fxamacker/cbor is a library for encoding and decoding CBOR and CBOR Sequences.

CBOR is a trusted alternative to JSON, MessagePack, Protocol Buffers, etc.  CBOR is an Internet Standard defined by IETF STD 94 (RFC 8949) and is designed to be relevant for decades.

fxamacker/cbor is used in projects by Arm Ltd., Cisco, Dapper Labs, EdgeX Foundry, Fraunhofer‑AISEC, Let's Encrypt (ISRG), Linux Foundation, Microsoft, Mozilla, Oasis Protocol, Tailscale, Teleport, and others.

See Quick Start and Releases. 🆕 UnmarshalFirst and DiagnoseFirst can decode CBOR Sequences.

fxamacker/cbor is a CBOR codec in full conformance with IETF STD 94 (RFC 8949). It also supports CBOR Sequences (RFC 8742) and Extended Diagnostic Notation (Appendix G of RFC 8610).

Features include full support for CBOR tags, Core Deterministic Encoding, duplicate map key detection, etc.

Design balances trade-offs between security, speed, concurrency, encoded data size, usability, etc.

fxamacker/cbor has configurable limits, etc. that defend against malicious CBOR data.

By contrast, encoding/gob is not designed to be hardened against adversarial inputs.

// Example of encoding/gob having "fatal error: runtime: out of memory"
// while decoding 181 bytes.
package main
import (
	"bytes"
	"encoding/gob"
	"encoding/hex"
	"fmt"
)

// Example data is from https://github.com/golang/go/issues/24446
// (shortened to 181 bytes).
const data = "4dffb503010102303001ff30000109010130010800010130010800010130" +
	"01ffb80001014a01ffb60001014b01ff860001013001ff860001013001ff" +
	"860001013001ff860001013001ffb80000001eff850401010e3030303030" +
	"30303030303030303001ff3000010c0104000016ffb70201010830303030" +
	"3030303001ff3000010c000030ffb6040405fcff00303030303030303030" +
	"303030303030303030303030303030303030303030303030303030303030" +
	"30"

type X struct {
	J *X
	K map[string]int
}

func main() {
	raw, _ := hex.DecodeString(data)
	decoder := gob.NewDecoder(bytes.NewReader(raw))

	var x X
	decoder.Decode(&x) // fatal error: runtime: out of memory
	fmt.Println("Decoding finished.")
}

fxamacker/cbor is fast at rejecting malformed CBOR data. E.g. attempts to
decode 10 bytes of malicious CBOR data to []byte (with default settings):

Struct tags (toarray, keyasint, omitempty) reduce encoded size of structs.

// Example encoding nested struct (with omitempty tag)
// - encoding/json:  18 byte JSON
// - fxamacker/cbor:  1 byte CBOR
package main

import (
	"encoding/hex"
	"encoding/json"
	"fmt"

	"github.com/fxamacker/cbor/v2"
)

type GrandChild struct {
	Quux int `json:",omitempty"`
}

type Child struct {
	Baz int        `json:",omitempty"`
	Qux GrandChild `json:",omitempty"`
}

type Parent struct {
	Foo Child `json:",omitempty"`
	Bar int   `json:",omitempty"`
}

func cb() {
	results, _ := cbor.Marshal(Parent{})
	fmt.Println("hex(CBOR): " + hex.EncodeToString(results))

	text, _ := cbor.Diagnose(results) // Diagnostic Notation
	fmt.Println("DN: " + text)
}

func js() {
	results, _ := json.Marshal(Parent{})
	fmt.Println("hex(JSON): " + hex.EncodeToString(results))

	text := string(results) // JSON
	fmt.Println("JSON: " + text)
}

func main() {
	cb()
	fmt.Println("-------------")
	js()
}

hex(CBOR): a0
DN: {}
-------------
hex(JSON): 7b22466f6f223a7b22517578223a7b7d7d7d
JSON: {"Foo":{"Qux":{}}}

Example using different struct tags together:

API is mostly same as encoding/json, plus interfaces that simplify concurrency for CBOR options.

Install: go get github.com/fxamacker/cbor/v2 and import "github.com/fxamacker/cbor/v2".

This library can encode and decode CBOR (RFC 8949) and CBOR Sequences (RFC 8742).

• CBOR data item is a single piece of CBOR data and its structure may contain 0 or more nested data items.
• CBOR sequence is a concatenation of 0 or more encoded CBOR data items.

Configurable limits and options can be used to balance trade-offs.

• Encoding and decoding modes are created from options (settings).
• Modes can be created at startup and reused.
• Modes are safe for concurrent use.

Package level functions only use this library's default settings.
They provide the "default mode" of encoding and decoding.

// API matches encoding/json for Marshal, Unmarshal, Encode, Decode, etc.
b, err = cbor.Marshal(v)        // encode v to []byte b
err = cbor.Unmarshal(b, &v)     // decode []byte b to v
decoder = cbor.NewDecoder(r)    // create decoder with io.Reader r
err = decoder.Decode(&v)        // decode a CBOR data item to v

// v2.5.0 added new functions that return remaining bytes.

// UnmarshalFirst decodes first CBOR data item and returns remaining bytes.
rest, err = cbor.UnmarshalFirst(b, &v)   // decode []byte b to v

// DiagnoseFirst translates first CBOR data item to text and returns remaining bytes.
text, rest, err = cbor.DiagnoseFirst(b)  // decode []byte b to Diagnostic Notation text

// NOTE: Unmarshal returns ExtraneousDataError if there are remaining bytes,
// but new funcs UnmarshalFirst and DiagnoseFirst do not.

IMPORTANT: 👉 CBOR settings allow trade-offs between speed, security, encoding size, etc.

• Different CBOR libraries may use different default settings.
• CBOR-based formats or protocols usually require specific settings.

For example, WebAuthn uses "CTAP2 Canonical CBOR" which is available as a preset.

Presets can be used as-is or as a starting point for custom settings.

// EncOptions is a struct of encoder settings.
func CoreDetEncOptions() EncOptions              // RFC 8949 Core Deterministic Encoding
func PreferredUnsortedEncOptions() EncOptions    // RFC 8949 Preferred Serialization
func CanonicalEncOptions() EncOptions            // RFC 7049 Canonical CBOR
func CTAP2EncOptions() EncOptions                // FIDO2 CTAP2 Canonical CBOR

Presets are used to create custom modes.

Modes are created from settings. Once created, modes have immutable settings.

💡 Create the mode at startup and reuse it. It is safe for concurrent use.

// Create encoding mode.
opts := cbor.CoreDetEncOptions()   // use preset options as a starting point
opts.Time = cbor.TimeUnix          // change any settings if needed
em, err := opts.EncMode()          // create an immutable encoding mode

// Reuse the encoding mode. It is safe for concurrent use.

// API matches encoding/json.
b, err := em.Marshal(v)            // encode v to []byte b
encoder := em.NewEncoder(w)        // create encoder with io.Writer w
err := encoder.Encode(v)           // encode v to io.Writer w

Default mode and custom modes automatically apply struct tags.

Struct tags (toarray, keyasint, omitempty) reduce encoded size of structs.

// Example encoding nested struct (with omitempty tag)
// - encoding/json:  18 byte JSON
// - fxamacker/cbor:  1 byte CBOR
package main

import (
	"encoding/hex"
	"encoding/json"
	"fmt"

	"github.com/fxamacker/cbor/v2"
)

type GrandChild struct {
	Quux int `json:",omitempty"`
}

type Child struct {
	Baz int        `json:",omitempty"`
	Qux GrandChild `json:",omitempty"`
}

type Parent struct {
	Foo Child `json:",omitempty"`
	Bar int   `json:",omitempty"`
}

func cb() {
	results, _ := cbor.Marshal(Parent{})
	fmt.Println("hex(CBOR): " + hex.EncodeToString(results))

	text, _ := cbor.Diagnose(results) // Diagnostic Notation
	fmt.Println("DN: " + text)
}

func js() {
	results, _ := json.Marshal(Parent{})
	fmt.Println("hex(JSON): " + hex.EncodeToString(results))

	text := string(results) // JSON
	fmt.Println("JSON: " + text)
}

func main() {
	cb()
	fmt.Println("-------------")
	js()
}

hex(CBOR): a0
DN: {}
-------------
hex(JSON): 7b22466f6f223a7b22517578223a7b7d7d7d
JSON: {"Foo":{"Qux":{}}}

Struct tags simplify use of CBOR-based protocols that require CBOR arrays or maps with integer keys.

CBOR tags are specified in a TagSet.

Custom modes can be created with a TagSet to handle CBOR tags.

em, err := opts.EncMode()                  // no CBOR tags
em, err := opts.EncModeWithTags(ts)        // immutable CBOR tags
em, err := opts.EncModeWithSharedTags(ts)  // mutable shared CBOR tags

TagSet and modes using it are safe for concurrent use. Equivalent API is available for DecMode.

// Use signedCWT struct defined in "Decoding CWT" example.

// Create TagSet (safe for concurrency).
tags := cbor.NewTagSet()
// Register tag COSE_Sign1 18 with signedCWT type.
tags.Add(	
	cbor.TagOptions{EncTag: cbor.EncTagRequired, DecTag: cbor.DecTagRequired}, 
	reflect.TypeOf(signedCWT{}), 
	18)

// Create DecMode with immutable tags.
dm, _ := cbor.DecOptions{}.DecModeWithTags(tags)

// Unmarshal to signedCWT with tag support.
var v signedCWT
if err := dm.Unmarshal(data, &v); err != nil {
	return err
}

// Create EncMode with immutable tags.
em, _ := cbor.EncOptions{}.EncModeWithTags(tags)

// Marshal signedCWT with tag number.
if data, err := cbor.Marshal(v); err != nil {
	return err
}

🔒 Use Go's io.LimitReader to limit size when decoding very large or indefinite size data.

Default limits may need to be increased for systems handling very large data (e.g. blockchains).

DecOptions can be used to modify default limits for MaxArrayElements, MaxMapPairs, and MaxNestedLevels.

v2.6.0 (February 2024) adds important new features, optimizations, and bug fixes. It is especially useful to systems that need to convert data between CBOR and JSON. New options and optimizations improve handling of bignum, integers, maps, and strings.

For more details, see release notes.

v2.5.0 was released on Sunday, August 13, 2023 with new features and important bug fixes. It is fuzz tested and production quality after extended beta v2.5.0-beta (Dec 2022) -> v2.5.0 (Aug 2023).

IMPORTANT: 👉 Before upgrading from v2.4 or older release, please read the notable changes highlighted in the release notes. v2.5.0 is a large release with bug fixes to error handling for extraneous data in Unmarshal, etc. that should be reviewed before upgrading.

See v2.5.0 release notes for list of new features, improvements, and bug fixes.

See "Version and API Changes" section for more info about version numbering, etc.

fxamacker/cbor is used in projects by Arm Ltd., Berlin Institute of Health at Charité, Chainlink, Cisco, Confidential Computing Consortium, ConsenSys, Dapper Labs, EdgeX Foundry, F5, FIDO Alliance, Fraunhofer‑AISEC, Let's Encrypt (ISRG), Linux Foundation, Matrix.org, Microsoft, Mozilla, National Cybersecurity Agency of France (govt), Netherlands (govt), Oasis Protocol, Smallstep, Tailscale, Taurus SA, Teleport, TIBCO, and others.

fxamacker/cbor passed multiple confidential security assessments. A nonconfidential security assessment (prepared by NCC Group for Microsoft Corporation) includes a subset of fxamacker/cbor v2.4.0 in its scope.

fxamacker/cbor is a CBOR codec in full conformance with IETF STD 94 (RFC 8949). It also supports CBOR Sequences (RFC 8742) and Extended Diagnostic Notation (Appendix G of RFC 8610).

Notable CBOR features include:

Known limitations are noted in the Limitations section.

Go nil values for slices, maps, pointers, etc. are encoded as CBOR null. Empty slices, maps, etc. are encoded as empty CBOR arrays and maps.

Decoder checks for all required well-formedness errors, including all "subkinds" of syntax errors and too little data.

After well-formedness is verified, basic validity errors are handled as follows:

• Invalid UTF-8 string: Decoder has option to check and return invalid UTF-8 string error. This check is enabled by default.
• Duplicate keys in a map: Decoder has options to ignore or enforce rejection of duplicate map keys.

When decoding well-formed CBOR arrays and maps, decoder saves the first error it encounters and continues with the next item. Options to handle this differently may be added in the future.

By default, decoder treats time values of floating-point NaN and Infinity as if they are CBOR Null or CBOR Undefined.

Click to expand topic:

If any of these limitations prevent you from using this library, please open an issue along with a link to your project.

• CBOR Undefined (0xf7) value decodes to Go's nil value. CBOR Null (0xf6) more closely matches Go's nil.
• CBOR map keys with data types not supported by Go for map keys are ignored and an error is returned after continuing to decode remaining items.
• When decoding registered CBOR tag data to interface type, decoder creates a pointer to registered Go type matching CBOR tag number. Requiring a pointer for this is a Go limitation.

Code coverage is always 95% or higher (with go test -cover) when tagging a release.

Coverage-guided fuzzing must pass billions of execs using before tagging a release. Fuzzing is done using nonpublic code which may eventually get merged into this project. Until then, reports like OpenSSF Scorecard can't detect fuzz tests being used by this project.

This project uses Semantic Versioning, so the API is always backwards compatible unless the major version number changes.

These functions have signatures identical to encoding/json and their API will continue to match encoding/json even after major new releases:
Marshal, Unmarshal, NewEncoder, NewDecoder, (*Encoder).Encode, and (*Decoder).Decode.

Exclusions from SemVer:

• Newly added API documented as "subject to change".
• Newly added API in the master branch that has never been tagged in non-beta release.
• If function parameters are unchanged, bug fixes that change behavior (e.g. return error for edge case was missed in prior version). We try to highlight these in the release notes and add extended beta period. E.g. v2.5.0-beta (Dec 2022) -> v2.5.0 (Aug 2023).

This project avoids breaking changes to behavior of encoding and decoding functions unless required to improve conformance with supported RFCs (e.g. RFC 8949, RFC 8742, etc.) Visible changes that don't improve conformance to standards are typically made available as new opt-in settings or new functions.

This project has adopted the Contributor Covenant Code of Conduct. Contact [email protected] with any questions or comments.

Please open an issue before beginning work on a PR. The improvement may have already been considered, etc.

For more info, see How to Contribute.

Security fixes are provided for the latest released version of fxamacker/cbor.

For the full text of the Security Policy, see SECURITY.md.

Many thanks to all the contributors on this project!

I'm especially grateful to Bastian Müller and Dieter Shirley for suggesting and collaborating on CBOR stream mode, and much more.

I'm very grateful to Stefan Tatschner, Yawning Angel, Jernej Kos, x448, ZenGround0, and Jakob Borg for their contributions or support in the very early days.

This library clearly wouldn't be possible without Carsten Bormann authoring CBOR RFCs.

Special thanks to Laurence Lundblade and Jeffrey Yasskin for their help on IETF mailing list or at 7049bis.

Huge thanks to The Go Authors for creating a fun and practical programming language with batteries included!

This library uses x448/float16 which used to be included. As a standalone package, x448/float16 is useful to other projects as well.

Copyright © 2019-2024 Faye Amacker.

fxamacker/cbor is licensed under the MIT License. See LICENSE for the full license text.