fix: add blur op for mature deviations with blur-constrained tokens

Mature deviations are signed with a watermark-service token whose obj
carries a "blur": ">=N" constraint. The composed wixmp /v1/fit
transform had no blur operation, so wixmp rejected it with 403, which
the proxy passed through as a broken image.

buildMediaURL now decodes the token and, when it demands a blur, appends
a matching blur_N op to the transform (e.g. w_1280,h_1920,blur_10).
Unconstrained media and tokens that don't parse are left untouched, so
only media that requires it is affected.

Closes #14

1 files changed, 73 insertions, 0 deletions

diff --git a/app/cache.go b/app/cache.go
index e4822db..6142178 100755
--- a/app/cache.go
+++ b/app/cache.go
@@ -4,11 +4,14 @@ package app
 
 import (
 	"crypto/sha1" //nolint:gosec // G505: SHA-1 is a cache-key hash here, not a security primitive
+	"encoding/base64"
 	"encoding/hex"
+	"encoding/json"
 	"io"
 	"net/url"
 	"os"
 	"regexp"
+	"strconv"
 	"strings"
 	"sync"
 	"syscall"
@@ -88,6 +91,69 @@ func ageMemCache() {
 // than escaped, because this label is what selects the host to fetch from.
 var mediaSubdomain = regexp.MustCompile(`^[a-zA-Z0-9-]+$`)
 
+// blurConstraint reports the minimum blur radius a wixmp media token demands, or
+// 0 if it demands none.
+//
+// DeviantArt signs mature-content media with a watermark-service token whose obj
+// carries a "blur": ">=N" constraint. wixmp then rejects a plain /v1/fit
+// transform with 403 unless it includes a matching blur_N operation, so this is
+// what tells buildMediaURL when to add one. A token it cannot parse yields 0,
+// leaving the URL untouched — the same behaviour as before this check existed.
+func blurConstraint(token string) int {
+	// A JWT is header.payload.signature; the claims are the middle segment,
+	// base64url-encoded without padding.
+	parts := strings.SplitN(token, ".", 3)
+	if len(parts) < 2 {
+		return 0
+	}
+	payload, err := base64.RawURLEncoding.DecodeString(parts[1])
+	if err != nil {
+		return 0
+	}
+
+	var claims struct {
+		Obj [][]struct {
+			Blur string `json:"blur"`
+		} `json:"obj"`
+	}
+	if json.Unmarshal(payload, &claims) != nil ||
+		len(claims.Obj) == 0 || len(claims.Obj[0]) == 0 {
+		return 0
+	}
+
+	// The constraint reads like ">=10"; take its digits as the radius, which is
+	// the minimum the token accepts.
+	n := 0
+	for _, c := range claims.Obj[0][0].Blur {
+		if c >= '0' && c <= '9' {
+			n = n*10 + int(c-'0')
+		}
+	}
+	return n
+}
+
+// addBlurToTransform inserts a blur_n operation into a wixmp /v1/fit transform,
+// turning e.g. w_1280,h_1920 into w_1280,h_1920,blur_n. It returns path
+// unchanged when it carries no /v1/fit transform (GIFs and oversized originals
+// are served without one) or already blurs.
+func addBlurToTransform(path string, n int) string {
+	const marker = "/v1/fit/"
+	start := strings.Index(path, marker)
+	if start < 0 {
+		return path
+	}
+	ops := start + len(marker)
+	end := strings.IndexByte(path[ops:], '/')
+	if end < 0 {
+		return path
+	}
+	end += ops
+	if strings.Contains(path[ops:end], "blur_") {
+		return path
+	}
+	return path[:end] + ",blur_" + strconv.Itoa(n) + path[end:]
+}
+
 // buildMediaURL returns the wixmp CDN URL for one media item, reporting false
 // when subdomain is not a bare hostname label.
 //
@@ -102,6 +168,13 @@ func buildMediaURL(subdomain, path, token string) (string, bool) {
 		return "", false
 	}
 
+	// Mature media is signed with a token that only authorizes a blurred render;
+	// without a matching blur op in the transform wixmp answers 403. Add the op
+	// the token demands, and only then, so unconstrained media is left as-is.
+	if n := blurConstraint(token); n > 0 {
+		path = addBlurToTransform(path, n)
+	}
+
 	// Fields rather than concatenation: String escapes the path, so a decoded
 	// "#" or "?" in it stays part of the path instead of ending it. The host is
 	// checked above rather than escaped, because url.URL passes it through