262 lines
11 KiB
Markdown
262 lines
11 KiB
Markdown
---
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no_site_title: true
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title: "Preserves: Binary Syntax"
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---
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Tony Garnock-Jones <tonyg@leastfixedpoint.com>
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{{ site.version_date }}. Version {{ site.version }}.
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[varint]: https://developers.google.com/protocol-buffers/docs/encoding#varints
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[LEB128]: https://en.wikipedia.org/wiki/LEB128
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[canonical]: canonical-binary.html
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*Preserves* is a data model, with associated serialization formats. This
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document defines one of those formats: a binary syntax for `Value`s from
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the [Preserves data model](preserves.html) that is easy for computer
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software to read and write. An [equivalent human-readable text
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syntax](preserves-text.html) also exists.
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## Machine-Oriented Binary Syntax
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A `Repr` is a binary-syntax encoding, or representation, of a `Value`.
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For a value `v`, we write `«v»` for the `Repr` of v.
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### Type and Length representation.
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Each `Repr` starts with a tag byte, describing the kind of information
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represented. Depending on the tag, a length indicator, further encoded
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information, and/or an ending tag may follow.
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tag (simple atomic data)
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tag ++ length ++ binarydata (doubles, integers, strings, symbols, and binary)
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tag ++ repr ++ ... ++ endtag (compound data)
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The unique end tag is byte value `0x84`.
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If present after a tag, the length of a following piece of binary data
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is formatted as a [base 128 varint][varint].[^see-also-leb128] We
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write `varint(m)` for the varint-encoding of `m`. Quoting the
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[Google Protocol Buffers][varint] definition,
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[^see-also-leb128]: Also known as [LEB128][] encoding, for unsigned
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integers. Varints and LEB128-encoded integers differ only for
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negative numbers, which cannot appear as length indicators and are
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thus not used in Preserves.
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> Each byte in a varint, except the last byte, has the most
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> significant bit (msb) set – this indicates that there are further
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> bytes to come. The lower 7 bits of each byte are used to store the
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> two's complement representation of the number in groups of 7 bits,
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> least significant group first.
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For example, `varint(15)` is `[0x0F]`, and `varint(1000000000)` is `[0x80, 0x94, 0xeb, 0xdc,
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0x03]`.
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It is an error for a varint-encoded `m` in a `Repr` to be anything
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other than the unique shortest encoding for that `m`. That is, a
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varint-encoding of `m` *MUST NOT* end in `0` unless `m`=0.
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### Records, Sequences, Sets and Dictionaries.
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«<L F_1...F_m>» = [0xB4] ++ «L» ++ «F_1» ++...++ «F_m» ++ [0x84]
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«[X_1...X_m]» = [0xB5] ++ «X_1» ++...++ «X_m» ++ [0x84]
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«#{E_1...E_m}» = [0xB6] ++ «E_1» ++...++ «E_m» ++ [0x84]
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«{K_1:V_1...K_m:V_m}» = [0xB7] ++ «K_1» ++ «V_1» ++...++ «K_m» ++ «V_m» ++ [0x84]
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There is *no* ordering requirement on the `E_i` elements or
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`K_i`/`V_i` pairs.[^no-sorting-rationale] They may appear in any
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order. However, the `E_i` and `K_i` *MUST* be pairwise distinct. In
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addition, implementations *SHOULD* default to writing set elements and
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dictionary key/value pairs in order sorted lexicographically by their
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`Repr`s[^not-sorted-semantically], and *MAY* offer the option of
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serializing in some other implementation-defined order.
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[^no-sorting-rationale]: In the BitTorrent encoding format,
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[bencoding](http://www.bittorrent.org/beps/bep_0003.html#bencoding),
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dictionary key/value pairs must be sorted by key. This is a
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necessary step for ensuring serialization of `Value`s is
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canonical. We encourage, but do not require that key/value pairs (or set
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elements) be in sorted order for serialized `Value`s; however, a
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[canonical form][canonical] for `Repr`s does exist where a sorted
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ordering is required.
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[^not-sorted-semantically]: It's important to note that the sort
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ordering for writing out set elements and dictionary key/value pairs
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is *not* the same as the sort ordering implied by the semantic
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ordering of those elements or keys. For example, the `Repr` of a
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negative number sorts lexicographically *after* the `Repr` of zero,
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despite being semantically *less than* zero.
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**Rationale**. This is for ease-of-implementation reasons: not all
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languages can easily represent sorted sets or sorted dictionaries,
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but encoding and then sorting byte strings is much more likely to
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be within easy reach.
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### SignedIntegers, Strings, ByteStrings and Symbols.
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«S» = [0xB0] ++ varint(|intbytes(S)|) ++ intbytes(S) if S ∈ SignedInteger
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[0xB1] ++ varint(|utf8(S)|) ++ utf8(S) if S ∈ String
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[0xB2] ++ varint(|S|) ++ S if S ∈ ByteString
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[0xB3] ++ varint(|utf8(S)|) ++ utf8(S) if S ∈ Symbol
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For `String` and `Symbol`, the data following the tag and length is a
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UTF-8 encoding of the `Value`. For `ByteString`, it is the raw data
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contained within the `Value` unmodified. For `SignedInteger`, it is
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the big-endian two's-complement binary representation of the number,
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taking exactly as many whole bytes as needed to unambiguously identify
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the value and its sign. `intbytes(0)` is special-cased to be the empty
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byte sequence;[^empty-intbytes-sequence] otherwise, the
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most-significant bit of the first byte is the sign bit. (See
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[SignedInteger examples](#signedinteger-examples) in the appendix
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below.)
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[^empty-intbytes-sequence]: Without the special case of
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`intbytes(0)` yielding the empty byte sequence, no input to
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`intbytes` would result in an empty sequence. In principle, either
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`0` or `-1` could be special-cased to the empty sequence; here, we
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arbitrarily choose `0`.
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### Booleans.
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«#f» = [0x80]
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«#t» = [0x81]
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### Doubles.
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«D» = [0x87, 0x08] ++ binary64(D) if D ∈ Double
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The function `binary64(D)` yields the big-endian 8-byte IEEE 754 binary
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representation of `D`.
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### Embeddeds.
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«#:V» = [0x86] ++ «V»
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The `Repr` of an `Embedded` is the `Repr` of a `Value` chosen to
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represent the denoted object, prefixed with `[0x86]`.
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### Annotations.
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«@W V» = [0x85] ++ «W» «V»
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Each annotation `W` precedes the `Value` `V` it
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annotates.[^annotation-syntax-rationale] Both `W` and `V` *MAY*
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themselves be further annotated. Implementations *SHOULD* default to
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omitting annotations from `Repr`s. See [examples in the
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appendix](#annotation-examples).
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[^annotation-syntax-rationale]: **Rationale.** The syntax for
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annotated values is unlike any other compound syntax defined here.
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It allows parsers to *statelessly* skip annotations: while `0x85` is
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the next input byte, skip it, skip a single `Repr`, and then try
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again; otherwise, parse the input stream as usual.
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By contrast, one might imagine using a parenthesized syntax like the syntax of
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compound data. In that case, parsers would have to remember
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information about nesting of annotations. This would not necessarily
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be an issue for recursive ("DOM"-like) parsers, but streaming
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("SAX"-like) parsers would have to either take on complexity
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internally or force it on their clients, even when those clients did
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not care about annotations.
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## Security Considerations
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**Annotations.** In modes where a `Value` is being read while
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annotations are skipped, an endless series of annotations may give an
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illusion of progress.
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**Canonical form for cryptographic hashing and signing.** No canonical
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*textual* encoding of a `Value` is specified. However, a [canonical
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form][canonical] exists for binary encoded `Value`s, and
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implementations *SHOULD* produce canonical binary encodings by
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default; however, an implementation *MAY* permit two serializations of
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the same `Value` to yield different binary `Repr`s.
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## Appendix. Autodetection of textual or binary syntax
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Every tag byte in a binary Preserves `Document` falls within the range
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[`0x80`, `0xBF`]. These bytes, interpreted as UTF-8, are *continuation
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bytes*, and will never occur as the first byte of a UTF-8 encoding. This
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means no binary-encoded document can be misinterpreted as valid UTF-8.
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Conversely, a UTF-8 document must start with a valid scalar value,
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meaning in particular that it must not start with a byte in the range
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[`0x80`, `0xBF`]. This means that no UTF-8 encoded textual-syntax
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Preserves document can be misinterpreted as a binary-syntax document.
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Examination of the top two bits of the first byte of a document gives
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its syntax: if the top two bits are `10`, it should be interpreted as
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a binary-syntax document; otherwise, it should be interpreted as text.
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## Appendix. Table of tag values
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80 - False
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81 - True
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84 - End marker
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85 - Annotation
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86 - Embedded
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87 - Double
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B0 - Integer
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B1 - String
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B2 - ByteString
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B3 - Symbol
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B4 - Record
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B5 - Sequence
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B6 - Set
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B7 - Dictionary
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All tags fall in the range [`0x80`, `0xBF`].
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Tag values `82`, `83`, `88`...`AF`, and `B8`...`BF` are **reserved**.
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## Appendix. Binary SignedInteger representation
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Languages that provide fixed-width machine word types may find the
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following table useful in encoding and decoding binary `SignedInteger`
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values.
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| Integer range | Bytes required | Encoding (hex) |
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| --- | --- | --- |
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| 0 | 2 | `B0` `00` |
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| -2<sup>7</sup> ≤ n < 2<sup>7</sup> (i8) | 3 | `B0` `01` `XX` |
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| -2<sup>15</sup> ≤ n < 2<sup>15</sup> (i16) | 4 | `B0` `02` `XX` `XX` |
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| -2<sup>23</sup> ≤ n < 2<sup>23</sup> (i24) | 5 | `B0` `03` `XX` `XX` `XX` |
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| -2<sup>31</sup> ≤ n < 2<sup>31</sup> (i32) | 6 | `B0` `04` `XX` `XX` `XX` `XX` |
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| -2<sup>39</sup> ≤ n < 2<sup>39</sup> (i40) | 7 | `B0` `05` `XX` `XX` `XX` `XX` `XX` |
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| -2<sup>47</sup> ≤ n < 2<sup>47</sup> (i48) | 8 | `B0` `06` `XX` `XX` `XX` `XX` `XX` `XX` |
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| -2<sup>55</sup> ≤ n < 2<sup>55</sup> (i56) | 9 | `B0` `07` `XX` `XX` `XX` `XX` `XX` `XX` `XX` |
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| -2<sup>63</sup> ≤ n < 2<sup>63</sup> (i64) | 10 | `B0` `08` `XX` `XX` `XX` `XX` `XX` `XX` `XX` `XX` |
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## Appendix. Examples
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### <a id="signedinteger-examples"></a>SignedInteger examples
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«-257» = B0 02 FE FF «-2» = B0 01 FE «255» = B0 02 00 FF
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«-256» = B0 02 FF 00 «-1» = B0 01 FF «256» = B0 02 01 00
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«-255» = B0 02 FF 01 «0» = B0 00 «32767» = B0 02 7F FF
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«-129» = B0 02 FF 7F «1» = B0 01 01 «32768» = B0 03 00 80 00
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«-128» = B0 01 80 «127» = B0 01 7F «65535» = B0 03 00 FF FF
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«-127» = B0 01 81 «128» = B0 02 00 80 «65536» = B0 03 01 00 00
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«87112285931760246646623899502532662132736»
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= B0 12 01 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00
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00 00
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### <a id="annotation-examples"></a>Annotation examples
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The `Repr` corresponding to textual syntax `@a@b[]`, i.e. an empty
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sequence annotated with two symbols, `a` and `b`, is
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«@a @b []» = [0x85] ++ «a» ++ [0x85] ++ «b» ++ «[]»
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= [0x85, 0xB3, 0x01, 0x61, 0x85, 0xB3, 0x01, 0x62, 0xB5, 0x84]
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Annotations may themselves be annotated. Here, `c` is annotated with
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`b`, which itself is annotated with `a`:
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«@ @a b c» = [0x85] ++ [0x85] ++ «a» ++ «b» ++ «c»>
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<!-- Heading to visually offset the footnotes from the main document: -->
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## Notes
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