don't generate biased prime numbers

since the current algorithm selects a number and then goes up, primes
for which the previous prime is far away are more likely to be selected,
similarly the bigger of the twin primes is less likely to be selected

make the prime selection a little bit more uniform by just modifying
the randomly selecting number to be odd, if an even is selected

Author: tomato42

tlslite/utils/cryptomath.py

@@ -387,16 +387,15 @@ def getRandomPrime(bits, display=False):
     #29 % 30 and keep them there
     low = ((2 ** (bits-1)) * 3) // 2
     high = 2 ** bits - 30
-    p = getRandomNumber(low, high)
-    p += 29 - (p % 30)
-    while 1:
-        if display: print(".", end=' ')
-        p += 30
-        if p >= high:
-            p = getRandomNumber(low, high)
-            p += 29 - (p % 30)
-        if isPrime(p, display=display):
-            return p
+    while True:
+        if display:
+            print(".", end=' ')
+        cand_p = getRandomNumber(low, high)
+        # make odd
+        if cand_p % 2 == 0:
+            cand_p += 1
+        if isPrime(cand_p, display=display):
+            return cand_p
 
 
 #Unused at the moment...