private key hex string is now always 64 characters long even when small

private key is random and as such can start by 0. This was not handled
properly and resulting hexa string was shorter than expected

src/main/java/io/proximax/core/crypto/PrivateKey.java

@@ -95,6 +95,6 @@ public boolean equals(final Object obj) {
 
     @Override
     public String toString() {
-        return HexEncoder.getString(this.value.toByteArray());
+        return HexEncoder.getString(this.value.toByteArray(), 32);
     }
 }

src/main/java/io/proximax/core/utils/HexEncoder.java

@@ -55,6 +55,21 @@ public static byte[] getBytes(final String hexString) {
       }
    }
 
+   /**
+    * <p>Converts a byte array to a hex string of at minimum specified length prefixed by 0 characters as needed</p>
+    *
+    * <p>This implementation is specifically intended to make sure that if key is 32 bytes then output is 64
+    * hexadecimal characters even if first bytes were 0 and input was shorter</p>
+    * 
+    * @param bytes The input byte array
+    * @param targetByteCount prefix 0 to make the source desired size
+    * @return The output hex string.
+    */
+   public static String getString(final byte[] bytes, int targetByteCount) {
+      String hexString = getString(bytes);
+      return StringUtils.repeat("00", targetByteCount - bytes.length) + hexString;
+   }
+
    /**
     * Converts a byte array to a hex string.
     *

src/main/java/io/proximax/core/utils/StringUtils.java

@@ -20,6 +20,10 @@
  * Static class that contains string utility functions.
  */
 public class StringUtils {
+
+   private StringUtils() {
+      // hiding implicit constructor for utility class
+   }
 
     /**
      * Determines if the specified string is null or empty.
@@ -51,6 +55,27 @@ public static boolean isNullOrWhitespace(final String str) {
         return true;
     }
 
+    /**
+     * <p>Repeat a String {@code repeat} times to form a new String.</p>
+     *
+     * <pre>
+     * StringUtils.repeat(null, 2) = null
+     * StringUtils.repeat("", 0)   = ""
+     * StringUtils.repeat("", 2)   = ""
+     * StringUtils.repeat("a", 3)  = "aaa"
+     * StringUtils.repeat("ab", 2) = "abab"
+     * StringUtils.repeat("a", -2) = ""
+     * </pre>
+     *
+     * @param str  the String to repeat, may be null
+     * @param repeat  number of times to repeat str, negative treated as zero
+     * @return a new String consisting of the original String repeated,
+     *  {@code null} if null String input
+     */
+    public static String repeat(String str, int repeat) {
+       return org.apache.commons.lang3.StringUtils.repeat(str, repeat);
+    }
+
     /**
      * Replaces a variable contained in a string with a value. A variable is defined as ${variable}.
      * This pattern is replaced by the given value.

src/test/java/io/proximax/core/crypto/PrivateKeyTest.java

@@ -16,6 +16,7 @@
 
 package io.proximax.core.crypto;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.math.BigInteger;
@@ -134,9 +135,9 @@ public void hashCodesAreEqualForEquivalentObjects() {
 
     @Test
     public void toStringReturnsHexRepresentation() {
-        // Assert:
-        MatcherAssert.assertThat(PrivateKey.fromHexString("2275").toString(), IsEqual.equalTo("2275"));
-        MatcherAssert.assertThat(PrivateKey.fromDecimalString("2275").toString(), IsEqual.equalTo("08e3"));
+       // private key is 32 bytes => 64 hexadecimal characters
+       assertEquals("0000000000000000000000000000000000000000000000000000000000002275", PrivateKey.fromHexString("2275").toString());
+       assertEquals("00000000000000000000000000000000000000000000000000000000000008e3", PrivateKey.fromDecimalString("2275").toString());
     }
 
     //endregion

src/test/java/io/proximax/core/utils/HexEncoderTest.java

@@ -16,6 +16,8 @@
 
 package io.proximax.core.utils;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 import org.hamcrest.MatcherAssert;
 import org.hamcrest.core.IsEqual;
 import org.junit.jupiter.api.Test;
@@ -84,5 +86,10 @@ public void getStringCanConvertBytesWithLeadingZerosToHexString() {
                 "00000d465457");
     }
 
-    //endregion
+    @Test
+    void getStringOfGivenLength() {
+       assertEquals("01", HexEncoder.getString(new byte[] {1}, 1));
+       assertEquals("0001", HexEncoder.getString(new byte[] {1}, 2));
+       assertEquals("010101", HexEncoder.getString(new byte[] {1, 1, 1}, 2));
+    }
 }

src/test/java/io/proximax/core/utils/StringUtilsTest.java

@@ -16,6 +16,8 @@
 
 package io.proximax.core.utils;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 import org.hamcrest.MatcherAssert;
 import org.hamcrest.core.IsEqual;
 import org.junit.jupiter.api.Test;
@@ -85,4 +87,14 @@ public void replaceVariableCanReplaceMultipleOccurrencesOfVariable() {
         MatcherAssert.assertThat(StringUtils.replaceVariable("Buffalo ${} Buffalo ${} ${} ${} Buffalo ${}", "", "buffalo"),
                 IsEqual.equalTo("Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo"));
     }
+
+    @Test
+    void repeat() {
+       assertEquals("", StringUtils.repeat("hello", 0));
+       assertEquals("", StringUtils.repeat("hello", -2));
+       assertEquals("", StringUtils.repeat("", 7));
+       assertEquals("aaa", StringUtils.repeat("a", 3));
+       assertEquals("abab", StringUtils.repeat("ab", 2));
+       assertEquals(null, StringUtils.repeat(null, 2));
+    }
 }

src/test/java/io/proximax/sdk/model/account/AccountTest.java

@@ -74,7 +74,7 @@ void generateNewAccountTest(){
         Account account = Account.generateNewAccount(NetworkType.MIJIN_TEST);
         assertNotEquals(account.getPrivateKey(),null);
         assertNotEquals(account.getPublicKey(), null);
-        assertEquals(64, account.getPrivateKey().toString().length());
+        assertEquals(64, account.getPrivateKey().length());
     }
 
     @Test