Adding MixedCaseChecksum feature implementing EIP55 standard

crypto-key-common/src/main/java/com/syntifi/crypto/key/checksum/MixedCaseChecksum.java

@@ -0,0 +1,38 @@
+package com.syntifi.crypto.key.checksum;
+
+import org.bouncycastle.jcajce.provider.digest.Keccak;
+import org.bouncycastle.util.encoders.Hex;
+
+/**
+ * Implementation of the EIP-55: Mixed-case checksum address encoding
+ * Documentation available at: https://eips.ethereum.org/EIPS/eip-55
+ *
+ * @author Alexandre Carvalho
+ * @author Andre Bertolace
+ * @since 0.5.0
+ */
+public class MixedCaseChecksum {
+
+    public static String kekkac256(String value) {
+        Keccak.DigestKeccak kekkac256 = new Keccak.Digest256();
+        byte[] hash = kekkac256.digest(value.getBytes());
+        return Hex.toHexString(hash);
+    }
+
+    public static String checksumEncode(String address) {
+        Boolean withPrefix = address.startsWith("0x");
+        String value = address.toLowerCase().replace("0x", "");
+        char[] hash = kekkac256(value).toCharArray();
+        char[] chars = value.toCharArray();
+        char[] encoded = new char[chars.length];
+        for (int i = 0; i < chars.length; i++) {
+            encoded[i] = Character.digit(hash[i], 16) > 7
+                    ? Character.toUpperCase(chars[i])
+                    : chars[i];
+        }
+        return withPrefix
+                ? "0x" + String.valueOf(encoded)
+                : String.valueOf(encoded);
+    }
+
+}

crypto-key-common/src/test/java/com/syntifi/crypto/key/checksum/MixedCaseChecksumTest.java

@@ -0,0 +1,25 @@
+package com.syntifi.crypto.key.checksum;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class MixedCaseChecksumTest {
+
+    @Test
+    void shouldMatchLowerCaseAddress() {
+        assertEquals("0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359",
+                MixedCaseChecksum.checksumEncode("0xfb6916095ca1df60bb79ce92ce3ea74c37c5d359"));
+    }
+
+    @Test
+    void shouldMatchUpperCaseAddress() {
+        assertEquals("0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359",
+                MixedCaseChecksum.checksumEncode("0xFB6916095CA1DF60BB79CE92CE3EA74C37C5D359"));
+    }
+
+    @Test
+    void kekkac256ShouldMatch() {
+        assertEquals("a9059cbb2ab09eb219583f4a59a5d0623ade346d962bcd4e46b11da047c9049b",
+                MixedCaseChecksum.kekkac256("transfer(address,uint256)"));
+    }
+}