pixa/internal/encurl/encurl.go

// Package encurl provides encrypted URL generation and parsing for pixa.
package encurl

import (
	"errors"
	"time"

	"github.com/fxamacker/cbor/v2"

	"sneak.berlin/go/pixa/internal/imagerequest"
	"sneak.berlin/go/pixa/internal/seal"
)

// Default values for optional fields.
const (
	DefaultQuality = 85
	DefaultFormat  = imagerequest.FormatJXL
	DefaultFitMode = imagerequest.FitCover

	// HKDF salt for URL encryption key derivation
	urlKeySalt = "pixa-urlenc-v1"
)

// Errors returned by encurl operations.
var (
	ErrExpired       = errors.New("encrypted URL has expired")
	ErrInvalidFormat = errors.New("invalid encrypted URL format")
	ErrDecryptFailed = errors.New("failed to decrypt URL")
)

// payload contains all image request parameters in a compact format.
// Field names are short to minimize encoded size.
// Fields with omitempty use sane defaults when absent.
type payload struct {
	SourceHost  string              `cbor:"h"`            // required
	SourcePath  string              `cbor:"p"`            // required
	SourceQuery string              `cbor:"q,omitempty"`  // optional
	Width       int                 `cbor:"w,omitempty"`  // 0 = original
	Height      int                 `cbor:"ht,omitempty"` // 0 = original
	Format      imagerequest.Format `cbor:"f,omitempty"`  // default: jxl
	Quality     int                 `cbor:"ql,omitempty"` // default: 85
	FitMode     imagerequest.Fit    `cbor:"fm,omitempty"` // default: cover
	ExpiresAt   int64               `cbor:"e,omitempty"`  // 0 = never expires
}

// Generator creates and parses encrypted URL tokens.
type Generator struct {
	key [seal.KeySize]byte
}

// NewGenerator creates an encrypted URL generator with a key derived
// from the signing key.
func NewGenerator(signingKey string) (*Generator, error) {
	key, err := seal.DeriveKey([]byte(signingKey), urlKeySalt)
	if err != nil {
		return nil, err
	}

	return &Generator{key: key}, nil
}

// Generate creates an encrypted URL token from the request's source,
// transform and expiry, each as the request holds it; an empty format, a
// quality of 0 or an empty fit mode is left out of the token, and a zero
// Expires makes a token that never expires. The token is CBOR-encoded,
// encrypted with NaCl secretbox, and base64url-encoded.
func (g *Generator) Generate(req *imagerequest.Request) (string, error) {
	p := &payload{
		SourceHost:  req.Source.Host,
		SourcePath:  req.Source.Path,
		SourceQuery: req.Source.Query,
		Width:       req.Transform.Size.Width,
		Height:      req.Transform.Size.Height,
		Format:      req.Transform.Format,
		Quality:     req.Transform.Quality,
		FitMode:     req.Transform.FitMode,
	}

	if !req.Expires.IsZero() {
		p.ExpiresAt = req.Expires.Unix()
	}

	// CBOR encode the payload
	data, err := cbor.Marshal(p)
	if err != nil {
		return "", err
	}

	// Encrypt and base64url encode
	return seal.Encrypt(g.key, data)
}

// Parse decrypts and validates an encrypted URL token and returns the
// request it holds, with default values for omitted optional fields. A token
// that never expires gives the zero Expires.
// Returns ErrExpired if the token has expired, or other errors for invalid tokens.
func (g *Generator) Parse(token string) (*imagerequest.Request, error) {
	// Decrypt
	data, err := seal.Decrypt(g.key, token)
	if err != nil {
		if errors.Is(err, seal.ErrDecryptionFailed) ||
			errors.Is(err, seal.ErrInvalidPayload) {
			return nil, ErrDecryptFailed
		}

		return nil, err
	}

	// CBOR decode
	var p payload

	err = cbor.Unmarshal(data, &p)
	if err != nil {
		return nil, ErrInvalidFormat
	}

	// Check expiration (0 = never expires)
	if p.ExpiresAt != 0 && time.Now().Unix() > p.ExpiresAt {
		return nil, ErrExpired
	}

	format := p.Format
	if format == "" {
		format = DefaultFormat
	}

	quality := p.Quality
	if quality == 0 {
		quality = DefaultQuality
	}

	fitMode := p.FitMode
	if fitMode == "" {
		fitMode = DefaultFitMode
	}

	var expires time.Time
	if p.ExpiresAt != 0 {
		expires = time.Unix(p.ExpiresAt, 0)
	}

	return &imagerequest.Request{
		Source: imagerequest.Source{
			Host:  p.SourceHost,
			Path:  p.SourcePath,
			Query: p.SourceQuery,
		},
		Transform: imagerequest.Transform{
			Size:    imagerequest.Size{Width: p.Width, Height: p.Height},
			Format:  format,
			Quality: quality,
			FitMode: fitMode,
		},
		Expires: expires,
	}, nil
}