challenger

commit 1ec9222b606e8ad3e1270c17a627e393068e7ae8
parent 59aac5395b7f1e00d8c85e8b2c43758cb8cb86fb
Author: Christian Grothoff <christian@grothoff.org>
Date:   Mon,  6 Jul 2026 11:05:20 +0200

use constant-time string equality check for authorization codes

Diffstat:
M
src/challenger/challenger-httpd_token.c
 | 
39
+++++++++++++++++++++++++++++++++------

1 file changed, 33 insertions(+), 6 deletions(-)
diff --git a/src/challenger/challenger-httpd_token.c b/src/challenger/challenger-httpd_token.c
@@ -33,6 +33,33 @@
 
 
 /**
+ * Compare two NUL-terminated strings @a a and @a b in constant time
+ * with respect to their contents.  Used for comparing secret/MAC
+ * material (authorization codes, PKCE challenges) to avoid leaking
+ * information via a timing oracle, as plain strcmp() short-circuits
+ * at the first differing byte.  Note that the string lengths may
+ * still leak, which is acceptable here as the token length is not
+ * a secret at all.
+ *
+ * @param a first string
+ * @param b second string
+ * @return 0 if the strings are equal, non-zero otherwise
+ */
+static int
+ct_strcmp (const char *a,
+           const char *b)
+{
+  size_t alen = strlen (a);
+
+  if (strlen (b) != alen)
+    return 1;
+  return GNUNET_memcmp_ct_ (a,
+                            b,
+                            alen);
+}
+
+
+/**
  * Context for a /token operation.
  */
 struct TokenContext
@@ -513,8 +540,8 @@ CH_handler_token (struct CH_HandlerContext *hc,
               "Failed to encode hash to Base64 URL");
           }
 
-          if (0 != strcmp (encoded_hash,
-                           code_challenge))
+          if (0 != ct_strcmp (encoded_hash,
+                              code_challenge))
           {
             GNUNET_break_op (0);
             json_decref (address);
@@ -536,8 +563,8 @@ CH_handler_token (struct CH_HandlerContext *hc,
         break;
       case CHALLENGER_CM_PLAIN:
         {
-          if (0 != strcmp (bc->code_verifier,
-                           code_challenge))
+          if (0 != ct_strcmp (bc->code_verifier,
+                              code_challenge))
           {
             GNUNET_break_op (0);
             json_decref (address);
@@ -599,8 +626,8 @@ CH_handler_token (struct CH_HandlerContext *hc,
     GNUNET_free (client_redirect_uri);
     GNUNET_free (client_state);
     GNUNET_free (code_challenge);
-    if (0 != strcmp (code,
-                     bc->code))
+    if (0 != ct_strcmp (code,
+                        bc->code))
     {
       GNUNET_break_op (0);
       GNUNET_free (code);