use non-blocking PRNG to generate secp256k1 context (#51)

This uses
  java.util.Random.nextBytes
instead of
  java.security.SecureRandom.generateSeed

to generate the seed used to "randomize" the secp256k1 context. The
later might block indefinitely on some platforms when the OS' RNG runs
out of entropy.

java.util.Random is a much weaker source of randomness (2^48 cycle,
much less under real world conditions due to imperfect seed). It is
however sufficient in this case: The context randomization is only
used as an additional precaution against side channel attacks in case
the compiler ruins the constant time property of the underlying code.

Signed-off-by: Taccat Isid <[email protected]>

secp256k1/src/main/java/org/hyperledger/besu/nativelib/secp256k1/LibSecp256k1.java

@@ -16,7 +16,7 @@
 package org.hyperledger.besu.nativelib.secp256k1;
 
 import java.nio.ByteBuffer;
-import java.security.SecureRandom;
+import java.util.Random;
 
 import com.sun.jna.Callback;
 import com.sun.jna.Library;
@@ -46,7 +46,9 @@ private static PointerByReference createContext() {
           secp256k1_context_create(SECP256K1_CONTEXT_VERIFY | SECP256K1_CONTEXT_SIGN);
       if (Boolean.parseBoolean(System.getProperty("secp256k1.randomize", "true"))) {
         // randomization requested or not explicitly disabled
-        if (secp256k1_context_randomize(context, new SecureRandom().generateSeed(32)) != 1) {
+        byte[] seed = new byte[32];
+        (new Random()).nextBytes(seed);
+        if (secp256k1_context_randomize(context, seed) != 1) {
           // there was an error, don't preserve the context
           return null;
         }