SecureRandom impl that XORs the outputs of other impls (not yet used).

This can be used to combine e.g. the platform's SecureRandom
implementation with our own, so that a weakness in either source doesn't
harm security as long as the other source is strong.

briar-core/src/org/briarproject/crypto/CombinedSecureRandom.java

+package org.briarproject.crypto;
+
+import java.security.Provider;
+import java.security.SecureRandom;
+import java.security.SecureRandomSpi;
+
+/**
+ * A {@link java.security.SecureRandom SecureRandom} implementation that
+ * combines the outputs of two or more other implementations using XOR.
+ */
+class CombinedSecureRandom extends SecureRandom {
+
+	private static final long serialVersionUID = -605269609862397104L;
+	private static final Provider PROVIDER = new CombinedProvider();
+
+	CombinedSecureRandom(SecureRandom... randoms) {
+		super(new CombinedSecureRandomSpi(randoms), PROVIDER);
+	}
+
+	private static class CombinedSecureRandomSpi extends SecureRandomSpi {
+
+		private static final long serialVersionUID = 483801767899979081L;
+
+		private final SecureRandom[] randoms;
+
+		private CombinedSecureRandomSpi(SecureRandom... randoms) {
+			if(randoms.length < 2) throw new IllegalArgumentException();
+			this.randoms = randoms;
+		}
+
+		@Override
+		protected byte[] engineGenerateSeed(int numBytes) {
+			byte[] combined = new byte[numBytes];
+			for(SecureRandom random : randoms) {
+				byte[] b = random.generateSeed(numBytes);
+				int length = Math.min(numBytes, b.length);
+				for(int i = 0; i < length; i++)
+					combined[i] = (byte) (combined[i] ^ b[i]);
+			}
+			return combined;
+		}
+
+		@Override
+		protected void engineNextBytes(byte[] b) {
+			byte[] temp = new byte[b.length];
+			for(SecureRandom random : randoms) {
+				random.nextBytes(temp);
+				for(int i = 0; i < b.length; i++)
+					b[i] = (byte) (b[i] ^ temp[i]);
+			}
+		}
+
+		@Override
+		protected void engineSetSeed(byte[] seed) {
+			for(SecureRandom random : randoms) random.setSeed(seed);
+		}
+	}
+
+	private static class CombinedProvider extends Provider {
+
+		private static final long serialVersionUID = -4678501890053703844L;
+
+		private CombinedProvider() {
+			super("Combined", 1.0, "");
+		}
+	}
+}