Internet Engineering Task Force P. M. Hallam-Baker
Internet-Draft Comodo Group Inc.
Intended status: Standards Track June 11, 2013
Expires: December 13, 2013

Binary Encodings for JavaScript Object Notation: JSON-B, JSON-C, JSON-D


Three binary encodings for JavaScript Object Notation (JSON) are presented. JSON-B (Binary) is a strict superset of the JSON encoding that permits efficient binary encoding of intrinsic JavaScript data types. JSON-C (Compact) is a strict superset of JSON-B that supports compact representation of repeated data strings with short numeric codes. JSON-D (Data) supports additional binary data types for integer and floating point representations for use in scientific applications where conversion between binary and decimal representations would cause a loss of precision.

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Table of Contents

1. Definitions

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Introduction

JavaScript Object Notation (JSON) is a simple text encoding for the JavaScript Data model that has found wide application beyond its original field of use. In particular JSON has rapidly become a preferred encoding for Web Services.

JSON encoding supports just four fundamental data types (integer, floating point, string and boolean), arrays and objects which consist of a list of tag-value pairs.

Although the JSON encoding is sufficient for many purposes it is not always efficient. In particular there is no efficient representation for blocks of binary data. Use of base64 encoding increases data volume by 33%. This overhead increases exponentially in applications where nested binary encodings are required making use of JSON encoding unsatisfactory in cryptographic applications where nested binary structures are frequently required.

Another source of inefficiency in JSON encoding is the repeated occurrence of object tags. A JSON encoding containing an array of a hundred objects such as {"first":1,"second":2} will contain a hundred occurrences of the string "first" (seven bytes) and a hundred occurrences of the string "second" (eight bytes). Using two byte code sequences in place of strings allows a saving of 11 bytes per object without loss of information, a saving of 50%.

A third objection to the use of JSON encoding is that floating point numbers can only be represented in decimal form and this necessarily involves a loss of precision when converting between binary and decimal representations. While such issues are rarely important in network applications they can be critical in scientific applications. It is not acceptable for saving and restoring a data set to change the result of a calculation.

2.1. Objectives

The following were identified as core objectives for a binary JSON encoding:

Three binary encodings are defined:

JSON-B (Binary)
Simply encodes JSON data in binary. Only the JavaScript data model is supported (i.e. atomic types are integers, double or string). Integers may be 8, 16, 32 or 64 bits either signed or unsigned. Floating points are IEEE 754 binary64 format [IEEE-754]. Supports chunked encoding for binary and UTF-8 string types.
JSON-C (Compact)
As JSON-B but with support for representing JSON tags in numeric code form (16 bit code space). This is done for both compact encoding and to allow simplification of encoders/decoders in constrained environments. Codes may be defined inline or by reference to a known dictionary of codes referenced via a digest value.
JSON-D (Data)
As JSON-C but with support for representing additional data types without loss of precision. In particular other IEEE 754 floating point formats, both binary and decimal and Intel's 80 bit floating point, plus 128 bit integers and bignum integers.

3. Extended JSON Grammar

The JSON-B, JSON-C and JSON-D encodings are all based on the JSON grammar [RFC4627] using the same syntactic structure but different lexical encodings.

JSON-B0 and JSON-C0 replace the JSON lexical encodings for strings and numbers with binary encodings. JSON-B1 and JSON-C1 allow either lexical encoding to be used. Thus any valid JSON encoding is a valid JSON-B1 or JSON-C1 encoding.

The grammar of JSON-B, JSON-C and JSON-D is a superset of the JSON grammar. The following productions are added to the grammar:

Binary encodings for data values. As the binary value encodings are all self delimiting
An object member where the value is specified as an X-value and thus does not require a value-separator.
Binary data encodings defined in JSON-B.
Defined length string encoding defined in JSON-B.
Tag code definition defined in JSON-C. These may only appear before the beginning of an Object or Array and before any preceeding white space.
Tag code value defined in JSON-C.
Additional binary data encodings defined in JSON-D for use in scientific data applications.

The JSON grammar is modified to permit the use of x-value productions in place of ( value value-separator ) :

JSON-text = (object / array)
    object = *cdef begin-object [  
        *( member value-separator | x-member ) 
          (member | x-member) ] end-object

    member = tag value
    x-member = tag x-value

    tag = string name-separator | b-string | c-tag

    array = *cdef begin-array [  *( value value-separator | x-value ) 
        (value | x-value) ] end-array

    x-value = b-value / d-value 
    value = false / null / true / object / array / number / string  

    name-separator  = ws %x3A ws  ; : colon
    value-separator = ws %x2C ws  ; , comma

The following lexical values are unchanged:

    begin-array     = ws %x5B ws  ; [ left square bracket
    begin-object    = ws %x7B ws  ; { left curly bracket
    end-array       = ws %x5D ws  ; ] right square bracket
    end-object      = ws %x7D ws  ; } right curly bracket

    ws = *( %x20 %x09 %x0A  %x0D )
    false = %x66.61.6c.73.65   ; false
    null  = %x6e.75.6c.6c      ; null
    true  = %x74.72.75.65      ; true   

The productions number and string are defined as before:

    number = [ minus ] int [ frac ] [ exp ]
    decimal-point = %x2E       ; .
    digit1-9 = %x31-39         ; 1-9
    e = %x65 / %x45            ; e E
    exp = e [ minus / plus ] 1*DIGIT
    frac = decimal-point 1*DIGIT
    int = zero / ( digit1-9 *DIGIT )
    minus = %x2D               ; -
    plus = %x2B                ; +
    zero = %x30                ; 0
    string = quotation-mark *char quotation-mark
    char = unescaped /
           escape ( %x22 / %x5C / %x2F / %x62 / %x66 / 
                    %x6E / %x72 / %x74 /  %x75 4HEXDIG ) 

    escape = %x5C              ; \
    quotation-mark = %x22      ; "
    unescaped = %x20-21 / %x23-5B / %x5D-10FFFF


The JSON-B encoding defines the b-value and b-string productions:

    b-value = b-atom | b-string | b-data | b-integer | 
    b-string = *( string-chunk ) string-term
    b-data = *( data-chunk ) data-last        
    b-integer = p-int8 | p-int16 | p-int32 | p-int64 | p-bignum16 |
                n-int8 | n-int16 | n-int32 | n-int64 | n-bignum16

    b-float = binary64

The lexical encodings of the productions are defined in the following table where the column 'tag' specifies the byte code that begins the production, 'Fixed' specifies the number of data bytes that follow and 'Length' specifies the number of bytes used to define the length of a variable length field following the data bytes:

JSON-B Lexical Encodings
Production Tag Fixed Length Data Description
string-term x80 - 1 Terminal String 8 bit length
string-term x81 - 2 Terminal String 16 bit length
string-term x82 - 4 Terminal String 32 bit length
string-term x83 - 8 Terminal String 64 bit length
string-chunk x84 - 1 Non-Terminal String 8 bit length
string-chunk x85 - 2 Non-Terminal String 16 bit length
string-chunk x86 - 4 Non-Terminal String 32 bit length
string-chunk x87 - 8 Non-Terminal String 64 bit length
data-term x88 - 1 Terminal Data 8 bit length
data-term x89 - 2 Terminal Data 16 bit length
data-term x8A - 4 Terminal Data 32 bit length
data-term x8B - 8 Terminal Data 64 bit length
data-chunk x8C - 1 Non-Terminal Data 8 bit length
data-chunk x8D - 2 Non-Terminal Data 16 bit length
data-chunk x8E - 4 Non-Terminal Data 32 bit length
data-chunk x8F - 8 Non-Terminal String 64 bit length
p-int8 xA0 1 - Positive 8 bit Integer
p-int16 xA1 2 - Positive 16 bit Integer
p-int32 xA2 4 - Positive 32 bit Integer
p-int64 xA3 8 - Positive 64 bit Integer
p-bignum16 xA5 - 2 Positive Bignum 16 bit length
n-int8 xA8 1 - Negative 8 bit Integer
n-int16 xA9 2 - Negative 16 bit Integer
n-int32 xAA 4 - Negative 32 bit Integer
n-int64 xAB 8 - Negative 64 bit Integer
n-bignum16 xAD - 2 Negative Bignum 16 bit length
binary64 x92 8 - IEEE 754 Floating Point binary64
b-value xB0 - - True
b-value xB1 - - False
b-value xB2 - - Null

A data type commonly used in networking that is not defined in this scheme is a datetime representation.

4.1. JSON-B Examples

The following examples show examples of using JSON-B encoding:

            Binary Encoding                  JSON Equivalent

A0 2A                            42 (as 8 bit integer)
A1 00 2A                         42 (as 16 bit integer)
A2 00 00 00 2A                   42 (as 32 bit integer)
A3 00 00 00 00 00 00 00 2A       42 (as 64 bit integer)
A5 00 01 42                      42 (as Bignum)

80 05 48 65 6c 6c 6f             "Hello" (single chunk)
81 00 05 48 65 6c 6c 6f          "Hello" (single chunk)
84 05 48 65 6c 6c 6f 80 00       "Hello" (as two chunks)

92 3f f0 00 00 00 00 00 00       1.0
92 40 24 00 00 00 00 00 00       10.0
92 40 09 21 fb 54 44 2e ea       3.14159265359
92 bf f0 00 00 00 00 00 00       -1.0

B0                               true
B1                               false
B2                               null



JSON-C (Compressed) permits numeric code values to be substituted for strings and binary data. Tag codes MAY be 8, 16 or 32 bits long encoded in network byte order.

Tag codes MUST be defined before they are referenced. A Tag code MAY be defined before the corresponding data or string value is used or at the same time that it is used.

A dictionary is a list of tag code definitions. An encoding MAY incorporate definitions from a dictionary using the dict-hash production. The dict hash production specifies a (positive) offset value to be added to the entries in the dictionary and a hash code identifier consisting of the ASN.1 OID value sequence for the cryptographic digest used to compute the hash value followed by the hash value in network byte order.

JSON-C Lexical Encodings
Production Tag Fixed Length Data Description
c-tag xC0 1 - 8 bit tag code
c-tag xC1 2 - 16 bit tag code
c-tag xC2 4 - 32 bit tag code
c-def xC4 1 - 8 bit tag definition
c-def xC5 2 - 16 bit tag definition
c-def xC6 4 - 32 bit tag definition
c-tag xC8 1 - 8 bit tag code & definition
c-tag xC9 2 - 16 bit tag code & definition
c-tag xCA 4 - 32 bit tag code & definition
c-def xCC 1 - 8 bit tag dictionary definition
c-tag xCD 2 - 16 bit tag dictionary definition
c-tag xCE 4 - 32 bit tag dictionary definition
dict-hash xD0 4 1 Hash of dictionary

All integer values are encoded in Network Byte Order (most significant byte first).

5.1. JSON-C Examples

The following examples show examples of using JSON-C encoding:

JSON-C                           Value      Define

C8 20 80 05 48 65 6c 6c 6f       "Hello"    20 = "Hello"
C4 21 80 05 48 65 6c 6c 6f                  21 = "Hello"
C0 20                            "Hello"
C1 00 20                         "Hello"

D0 00 00 01 00 1B                           277 = "Hello"
   06 09 60 86 48 01 65 03 
   04 02 01                      OID for SHA-2-256
   e3 b0 c4 42 98 fc 1c 14 
   9a fb f4 c8 99 6f b9 24   
   27 ae 41 e4 64 9b 93 4c 
   a4 95 99 1b 78 52 b8 55       SHA-256(C4 21 80 05 48 65 6c 6c 6f)



6. JSON-D (Data)

JSON-B and JSON-C only support the two numeric types defined in the JavaScript data model: Integers and 64 bit floating point values. JSON-D (Data) defines binary encodings for additional data types that are commonly used in scientific applications. These comprise positive and negative 128 bit integers, six additional floating point representations defined by IEEE 754 [RFC2119] and the Intel extended precision 80 bit floating point representation.

Should the need arise, even bigger bignums could be defined with the length specified as a 32 bit value permitting bignums of up to 2^35 bits to be represented.

              d-value = d-integer | d-float

    d-float = binary16 | binary32 | binary128 | binary80 |
              decimal32 | decimal64 | decimal 128


JSON-D Lexical Encodings
Production Tag Fixed Length Data Description
p-int128 xA4 16 - Positive 128 bit Integer
n-in7128 xAC 16 - Negative 128 bit Integer
binary16 x90 2 - IEEE 754 Floating Point binary16
binary32 x91 4 - IEEE 754 Floating Point binary32
binary128 x94 16 - IEEE 754 Floating Point binary128
intel80 x95 10 - Intel 80 bit extended binary Floating Point
decimal32 x96 4 - IEEE 754 Floating Point decimal32
decimal64 x97 8 - IEEE 754 Floating Point decimal64
decimal128 x98 18 - IEEE 754 Floating Point decimal128

7. Acknowledgements

Nico Williams, etc

8. Security Considerations

9. IANA Considerations

[TBS list out all the code points that require an IANA registration]

10. Normative References

, "
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4627] Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[IEEE-754]Information technology -- Microprocessor Systems -- Floating-Point arithmetic", ISO/IEC/IEEE 60559:2011, July 2011.

Author's Address

Phillip Hallam-Baker Comodo Group Inc. EMail: