diff --git a/lib/misc.c b/lib/misc.c
index f9cacd9..c8707ee 100644
--- a/lib/misc.c
+++ b/lib/misc.c
@@ -15,8 +15,10 @@
  * limitations under the License.
  */
 
+#define _GNU_SOURCE
 #include "misc.h"
 #include <jose/b64.h>
+#include <jose/cfg.h>
 #include <string.h>
 #include "hooks.h"
 
@@ -43,6 +45,51 @@ zero(void *mem, size_t len)
     memset(mem, 0, len);
 }
 
+/* Decode the base64url-encoded protected header and load it as JSON only
+ * when it can contain a "zip" key. The full JSON parse is the expensive
+ * step (the base64url decode is comparatively cheap), so we gate it behind
+ * a substring scan for "zip" over the *decoded* bytes: a "zip" key is only
+ * possible if the literal bytes "zip" appear in the decoded JSON text.
+ *
+ * The scan must run on the decoded bytes, NOT on the base64url-encoded
+ * string: a "zip" key does not survive base64url encoding as the literal
+ * substring "zip", so scanning the encoded form yields false negatives.
+ *
+ * When "zip" is definitely absent, *no_zip is set to true and NULL is
+ * returned without parsing. On decode error, *no_zip stays false and NULL
+ * is returned (callers then treat it the same as a header with no zip). */
+static json_t *
+load_protected_check_zip(const json_t *prt, bool *no_zip)
+{
+    uint8_t *buf = NULL;
+    json_t *out = NULL;
+    size_t size = 0;
+
+    *no_zip = false;
+
+    size = jose_b64_dec(prt, NULL, 0);
+    if (size == SIZE_MAX)
+        return NULL;
+
+    buf = jose_calloc(1, size);
+    if (!buf)
+        return NULL;
+
+    if (jose_b64_dec(prt, buf, size) != size) {
+        zero(buf, size);
+        jose_free(buf);
+        return NULL;
+    }
+
+    if (memmem(buf, size, "zip", 3))
+        out = json_loadb((char *) buf, size, JSON_DECODE_ANY, NULL);
+    else
+        *no_zip = true;
+
+    zero(buf, size);
+    jose_free(buf);
+    return out;
+}
 
 bool
 handle_zip_enc(json_t *json, const void *in, size_t len, void **data, size_t *datalen)
@@ -54,8 +101,19 @@ handle_zip_enc(json_t *json, const void *in, size_t len, void **data, size_t *da
     jose_io_auto_t *zipdata = NULL;
 
     prt = json_object_get(json, "protected");
-    if (prt && json_is_string(prt))
-        prt = jose_b64_dec_load(prt);
+    if (prt && json_is_string(prt)) {
+        bool no_zip = false;
+        json_t *loaded = load_protected_check_zip(prt, &no_zip);
+        if (no_zip) {
+            /* No zip; skip the full parse. (prt is a borrowed reference,
+             * so clear it before returning to avoid a spurious decref.) */
+            prt = NULL;
+            *data = (void*)in;
+            *datalen = len;
+            return true;
+        }
+        prt = loaded;
+    }
 
     /* Check if we have "zip" in the protected header. */
     if (json_unpack(prt, "{s:s}", "zip", &z) == -1) {
@@ -89,8 +147,17 @@ zip_in_protected_header(json_t *json)
     char *z = NULL;
 
     prt = json_object_get(json, "protected");
-    if (prt && json_is_string(prt))
-        prt = jose_b64_dec_load(prt);
+    if (prt && json_is_string(prt)) {
+        bool no_zip = false;
+        json_t *loaded = load_protected_check_zip(prt, &no_zip);
+        if (no_zip) {
+            /* No zip; skip the full parse. (prt is a borrowed reference,
+             * so clear it before returning to avoid a spurious decref.) */
+            prt = NULL;
+            return false;
+        }
+        prt = loaded;
+    }
 
     /* Check if we have "zip" in the protected header. */
     if (json_unpack(prt, "{s:s}", "zip", &z) == -1)