gomail/internal/webauthn/cbor.go

package webauthn

import (
	"encoding/binary"
	"fmt"
)

// cborDecode is a minimal CBOR (RFC 8949) decoder covering only the subset
// WebAuthn actually uses: unsigned/negative integers, byte strings, text
// strings, arrays, maps, and the true/false/null simple values —
// definite-length items only (WebAuthn's attestationObject/COSE keys never
// use CBOR's indefinite-length encoding). This is not a general-purpose
// CBOR library, the same way internal/dnsutil is not a general DNS library.
//
// All integers (major types 0 and 1) decode to Go int64 — COSE key labels
// mix small positive (kty=1, alg=3) and negative (crv=-1, x=-2, y=-3)
// values, and normalizing to one Go type avoids uint64-vs-int64 mismatches
// when looking values up in a decoded map.
func cborDecode(data []byte) (any, error) {
	d := &cborDecoder{data: data}
	return d.decodeValue()
}

// cborDecodeWithLength is cborDecode plus how many bytes were consumed —
// needed when a CBOR item (a COSE key) is embedded inside a larger binary
// structure (authData) followed by more data, not the whole buffer.
func cborDecodeWithLength(data []byte) (any, int, error) {
	d := &cborDecoder{data: data}
	v, err := d.decodeValue()
	if err != nil {
		return nil, 0, err
	}
	return v, d.pos, nil
}

type cborDecoder struct {
	data []byte
	pos  int
}

func (d *cborDecoder) readByte() (byte, error) {
	if d.pos >= len(d.data) {
		return 0, fmt.Errorf("cbor: unexpected end of data")
	}
	b := d.data[d.pos]
	d.pos++
	return b, nil
}

func (d *cborDecoder) readBytes(n int) ([]byte, error) {
	if n < 0 || d.pos+n > len(d.data) {
		return nil, fmt.Errorf("cbor: unexpected end of data (need %d bytes, have %d)", n, len(d.data)-d.pos)
	}
	b := d.data[d.pos : d.pos+n]
	d.pos += n
	return b, nil
}

// readLength reads the additional-info length/value encoding shared by
// every major type: 0-23 is a literal value, 24/25/26/27 mean 1/2/4/8
// following bytes hold it. Indefinite length (additional info 31) is
// rejected — not used by anything this package parses.
func (d *cborDecoder) readLength(additionalInfo byte) (uint64, error) {
	switch {
	case additionalInfo < 24:
		return uint64(additionalInfo), nil
	case additionalInfo == 24:
		b, err := d.readByte()
		return uint64(b), err
	case additionalInfo == 25:
		b, err := d.readBytes(2)
		if err != nil {
			return 0, err
		}
		return uint64(binary.BigEndian.Uint16(b)), nil
	case additionalInfo == 26:
		b, err := d.readBytes(4)
		if err != nil {
			return 0, err
		}
		return uint64(binary.BigEndian.Uint32(b)), nil
	case additionalInfo == 27:
		b, err := d.readBytes(8)
		if err != nil {
			return 0, err
		}
		return binary.BigEndian.Uint64(b), nil
	default:
		return 0, fmt.Errorf("cbor: indefinite-length items are not supported (additional info %d)", additionalInfo)
	}
}

func (d *cborDecoder) decodeValue() (any, error) {
	head, err := d.readByte()
	if err != nil {
		return nil, err
	}
	majorType := head >> 5
	additionalInfo := head & 0x1F

	switch majorType {
	case 0: // unsigned integer
		n, err := d.readLength(additionalInfo)
		if err != nil {
			return nil, err
		}
		return int64(n), nil
	case 1: // negative integer: value = -1 - n
		n, err := d.readLength(additionalInfo)
		if err != nil {
			return nil, err
		}
		return -1 - int64(n), nil
	case 2: // byte string
		n, err := d.readLength(additionalInfo)
		if err != nil {
			return nil, err
		}
		return d.readBytes(int(n))
	case 3: // text string
		n, err := d.readLength(additionalInfo)
		if err != nil {
			return nil, err
		}
		b, err := d.readBytes(int(n))
		if err != nil {
			return nil, err
		}
		return string(b), nil
	case 4: // array
		n, err := d.readLength(additionalInfo)
		if err != nil {
			return nil, err
		}
		arr := make([]any, n)
		for i := range arr {
			v, err := d.decodeValue()
			if err != nil {
				return nil, err
			}
			arr[i] = v
		}
		return arr, nil
	case 5: // map
		n, err := d.readLength(additionalInfo)
		if err != nil {
			return nil, err
		}
		m := make(map[any]any, n)
		for i := uint64(0); i < n; i++ {
			k, err := d.decodeValue()
			if err != nil {
				return nil, err
			}
			v, err := d.decodeValue()
			if err != nil {
				return nil, err
			}
			m[k] = v
		}
		return m, nil
	case 7: // simple values: only false/true/null are meaningful here
		switch additionalInfo {
		case 20:
			return false, nil
		case 21:
			return true, nil
		case 22, 23:
			return nil, nil
		default:
			return nil, fmt.Errorf("cbor: unsupported simple/float value (additional info %d)", additionalInfo)
		}
	default:
		return nil, fmt.Errorf("cbor: unsupported major type %d", majorType)
	}
}