocol in such a way that changing the key/value pair order in a map would change the semantics, apart from trivial aspects (cache usage etc.). (A CBOR-based protocol can prescribe a specific order of serialization, such as for canonicalization.)

Applications for constrained devices that have maps with 24 or fewer frequently used keys should consider using small integers (and those with up to 48 frequently used keys should consider also using small negative integers) because the keys can then be encoded in a single byte.

3.8. Undefined Values

In some CBOR-based protocols, the simple value (Section 2.3) of Undefined might be used by an encoder as a substitute for a data item with an encoding problem, in order to allow the rest of the enclosing data items to be encoded without harm.

3.9. Canonical CBOR

Some protocols may want encoders to only emit CBOR in a particular canonical format; those protocols might also have the decoders check that their input is canonical. Those protocols are free to define what they mean by a canonical format and what encoders and decoders are expected to do. This section lists some suggestions for such protocols.

If a protocol considers “canonical” to mean that two encoder implementations starting with the same input data will produce the same CBOR output, the following four rules would suffice:

• Integers must be as small as possible.
  • 0 to 23 and -1 to -24 must be expressed in the same byte as the major type;
  • 24 to 255 and -25 to -256 must be expressed only with an additional uint8_t;
  • 256 to 65535 and -257 to -65536 must be expressed only with an additional uint16_t;
  • 65536 to 4294967295 and -65537 to -4294967296 must be expressed only with an additional uint32_t.
• The expression of lengths in major types 2 through 5 must be as short as possible. The rules for these lengths follow the above rule for integers.
• The keys in every map must be sorted lowest value to highest. Sorting is performed on the bytes of the representation of the key data items without paying attention to the 3/5 bit splitting for major types. (Note that this rule allows maps that have keys of different types, even though that is probably a bad practice that could lead to errors in some canonicalization implementations.) The sorting rules are:
  • If two keys have different lengths, the shorter one sorts earlier;
  • If two keys have the same length, the one with the lower value in (byte-wise) lexical order sorts earlier.
• Indefinite length items must be made into definite length items.

If a protocol allows for IEEE floats, then additional canonicalization rules might need to be added. One example rule might be to have all floats start as a 64-bit float, then do a test conversion to a 32-bit float; if the result is the same numeric value, use the shorter value and repeat the process with a test conversion to a 16-bit float. (This rule selects 16-bit float for positive and negative infinity as well.) Also, there are many representations for NaN. If NaN is an allowed value, it must always be represented as 0xf97e00.

CBOR tags present additional considerations for the canonicalization. The absence or presence of tags in a canonical format is determined by the optionality of the tags in the protocol. In a CBOR-based protocol that allows optional tagging anywhere, the canonical format must not allow them. In a protocol that requires tags in certain places, the tag needs to appear in the canonical format. A CBOR-based protocol that uses canonicalization might instead say that all tags that appear in a message must be retained regardless of whether they are optional.

3.10. Strict Mode

Some areas of application of CBOR do not require canonicalization (Section 3.9), but may require that different decoders reach the same (semantically equivalent) results, even in the presence of potentially malicious data. This can be required if one application (such as a firewall or other protecting entity) makes a decision based on the data that another application, which independently decodes the data, relies on.

Normally, it is the responsibility of the sender to avoid ambiguously decodable data. However, the sender might be an attacker specially making up CBOR data such that it will be interpreted differently by different decoders in an attempt to exploit that as a vulnerability. Generic decoders used in applications where this might be a problem need to support a strict mode in which it is also the responsibility of the receiver to reject ambiguously decodable data. It is expected that firewalls and other security systems that decode CBOR will only decode in strict mode.

A decoder in strict mode will reliably reject any data that could be interpreted by other decoders in different ways. It will reliably reject data items with syntax errors (Section 3.3). It will also expend the effort to reliably detect other decoding errors Section 3.4. In particular, a strict decoder needs to have an API that reports an error (and does not return data) for a CBOR data item that contains any of the following:

• A map (major type 5) that has more than one entry with the same key
• A tag that is used on a data item of the incorrect type
• A data item that is incorrectly formatted for the type given to it, such as invalid UTF-8 or data that cannot be interpreted with the specific tag that it has been tagged with

A decoder in strict mode can do one of two things when it encounters a tag or simple value that it does not recognize:

• It can report an error (and not return data).
• It can emit the unknown item (type, value, and, for tags, the decoded tagged data item) to the application calling the decoder with an indication that the decoder did not recognize that tag or simple value.

The latter approach, which is also appropriate for non-strict decoders, supports forward compatibility with newly registered tags and simple values without the requirement to update the encoder at the same time as the calling application. (For this, the API for the decoder needs to have a way to mark unknown items so that the calling application can handle them in a manner appropriate for the program.)

Since some of this processing may have an appreciable cost (in particular with duplicate detection for maps), support of strict mode is not a requirement placed on all CBOR decoders.

Some encoders will rely on their applications to provide input data in such a way that unambiguously decodable CBOR results. A generic encoder also may want to provide a strict mode where it reliably limits its output to unambiguously decodable CBOR, independent of whether its application is providing API-conformant data or not.

4. Converting Data Between CBOR and JSON

This section gives non-normative advice about converting between CBOR and JSON. Implementations of converters are free to use whichever advice here they want.

It is worth noting that a JSON text is a sequence of characters, not an encoded sequence of bytes, while a CBOR data item consist of bytes, not characters.

4.1. Converting From CBOR to JSON

Most of the types in CBOR have direct analogs in JSON. However, some do not, and someone implementing a CBOR-to-JSON converter has to consider what to do in those cases. The following non-normative suggestion deals with these by converting them to a single substitute value, such as a JSON null.

• An Integer (major type 0 or 1) becomes a JSON number.
• A byte string (major type 2) that is not embedded in a tag that specifies a proposed encoding is encoded in Base64url without padding and becomes a JSON string.
• A UTF-8 string (major type 3) becomes a JSON string. Note that JSON requires escaping certain characters (RFC 4627, section 2.5): quotation mark (U+0022), reverse solidus (U+005C), and the “C0 control characters” (U+0000 through U+001F). All other characters are copied unchanged into the JSON UTF-8 string.
• An array (major type 4) becomes a JSON array.
• A map (major type 5) becomes a JSON object. This is possible directly only if all keys are UTF-8 strings. A converter might also convert other keys into UTF-8 strings (such as by converting integers into strings containing their decimal representation); however, doing so introduces a danger of key collision.
• False (major type 7, additional information 20) becomes a JSON false.
• True (major type 7, additional information 21) becomes a JSON true.
• Null (major type 7, additional information 22) becomes a JSON null.
• A floating point value (major type 7, additional information 25 through 27) becomes a JSON number if it is finite (that is, it can be represented in a JSON number); if the value is non-finite (NaN, or positive or negative Infinity), it is represented by the substitute value.
• Any other simple value (Major type 7, any additional information value not yet discussed) is represented by the substitute value.
• A bignum (major type 6, tag value 2 or 3) is represented by encoding its byte string in Base64url without padding and becomes a JSON string. For tag value 3 (negative bignum), a “~” (ASCII tilde) is inserted before the base-encoded value. (The conversion to a binary blob instead of a number is to prevent a likely numeric overflow for the JSON decoder.)
• A byte string with an encoding hint (major type 6, tag value 21 through 23) is encoded as described and becomes a JSON string.
• For all other tags (major type 6, any other tag value), the embedded CBOR item is represented as a JSON value; the tag value is ignored.
• Indefinite length items are made definite before conversion.

4.2. Converting From JSON to CBOR

All JSON values, once decoded, directly map into one or more CBOR values. As with any kind of CBOR generation, decisions have to be made with respect to number representation. In a suggested conversion:

• JSON numbers without fractional parts (integer numbers) are represented as integers (major types 0 and 1, possibly major type 6 tag value 2 and 3), choosing the shortest form; integers longer than an implementation-defined threshold (which is usually either 32 or 64 bits) may instead be represented as floating point values. (If the JSON was generated from a JavaScript implementation, its precision is already limited to 53 bits maximum.)
• Numbers with fractional parts are represented as floating point values. Preferably, the shortest exact floating point representation is used; for instance, 1.5 is represented in a 16-bit floating point value (not all implementations will be efficiently capable of finding the minimum form, though). There may be an implementation-defined limit to the precision that will affect the precision of the represented values. Decimal representation should only be used if that is specified in a protocol.

CBOR has been designed to generally provide a more compact encoding than JSON. One implementation strategy that might come to mind is to perform a JSON to CBOR encoding in place in a single buffer. This strategy would need to carefully consider a number of pathological case, such as that some strings represented with no or very few escapes and longer (or much longer) than 255 may expand when encoded as UTF-8 strings in CBOR. Similarly, a few of the binary floating point representations might cause expansion from some short decimal representations (1.1, 1e9) in JSON. This may be hard to get right and any ensuing vulnerabilities may be exploited by an attacker.

5. Future Evolution of CBOR

Successful protocols evolve over time. New ideas appear, implementation platforms improve, related protocols are developed and evolve, and new requirements from applications and protocols are added. Facilitating protocol evolution is therefore an important design consideration for any protocol development.

For protocols that will use CBOR, CBOR provides some useful mechanisms to facilitate their evolution. Best practices for this are well known, particularly from JSON format development of JSON-based protocols. Therefore, such best practices are outside the scope of this specification.

However, facilitating the evolution of CBOR itself is very well within its scope. CBOR is designed to both provide a stable basis for development of CBOR-based protocols and to be able to evolve. Since a successful protocol may live for decades, CBOR needs to be designed for decades of use and evolution. This section provides some guidance for the evolution of CBOR. It is necessarily more subjective than other parts of this document. It is also necessarily incomplete, lest it turn into a textbook on protocol development.

5.1. Extension Points

In a protocol design, opportunities for evolution are often included in the form of extension points. For example, there may be a code point space that is not fully allocated from the outset, and the protocol is designed to tolerate and embrace implementations that start using more code points than initially allocated.

Sizing the code point space may be difficult because the range required may be hard to predict. An attempt should be made to make the codepoint space large enough so that it can slowly be filled over the intended lifetime of the protocol.

CBOR has three major extension points:

• the “simple” space (values in major type 7). Of the 24 efficient (and 224 slightly less efficient) values, only a small number have been allocated. Implementations receiving an unknown simple data item may be able to process it as such, given that the structure of the value is indeed simple. An IANA registry is appropriate here.
• the “tag” space (values in major type 6). Again, only a small part of the code point space has been allocated, and the space is abundant (although the early numbers are more efficient than the later ones). Implementations receiving an unknown tag can choose to simply ignore it, or to process it as an unknown tag wrapping the following data item. An IANA registry is appropriate here.
• the “additional information” space. An implementation receiving an unknown additional information has no way to continue parsing, so allocating codepoints to this space is a major step. There are also very few codepoints left.

5.2. Curating the Additional Information Space

The human mind is sometimes drawn to filling in little perceived gaps to make something neat. We expect the remaining gaps in the code point space for the additional information values to be an attractor for new ideas, just because they are there.

The present specification does not manage the additional information code point space by an IANA registry. Instead, allocations out of this space can only be done by updating this specification.

For an additional information value of n ≥ 24, the size of the additional data typically is 2**(n-24) bytes. Therefore, additional information values 28 and 29 should be viewed as candidates for 128-bit and 256-bit quantities, in case a need arises to add them to the protocol. Additional information value 30 is then the only additional information value available for general allocation, and there should be a very good reason for allocating it before assigning it through an update of this protocol.

6. Diagnostic Notation

CBOR is a binary interchange format. To facilitate documentation and debugging, and in particular to facilitate communication between entities cooperating in debugging, this section defines a simple human-readable diagnostic notation. All actual interchange always happens in the binary format.

Note that this truly is a diagnostic format; it is not meant to be parsed. Therefore, no formal definition (as in ABNF) is given in this document. (Implementers looking for a text-based format for representing CBOR data items in configuration files may also want to consider YAML [YAML].)

The diagnostic notation is loosely based on JSON as it is defined in RFC 4627, extending it where needed.

The notation borrows the JSON syntax for numbers (integer and floating point), True (>true<), False (>false<), Null (>null<), UTF-8 strings, arrays and maps (maps are called objects in JSON; the diagnostic notation extends JSON here by allowing any data item in the key position). Undefined is written >undefined< as in JavaScript. The non-finite floating point numbers Infinity, -Infinity, and NaN are written exactly as in this sentence (this is also a way they can be written in JavaScript, although JSON does not allow them). A tagged item is written as an integer number for the tag followed by the item in parentheses; for instance, an RFC 3339 (ISO 8601) date could be notated as:

• 0(“2013-03-21T20:04:00Z”)

or the equivalent relative time as

• 1(1363896240)

Byte strings are notated in one of the base encodings, without padding, enclosed in single quotes, prefixed by >h< for base16, >b32< for base32, >h32< for base32hex, >b64< for base64 or base64url (the actual encodings do not overlap, so the string remains unambiguous). For example, the byte string 0x12345678 could be written h’12345678’, b32’CI2FM6A’, or b64’EjRWeA’.

Unassigned simple values are given as “simple()” with the appropriate integer in the parentheses. For example, “simple(42)” indicates major type 7, value 42.

6.1. Encoding indicators

Sometimes it is useful to indicate in the diagnostic notation which of several alternative representations were actually used; for example, a data item written >1.5< by a diagnostic decoder might have been encoded as a half-, single-, or double-precision float.

The convention for encoding indicators is that anything starting with an underscore and all following characters that are alphanumeric or underscore, is an encoding indicator, and can be ignored by anyone not interested in this information. Encoding indicators are always optional.

A single underscore can be written after the opening brace of a map or the opening bracket of an array to indicate that the data item was represented in indefinite length format. For example, [_ 1, 2] contains a indicator that an indefinite length representation was used to represent the data item [1, 2].

An underscore followed by a decimal digit n indicates that the preceding item (or, for arrays and maps, the item starting with the preceding bracket or brace) was encoded with an additional information value of 24+n. For example, 1.5_1 is a half precision floating point number, while 1.5_3 is encoded as double precision. (This encoding indicator is not shown in Appendix A.) (Note that the encoding indicator “_” is thus an abbreviation of the full form “_7”, which is not used.)

As a special case, byte and text strings of indefinite length can be notated in the form (_ h’0123’, h’4567’) and (_ “foo”, “bar”).

7. IANA Considerations

IANA will create two registries for new CBOR values. The registries will be separate, not under an umbrella registry. The registries will follow the rules in [RFC5226]. IANA will also assign a new MIME media type and an associated CoAP Content-Format entry.

7.1. Simple Values Registry

A registry called “CBOR Simple Values” will be created. The initial values are shown in Table 2.

New entries in the range 0 to 19 will be assigned by Standards Action. It is suggested that these Standards Actions allocate values starting with the number 16 in order to reserve the lower numbers for any contiguous block.

New entries in the range 32 to 255 will be assigned by Specification Required.

7.2. Tags Registry

A registry called “CBOR Tags” will be created. The initial values are shown in Table 3.

New entries in the range 0 to 23 will be assigned by Standards Action. New entries in the range 24 to 255 will be assigned by Specification Required. New entries in the range 256 to 18446744073709551615 will be assigned by First Come First Served. The template for First Come First Served will include point of contact and an optional field for URL to a description of the semantics of the tag; the latter can be something like an Internet-Draft or a web page.

7.3. Media Type (“MIME Type”)

The Internet media type [RFC6838] for CBOR data is application/cbor.

Type name: application

Subtype name: cbor

Required parameters: n/a

Optional parameters: n/a

Encoding considerations:

binary

Security considerations:

See Section 8 of this document

Interoperability considerations: n/a

Published specification: This document

Applications that use this media type:

None yet, but it is expected that this format will be deployed in protocols and applications.

Additional information:
  Magic number(s): n/a
  File extension(s): .cbor
  Macintosh file type code(s): n/a

Person & email address to contact for further information:
  Carsten Bormann
  cabo@tzi.org

Intended usage: COMMON

Restrictions on usage: none

Author:
  Carsten Bormann <cabo@tzi.org>

Change controller:
  The IESG <iesg@ietf.org>

7.4. CoAP Content-Format

Media Type: application/cbor

Encoding: -

Id: 60

Reference: [RFC-THIS-SPEC]

7.5. The +cbor Structured Syntax Suffix Registration

Name: Concise Binary Object Representation (CBOR)

+suffix: +cbor

References: [RFC-THIS-SPEC]

Encoding considerations: CBOR is a binary format.

Fragment identifier considerations:
  The syntax and semantics of fragment identifiers specified for
  +cbor SHOULD be as specified for "application/cbor".  (At
  publication of this document, there is no fragment identification
  syntax defined for "application/cbor".)

  The syntax and semantics for fragment identifiers for a specific
  "xxx/yyy+cbor" SHOULD be processed as follows:

  For cases defined in +cbor, where the fragment identifier resolves
  per the +cbor rules, then process as specified in +cbor.

  For cases defined in +cbor, where the fragment identifier does
  not resolve per the +cbor rules, then process as specified in
  "xxx/yyy+cbor".

  For cases not defined in +cbor, then process as specified in
  "xxx/yyy+cbor".

Interoperability considerations: n/a

Security considerations:

See Section 8 of this document

Contact:
  Apps Area Working Group (apps-discuss at ietf.org)

Author/Change controller:
  The Apps Area Working Group.
  The IESG has change control over this registration.

8. Security Considerations

A network-facing application can exhibit vulnerabilities in its processing logic for incoming data. Complex parsers are well known as a likely source of such vulnerabilities, such as the ability to remotely crash a node, or even remotely execute arbitrary code on it. CBOR attempts to narrow the opportunities for introducing such vulnerabilities by reducing parser complexity, by giving the entire range of encodable values a meaning where possible.

Resource exhaustion attacks might attempt to lure a decoder into allocating very big data items (strings, arrays, maps) or exhaust the stack depth by setting up deeply nested items. Decoders need to have appropriate resource management to mitigate these attacks. (Items for which very large sizes are given can also attempt to exploit integer overflow vulnerabilities.)

Applications where CBOR data items are examined by a gatekeeper function and later used by a different application may exhibit vulnerabilities when multiple interpretations of the data item are possible. For example, an attacker could make use of duplicate keys in maps and precision issues in numbers to make the gatekeeper base its decisions on a different interpretation than the one that will be used by the second application. Protocols that are used in a security context should be defined in such a way that these multiple interpretations are reliably reduced to a single one. To facilitate this, encoder and decoder implementations used in such contexts should provide at least one strict mode of operation (Section 3.10).

9. Acknowledgements

CBOR was inspired by MessagePack. MessagePack was developed and promoted by Sadayuki Furuhashi (“frsyuki”). This reference to MessagePack is solely for attribution; CBOR is not intended as a version of or replacement for MessagePack, as it has different design goals and requirements.

The need for functionality beyond the original MessagePack Specification became obvious to many people at about the same time around the year 2012. BinaryPack is a minor derivation of MessagePack that was developed by Eric Zhang for the binaryjs project. A similar, but different extension was made by Tim Caswell for his msgpack-js and msgpack-js-browser projects. Many people have contributed to the recent discussion about extending MessagePack to separate text string representation from byte string representation.

The encoding of the additional information in CBOR was inspired by the encoding of length information designed by Klaus Hartke for CoAP.

This document also incorporates suggestions made by many people, notably Dan Frost, James Manger, Joe Hildebrand, Keith Moore, Matthew Lepinski, Nico Williams, Phillip Hallam-Baker, Ray Polk, Tim Bray, Tony Finch, Tony Hansen, and Yaron Sheffer.

10. References

10.1. Normative References

10.2. Informative References

Appendix A. Examples

The following table provides some CBOR encoded values in hexadecimal (right column), together with diagnostic notation for these values (left column). Note that the string “\u00fc” is one form of diagnostic notation for a UTF-8 string containing the single Unicode character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (u umlaut). Similarly, “\u6c34” is a UTF-8 string in diagnostic notation with a single character U+6C34 (CJK UNIFIED IDEOGRAPH-6C34, often representing “water”), and “\ud800\udd51” is a UTF-8 string in diagnostic notation with a single character U+10151 (GREEK ACROPHONIC ATTIC FIFTY STATERS). (Note that all these single-character strings could also be represented in native UTF-8 in diagnostic notation, just not in an ASCII-only specification like the present one.) In the diagnostic notation provided for bignums, their intended numeric value is shown as a decimal number (such as 18446744073709551616) instead of showing a tagged byte string (such as 2(h’010000000000000000’)).

Appendix B. Jump Table

For brevity, this jump table does not show initial bytes that are reserved for future extension. It also only shows a selection of the initial bytes that can be used for optional features. (All unsigned integers are in network byte order.)

well_formed (breakable = false) {
  // process initial bytes
  ib = uint(take(1));
  mt = ib >> 5;
  val = ai = ib & 0x1f;
  switch (ai) {
    case 24: val = uint(take(1)); break;
    case 25: val = uint(take(2)); break;
    case 26: val = uint(take(4)); break;
    case 27: val = uint(take(8)); break;
    case 28: case 29: case 30: fail();
    case 31:
      return well_formed_indefinite(mt, breakable);
  }
  // process content
  switch (mt) {
    // case 0, 1, 7 do not have content; just use val
    case 2: case 3: take(val); break; // bytes/UTF-8
    case 4: for (i = 0; i < val; i++) well_formed(); break;
    case 5: for (i = 0; i < val*2; i++) well_formed(); break;
    case 6: well_formed(); break;     // 1 embedded data item
  }
  return mt;                    // finite data item
}

well_formed_indefinite(mt, breakable) {
  switch (mt) {
    case 2: case 3:
      while ((it = well_formed(true)) != -1)
        if (it != mt)           // need finite embedded
          fail();               //    of same type
      break;
    case 4: while (well_formed(true) != -1); break;
    case 5: while (well_formed(true) != -1) well_formed(); break;
    case 7:
      if (breakable)
        return -1;              // signal break out
      else fail();              // no enclosing indefinite
    default: fail();            // wrong mt
  }
  return 0;                     // no break out
}

void encode_sint(int64_t n) {
  uint64t ui = n >> 63;    // extend sign to whole length
  mt = ui & 0x20;          // extract major type
  ui ^= n;                 // complement negatives
  if (ui < 24)
    *p++ = mt + ui;
  else if (ui < 256) {
    *p++ = mt + 24;
    *p++ = ui;
  } else
       ...

#include <math.h>

double decode_half(unsigned char *halfp) {
  int half = (halfp[0] << 8) + halfp[1];
  int exp = (half >> 10) & 0x1f;
  int mant = half & 0x3ff;
  double val;
  if (exp == 0) val = ldexp(mant, -24);
  else if (exp != 31) val = ldexp(mant + 1024, exp - 25);
  else val = mant == 0 ? INFINITY : NAN;
  return half & 0x8000 ? -val : val;
}

import struct
from math import ldexp

def decode_single(single):
    return struct.unpack("!f", struct.pack("!I", single))[0]

def decode_half(half):
    valu = (half & 0x7fff) << 13 | (half & 0x8000) << 16
    if ((half & 0x7c00) != 0x7c00):
        return ldexp(decode_single(valu), 112)
    return decode_single(valu | 0x7f800000)