#json #parser #cddl

bin+lib cddl

Parser for the Concise data definition language (CDDL)

42 releases

0.9.0-beta.0 Sep 13, 2021
0.8.6 Jan 25, 2021
0.8.5 Dec 4, 2020
0.8.4 Nov 12, 2020
0.3.4 Jul 25, 2019

#44 in Parser implementations

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MIT license

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This crate was originally developed as a personal learning exercise for getting acquainted with Rust and parsing in general. There are likely more performant and stable libraries out there for parsing CDDL. While there are some examples of this crate being used in production, careful consideration should be made prior to using this crate as such.

A Rust implementation of the Concise data definition language (CDDL). CDDL is an IETF standard that "proposes a notational convention to express CBOR and JSON data structures." As of 2019-06-12, it is published as RFC 8610 (Proposed Standard) at https://tools.ietf.org/html/rfc8610.

This crate includes a handwritten parser and lexer for CDDL, and its development has been heavily inspired by the techniques outlined in Thorsten Ball's book "Writing An Interpretor In Go". The AST has been built to closely match the rules defined by the ABNF grammar in Appendix B. of the spec. All CDDL must use UTF-8 for its encoding per the spec.

This crate supports validation of both CBOR and JSON data structures. An extremely basic REPL is included as well. This crate's minimum supported Rust version (MSRV) is 1.46.0.

Also bundled into this repository is a basic language server implementation and extension for Visual Studio Code for editing CDDL. The implementation is backed by the compiled WebAssembly target included in this crate.


  • Parse CDDL documents into an AST
  • Verify conformance of CDDL documents against RFC 8610
  • Validate CBOR data structures
  • Validate JSON documents
  • Basic REPL
  • Generate dummy JSON from conformant CDDL
  • As close to zero-copy as possible
  • Compile WebAssembly target for browser and Node.js
  • no_std support (lexing and parsing only)
  • Language server implementation and Visual Studio Code Extension


  • Performance (if this crate gains enough traction, it may be prudent to conduct more formal profiling and/or explore using a parser-combinator framework like nom)
  • Support CBOR diagnostic notation
  • I-JSON compatibility

Why Rust?

Rust is a systems programming language designed around safety and is ideally-suited for resource-constrained systems. CDDL and CBOR are designed around small code and message sizes and constrained nodes, scenarios for which Rust has also been designed.


A CLI is available for various platforms. The tool supports parsing of .cddl files for verifying conformance against RFC 8610. It can also be used to validate .json documents and .cbor binary files against .cddl documents. Detailed information about the JSON and CBOR validation implementation can be found in the sections below.


GitHub Releases

Binaries for Linux, macOS and Windows can be downloaded from GitHub Releases.


cargo install cddl


docker pull ghcr.io/anweiss/cddl-cli:latest

CLI usage

Instructions for using the tool can be viewed by executing the help subcommand:

cddl help

If using Docker:

Replace <version> with an appropriate release tag. Requires use of the --volume argument for mounting .cddl documents into the container when executing the command. .json or .cbor files can either be included in the volume mount or passed into the command via STDIN.

docker run -it --rm -v $PWD:/cddl -w /cddl ghcr.io/anweiss/cddl-cli:<version> help

You can validate JSON documents:

cddl validate --cddl <FILE.cddl> [FILE.json]...

You can validate CBOR files:

cddl validate --cddl <FILE.cddl> [FILE.cbor]...

It also supports validating files from STDIN (if it detects the input as valid UTF-8, it will attempt to validate the input as JSON, otherwise it will treat it as CBOR):

cat reputon.json | cddl validate --cddl reputon.cddl --stdin
cat reputon.cbor | cddl validate --cddl reputon.cddl --stdin

or using Docker:

docker run -i --rm -v $PWD:/cddl -w /cddl ghcr.io/anweiss/cddl-cli:latest validate --cddl reputon.cddl --stdin < reputon.json


You can also find a simple RFC 8610 conformance tool at https://cddl.anweiss.tech. This same codebase has been compiled for use in the browser via WebAssembly.

Visual Studio Code extension

An extension for editing CDDL documents with Visual Studio Code has been published to the Marketplace here. You can find more information in the README.

Supported features

  • maps
    • structs
    • tables
    • cuts
  • groups
  • arrays
  • values
  • choices
  • ranges
  • enumeration (building a choice from a group)
  • root type
  • occurrence
  • predefined types
  • tags
  • unwrapping
  • controls
  • socket/plug
  • generics
  • operator precedence
  • comments
  • numerical int/uint values
  • numerical hexfloat values
  • numerical values with exponents
  • unprefixed byte strings
  • prefixed byte strings


Simply add the dependency to Cargo.toml:

cddl = "0.9.0-beta.0"

Both JSON and CBOR validation require std.

Feature flags

A few convenience features have been included to make the AST more concise and for enabling additional functionality. You can build with default-features = false for a no_std build and selectively enable any of the features below.

--feature ast-span

Add the Span type to the AST for keeping track of the position of the lexer and parser. Enabled by default.

--feature ast-comments

Include comment strings in the AST. Enabled by default.

--feature json

Enable JSON validation. Enabled by default.

--feature cbor

Enable CBOR validation. Enabled by default.

--feature additional-controls

Enable validation support for the additional control operators proposed in https://datatracker.ietf.org/doc/html/draft-ietf-cbor-cddl-control-05. Enabled by default.

Parsing CDDL

use cddl::{lexer_from_str, parser::cddl_from_str};

let input = r#"myrule = int"#;
assert!(cddl_from_str(&mut lexer_from_str(input), input, true).is_ok())

Validating JSON

use cddl::validate_json_from_str;

let cddl = r#"person = {
  name: tstr,
  age: uint,
  address: tstr,

let json = r#"{
  "name": "John",
  "age": 50,
  "address": "1234 Lakeshore Dr"

assert!(validate_json_from_str(cddl, json).is_ok())

This crate uses the Serde framework, and more specifically, the serde_json crate, for parsing and validating JSON. Serde was chosen due to its maturity in the ecosystem and its support for serializing and deserializing CBOR via the serde_cbor crate.

As outlined in Appendix E. of the standard, only the JSON data model subset of CBOR can be used for validation. The limited prelude from the spec has been included below for brevity:

any = #

uint = #0
nint = #1
int = uint / nint

tstr = #3
text = tstr

number = int / float

float16 = #7.25
float32 = #7.26
float64 = #7.27
float16-32 = float16 / float32
float32-64 = float32 / float64
float = float16-32 / float64

false = #7.20
true = #7.21
bool = false / true
nil = #7.22
null = nil

Furthermore, the following data types from the standard prelude can be used for validating JSON strings and numbers:

tdate = #6.0(tstr)
uri = #6.32(tstr)
b64url = #6.33(tstr)
time = #6.1(number)

The first non-group rule defined by a CDDL data structure definition determines the root type, which is subsequently used for validating the top-level JSON data type.

Supported JSON validation features

The following types and features of CDDL are supported by this crate for validating JSON:

structs objects
arrays arrays1
text / tstr string
uri string (valid RFC3986 URI)
tdate string (valid RFC3339 date/time)
b64url string (base64url-encoded)
time number (valid UNIX timestamp integer in seconds)
number / int / float number2
bool / true / false boolean
null / nil null
any any valid JSON
byte strings not yet implemented
unwrap (~) any JSON that matches unwrapped type from map, array or tag

CDDL groups, generics, sockets/plugs and group-to-choice enumerations can all be used when validating JSON.

Since JSON objects only support keys whose types are JSON strings, when validating JSON, member keys defined in CDDL structs must use either the colon syntax (mykey: tstr or "mykey": tstr) or the double arrow syntax provided that the member key is either a text string value ("mykey" => tstr) or a bareword that resolves to either a string data type (text or tstr) or another text string value (* tstr => any).

Occurrence indicators can be used to validate key/value pairs in a JSON object and the number of elements in a JSON array; depending on how the indicators are defined in a CDDL data definition.

Below is the table of supported control operators:

Control operator Supported
.pcre ✔️3
.regex ✔️3 (alias for .pcre)
.size ✔️
.bits Ignored when validating JSON
.cbor Ignored when validating JSON
.cborseq Ignored when validating JSON
.within ✔️
.and ✔️
.lt ✔️
.le ✔️
.gt ✔️
.ge ✔️
.eq ✔️
.ne ✔️
.default ✔️

1: When groups with multiple group entries are used to validate arrays, occurrence indicators are "greedy" in that only the first occurrence indicator that is come across is used in the validation. Subsequent entries with occurrence indicators are ignored due to complexities involved with processing these ambiguities. For proper JSON validation, avoid writing CDDL that looks like the following: [ * a: int, b: tstr, ? c: int ].

2: While JSON itself does not distinguish between integers and floating-point numbers, this crate does provide the ability to validate numbers against a more specific numerical CBOR type, provided that its equivalent representation is allowed by JSON. Refer to Appendix E. of the standard for more details on the implications of using CDDL with JSON numbers.

3: Due to Perl-Compatible Regular Expressions (PCREs) being more widely used than XSD regular expressions, this crate also provides support for the proposed .pcre control extension in place of the .regexp operator (see Discussion and CDDL-Freezer proposal). Ensure that your regex string is properly JSON escaped when using this control.

If you've enabled the additional-controls feature, the table of controls below is also available for use:

Control operator Supported
.plus ✔️
.cat ✔️
.det ✔️
.abnf ✔️
.abnfb Ignored when validating JSON
.feature ✔️

You can activate features during validation as follows:

use cddl::validate_json_from_str;

let cddl = r#"
  v = JC<"v", 2>
  JC<J, C> =  C .feature "cbor" / J .feature "json"

let json = r#""v""#;

assert!(validate_json_from_str(cddl, json, Some(&["json"])).is_ok())

Comparing with JSON schema and JSON schema language

CDDL, JSON schema and JSON schema language can all be used to define JSON data structures. However, the approaches taken to develop each of these are vastly different. A good place to find past discussions on the differences between these formats is the IETF mail archive, specifically in the JSON and CBOR lists. The purpose of this crate is not to argue for the use of CDDL over any one of these formats, but simply to provide an example implementation in Rust.

Validating CBOR

use cddl::validate_cbor_from_slice;

let cddl = r#"rule = false"#;

let cbor = b"\xF4";

assert!(validate_cbor_from_slice(cddl, cbor).is_ok())

This crate also uses Serde and serde_cbor for validating CBOR data structures. CBOR validation is done via the loosely typed serde_cbor::Value enum. In addition to all of the same features implemented by the JSON validator, this crate also supports validating CBOR tags (e.g. #6.32(tstr)), CBOR major types (e.g. #1.2), table types (e.g. { [ + tstr ] => int }) and byte strings. The .bits, .cbor and .cborseq control operators are all supported as well.

The following tags are supported when validating CBOR:

Tag Supported
tdate = #6.0(tstr) ✔️
time = #6.1(number) ✔️
biguint = #6.2(bstr) ✔️
bignint = #6.3(bstr) ✔️
decfrac = #6.4([e10: int, m: integer]) not yet implemented
bigfloat = #6.5([e2: int, m: integer]) not yet implemented
eb64url = #6.21(any) ✔️
eb64legacy = #6.22(any) ✔️
eb16 = #6.23(any) ✔️
encoded-cbor = #6.24(bstr) ✔️
uri = #6.32(tstr) ✔️
b64url = #6.33(tstr) ✔️
b64legacy = #6.34(tstr) ✔️
regexp = #6.35(tstr) ✔️
mime-message = #6.36(tstr) ✔️
cbor-any = #6.55799(any) ✔️

If you've enabled the additional-controls feature, the table of controls below is also available for use:

Control operator Supported
.plus ✔️
.cat ✔️
.det ✔️
.abnf ✔️
.abnfb ✔️
.feature ✔️

You can activate features during validation by passing a slice of feature strings as follows:

use cddl::validate_cbor_from_slice;

let cddl = r#"
  v = JC<"v", 2>
  JC<J, C> =  C .feature "cbor" / J .feature "json"

let cbor = b"\x02";

assert!(validate_cbor_from_slice(cddl, cbor, Some(&["cbor"])).is_ok())

no_std support

Only the lexer and parser can be used in a no_std context provided that a heap allocator is available. This can be enabled by opting out of the default features in your Cargo.toml file as follows:

cddl = { version = "0.9.0-beta.0", default-features = false }

Zero-copy parsing is implemented to the extent that is possible. Allocation is required for error handling and diagnostics.

Both JSON and CBOR validation are dependent on their respective heap allocated Value types, but since these types aren't supported in a no_std context, they subsequently aren't supported by this crate in no_std.

Projects using this crate

Below are some known projects that leverage this crate:


~81K SLoC