use 0x7f as upper counter byte

fido2/u2f.c

@@ -247,7 +247,7 @@ static int16_t u2f_authenticate(struct u2f_authenticate_request * req, uint8_t c
 	}
 
     count = ctap_atomic_count(0);
-    hash[0] = (count >> 24) & 0xff;
+    hash[0] = 0x7f;
     hash[1] = (count >> 16) & 0xff;
     hash[2] = (count >> 8) & 0xff;
     hash[3] = (count >> 0) & 0xff;
@@ -264,7 +264,7 @@ static int16_t u2f_authenticate(struct u2f_authenticate_request * req, uint8_t c
     crypto_ecc256_sign(hash, 32, sig);
 
     u2f_response_writeback(&up,1);
-    hash[0] = (count >> 24) & 0xff;
+    hash[0] = 0x7f;
     hash[1] = (count >> 16) & 0xff;
     hash[2] = (count >> 8) & 0xff;
     hash[3] = (count >> 0) & 0xff;

tools/ctap_test.py

@@ -448,7 +448,7 @@ class Tester:
             if lastc:
                 assert (auth.counter - lastc) < 10
             lastc = auth.counter
-            if lastc > 0x100000:
+            if lastc > 0x80000000:
                 print("WARNING: counter is unusually high: %04x" % lastc)
                 assert 0