One image request type instead of five (closes #307)

The new package internal/imagerequest holds the request type, its two
parts Source and Transform, the Format and Fit types, the dimension and
fit checks, the /v1/image/ path parsing and the variant key; it imports
no other package of pixa. imgcache.ParsedURL, imgcache.ImageRequest,
signature.Request, imageprocessor.Request, the second Size, format and
fit types and the conversions between them are gone. The image
processor takes a Transform only. encurl keeps a private struct with the
token's short CBOR field names. VariantKey moves with the function that
makes it. Tests follow the moved and renamed code; the tests of the
removed conversions go with them.

Model: opus-5-5

internal/imagerequest/imagerequest.go

@@ -0,0 +1,147 @@
+// Package imagerequest holds the image request type, its parts, its checks,
+// the parsing of the /v1/image/ path and the variant key. It imports no other
+// package of this repo, so every package that handles an image request can
+// import it.
+package imagerequest
+
+import (
+	"errors"
+	"net/url"
+	"time"
+)
+
+// ErrInvalidFitMode is returned when an invalid fit mode is provided.
+var ErrInvalidFitMode = errors.New("invalid fit mode")
+
+// Format represents supported output image formats.
+type Format string
+
+// Supported image output formats.
+const (
+	FormatOriginal Format = "orig"
+	FormatJPEG     Format = "jpeg"
+	FormatPNG      Format = "png"
+	FormatWebP     Format = "webp"
+	FormatAVIF     Format = "avif"
+	FormatJXL      Format = "jxl"
+	FormatGIF      Format = "gif"
+
+	// FormatAuto stands for JPEG XL, AVIF, WebP or JPEG, chosen for each
+	// request from its Accept header once the URL's signature or token has
+	// been checked; it is never processed or cached as itself.
+	FormatAuto Format = "auto"
+)
+
+// Size represents requested image dimensions
+type Size struct {
+	Width  int
+	Height int
+}
+
+// OriginalSize returns true if this represents "keep original size"
+func (s Size) OriginalSize() bool {
+	return s.Width == 0 && s.Height == 0
+}
+
+// Fit represents how to fit image into requested dimensions.
+type Fit string
+
+// Supported image fit modes.
+const (
+	FitCover   Fit = "cover"
+	FitContain Fit = "contain"
+	FitFill    Fit = "fill"
+	FitInside  Fit = "inside"
+	FitOutside Fit = "outside"
+)
+
+// ValidateFitMode checks if the given fit mode is valid.
+// Returns ErrInvalidFitMode for unrecognized fit modes.
+func ValidateFitMode(fit Fit) error {
+	switch fit {
+	case FitCover, FitContain, FitFill, FitInside, FitOutside, "":
+		return nil
+	default:
+		return ErrInvalidFitMode
+	}
+}
+
+// ValidateImageRequest checks a request's width and height with
+// ValidateDimension and its fit mode with ValidateFitMode. Both the plain
+// /v1/image/ route and the encrypted /v1/e/ route validate through this
+// function so a request from either source enforces identical bounds.
+func ValidateImageRequest(req *Request) error {
+	err := ValidateDimension("width", req.Transform.Size.Width)
+	if err != nil {
+		return err
+	}
+
+	err = ValidateDimension("height", req.Transform.Size.Height)
+	if err != nil {
+		return err
+	}
+
+	return ValidateFitMode(req.Transform.FitMode)
+}
+
+// Request represents a request for a processed image: the source to fetch
+// and the transform to apply to it.
+type Request struct {
+	// Source is what to fetch
+	Source Source
+	// Transform is what to do to it
+	Transform Transform
+	// Signature is the HMAC signature for non-allowlisted hosts
+	Signature string
+	// Expires is when the URL expires: the exp of a signed URL, or the expiry
+	// of an encrypted URL; the zero time if it has none
+	Expires time.Time
+	// AllowHTTP indicates whether HTTP (non-TLS) is allowed for this request
+	AllowHTTP bool
+}
+
+// Source is the image a request fetches from the origin.
+type Source struct {
+	// Host is the origin host (e.g., "cdn.example.com")
+	Host string
+	// Path is the path on the origin with its percent-escapes decoded, as
+	// url.URL's Path holds it (e.g., "/photos/my cat.jpg")
+	Path string
+	// Query is the optional query string for the origin URL as the URL
+	// writes it, percent-escapes included, as url.URL's RawQuery holds it
+	Query string
+}
+
+// Transform is what a request does to its source; the image processor
+// takes it alone.
+type Transform struct {
+	// Size is the requested output dimensions
+	Size Size
+	// Format is the requested output format
+	Format Format
+	// Quality is the output quality (1-100) for lossy formats
+	Quality int
+	// FitMode is how to fit the image into requested dimensions
+	FitMode Fit
+}
+
+// SourceURL returns the full upstream URL to fetch: the source path
+// percent-encoded again, as net/url writes a path, so each source path is one
+// upstream path, and the source query as it is, with the characters
+// escapeQuery encodes percent-encoded. Uses http:// scheme when AllowHTTP is
+// true, otherwise https://.
+func (r *Request) SourceURL() string {
+	scheme := "https"
+	if r.AllowHTTP {
+		scheme = "http"
+	}
+
+	sourceURL := url.URL{
+		Scheme:   scheme,
+		Host:     r.Source.Host,
+		Path:     r.Source.Path,
+		RawQuery: escapeQuery(r.Source.Query),
+	}
+
+	return sourceURL.String()
+}

internal/imagerequest/sourceurl_internal_test.go

@@ -0,0 +1,58 @@
+package imagerequest
+
+import "testing"
+
+func TestRequest_SourceURL_DefaultHTTPS(t *testing.T) {
+	t.Parallel()
+
+	req := &Request{
+		Source: Source{
+			Host:  testHostCDN,
+			Path:  testPathCat,
+			Query: "v=2",
+		},
+	}
+
+	got := req.SourceURL()
+
+	want := "https://cdn.example.com/photos/cat.jpg?v=2"
+	if got != want {
+		t.Errorf("SourceURL() = %q, want %q", got, want)
+	}
+}
+
+func TestRequest_SourceURL_AllowHTTP(t *testing.T) {
+	t.Parallel()
+
+	req := &Request{
+		Source: Source{
+			Host: "localhost:8080",
+			Path: testPathCat,
+		},
+		AllowHTTP: true,
+	}
+
+	got := req.SourceURL()
+
+	want := "http://localhost:8080/photos/cat.jpg"
+	if got != want {
+		t.Errorf("SourceURL() = %q, want %q", got, want)
+	}
+}
+
+func TestRequest_SourceURL_AllowHTTPFalse(t *testing.T) {
+	t.Parallel()
+
+	req := &Request{
+		Source: Source{
+			Host: testHostCDN,
+			Path: "/img.jpg",
+		},
+		AllowHTTP: false,
+	}
+
+	got := req.SourceURL()
+	if got != "https://cdn.example.com/img.jpg" {
+		t.Errorf("SourceURL() = %q, want https scheme", got)
+	}
+}

internal/imagerequest/urlparser.go

@@ -0,0 +1,357 @@
+package imagerequest
+
+import (
+	"errors"
+	"fmt"
+	"net/netip"
+	"net/url"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// URL parsing errors.
+var (
+	ErrInvalidPath       = errors.New("invalid image path")
+	ErrMissingHost       = errors.New("missing source host")
+	ErrInvalidHost       = errors.New("invalid source host")
+	ErrMissingSize       = errors.New("missing size specification")
+	ErrInvalidSize       = errors.New("invalid size format")
+	ErrInvalidFormat     = errors.New("invalid or unsupported format")
+	ErrDimensionTooLarge = errors.New("dimension exceeds maximum")
+	ErrPathTraversal     = errors.New("path traversal detected")
+)
+
+// MaxDimension is the maximum allowed width or height, the same as the image
+// processor's maximum for an output image (imageprocessor.MaxOutputDimension).
+const MaxDimension = 8192
+
+// ValidateDimension checks one requested width or height; name ("width" or
+// "height") appears in the error. 0 is valid: that side is then worked out from
+// the source's proportions and the other requested side, rounded down but at
+// least 1, or is the source's own side when the other side is 0 too.
+func ValidateDimension(name string, value int) error {
+	if value < 0 {
+		return fmt.Errorf("%w: %s is negative", ErrInvalidSize, name)
+	}
+
+	if value > MaxDimension {
+		return fmt.Errorf("%w: %s is above %d",
+			ErrDimensionTooLarge, name, MaxDimension)
+	}
+
+	return nil
+}
+
+// sizeFormatRegex matches patterns like "800x600.webp", "0x0.jpeg", "orig.png",
+// and a size with no format, such as "800x600" or "orig"
+var sizeFormatRegex = regexp.MustCompile(
+	`^(\d+)x(\d+)(?:\.(\w+))?$|^(orig)(?:\.(\w+))?$`)
+
+// percentEscapeRegex matches one percent-escape, such as "%2e"
+var percentEscapeRegex = regexp.MustCompile(`%[0-9A-Fa-f]{2}`)
+
+// queryEscapeRegex matches a percent-escape, or one character that a browser
+// percent-encodes in the query of an http or https URL: one outside the
+// printable ASCII characters "!" to "~", such as a space, a control character
+// or a character outside ASCII, or one of `"#'<>`. A "%" that does not begin a
+// percent-escape matches too.
+var queryEscapeRegex = regexp.MustCompile(`%[0-9A-Fa-f]{2}|[^!-~]|["#'<>%]`)
+
+// hostRegex matches the forms the host of an image URL may take: a host name
+// or an IPv4 address (letters, digits, "-", "_" and "."), or an IPv6 address
+// in brackets (hex digits, ":" and "."), either one optionally followed by ":"
+// and a port, which may be empty. Its one group holds the text in brackets,
+// which parseImageComponents also checks is an IPv6 address.
+var hostRegex = regexp.MustCompile(
+	`^(?:[A-Za-z0-9_.-]+|\[([0-9A-Fa-f:.]+)\])(?::[0-9]*)?$`)
+
+// ParseImagePath parses an image URL's path after /v1/image/, decoded once:
+// <host>/<path>/<size>.<format>, or <host>/<path>/<size> for JPEG XL.
+// Examples:
+//   - cdn.example.com/photos/cat.jpg/800x600.webp
+//   - cdn.example.com/photos/cat.jpg/0x0.jpeg
+//   - cdn.example.com/photos/cat.jpg/orig.png
+//   - cdn.example.com/photos/cat.jpg/800x600
+func ParseImagePath(path string) (*Request, error) {
+	// Strip leading slash if present (chi may include it)
+	path = strings.TrimPrefix(path, "/")
+	if path == "" {
+		return nil, ErrMissingHost
+	}
+
+	return parseImageComponents(path)
+}
+
+// ParseImageURL parses an image URL's path, decoded once as r.URL.Path holds
+// it: /v1/image/<host>/<path>/<size>.<format>, or /v1/image/<host>/<path>/<size>
+// for JPEG XL. A source query follows the source path after a "?", which the
+// URL writes as %3F.
+func ParseImageURL(urlPath string) (*Request, error) {
+	// Remove the /v1/image/ prefix
+	const prefix = "/v1/image/"
+	if !strings.HasPrefix(urlPath, prefix) {
+		return nil, ErrInvalidPath
+	}
+
+	remainder := strings.TrimPrefix(urlPath, prefix)
+	if remainder == "" {
+		return nil, ErrMissingHost
+	}
+
+	return parseImageComponents(remainder)
+}
+
+// parseImageComponents parses <host>/<path>/<size>.<format>, or
+// <host>/<path>/<size> for JPEG XL.
+func parseImageComponents(remainder string) (*Request, error) {
+	// Find the last path segment, which holds "size" or "size.format"
+	lastSlash := strings.LastIndex(remainder, "/")
+	if lastSlash == -1 {
+		return nil, ErrMissingSize
+	}
+
+	sizeFormat := remainder[lastSlash+1:]
+	hostAndPath := remainder[:lastSlash]
+
+	if hostAndPath == "" {
+		return nil, ErrMissingHost
+	}
+
+	// Parse size and format from the last segment
+	size, format, err := parseSizeFormat(sizeFormat)
+	if err != nil {
+		return nil, err
+	}
+
+	// Split host from path
+	// The first segment is the host, everything after is the path
+	firstSlash := strings.Index(hostAndPath, "/")
+
+	var host, path, query string
+
+	if firstSlash == -1 {
+		// No path, just host (unusual but valid)
+		host = hostAndPath
+		path = "/"
+	} else {
+		host = hostAndPath[:firstSlash]
+		path = hostAndPath[firstSlash:]
+	}
+
+	if host == "" {
+		return nil, ErrMissingHost
+	}
+
+	// Only the forms hostRegex describes are accepted. A "?", "#" or "@" in
+	// the host would make Go's URL parser, which the allowlist check and the
+	// fetcher use, read a different host from the one allowlisted and cached
+	hostMatch := hostRegex.FindStringSubmatch(host)
+	if hostMatch == nil {
+		return nil, fmt.Errorf("%w: %q", ErrInvalidHost, host)
+	}
+
+	// Go's URL parser refuses brackets around anything but an IPv6 address
+	if inBrackets := hostMatch[1]; inBrackets != "" {
+		addr, err := netip.ParseAddr(inBrackets)
+		if err != nil || !addr.Is6() {
+			return nil, fmt.Errorf("%w: %q", ErrInvalidHost, host)
+		}
+	}
+
+	// Host names do not depend on letter case, so the allowlist, the upstream
+	// connection limits, the cache and the signature see one spelling of each
+	// host
+	host = strings.ToLower(host)
+
+	// Go's URL parser, which the fetcher uses, reads a "#" as the start of a
+	// fragment and drops the rest, so each text after it would be another
+	// spelling, cached under its own keys, of one upstream request
+	if strings.Contains(path, "#") {
+		return nil, fmt.Errorf(
+			`%w: the source path or query holds a "#"`, ErrInvalidPath)
+	}
+
+	// Extract query string if present in path. It is the source URL's query
+	// as that URL writes it. A character a URL's query cannot hold as it is,
+	// such as a space written %20 in the image URL, is percent-encoded here,
+	// so the query is signed and cached as it is sent upstream
+	if qIndex := strings.Index(path, "?"); qIndex != -1 {
+		query = escapeQuery(path[qIndex+1:])
+		path = path[:qIndex]
+	}
+
+	// The source query names no file on the upstream, so only the host and
+	// the source path are checked for path traversal
+	err = checkPathTraversal(host + path)
+	if err != nil {
+		return nil, err
+	}
+
+	// Ensure path starts with /
+	if !strings.HasPrefix(path, "/") {
+		path = "/" + path
+	}
+
+	return &Request{
+		Source:    Source{Host: host, Path: path, Query: query},
+		Transform: Transform{Size: size, Format: format},
+	}, nil
+}
+
+// checkPathTraversal detects path traversal attempts in a URL path.
+// It checks for various attack vectors including:
+// - Direct ../ sequences
+// - URL-encoded variants (%2e%2e, %252e%252e)
+// - Backslash variants (..\)
+// - Null byte injection (%00)
+func checkPathTraversal(path string) error {
+	// First, URL-decode the path to catch encoded attacks
+	// Decode multiple times to catch double-encoding
+	decoded := path
+	for range 3 {
+		newDecoded := decodePercentEscapes(decoded)
+		if newDecoded == decoded {
+			break
+		}
+
+		decoded = newDecoded
+	}
+
+	// Normalize backslashes to forward slashes
+	normalized := strings.ReplaceAll(decoded, "\\", "/")
+
+	// Check for null bytes
+	if strings.Contains(normalized, "\x00") {
+		return ErrPathTraversal
+	}
+
+	// Check for parent directory traversal
+	// Look for "/.." or "../" patterns
+	if strings.Contains(normalized, "/../") ||
+		strings.Contains(normalized, "/..") ||
+		strings.HasPrefix(normalized, "../") ||
+		strings.HasSuffix(normalized, "/..") ||
+		normalized == ".." {
+		return ErrPathTraversal
+	}
+
+	// Also check for ".." as a path segment in the original path
+	// This catches cases where the path hasn't been normalized
+	for seg := range strings.SplitSeq(path, "/") {
+		// URL decode the segment
+		decodedSeg, _ := url.PathUnescape(seg)
+		decodedSeg = strings.ReplaceAll(decodedSeg, "\\", "/")
+
+		if decodedSeg == ".." {
+			return ErrPathTraversal
+		}
+	}
+
+	return nil
+}
+
+// decodePercentEscapes decodes every percent-escape in s, such as %2e, and
+// keeps a "%" that does not start one as it is, where url.PathUnescape would
+// refuse the whole string. A source path holding %25, a literal "%", has such
+// a "%" after one more decoding.
+func decodePercentEscapes(s string) string {
+	return percentEscapeRegex.ReplaceAllStringFunc(s, func(escape string) string {
+		decoded, _ := url.PathUnescape(escape)
+
+		return decoded
+	})
+}
+
+// escapeQuery returns a source query as a URL's query holds it: each
+// character that queryEscapeRegex names, such as a space, percent-encoded,
+// and the rest, the query's percent-escapes included, kept as they are. The
+// result is unchanged by a second escapeQuery.
+func escapeQuery(query string) string {
+	return queryEscapeRegex.ReplaceAllStringFunc(query, func(match string) string {
+		if percentEscapeRegex.MatchString(match) {
+			return match
+		}
+
+		return url.PathEscape(match)
+	})
+}
+
+// parseSizeFormat parses strings like "800x600.webp", "orig.png" or "800x600"
+func parseSizeFormat(s string) (Size, Format, error) {
+	matches := sizeFormatRegex.FindStringSubmatch(s)
+	if matches == nil {
+		return Size{}, "", ErrInvalidSize
+	}
+
+	var (
+		size      Size
+		formatStr string
+	)
+
+	if matches[4] == "orig" {
+		// "orig" or "orig.format" pattern
+		size = Size{Width: 0, Height: 0}
+		formatStr = matches[5]
+	} else {
+		// "WxH" or "WxH.format" pattern
+		width, err := strconv.Atoi(matches[1])
+		if err != nil {
+			return Size{}, "", ErrInvalidSize
+		}
+
+		height, err := strconv.Atoi(matches[2])
+		if err != nil {
+			return Size{}, "", ErrInvalidSize
+		}
+
+		size = Size{Width: width, Height: height}
+		formatStr = matches[3]
+	}
+
+	err := ValidateDimension("width", size.Width)
+	if err != nil {
+		return Size{}, "", err
+	}
+
+	err = ValidateDimension("height", size.Height)
+	if err != nil {
+		return Size{}, "", err
+	}
+
+	// A URL that names no format is served, and signed, as JPEG XL
+	if formatStr == "" {
+		return size, FormatJXL, nil
+	}
+
+	format, err := parseFormat(formatStr)
+	if err != nil {
+		return Size{}, "", err
+	}
+
+	return size, format, nil
+}
+
+// parseFormat converts a format string to Format.
+func parseFormat(s string) (Format, error) {
+	switch strings.ToLower(s) {
+	case "orig", "original":
+		return FormatOriginal, nil
+	case "jpg", "jpeg":
+		return FormatJPEG, nil
+	case "png":
+		return FormatPNG, nil
+	case "webp":
+		return FormatWebP, nil
+	case "avif":
+		return FormatAVIF, nil
+	case "jxl":
+		return FormatJXL, nil
+	case "gif":
+		return FormatGIF, nil
+	case "auto":
+		return FormatAuto, nil
+	default:
+		return "", fmt.Errorf("%w: %s", ErrInvalidFormat, s)
+	}
+}

internal/imagerequest/urlparser_internal_test.go

@@ -0,0 +1,486 @@
+package imagerequest
+
+import (
+	"errors"
+	"testing"
+)
+
+// Shared test fixture strings.
+const (
+	testHostCDN     = "cdn.example.com"
+	testHostExample = "example.com"
+	testPathCat     = "/photos/cat.jpg"
+)
+
+// assertRequest compares the source host, path and query and the size and
+// format of got against want.
+func assertRequest(t *testing.T, got, want *Request) {
+	t.Helper()
+
+	if got.Source.Host != want.Source.Host {
+		t.Errorf("Host = %q, want %q", got.Source.Host, want.Source.Host)
+	}
+
+	if got.Source.Path != want.Source.Path {
+		t.Errorf("Path = %q, want %q", got.Source.Path, want.Source.Path)
+	}
+
+	if got.Source.Query != want.Source.Query {
+		t.Errorf("Query = %q, want %q", got.Source.Query, want.Source.Query)
+	}
+
+	if got.Transform.Size != want.Transform.Size {
+		t.Errorf("Size = %v, want %v", got.Transform.Size, want.Transform.Size)
+	}
+
+	if got.Transform.Format != want.Transform.Format {
+		t.Errorf("Format = %q, want %q",
+			got.Transform.Format, want.Transform.Format)
+	}
+}
+
+func TestParseImageURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		input string
+		want  *Request
+	}{
+		{
+			name:  "basic path with size",
+			input: "/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp",
+			want: &Request{
+				Source:    Source{Host: testHostCDN, Path: testPathCat},
+				Transform: Transform{Size: Size{Width: 800, Height: 600}, Format: FormatWebP},
+			},
+		},
+		{
+			name:  "original size with 0x0",
+			input: "/v1/image/cdn.example.com/photos/cat.jpg/0x0.jpeg",
+			want: &Request{
+				Source:    Source{Host: testHostCDN, Path: testPathCat},
+				Transform: Transform{Size: Size{Width: 0, Height: 0}, Format: FormatJPEG},
+			},
+		},
+		{
+			name:  "original size with orig keyword",
+			input: "/v1/image/cdn.example.com/photos/cat.jpg/orig.png",
+			want: &Request{
+				Source:    Source{Host: testHostCDN, Path: testPathCat},
+				Transform: Transform{Size: Size{Width: 0, Height: 0}, Format: FormatPNG},
+			},
+		},
+		{
+			name:  "path with query string",
+			input: "/v1/image/cdn.example.com/photos/cat.jpg?arg1=val1&arg2=val2/800x600.webp",
+			want: &Request{
+				Source: Source{
+					Host: testHostCDN, Path: testPathCat, Query: "arg1=val1&arg2=val2",
+				},
+				Transform: Transform{Size: Size{Width: 800, Height: 600}, Format: FormatWebP},
+			},
+		},
+		{
+			name:  "deep nested path",
+			input: "/v1/image/cdn.example.com/a/b/c/d/image.jpg/1920x1080.avif",
+			want: &Request{
+				Source:    Source{Host: testHostCDN, Path: "/a/b/c/d/image.jpg"},
+				Transform: Transform{Size: Size{Width: 1920, Height: 1080}, Format: FormatAVIF},
+			},
+		},
+		{
+			name:  "jpg alias for jpeg",
+			input: "/v1/image/example.com/img.png/100x100.jpg",
+			want: &Request{
+				Source:    Source{Host: testHostExample, Path: "/img.png"},
+				Transform: Transform{Size: Size{Width: 100, Height: 100}, Format: FormatJPEG},
+			},
+		},
+		{
+			name:  "gif format",
+			input: "/v1/image/example.com/animated.gif/200x200.gif",
+			want: &Request{
+				Source:    Source{Host: testHostExample, Path: "/animated.gif"},
+				Transform: Transform{Size: Size{Width: 200, Height: 200}, Format: FormatGIF},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := ParseImageURL(tt.input)
+			if err != nil {
+				t.Fatalf("ParseImageURL() unexpected error = %v", err)
+			}
+
+			assertRequest(t, got, tt.want)
+		})
+	}
+}
+
+// TestParseImageURL_AutoFormat verifies that the format auto in an image URL
+// parses as FormatAuto.
+func TestParseImageURL_AutoFormat(t *testing.T) {
+	t.Parallel()
+
+	got, err := ParseImageURL("/v1/image/example.com/photo.jpg/200x200.auto")
+	if err != nil {
+		t.Fatalf("ParseImageURL() error = %v", err)
+	}
+
+	if got.Transform.Format != FormatAuto {
+		t.Errorf("Format = %q, want %q", got.Transform.Format, FormatAuto)
+	}
+}
+
+func TestParseImageURL_Errors(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		input   string
+		wantErr error
+	}{
+		{
+			name:    "missing prefix",
+			input:   "/image/cdn.example.com/photo.jpg/800x600.webp",
+			wantErr: ErrInvalidPath,
+		},
+		{
+			name:    "empty path after prefix",
+			input:   "/v1/image/",
+			wantErr: ErrMissingHost,
+		},
+		{
+			name:    "host only, no path or size",
+			input:   "/v1/image/cdn.example.com",
+			wantErr: ErrMissingSize,
+		},
+		{
+			name:    "invalid size format",
+			input:   "/v1/image/cdn.example.com/photo.jpg/invalid.webp",
+			wantErr: ErrInvalidSize,
+		},
+		{
+			name:    "unsupported format",
+			input:   "/v1/image/cdn.example.com/photo.jpg/800x600.bmp",
+			wantErr: ErrInvalidFormat,
+		},
+		{
+			name:    "dimension too large",
+			input:   "/v1/image/cdn.example.com/photo.jpg/10000x600.webp",
+			wantErr: ErrDimensionTooLarge,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := ParseImageURL(tt.input)
+			if err == nil {
+				t.Fatalf("ParseImageURL() error = nil, wantErr %v", tt.wantErr)
+			}
+
+			if !errorIs(err, tt.wantErr) {
+				t.Errorf("ParseImageURL() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestParseImagePath(t *testing.T) {
+	t.Parallel()
+
+	// ParseImagePath is for chi wildcard capture (no /v1/image/ prefix)
+	tests := []struct {
+		name    string
+		input   string
+		want    *Request
+		wantErr bool
+	}{
+		{
+			name:  "chi wildcard capture",
+			input: "cdn.example.com/photos/cat.jpg/800x600.webp",
+			want: &Request{
+				Source: Source{
+					Host: testHostCDN,
+					Path: testPathCat,
+				},
+				Transform: Transform{
+					Size:   Size{Width: 800, Height: 600},
+					Format: FormatWebP,
+				},
+			},
+		},
+		{
+			name:  "with leading slash from chi",
+			input: "/cdn.example.com/photos/cat.jpg/800x600.webp",
+			want: &Request{
+				Source: Source{
+					Host: testHostCDN,
+					Path: testPathCat,
+				},
+				Transform: Transform{
+					Size:   Size{Width: 800, Height: 600},
+					Format: FormatWebP,
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := ParseImagePath(tt.input)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ParseImagePath() error = %v, wantErr %v", err, tt.wantErr)
+
+				return
+			}
+
+			if err != nil {
+				return
+			}
+
+			assertRequest(t, got, tt.want)
+		})
+	}
+}
+
+// TestParseImagePath_LowerCasesHost verifies that the host is lower-cased, so
+// that every spelling of its letter case is the same host to the allowlist,
+// the upstream connection limits and the cache, and that the path keeps its
+// case.
+func TestParseImagePath_LowerCasesHost(t *testing.T) {
+	t.Parallel()
+
+	got, err := ParseImagePath("CDN.Example.COM/Photos/Cat.jpg/800x600.webp")
+	if err != nil {
+		t.Fatalf("ParseImagePath() error = %v", err)
+	}
+
+	t.Logf("parsed host %q, path %q", got.Source.Host, got.Source.Path)
+
+	if got.Source.Host != testHostCDN {
+		t.Errorf("Host = %q, want %q", got.Source.Host, testHostCDN)
+	}
+
+	if got.Source.Path != "/Photos/Cat.jpg" {
+		t.Errorf("Path = %q, want %q", got.Source.Path, "/Photos/Cat.jpg")
+	}
+}
+
+// TestParseImagePath_RefusesUserinfo verifies that a host holding "@" is
+// refused: a URL reads what comes before the "@" as a user name and password
+// and the rest as the host.
+func TestParseImagePath_RefusesUserinfo(t *testing.T) {
+	t.Parallel()
+
+	for _, input := range []string{
+		"user@cdn.example.com/photos/cat.jpg/800x600.webp",
+		"user:password@cdn.example.com/photos/cat.jpg/800x600.webp",
+	} {
+		got, err := ParseImagePath(input)
+		t.Logf("ParseImagePath(%q) = %+v, error = %v", input, got, err)
+
+		if !errors.Is(err, ErrInvalidHost) {
+			t.Errorf("ParseImagePath(%q) error = %v, want %v",
+				input, err, ErrInvalidHost)
+		}
+	}
+}
+
+func TestParseImageURL_PathTraversal(t *testing.T) {
+	t.Parallel()
+
+	// All path traversal attempts should be rejected
+	tests := []struct {
+		name  string
+		input string
+	}{
+		{
+			name:  "simple parent directory",
+			input: "/v1/image/cdn.example.com/../etc/passwd/800x600.jpeg",
+		},
+		{
+			name:  "double parent directory",
+			input: "/v1/image/cdn.example.com/../../etc/passwd/800x600.jpeg",
+		},
+		{
+			name:  "parent in middle of path",
+			input: "/v1/image/cdn.example.com/photos/../../../etc/passwd/800x600.jpeg",
+		},
+		{
+			name:  "encoded parent directory",
+			input: "/v1/image/cdn.example.com/photos/%2e%2e/secret/800x600.jpeg",
+		},
+		{
+			name:  "double encoded parent",
+			input: "/v1/image/cdn.example.com/photos/%252e%252e/secret/800x600.jpeg",
+		},
+		{
+			name:  "backslash traversal",
+			input: "/v1/image/cdn.example.com/photos/..\\secret/800x600.jpeg",
+		},
+		{
+			name:  "mixed slashes",
+			input: "/v1/image/cdn.example.com/photos/../\\../secret/800x600.jpeg",
+		},
+		{
+			name:  "null byte injection",
+			input: "/v1/image/cdn.example.com/photos/image.jpg%00.png/800x600.jpeg",
+		},
+		{
+			name:  "parent at start of path",
+			input: "/v1/image/cdn.example.com/../800x600.jpeg",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := ParseImageURL(tt.input)
+			if err == nil {
+				t.Error("ParseImageURL() should reject path traversal attempts")
+			}
+
+			if !errors.Is(err, ErrPathTraversal) {
+				t.Errorf("ParseImageURL() error = %v, want ErrPathTraversal", err)
+			}
+		})
+	}
+}
+
+func TestParseImagePath_PathTraversal(t *testing.T) {
+	t.Parallel()
+
+	// Test path traversal via ParseImagePath (chi wildcard)
+	tests := []struct {
+		name  string
+		input string
+	}{
+		{
+			name:  "parent directory in path",
+			input: "cdn.example.com/photos/../secret/800x600.jpeg",
+		},
+		{
+			name:  "encoded traversal",
+			input: "cdn.example.com/photos/%2e%2e/secret/800x600.jpeg",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := ParseImagePath(tt.input)
+			if err == nil {
+				t.Error("ParseImagePath() should reject path traversal attempts")
+			}
+
+			if !errors.Is(err, ErrPathTraversal) {
+				t.Errorf("ParseImagePath() error = %v, want ErrPathTraversal", err)
+			}
+		})
+	}
+}
+
+// TestParseImageURL_SourceURLHoldingPercent25 verifies that a source path and
+// a source query holding %25, the percent-escape of a literal "%", are
+// accepted as they are, although decoding them once more leaves a "%" that
+// does not start a percent-escape.
+func TestParseImageURL_SourceURLHoldingPercent25(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		input string
+		want  *Request
+	}{
+		{
+			name:  "path",
+			input: "/v1/image/cdn.example.com/sale/50%25off.jpg/800x600.webp",
+			want: &Request{
+				Source:    Source{Host: testHostCDN, Path: "/sale/50%25off.jpg"},
+				Transform: Transform{Size: Size{Width: 800, Height: 600}, Format: FormatWebP},
+			},
+		},
+		{
+			name:  "query",
+			input: "/v1/image/cdn.example.com/photos/cat.jpg?discount=50%25/800x600.webp",
+			want: &Request{
+				Source: Source{
+					Host: testHostCDN, Path: testPathCat, Query: "discount=50%25",
+				},
+				Transform: Transform{Size: Size{Width: 800, Height: 600}, Format: FormatWebP},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := ParseImageURL(tt.input)
+			if err != nil {
+				t.Fatalf("ParseImageURL() unexpected error = %v", err)
+			}
+
+			assertRequest(t, got, tt.want)
+		})
+	}
+}
+
+// TestParseImageURL_DoubleEncodedTraversalNextToPercent verifies that a
+// double-encoded ".." is still refused when the path also holds a "%" that
+// does not start a percent-escape, either as written or after one more
+// decoding.
+func TestParseImageURL_DoubleEncodedTraversalNextToPercent(t *testing.T) {
+	t.Parallel()
+
+	for _, input := range []string{
+		"/v1/image/cdn.example.com/photos/%252e%252e/secret/50%25off.jpg/800x600.jpeg",
+		"/v1/image/cdn.example.com/photos/%252e%252e/secret/100%/800x600.jpeg",
+	} {
+		_, err := ParseImageURL(input)
+		t.Logf("ParseImageURL(%q) error = %v", input, err)
+
+		if !errors.Is(err, ErrPathTraversal) {
+			t.Errorf("ParseImageURL(%q) error = %v, want ErrPathTraversal",
+				input, err)
+		}
+	}
+}
+
+// errorIs checks if err matches target (handles wrapped errors).
+func errorIs(err, target error) bool {
+	if errors.Is(err, target) {
+		return true
+	}
+	// Check if error message contains target message for wrapped errors
+	if err != nil && target != nil {
+		return contains(err.Error(), target.Error())
+	}
+
+	return false
+}
+
+func contains(s, substr string) bool {
+	return len(s) >= len(substr) && (s == substr || len(s) > 0 && containsAt(s, substr))
+}
+
+func containsAt(s, substr string) bool {
+	for i := 0; i <= len(s)-len(substr); i++ {
+		if s[i:i+len(substr)] == substr {
+			return true
+		}
+	}
+
+	return false
+}

internal/imagerequest/validate_internal_test.go

@@ -0,0 +1,72 @@
+package imagerequest
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestValidateImageRequest(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		req     Request
+		wantErr error
+	}{
+		{
+			name: "within bounds",
+			req: Request{Transform: Transform{
+				Size: Size{Width: 800, Height: 600}, FitMode: FitCover,
+			}},
+		},
+		{
+			name: "original size and empty fit",
+			req:  Request{Transform: Transform{Size: Size{Width: 0, Height: 0}}},
+		},
+		{
+			name: "width over limit",
+			req: Request{Transform: Transform{
+				Size: Size{Width: MaxDimension + 1, Height: 600},
+			}},
+			wantErr: ErrDimensionTooLarge,
+		},
+		{
+			name: "height over limit",
+			req: Request{Transform: Transform{
+				Size: Size{Width: 800, Height: MaxDimension + 1},
+			}},
+			wantErr: ErrDimensionTooLarge,
+		},
+		{
+			name:    "negative width",
+			req:     Request{Transform: Transform{Size: Size{Width: -1, Height: 600}}},
+			wantErr: ErrInvalidSize,
+		},
+		{
+			name: "invalid fit mode",
+			req: Request{Transform: Transform{
+				Size: Size{Width: 800, Height: 600}, FitMode: "bogus",
+			}},
+			wantErr: ErrInvalidFitMode,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := ValidateImageRequest(&tt.req)
+			if tt.wantErr == nil {
+				if err != nil {
+					t.Fatalf("ValidateImageRequest() error = %v, want nil", err)
+				}
+
+				return
+			}
+
+			if !errors.Is(err, tt.wantErr) {
+				t.Fatalf("ValidateImageRequest() error = %v, want %v", err, tt.wantErr)
+			}
+		})
+	}
+}

internal/imagerequest/variantkey.go

@@ -0,0 +1,39 @@
+package imagerequest
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"strings"
+)
+
+// VariantKey is a SHA256 hash identifying a specific image variant (hex-encoded).
+type VariantKey string
+
+// CacheKey generates a unique key for a request variant.
+// Format: sha256(host:path:query:width:height:format:quality:fit_mode)
+// The path and query go through escapePercentAndColon, so that a ":" in
+// either cannot be taken for the one between them.
+func CacheKey(req *Request) VariantKey {
+	data := fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d:%s",
+		req.Source.Host,
+		escapePercentAndColon(req.Source.Path),
+		escapePercentAndColon(req.Source.Query),
+		req.Transform.Size.Width,
+		req.Transform.Size.Height,
+		req.Transform.Format,
+		req.Transform.Quality,
+		req.Transform.FitMode,
+	)
+	h := sha256.Sum256([]byte(data))
+
+	return VariantKey(hex.EncodeToString(h[:]))
+}
+
+// escapePercentAndColon returns s with each "%" written as "%25" and each
+// ":" as "%3A", so that the result holds no ":".
+func escapePercentAndColon(s string) string {
+	s = strings.ReplaceAll(s, "%", "%25")
+
+	return strings.ReplaceAll(s, ":", "%3A")
+}

internal/imagerequest/variantkey_internal_test.go

@@ -0,0 +1,142 @@
+package imagerequest
+
+import "testing"
+
+func TestCacheKey(t *testing.T) {
+	t.Parallel()
+
+	req1 := &Request{
+		Source: Source{Host: testHostCDN, Path: testPathCat, Query: ""},
+		Transform: Transform{
+			Size: Size{Width: 800, Height: 600}, Format: FormatWebP,
+			Quality: 85, FitMode: FitCover,
+		},
+	}
+
+	req2 := &Request{
+		Source: Source{Host: testHostCDN, Path: testPathCat, Query: ""},
+		Transform: Transform{
+			Size: Size{Width: 800, Height: 600}, Format: FormatWebP,
+			Quality: 85, FitMode: FitCover,
+		},
+	}
+
+	// Same request should produce same key
+	key1 := CacheKey(req1)
+	key2 := CacheKey(req2)
+
+	if key1 != key2 {
+		t.Errorf("CacheKey() not deterministic: %s vs %s", key1, key2)
+	}
+
+	// Key should be 64 hex chars
+	if len(string(key1)) != 64 {
+		t.Errorf("CacheKey() length = %d, want 64", len(string(key1)))
+	}
+
+	// Different size should produce different key
+	req3 := &Request{
+		Source: Source{Host: testHostCDN, Path: testPathCat, Query: ""},
+		Transform: Transform{
+			Size: Size{Width: 400, Height: 300}, Format: FormatWebP, // Different size
+			Quality: 85, FitMode: FitCover,
+		},
+	}
+
+	key3 := CacheKey(req3)
+	if key1 == key3 {
+		t.Error("CacheKey() produced same key for different sizes")
+	}
+
+	// Different format should produce different key
+	req4 := &Request{
+		Source: Source{Host: testHostCDN, Path: testPathCat, Query: ""},
+		Transform: Transform{
+			Size: Size{Width: 800, Height: 600}, Format: FormatPNG, // Different format
+			Quality: 85, FitMode: FitCover,
+		},
+	}
+
+	key4 := CacheKey(req4)
+	if key1 == key4 {
+		t.Error("CacheKey() produced same key for different formats")
+	}
+
+	// Different quality should produce different key
+	req5 := &Request{
+		Source: Source{Host: testHostCDN, Path: testPathCat, Query: ""},
+		Transform: Transform{
+			Size: Size{Width: 800, Height: 600}, Format: FormatWebP,
+			Quality: 50, FitMode: FitCover, // Different quality
+		},
+	}
+
+	key5 := CacheKey(req5)
+	if key1 == key5 {
+		t.Error("CacheKey() produced same key for different quality")
+	}
+}
+
+// TestCacheKey_ColonOrPercentInPathOrQuery checks that two different source
+// URLs whose path or query holds ":" or "%" get different keys.
+func TestCacheKey_ColonOrPercentInPathOrQuery(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name          string
+		path1, query1 string
+		path2, query2 string
+	}{
+		{
+			name:  "colon moved from path to query",
+			path1: "/a:b", query1: "",
+			path2: "/a", query2: "b:",
+		},
+		{
+			name:  "colon in path and its percent-encoding",
+			path1: "/a:b", query1: "",
+			path2: "/a%3Ab", query2: "",
+		},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			key1 := CacheKey(&Request{
+				Source: Source{
+					Host:  testHostCDN,
+					Path:  tt.path1,
+					Query: tt.query1,
+				},
+				Transform: Transform{
+					Size:    Size{Width: 800, Height: 600},
+					Format:  FormatWebP,
+					Quality: 85,
+					FitMode: FitCover,
+				},
+			})
+			key2 := CacheKey(&Request{
+				Source: Source{
+					Host:  testHostCDN,
+					Path:  tt.path2,
+					Query: tt.query2,
+				},
+				Transform: Transform{
+					Size:    Size{Width: 800, Height: 600},
+					Format:  FormatWebP,
+					Quality: 85,
+					FitMode: FitCover,
+				},
+			})
+
+			t.Logf("CacheKey() of path %q query %q = %s", tt.path1, tt.query1, key1)
+			t.Logf("CacheKey() of path %q query %q = %s", tt.path2, tt.query2, key2)
+
+			if key1 == key2 {
+				t.Errorf("CacheKey() gave path %q query %q and path %q query %q "+
+					"the same key", tt.path1, tt.query1, tt.path2, tt.query2)
+			}
+		})
+	}
+}