CBOR Codec

[fxamacker/cbor](https://github.com/fxamacker/cbor) is a library for encoding and decoding [CBOR](https://www.rfc-editor.org/info/std94) and [CBOR Sequences](https://www.rfc-editor.org/rfc/rfc8742.html). CBOR is a [trusted alternative](https://www.rfc-editor.org/rfc/rfc8949.html#name-comparison-of-other-binary-) to JSON, MessagePack, Protocol Buffers, etc. CBOR is an Internet Standard defined by [IETF STD 94 (RFC 8949)](https://www.rfc-editor.org/info/std94) and is designed to be relevant for decades. `fxamacker/cbor` is used in projects by Arm Ltd., EdgeX Foundry, Flow Foundation, Fraunhofer‑AISEC, IBM, Kubernetes[*](https://github.com/search?q=org%3Akubernetes%20fxamacker%2Fcbor&type=code), Let's Encrypt, Linux Foundation, Microsoft, Oasis Protocol, Red Hat[*](https://github.com/search?q=org%3Aopenshift+fxamacker%2Fcbor&type=code), Tailscale[*](https://github.com/search?q=org%3Atailscale+fxamacker%2Fcbor&type=code), Veraison[*](https://github.com/search?q=org%3Averaison+fxamacker%2Fcbor&type=code), [etc](https://github.com/fxamacker/cbor#who-uses-fxamackercbor). See [Quick Start](#quick-start) and [Releases](https://github.com/fxamacker/cbor/releases/). 🆕 `UnmarshalFirst` and `DiagnoseFirst` can decode CBOR Sequences. `MarshalToBuffer` and `UserBufferEncMode` accepts user-specified buffer. ## fxamacker/cbor [![](https://github.com/fxamacker/cbor/workflows/ci/badge.svg)](https://github.com/fxamacker/cbor/actions?query=workflow%3Aci) [![](https://github.com/fxamacker/cbor/workflows/cover%20%E2%89%A597%25/badge.svg)](https://github.com/fxamacker/cbor/actions?query=workflow%3A%22cover+%E2%89%A597%25%22) [![CodeQL](https://github.com/fxamacker/cbor/actions/workflows/codeql-analysis.yml/badge.svg)](https://github.com/fxamacker/cbor/actions/workflows/codeql-analysis.yml) [![](https://img.shields.io/badge/fuzzing-passing-44c010)](#fuzzing-and-code-coverage) [![Go Report Card](https://goreportcard.com/badge/github.com/fxamacker/cbor)](https://goreportcard.com/report/github.com/fxamacker/cbor) [![](https://img.shields.io/ossf-scorecard/github.com/fxamacker/cbor?label=openssf%20scorecard)](https://github.com/fxamacker/cbor#fuzzing-and-code-coverage) `fxamacker/cbor` is a CBOR codec in full conformance with [IETF STD 94 (RFC 8949)](https://www.rfc-editor.org/info/std94). It also supports CBOR Sequences ([RFC 8742](https://www.rfc-editor.org/rfc/rfc8742.html)) and Extended Diagnostic Notation ([Appendix G of RFC 8610](https://www.rfc-editor.org/rfc/rfc8610.html#appendix-G)). Features include full support for CBOR tags, [Core Deterministic Encoding](https://www.rfc-editor.org/rfc/rfc8949.html#name-core-deterministic-encoding), duplicate map key detection, etc. API is mostly same as `encoding/json`, plus interfaces that simplify concurrency and CBOR options. Design balances trade-offs between security, speed, concurrency, encoded data size, usability, etc.

🔎 Highlights

__🚀 Speed__ Encoding and decoding is fast without using Go's `unsafe` package. Slower settings are opt-in. Default limits allow very fast and memory efficient rejection of malformed CBOR data. __🔒 Security__ Decoder has configurable limits that defend against malicious inputs. Duplicate map key detection is supported. By contrast, `encoding/gob` is [not designed to be hardened against adversarial inputs](https://pkg.go.dev/encoding/gob#hdr-Security). Codec passed multiple confidential security assessments in 2022. No vulnerabilities found in subset of codec in a [nonconfidential security assessment](https://github.com/veraison/go-cose/blob/v1.0.0-rc.1/reports/NCC_Microsoft-go-cose-Report_2022-05-26_v1.0.pdf) prepared by NCC Group for Microsoft Corporation. __🗜️ Data Size__ Struct tag options (`toarray`, `keyasint`, `omitempty`, `omitzero`) and field tag "-" automatically reduce size of encoded structs. Encoding optionally shrinks float64→32→16 when values fit. __:jigsaw: Usability__ API is mostly same as `encoding/json` plus interfaces that simplify concurrency for CBOR options. Encoding and decoding modes can be created at startup and reused by any goroutines. Presets include Core Deterministic Encoding, Preferred Serialization, CTAP2 Canonical CBOR, etc. __📆 Extensibility__ Features include CBOR [extension points](https://www.rfc-editor.org/rfc/rfc8949.html#section-7.1) (e.g. CBOR tags) and extensive settings. API has interfaces that allow users to create custom encoding and decoding without modifying this library.

### Secure Decoding with Configurable Settings `fxamacker/cbor` has configurable limits, etc. that defend against malicious CBOR data. Notably, `fxamacker/cbor` is fast at rejecting malformed CBOR data. > [!NOTE] > Benchmarks rejecting 10 bytes of malicious CBOR data decoding to `[]byte`: > > | Codec | Speed (ns/op) | Memory | Allocs | > | :---- | ------------: | -----: | -----: | > | fxamacker/cbor 2.7.0 | 47 ± 7% | 32 B/op | 2 allocs/op | > | ugorji/go 1.2.12 | 5878187 ± 3% | 67111556 B/op | 13 allocs/op | > > Faster hardware (overclocked DDR4 or DDR5) can reduce speed difference. > >

🔎 Benchmark details

> > Latest comparison for decoding CBOR data to Go `[]byte`: > - Input: `[]byte{0x9B, 0x00, 0x00, 0x42, 0xFA, 0x42, 0xFA, 0x42, 0xFA, 0x42}` > - go1.22.7, linux/amd64, i5-13600K (DDR4-2933, disabled e-cores) > - go test -bench=. -benchmem -count=20 > > #### Prior comparisons > > | Codec | Speed (ns/op) | Memory | Allocs | > | :---- | ------------: | -----: | -----: | > | fxamacker/cbor 2.5.0-beta2 | 44.33 ± 2% | 32 B/op | 2 allocs/op | > | fxamacker/cbor 0.1.0 - 2.4.0 | ~44.68 ± 6% | 32 B/op | 2 allocs/op | > | ugorji/go 1.2.10 | 5524792.50 ± 3% | 67110491 B/op | 12 allocs/op | > | ugorji/go 1.1.0 - 1.2.6 | 💥 runtime: | out of memory: | cannot allocate | > > - Input: `[]byte{0x9B, 0x00, 0x00, 0x42, 0xFA, 0x42, 0xFA, 0x42, 0xFA, 0x42}` > - go1.19.6, linux/amd64, i5-13600K (DDR4) > - go test -bench=. -benchmem -count=20 > >

In contrast, some codecs can crash or use excessive resources while decoding bad data. > [!WARNING] > Go's `encoding/gob` is [not designed to be hardened against adversarial inputs](https://pkg.go.dev/encoding/gob#hdr-Security). > >

🔎 gob fatal error (out of memory) 💥 decoding 181 bytes

> > ```Go > // Example of encoding/gob having "fatal error: runtime: out of memory" > // while decoding 181 bytes (all Go versions as of Dec. 8, 2024). > 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.") > } > ``` > > >

### Smaller Encodings with Struct Tag Options Struct tags automatically reduce encoded size of structs and improve speed. We can write less code by using struct tag options: - `toarray`: encode without field names (decode back to original struct) - `keyasint`: encode field names as integers (decode back to original struct) - `omitempty`: omit empty field when encoding - `omitzero`: omit zero-value field when encoding As a special case, struct field tag "-" omits the field. NOTE: When a struct uses `toarray`, the encoder will ignore `omitempty` and `omitzero` to prevent position of encoded array elements from changing. This allows decoder to match encoded elements to their Go struct field. ![alt text](https://github.com/fxamacker/images/raw/master/cbor/v2.3.0/cbor_struct_tags_api.svg?sanitize=1 "CBOR API and Go Struct Tags") > [!NOTE] > `fxamacker/cbor` can encode a 3-level nested Go struct to 1 byte! > - `encoding/json`: 18 bytes of JSON > - `fxamacker/cbor`: 1 byte of CBOR > >

🔎 Encoding 3-level nested Go struct with omitempty

> > https://go.dev/play/p/YxwvfPdFQG2 > > ```Go > // 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() > } > ``` > > Output (DN is Diagnostic Notation): > ``` > hex(CBOR): a0 > DN: {} > ------------- > hex(JSON): 7b22466f6f223a7b22517578223a7b7d7d7d > JSON: {"Foo":{"Qux":{}}} > ``` > >

## Quick Start __Install__: `go get github.com/fxamacker/cbor/v2` and `import "github.com/fxamacker/cbor/v2"`. > [!TIP] > > Tinygo users can try beta/experimental branch [feature/cbor-tinygo-beta](https://github.com/fxamacker/cbor/tree/feature/cbor-tinygo-beta). > >

🔎 More about tinygo feature branch

> > ### Tinygo > > Branch [feature/cbor-tinygo-beta](https://github.com/fxamacker/cbor/tree/feature/cbor-tinygo-beta) is based on fxamacker/cbor v2.7.0 and it can be compiled using tinygo v0.33 (also compiles with golang/go). > > It passes unit tests (with both go1.22 and tinygo v0.33) and is considered beta/experimental for tinygo. > > :warning: The `feature/cbor-tinygo-beta` branch does not get fuzz tested yet. > > Changes in this feature branch only affect tinygo compiled software. Summary of changes: > - default `DecOptions.MaxNestedLevels` is reduced to 16 (was 32). User can specify higher limit but 24+ crashes tests when compiled with tinygo v0.33. > - disabled decoding CBOR tag data to Go interface because tinygo v0.33 is missing needed feature. > - encoding error message can be different when encoding function type. > > Related tinygo issues: > - https://github.com/tinygo-org/tinygo/issues/4277 > - https://github.com/tinygo-org/tinygo/issues/4458 > >

### Key Points 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. ### Default Mode Package level functions only use this library's default settings. They provide the "default mode" of encoding and decoding. ```go // 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 // v2.7.0 added MarshalToBuffer() and UserBufferEncMode interface. err = cbor.MarshalToBuffer(v, b) // encode v to b instead of using built-in buf pool. // 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 // UnmarshalFirst() and DiagnoseFirst() allow trailing bytes. ``` > [!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 Presets can be used as-is or as a starting point for custom settings. ```go // 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. ### 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. ```Go // 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. ### User Specified Buffer for Encoding (v2.7.0) `UserBufferEncMode` interface extends `EncMode` interface to add `MarshalToBuffer()`. It accepts a user-specified buffer instead of using built-in buffer pool. ```Go em, err := myEncOptions.UserBufferEncMode() // create UserBufferEncMode mode var buf bytes.Buffer err = em.MarshalToBuffer(v, &buf) // encode v to provided buf ``` ### Struct Tags Struct tag options (`toarray`, `keyasint`, `omitempty`, `omitzero`) reduce encoded size of structs. As a special case, struct field tag "-" omits the field.

🔎 Example encoding with struct field tag "-"

https://go.dev/play/p/aWEIFxd7InX ```Go // https://github.com/fxamacker/cbor/issues/652 package main import ( "encoding/json" "fmt" "github.com/fxamacker/cbor/v2" ) // The `cbor:"-"` tag omits the Type field when encoding to CBOR. type Entity struct { _ struct{} `cbor:",toarray"` ID uint64 `json:"id"` Type string `cbor:"-" json:"typeOf"` Name string `json:"name"` } func main() { entity := Entity{ ID: 1, Type: "int64", Name: "Identifier", } c, _ := cbor.Marshal(entity) diag, _ := cbor.Diagnose(c) fmt.Printf("CBOR in hex: %x\n", c) fmt.Printf("CBOR in edn: %s\n", diag) j, _ := json.Marshal(entity) fmt.Printf("JSON: %s\n", string(j)) fmt.Printf("JSON encoding is %d bytes\n", len(j)) fmt.Printf("CBOR encoding is %d bytes\n", len(c)) // Output: // CBOR in hex: 82016a4964656e746966696572 // CBOR in edn: [1, "Identifier"] // JSON: {"id":1,"typeOf":"int64","name":"Identifier"} // JSON encoding is 45 bytes // CBOR encoding is 13 bytes } ```

🔎 Example encoding 3-level nested Go struct to 1 byte CBOR

https://go.dev/play/p/YxwvfPdFQG2 ```Go // 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() } ``` Output (DN is Diagnostic Notation): ``` hex(CBOR): a0 DN: {} ------------- hex(JSON): 7b22466f6f223a7b22517578223a7b7d7d7d JSON: {"Foo":{"Qux":{}}} ```

🔎 Example using struct tag options

![alt text](https://github.com/fxamacker/images/raw/master/cbor/v2.3.0/cbor_struct_tags_api.svg?sanitize=1 "CBOR API and Go Struct Tags")

Struct tag options simplify use of CBOR-based protocols that require CBOR arrays or maps with integer keys. ### CBOR Tags CBOR tags are specified in a `TagSet`. Custom modes can be created with a `TagSet` to handle CBOR tags. ```go 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`.

🔎 Example using TagSet and TagOptions

```go // 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 := em.Marshal(v); err != nil { return err } ```

👉 `fxamacker/cbor` allows user apps to use almost any current or future CBOR tag number by implementing `cbor.Marshaler` and `cbor.Unmarshaler` interfaces. Basically, `MarshalCBOR` and `UnmarshalCBOR` functions can be implemented by user apps and those functions will automatically be called by this CBOR codec's `Marshal`, `Unmarshal`, etc. The following [example](https://github.com/fxamacker/cbor/blob/master/example_embedded_json_tag_for_cbor_test.go) shows how to encode and decode a tagged CBOR data item with tag number 262. The tag content is a JSON object "embedded" as a CBOR byte string (major type 2).

🔎 Example using Embedded JSON Tag for CBOR (tag 262)

```go // https://github.com/fxamacker/cbor/issues/657 package cbor_test // NOTE: RFC 8949 does not mention tag number 262. IANA assigned // CBOR tag number 262 as "Embedded JSON Object" specified by the // document Embedded JSON Tag for CBOR: // // "Tag 262 can be applied to a byte string (major type 2) to indicate // that the byte string is a JSON Object. The length of the byte string // indicates the content." // // For more info, see Embedded JSON Tag for CBOR at: // https://github.com/toravir/CBOR-Tag-Specs/blob/master/embeddedJSON.md import ( "bytes" "encoding/json" "fmt" "github.com/fxamacker/cbor/v2" ) // cborTagNumForEmbeddedJSON is the CBOR tag number 262. const cborTagNumForEmbeddedJSON = 262 // EmbeddedJSON represents a Go value to be encoded as a tagged CBOR data item // with tag number 262 and the tag content is a JSON object "embedded" as a // CBOR byte string (major type 2). type EmbeddedJSON struct { any } func NewEmbeddedJSON(val any) EmbeddedJSON { return EmbeddedJSON{val} } // MarshalCBOR encodes EmbeddedJSON to a tagged CBOR data item with the // tag number 262 and the tag content is a JSON object that is // "embedded" as a CBOR byte string. func (v EmbeddedJSON) MarshalCBOR() ([]byte, error) { // Encode v to JSON object. data, err := json.Marshal(v) if err != nil { return nil, err } // Create cbor.Tag representing a tagged CBOR data item. tag := cbor.Tag{ Number: cborTagNumForEmbeddedJSON, Content: data, } // Marshal to a tagged CBOR data item. return cbor.Marshal(tag) } // UnmarshalCBOR decodes a tagged CBOR data item to EmbeddedJSON. // The byte slice provided to this function must contain a single // tagged CBOR data item with the tag number 262 and tag content // must be a JSON object "embedded" as a CBOR byte string. func (v *EmbeddedJSON) UnmarshalCBOR(b []byte) error { // Unmarshal tagged CBOR data item. var tag cbor.Tag if err := cbor.Unmarshal(b, &tag); err != nil { return err } // Check tag number. if tag.Number != cborTagNumForEmbeddedJSON { return fmt.Errorf("got tag number %d, expect tag number %d", tag.Number, cborTagNumForEmbeddedJSON) } // Check tag content. jsonData, isByteString := tag.Content.([]byte) if !isByteString { return fmt.Errorf("got tag content type %T, expect tag content []byte", tag.Content) } // Unmarshal JSON object. return json.Unmarshal(jsonData, v) } // MarshalJSON encodes EmbeddedJSON to a JSON object. func (v EmbeddedJSON) MarshalJSON() ([]byte, error) { return json.Marshal(v.any) } // UnmarshalJSON decodes a JSON object. func (v *EmbeddedJSON) UnmarshalJSON(b []byte) error { dec := json.NewDecoder(bytes.NewReader(b)) dec.UseNumber() return dec.Decode(&v.any) } func Example_embeddedJSONTagForCBOR() { value := NewEmbeddedJSON(map[string]any{ "name": "gopher", "id": json.Number("42"), }) data, err := cbor.Marshal(value) if err != nil { panic(err) } fmt.Printf("cbor: %x\n", data) var v EmbeddedJSON err = cbor.Unmarshal(data, &v) if err != nil { panic(err) } fmt.Printf("%+v\n", v.any) for k, v := range v.any.(map[string]any) { fmt.Printf(" %s: %v (%T)\n", k, v, v) } } ```

### Functions and Interfaces

🔎 Functions and interfaces at a glance

Common functions with same API as `encoding/json`: - `Marshal`, `Unmarshal` - `NewEncoder`, `(*Encoder).Encode` - `NewDecoder`, `(*Decoder).Decode` NOTE: `Unmarshal` will return `ExtraneousDataError` if there are remaining bytes because RFC 8949 treats CBOR data item with remaining bytes as malformed. - 💡 Use `UnmarshalFirst` to decode first CBOR data item and return any remaining bytes. Other useful functions: - `Diagnose`, `DiagnoseFirst` produce human-readable [Extended Diagnostic Notation](https://www.rfc-editor.org/rfc/rfc8610.html#appendix-G) from CBOR data. - `UnmarshalFirst` decodes first CBOR data item and return any remaining bytes. - `Wellformed` returns true if the CBOR data item is well-formed. Interfaces identical or comparable to Go `encoding` packages include: `Marshaler`, `Unmarshaler`, `BinaryMarshaler`, and `BinaryUnmarshaler`. The `RawMessage` type can be used to delay CBOR decoding or precompute CBOR encoding.

### Security Tips 🔒 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`. ## Status v2.9.1 (Mar 29-30, 2026) includes important bugfixes, defensive checks, improved code quality, and more tests. Although not public, the fuzzer was also improved by adding more fuzz tests. v2.9.1 passed fuzz tests and is production quality. The minimum version of Go required to build: - v2.8.0 and newer releases require go 1.20+. - v2.7.1 and older releases require go 1.17+. For more details, see [v2.9.1 release notes](https://github.com/fxamacker/cbor/releases). ### Prior Releases [v2.9.0](https://github.com/fxamacker/cbor/releases/tag/v2.9.0) (Jul 13, 2025) improved interoperability/transcoding between CBOR & JSON, refactored tests, and improved docs. It passed fuzz tests (billions of executions) and is production quality. [v2.8.0](https://github.com/fxamacker/cbor/releases/tag/v2.8.0) (March 30, 2025) is a small release primarily to add `omitzero` option to struct field tags and fix bugs. It passed fuzz tests (billions of executions) and is production quality. [v2.7.0](https://github.com/fxamacker/cbor/releases/tag/v2.7.0) (June 23, 2024) adds features and improvements that help large projects (e.g. Kubernetes) use CBOR as an alternative to JSON and Protocol Buffers. Other improvements include speedups, improved memory use, bug fixes, new serialization options, etc. It passed fuzz tests (5+ billion executions) and is production quality. [v2.6.0](https://github.com/fxamacker/cbor/releases/tag/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. [v2.5.0](https://github.com/fxamacker/cbor/releases/tag/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](https://github.com/fxamacker/cbor/releases/tag/v2.5.0-beta) (Dec 2022) -> [v2.5.0](https://github.com/fxamacker/cbor/releases/tag/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](https://github.com/fxamacker/cbor/releases/tag/v2.5.0) for list of new features, improvements, and bug fixes. See ["Version and API Changes"](https://github.com/fxamacker/cbor#versions-and-api-changes) section for more info about version numbering, etc. ## Who uses fxamacker/cbor `fxamacker/cbor` is used in projects by Arm Ltd., Berlin Institute of Health at Charité, Chainlink, Confidential Computing Consortium, ConsenSys, EdgeX Foundry, F5, Flow Foundation, Fraunhofer‑AISEC, IBM, Kubernetes, Let's Encrypt (ISRG), Linaro, Linux Foundation, Matrix.org, Microsoft, National Cybersecurity Agency of France (govt), Netherlands (govt), Oasis Protocol, Red Hat OpenShift, Smallstep, Tailscale, Taurus SA, TIBCO, Veraison, and others. `fxamacker/cbor` passed multiple confidential security assessments in 2022. A [nonconfidential security assessment](https://github.com/veraison/go-cose/blob/v1.0.0-rc.1/reports/NCC_Microsoft-go-cose-Report_2022-05-26_v1.0.pdf) (prepared by NCC Group for Microsoft Corporation) assessed a subset of fxamacker/cbor v2.4. ## Standards `fxamacker/cbor` is a CBOR codec in full conformance with [IETF STD 94 (RFC 8949)](https://www.rfc-editor.org/info/std94). It also supports CBOR Sequences ([RFC 8742](https://www.rfc-editor.org/rfc/rfc8742.html)) and Extended Diagnostic Notation ([Appendix G of RFC 8610](https://www.rfc-editor.org/rfc/rfc8610.html#appendix-G)). Notable CBOR features include: | CBOR Feature | Description | | :--- | :--- | | CBOR tags | API supports built-in and user-defined tags. | | Preferred serialization | Integers encode to fewest bytes. Optional float64 → float32 → float16. | | Map key sorting | Unsorted, length-first (Canonical CBOR), and bytewise-lexicographic (CTAP2). | | Duplicate map keys | Always forbid for encoding and option to allow/forbid for decoding. | | Indefinite length data | Option to allow/forbid for encoding and decoding. | | Well-formedness | Always checked and enforced. | | Basic validity checks | Optionally check UTF-8 validity and duplicate map keys. | | Security considerations | Prevent integer overflow and resource exhaustion (RFC 8949 Section 10). | Known limitations are noted in the [Limitations section](#limitations). 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:__

🔎 Duplicate Map Keys

This library provides options for fast detection and rejection of duplicate map keys based on applying a Go-specific data model to CBOR's extended generic data model in order to determine duplicate vs distinct map keys. Detection relies on whether the CBOR map key would be a duplicate "key" when decoded and applied to the user-provided Go map or struct. `DupMapKeyQuiet` turns off detection of duplicate map keys. It tries to use a "keep fastest" method by choosing either "keep first" or "keep last" depending on the Go data type. `DupMapKeyEnforcedAPF` enforces detection and rejection of duplidate map keys. Decoding stops immediately and returns `DupMapKeyError` when the first duplicate key is detected. The error includes the duplicate map key and the index number. APF suffix means "Allow Partial Fill" so the destination map or struct can contain some decoded values at the time of error. It is the caller's responsibility to respond to the `DupMapKeyError` by discarding the partially filled result if that's required by their protocol.

🔎 Tag Validity

This library checks tag validity for built-in tags (currently tag numbers 0, 1, 2, 3, and 55799): * Inadmissible type for tag content * Inadmissible value for tag content Unknown tag data items (not tag number 0, 1, 2, 3, or 55799) are handled in two ways: * When decoding into an empty interface, unknown tag data item will be decoded into `cbor.Tag` data type, which contains tag number and tag content. The tag content will be decoded into the default Go data type for the CBOR data type. * When decoding into other Go types, unknown tag data item is decoded into the specified Go type. If Go type is registered with a tag number, the tag number can optionally be verified. Decoder also has an option to forbid tag data items (treat any tag data item as error) which is specified by protocols such as CTAP2 Canonical CBOR. For more information, see [decoding options](#decoding-options-1) and [tag options](#tag-options).

## Limitations 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. ## Fuzzing and Code Coverage __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.

## Versions and API Changes This project uses [Semantic Versioning](https://semver.org), 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](https://github.com/fxamacker/cbor/releases/tag/v2.5.0-beta) (Dec 2022) -> [v2.5.0](https://github.com/fxamacker/cbor/releases/tag/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. ## Code of Conduct This project has adopted the [Contributor Covenant Code of Conduct](CODE_OF_CONDUCT.md). Contact [faye.github@gmail.com](mailto:faye.github@gmail.com) with any questions or comments. ## Contributing 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](CONTRIBUTING.md). ## Security Policy Security fixes are provided for the latest released version of fxamacker/cbor. For the full text of the Security Policy, see [SECURITY.md](SECURITY.md). ## Acknowledgements 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. Big thanks to Ben Luddy for his contributions in v2.6.0 and v2.7.0. 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](https://github.com/cbor-wg/CBORbis). 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. ## License Copyright © 2019-2024 [Faye Amacker](https://github.com/fxamacker). fxamacker/cbor is licensed under the MIT License. See [LICENSE](LICENSE) for the full license text.