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Binary Canonical Serialization (BCS)

BCS (formerly "Libra Canonical Serialization" or LCS) is a serialization format developed in the context of the Diem blockchain.

BCS was designed with the following main goals in mind:

• provide good performance and concise (binary) representations;
• support a rich set of data types commonly used in Rust;
• enforce canonical serialization, meaning that every value of a given type should have a single valid representation.

BCS also aims to mitigate the consequence of malicious inputs by enforcing well-defined limits on large or nested containers during (de)serialization.

Rust Implementation

This crate provides a Rust implementation of BCS as an encoding format for the Serde library. As such, this implementation covers most data types supported by Serde -- including user-defined structs, tagged variants (Rust enums), tuples, and maps -- excluding floats, single unicode characters (char), and sets.

BCS is also available in other programming languages, thanks to the separate project serde-reflection.

Application to Cryptography

The BCS format guarantees canonical serialization, meaning that for any given data type, there is a one-to-one correspondance between in-memory values and valid byte representations.

In the context of a cryptographic application, canonical serialization has several benefits:

• It provides a natural and reliable way to associate in-memory values to cryptographic hashes.
• It allows the signature of a message to be defined equivalently as the signature of the serialized bytes or as the signature of the in-memory value.

Note that BCS ensures canonical serialization for each data type separately. The data type of a serialized value must be enforced by the application itself. This requirement is typically fulfilled using unique hash seeds for each data type. (See Diem's cryptographic library for an example.)

Backwards Compatibility

By design, BCS does not provide implicit versioning or backwards/forwards compatibility, therefore applications must carefully plan in advance for adhoc extension points:

• Enums may be used for explicit versioning and backward compatibility (e.g. extensible query interfaces).
• In some cases, data fields of type Vec<u8> may also be added to allow (future) unknown payloads in serialized form.

Detailed Specifications

BCS supports the following data types:

• Booleans
• Signed 8-bit, 16-bit, 32-bit, 64-bit, and 128-bit integers
• Unsigned 8-bit, 16-bit, 32-bit, 64-bit, and 128-bit integers
• Option
• Unit (an empty value)
• Fixed and variable length sequences
• UTF-8 Encoded Strings
• Tuples
• Structures (aka "structs")
• Externally tagged enumerations (aka "enums")
• Maps

BCS is not a self-describing format. As such, in order to deserialize a message, one must know the message type and layout ahead of time.

Unless specified, all numbers are stored in little endian, two's complement format.

Recursion and Depth of BCS Data

Recursive data-structures (e.g. trees) are allowed. However, because of the possibility of stack overflow during (de)serialization, the container depth of any valid BCS data cannot exceed the constant MAX_CONTAINER_DEPTH. Formally, we define container depth as the number of structs and enums traversed during (de)serialization.

This definition aims to minimize the number of operations while ensuring that (de)serialization of a known BCS format cannot cause arbitrarily large stack allocations.

As an example, if v1 and v2 are values of depth n1 and n2,

• a struct value Foo { v1, v2 } has depth 1 + max(n1, n2);
• an enum value E::Foo { v1, v2 } has depth 1 + max(n1, n2);
• a pair (v1, v2) has depth max(n1, n2);
• the value Some(v1) has depth n1.

All string and integer values have depths 0.

Booleans and Integers

ULEB128-Encoded Integers

The BCS format also uses the ULEB128 encoding internally to represent unsigned 32-bit integers in two cases where small values are usually expected: (1) lengths of variable-length sequences and (2) tags of enum values (see the corresponding sections below).

In general, a ULEB128 encoding consists of a little-endian sequence of base-128 (7-bit) digits. Each digit is completed into a byte by setting the highest bit to 1, except for the last (highest-significance) digit whose highest bit is set to 0.

In BCS, the result of decoding ULEB128 bytes is required to fit into a 32-bit unsigned integer and be in canonical form. For instance, the following values are rejected:

• 80 80 80 80 80 01 (2^36) is too large.
• 80 80 80 80 10 (2^33) is too large.
• 80 00 is not a minimal encoding of 0.

Optional Data

Optional or nullable data either exists in its full representation or does not. BCS represents this as a single byte representing the presence 0x01 or absence 0x00 of data. If the data is present then the serialized form of that data follows. For example:

let some_data: Option<u8> = Some(8);
assert_eq!(to_bytes(&some_data)?, vec![1, 8]);

let no_data: Option<u8> = None;
assert_eq!(to_bytes(&no_data)?, vec![0]);

Fixed and Variable Length Sequences

Sequences can be made of up of any BCS supported types (even complex structures) but all elements in the sequence must be of the same type. If the length of a sequence is fixed and well known then BCS represents this as just the concatenation of the serialized form of each individual element in the sequence. If the length of the sequence can be variable, then the serialized sequence is length prefixed with a ULEB128-encoded unsigned integer indicating the number of elements in the sequence. All variable length sequences must be MAX_SEQUENCE_LENGTH elements long or less.

let fixed: [u16; 3] = [1, 2, 3];
assert_eq!(to_bytes(&fixed)?, vec![1, 0, 2, 0, 3, 0]);

let variable: Vec<u16> = vec![1, 2];
assert_eq!(to_bytes(&variable)?, vec![2, 1, 0, 2, 0]);

let large_variable_length: Vec<()> = vec![(); 9_487];
assert_eq!(to_bytes(&large_variable_length)?, vec![0x8f, 0x4a]);

Strings

Only valid UTF-8 Strings are supported. BCS serializes such strings as a variable length byte sequence, i.e. length prefixed with a ULEB128-encoded unsigned integer followed by the byte representation of the string.

// Note that this string has 10 characters but has a byte length of 24
let utf8_str = "çå∞≠¢õß∂ƒ∫";
let expecting = vec![
    24, 0xc3, 0xa7, 0xc3, 0xa5, 0xe2, 0x88, 0x9e, 0xe2, 0x89, 0xa0, 0xc2,
    0xa2, 0xc3, 0xb5, 0xc3, 0x9f, 0xe2, 0x88, 0x82, 0xc6, 0x92, 0xe2, 0x88, 0xab,
];
assert_eq!(to_bytes(&utf8_str)?, expecting);

Tuples

Tuples are typed composition of objects: (Type0, Type1)

Tuples are considered a fixed length sequence where each element in the sequence can be a different type supported by BCS. Each element of a tuple is serialized in the order it is defined within the tuple, i.e. [tuple.0, tuple.2].

let tuple = (-1i8, "diem");
let expecting = vec![0xFF, 4, b'd', b'i', b'e', b'm'];
assert_eq!(to_bytes(&tuple)?, expecting);

Structures

Structures are fixed length sequences consisting of fields with potentially different types. Each field within a struct is serialized in the order specified by the canonical structure definition. Structs can exist within other structs and as such, BCS recurses into each struct and serializes them in order. There are no labels in the serialized format, the struct ordering defines the organization within the serialization stream.

#[derive(Serialize)]
struct MyStruct {
    boolean: bool,
    bytes: Vec<u8>,
    label: String,
}

#[derive(Serialize)]
struct Wrapper {
    inner: MyStruct,
    name: String,
}

let s = MyStruct {
    boolean: true,
    bytes: vec![0xC0, 0xDE],
    label: "a".to_owned(),
};
let s_bytes = to_bytes(&s)?;
let mut expecting = vec![1, 2, 0xC0, 0xDE, 1, b'a'];
assert_eq!(s_bytes, expecting);

let w = Wrapper {
    inner: s,
    name: "b".to_owned(),
};
let w_bytes = to_bytes(&w)?;
assert!(w_bytes.starts_with(&s_bytes));

expecting.append(&mut vec![1, b'b']);
assert_eq!(w_bytes, expecting);

Externally Tagged Enumerations

An enumeration is typically represented as a type that can take one of potentially many different variants. In BCS, each variant is mapped to a variant index, a ULEB128-encoded 32-bit unsigned integer, followed by serialized data if the type has an associated value. An associated type can be any BCS supported type. The variant index is determined based on the ordering of the variants in the canonical enum definition, where the first variant has an index of 0, the second an index of 1, etc.

#[derive(Serialize)]
enum E {
    Variant0(u16),
    Variant1(u8),
    Variant2(String),
}

let v0 = E::Variant0(8000);
let v1 = E::Variant1(255);
let v2 = E::Variant2("e".to_owned());

assert_eq!(to_bytes(&v0)?, vec![0, 0x40, 0x1F]);
assert_eq!(to_bytes(&v1)?, vec![1, 0xFF]);
assert_eq!(to_bytes(&v2)?, vec![2, 1, b'e']);

If you need to serialize a C-style enum, you should use a primitive integer type.

Maps (Key / Value Stores)

Maps are represented as a variable-length, sorted sequence of (Key, Value) tuples. Keys must be unique and the tuples sorted by increasing lexicographical order on the BCS bytes of each key. The representation is otherwise similar to that of a variable-length sequence. In particular, it is preceded by the number of tuples, encoded in ULEB128.

let mut map = HashMap::new();
map.insert(b'e', b'f');
map.insert(b'a', b'b');
map.insert(b'c', b'd');

let expecting = vec![(b'a', b'b'), (b'c', b'd'), (b'e', b'f')];

assert_eq!(to_bytes(&map)?, to_bytes(&expecting)?);

Contributing

See the CONTRIBUTING file for how to help out.

License

This project is available under the terms of either the Apache 2.0 license.