diff --git a/common/crapto1/crapto1.c b/common/crapto1/crapto1.c
index 60ae72dad..4a2e9e791 100644
--- a/common/crapto1/crapto1.c
+++ b/common/crapto1/crapto1.c
@@ -430,10 +430,13 @@ int nonce_distance(uint32_t from, uint32_t to) {
  *   false = hardend prng
  */
 bool validate_prng_nonce(uint32_t nonce) {
-    // init prng table:
-    if (nonce_distance(nonce, nonce) == -1)
-        return false;
-    return ((65535 - dist[nonce >> 16] + dist[nonce & 0xffff]) % 65535) == 16;
+    uint16_t x = nonce >> 16;
+    x = (x & 0xff) << 8 | x >> 8;
+    for (uint8_t i = 0; i<16; i++) {
+        x = x >> 1 | (x ^ x >> 2 ^ x >> 3 ^ x >> 5) << 15;
+    }
+    x = (x & 0xff) << 8 | x >> 8;
+    return x == (nonce & 0xFFFF);
 }
 
 static uint32_t fastfwd[2][8] = {