pixa/internal/signature/signature.go

// Package signature provides HMAC-SHA256 signing and verification of image
// requests.
package signature

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"net/url"
	"strconv"
	"strings"
	"time"

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

// Signature errors.
var (
	ErrRequired          = errors.New("signature required for non-allowlisted host")
	ErrInvalid           = errors.New("invalid signature")
	ErrExpired           = errors.New("signature has expired")
	ErrMissingExpiration = errors.New("signature expiration is required")
)

// Signer handles HMAC-SHA256 signature generation and verification.
type Signer struct {
	secretKey []byte
}

// New creates a new Signer with the given secret key.
func New(secretKey string) *Signer {
	return &Signer{
		secretKey: []byte(secretKey),
	}
}

// Sign generates an HMAC-SHA256 signature for the given request.
// The signature covers: host + path + query + width + height + format +
// expiration + quality + fit. The quality and fit mode are the effective
// values the request resolves to: callers pass the defaults when the request
// omits those parameters, so an omitted quality or fit mode signs
// identically to that same value stated explicitly.
func (s *Signer) Sign(req *imagerequest.Request) string {
	data := s.buildSignatureData(req)
	mac := hmac.New(sha256.New, s.secretKey)
	mac.Write([]byte(data))
	sig := mac.Sum(nil)

	return base64.URLEncoding.EncodeToString(sig)
}

// Verify checks if the signature on the request is valid and not expired.
// Signatures are exact-match only: every component of the signed data
// (host, path, query, dimensions, format, expiration, quality, fit) must
// match exactly.
// No suffix matching, wildcard matching, or partial matching is supported.
// A signature for "cdn.example.com" will NOT verify for "example.com" or
// "other.cdn.example.com", and vice versa.
func (s *Signer) Verify(req *imagerequest.Request) error {
	// Check expiration first
	if req.Expires.IsZero() {
		return ErrMissingExpiration
	}

	if time.Now().After(req.Expires) {
		return ErrExpired
	}

	// Compute expected signature
	expected := s.Sign(req)

	// Constant-time comparison to prevent timing attacks
	if !hmac.Equal([]byte(req.Signature), []byte(expected)) {
		return ErrInvalid
	}

	return nil
}

// GenerateSignedURL creates a complete URL with signature and expiration.
// Returns the path portion that should be appended to the base URL.
func (s *Signer) GenerateSignedURL(
	req *imagerequest.Request, ttl time.Duration,
) (string, string, int64) {
	// Set expiration
	req.Expires = time.Now().Add(ttl)
	exp := req.Expires.Unix()

	// Generate signature
	sig := s.Sign(req)
	req.Signature = sig

	// Build the size component
	var sizeStr string
	if req.Transform.Size.Width == 0 && req.Transform.Size.Height == 0 {
		sizeStr = "orig"
	} else {
		sizeStr = fmt.Sprintf("%dx%d",
			req.Transform.Size.Width, req.Transform.Size.Height)
	}

	// Build the path. A source query follows the source path after a "?".
	// The image route reads the path decoded once, so the whole path is
	// written in Go's own percent-encoding of a URL path, which writes "?"
	// as %3F and "%" as %25, and leaves "/", ";", ",", "=" and "&" as they
	// are.
	sourcePathAndQuery := req.Source.Path
	if req.Source.Query != "" {
		sourcePathAndQuery += "?" + req.Source.Query
	}

	unescapedPath := fmt.Sprintf("/v1/image/%s%s/%s.%s",
		req.Source.Host, sourcePathAndQuery, sizeStr, req.Transform.Format)

	return (&url.URL{Path: unescapedPath}).EscapedPath(), sig, exp
}

// buildSignatureData creates the string to be signed.
// Format: "host:path:query:width:height:format:expiration:quality:fit"
// The path and query go through escapePercentAndColon, so that a ":" in
// either cannot be taken for the one between them; every other component
// is used verbatim. No normalization, suffix matching, or wildcard
// expansion is performed. Quality and fit are the effective transform
// values, so replaying a signed URL with a different quality or fit mode
// fails verification.
func (s *Signer) buildSignatureData(req *imagerequest.Request) string {
	return fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d:%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.Expires.Unix(),
		req.Transform.Quality,
		req.Transform.FitMode,
	)
}

// 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")
}

// ParseParams extracts signature and expiration from query parameters.
func ParseParams(sig, expStr string) (string, time.Time, error) {
	if expStr == "" {
		return sig, time.Time{}, nil
	}

	expUnix, err := strconv.ParseInt(expStr, 10, 64)
	if err != nil {
		return "", time.Time{}, fmt.Errorf("invalid expiration: %w", err)
	}

	return sig, time.Unix(expUnix, 0), nil
}