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Tony Garnock-Jones tonyg@leastfixedpoint.com
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Preserves is a data model, with associated serialization formats. This
document defines one of those formats: a binary syntax for Values from
the Preserves data model that is easy for computer
software to read and write. An equivalent human-readable text
syntax also exists.
Machine-Oriented Binary Syntax
A Repr is a binary-syntax encoding, or representation, of a Value.
For a value v, we write «v» for the Repr of v.
Type and Length representation.
Each Repr starts with a tag byte, describing the kind of information
represented. Depending on the tag, a length indicator, further encoded
information, and/or an ending tag may follow.
tag (simple atomic data)
tag ++ length ++ binarydata (floats, doubles, integers, strings, symbols, and binary)
tag ++ repr ++ ... ++ endtag (compound data and annotations)
The unique end tag is byte value 0x84.
If present after a tag, the length of a following piece of binary data
is formatted as a base 128 varint.1 We
write varint(m) for the varint-encoding of m. Quoting the
Google Protocol Buffers definition,
Each byte in a varint, except the last byte, has the most significant bit (msb) set – this indicates that there are further bytes to come. The lower 7 bits of each byte are used to store the two's complement representation of the number in groups of 7 bits, least significant group first.
For example, varint(15) is [0x0F], and varint(1000000000) is [0x80, 0x94, 0xeb, 0xdc, 0x03].
It is an error for a varint-encoded m in a Repr to be anything
other than the unique shortest encoding for that m. That is, a
varint-encoding of m MUST NOT end in 0 unless m=0.
Records, Sequences, Sets and Dictionaries.
«<L F_1...F_m>» = [0xB4] ++ «L» ++ «F_1» ++...++ «F_m» ++ [0x84]
«[X_1...X_m]» = [0xB5] ++ «X_1» ++...++ «X_m» ++ [0x84]
«#{E_1...E_m}» = [0xB6] ++ «E_1» ++...++ «E_m» ++ [0x84]
«{K_1:V_1...K_m:V_m}» = [0xB7] ++ «K_1» ++ «V_1» ++...++ «K_m» ++ «V_m» ++ [0x84]
There is no ordering requirement on the E_i elements or
K_i/V_i pairs.2 They may appear in any
order. However, the E_i and K_i MUST be pairwise distinct. In
addition, implementations SHOULD default to writing set elements and
dictionary key/value pairs in order sorted lexicographically by their
Reprs3, and MAY offer the option of
serializing in some other implementation-defined order.
SignedIntegers.
«x» = [0xB0] ++ varint(|intbytes(x)|) ++ intbytes(x) if x ∈ SignedInteger
The function intbytes(x) gives the big-endian two's-complement
binary representation of x, taking exactly as many whole bytes as
needed to unambiguously identify the value and its sign. The value 0
needs zero bytes to identify the value; non-zero values need at least
one byte, and the most-significant bit in the first byte is the sign
bit. See the examples in the appendix below.
Strings, ByteStrings and Symbols.
«S» = [0xB1] ++ varint(|utf8(S)|) ++ utf8(S) if S ∈ String
[0xB2] ++ varint(|S|) ++ S if S ∈ ByteString
[0xB3] ++ varint(|utf8(S)|) ++ utf8(S) if S ∈ Symbol
Syntax for these three types varies only in the tag used. For String
and Symbol, the data following the tag is a UTF-8 encoding of the
Value, while for ByteString it is the raw data contained within the
Value unmodified.
Booleans.
«#f» = [0x80]
«#t» = [0x81]
Floats and Doubles.
«F» = [0x87, 0x04] ++ binary32(F) if F ∈ Float
«D» = [0x87, 0x08] ++ binary64(D) if D ∈ Double
The functions binary32(F) and binary64(D) yield big-endian 4- and
8-byte IEEE 754 binary representations of F and D, respectively.
Embeddeds.
«#!V» = [0x86] ++ «V»
The Repr of an Embedded is the Repr of a Value chosen to
represent the denoted object, prefixed with [0x86].
Annotations.
«@V_1...@V_n V» = [0xBF] ++ «V» ++ «V_1» ++...++ «V_n» ++ [0x84]
V MUST NOT itself be annotated, but each V_i in V_1...V_n
MAY be annotated. Furthermore, n MUST be greater than zero. For
example, the Repr corresponding to textual syntax @a@b[], i.e. an
empty sequence annotated with two symbols, a and b, is
«@a @b []» = [0xBF] ++ «[]» ++ «a» ++ «b» ++ [0x84]
= [0xBF, 0xB5, 0x84, 0xB3, 0x01, 0x61, 0xB3, 0x01, 0x62, 0x84]
Implementations SHOULD default to omitting annotations from binary Reprs.
Security Considerations
Annotations. In modes where a Value is being read while
annotations are skipped, an endless nesting of annotations may give an
illusion of progress.
Canonical form for cryptographic hashing and signing. No canonical
textual encoding of a Value is specified. However, a canonical
form exists for binary encoded Values, and
implementations SHOULD produce canonical binary encodings by
default; however, an implementation MAY permit two serializations of
the same Value to yield different binary Reprs.
Appendix. Autodetection of textual or binary syntax
Every tag byte in a binary Preserves Document falls within the range
[0x80, 0xBF]. These bytes, interpreted as UTF-8, are continuation
bytes, and will never occur as the first byte of a UTF-8 encoding. This
means no binary-encoded document can be misinterpreted as valid UTF-8.
Conversely, a UTF-8 document must start with a valid scalar value,
meaning in particular that it must not start with a byte in the range
[0x80, 0xBF]. This means that no UTF-8 encoded textual-syntax
Preserves document can be misinterpreted as a binary-syntax document.
Examination of the top two bits of the first byte of a document gives
its syntax: if the top two bits are 10, it should be interpreted as
a binary-syntax document; otherwise, it should be interpreted as text.
Appendix. Table of tag values
80 - False
81 - True
84 - End marker
86 - Embedded
87 - Float and Double
B0 - Integer
B1 - String
B2 - ByteString
B3 - Symbol
B4 - Record
B5 - Sequence
B6 - Set
B7 - Dictionary
BF - Annotated Repr (not itself starting with BF) followed by annotations
All tags fall in the range [0x80, 0xBF].
Tag values 82, 83, 85, 88...AF, and B8...BE are reserved.
Appendix. Binary SignedInteger representation
Languages that provide fixed-width machine word types may find the
following table useful in encoding and decoding binary SignedInteger
values.
| Integer range | Bytes required | Encoding (hex) |
|---|---|---|
| 0 | 2 | B0 00 |
| -27 ≤ n < 27 (i8) | 3 | B0 01 XX |
| -215 ≤ n < 215 (i16) | 4 | B0 02 XX XX |
| -223 ≤ n < 223 (i24) | 5 | B0 03 XX XX XX |
| -231 ≤ n < 231 (i32) | 6 | B0 04 XX XX XX XX |
| -239 ≤ n < 239 (i40) | 7 | B0 05 XX XX XX XX XX |
| -247 ≤ n < 247 (i48) | 8 | B0 06 XX XX XX XX XX XX |
| -255 ≤ n < 255 (i56) | 9 | B0 07 XX XX XX XX XX XX XX |
| -263 ≤ n < 263 (i64) | 10 | B0 08 XX XX XX XX XX XX XX XX |
Appendix. Binary SignedInteger examples
«-257» = B0 02 FE FF «-2» = B0 01 FE «255» = B0 02 00 FF
«-256» = B0 02 FF 00 «-1» = B0 01 FF «256» = B0 02 01 00
«-255» = B0 02 FF 01 «0» = B0 00 «32767» = B0 02 7F FF
«-129» = B0 02 FF 7F «1» = B0 01 01 «32768» = B0 03 00 80 00
«-128» = B0 01 80 «127» = B0 01 7F «65535» = B0 03 00 FF FF
«-127» = B0 01 81 «128» = B0 02 00 80 «65536» = B0 03 01 00 00
«87112285931760246646623899502532662132736»
= B0 12 01 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00
Notes
-
Also known as LEB128 encoding, for unsigned integers. Varints and LEB128-encoded integers differ only for negative numbers, which cannot appear as length indicators and are thus not used in Preserves. ↩︎
-
In the BitTorrent encoding format, bencoding, dictionary key/value pairs must be sorted by key. This is a necessary step for ensuring serialization of
Values is canonical. We encourage, but do not require that key/value pairs (or set elements) be in sorted order for serializedValues; however, a canonical form forReprs does exist where a sorted ordering is required. ↩︎ -
It's important to note that the sort ordering for writing out set elements and dictionary key/value pairs is not the same as the sort ordering implied by the semantic ordering of those elements or keys. For example, the
Reprof a negative number very far from zero will start with byte that is greater than the byte which starts theReprof zero, making it sort lexicographically later byRepr, despite being semantically less than zero.Rationale. This is for ease-of-implementation reasons: not all languages can easily represent sorted sets or sorted dictionaries, but encoding and then sorting byte strings is much more likely to be within easy reach. ↩︎