diff --git a/src/main/java/com/example/bip39/entropy/EntropySource.java b/src/main/java/com/example/bip39/entropy/EntropySource.java
new file mode 100644
index 0000000..35e1442
--- /dev/null
+++ b/src/main/java/com/example/bip39/entropy/EntropySource.java
@@ -0,0 +1,7 @@
+package com.example.bip39.entropy;
+
+@FunctionalInterface
+public interface EntropySource {
+
+  void nextBytes(byte[] bytes);
+}
diff --git a/src/main/java/com/example/bip39/entropy/SecureEntropyGenerator.java b/src/main/java/com/example/bip39/entropy/SecureEntropyGenerator.java
new file mode 100644
index 0000000..582b5ee
--- /dev/null
+++ b/src/main/java/com/example/bip39/entropy/SecureEntropyGenerator.java
@@ -0,0 +1,36 @@
+package com.example.bip39.entropy;
+
+import com.example.bip39.api.Bip39Service;
+import com.example.bip39.error.Bip39ErrorCode;
+import com.example.bip39.error.Bip39Exception;
+import com.example.bip39.util.Bip39Constants;
+import java.security.SecureRandom;
+import java.util.Objects;
+
+public final class SecureEntropyGenerator {
+
+  private final EntropySource entropySource;
+
+  public SecureEntropyGenerator() {
+    this(new SecureRandom()::nextBytes);
+  }
+
+  SecureEntropyGenerator(EntropySource entropySource) {
+    this.entropySource = Objects.requireNonNull(entropySource, "entropySource must not be null");
+  }
+
+  public byte[] generateEntropy(int numBytes) {
+    if (!Bip39Constants.isAllowedEntropyLengthBytes(numBytes)) {
+      throw new Bip39Exception(Bip39ErrorCode.ERR_ENTROPY_LENGTH, "Unsupported entropy length");
+    }
+
+    byte[] entropy = new byte[numBytes];
+    entropySource.nextBytes(entropy);
+    return entropy;
+  }
+
+  public String generateMnemonic(int numBytes, Bip39Service bip39Service) {
+    Objects.requireNonNull(bip39Service, "bip39Service must not be null");
+    return bip39Service.entropyToMnemonic(generateEntropy(numBytes));
+  }
+}
diff --git a/src/main/java/com/example/bip39/normalize/MnemonicInputNormalizer.java b/src/main/java/com/example/bip39/normalize/MnemonicInputNormalizer.java
new file mode 100644
index 0000000..7afcea7
--- /dev/null
+++ b/src/main/java/com/example/bip39/normalize/MnemonicInputNormalizer.java
@@ -0,0 +1,52 @@
+package com.example.bip39.normalize;
+
+import java.text.Normalizer;
+import java.text.Normalizer.Form;
+import java.util.Locale;
+import java.util.Objects;
+
+public final class MnemonicInputNormalizer {
+
+  private MnemonicInputNormalizer() {}
+
+  public static String normalizeMnemonicInput(String text) {
+    Objects.requireNonNull(text, "text must not be null");
+
+    String trimmed = text.strip();
+    String whitespaceNormalized = replaceAsciiWhitespaceWithSpace(trimmed);
+    String singleSpaced = collapseSpaces(whitespaceNormalized);
+    String nfkdNormalized = Normalizer.normalize(singleSpaced, Form.NFKD);
+    return nfkdNormalized.toLowerCase(Locale.ROOT);
+  }
+
+  private static String replaceAsciiWhitespaceWithSpace(String text) {
+    StringBuilder normalized = new StringBuilder(text.length());
+    for (int index = 0; index < text.length(); index++) {
+      char character = text.charAt(index);
+      if (character == '\t' || character == '\n' || character == '\r') {
+        normalized.append(' ');
+      } else {
+        normalized.append(character);
+      }
+    }
+    return normalized.toString();
+  }
+
+  private static String collapseSpaces(String text) {
+    StringBuilder collapsed = new StringBuilder(text.length());
+    boolean previousWasSpace = false;
+    for (int index = 0; index < text.length(); index++) {
+      char character = text.charAt(index);
+      if (character == ' ') {
+        if (!previousWasSpace) {
+          collapsed.append(character);
+        }
+        previousWasSpace = true;
+      } else {
+        collapsed.append(character);
+        previousWasSpace = false;
+      }
+    }
+    return collapsed.toString();
+  }
+}
diff --git a/src/test/java/com/example/bip39/api/ErrorPriorityTest.java b/src/test/java/com/example/bip39/api/ErrorPriorityTest.java
new file mode 100644
index 0000000..25b90be
--- /dev/null
+++ b/src/test/java/com/example/bip39/api/ErrorPriorityTest.java
@@ -0,0 +1,62 @@
+package com.example.bip39.api;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import com.example.bip39.error.Bip39ErrorCode;
+import com.example.bip39.error.Bip39Exception;
+import org.junit.jupiter.api.Test;
+
+class ErrorPriorityTest {
+
+  private static final String INVALID_WORD_COUNT_WITH_UNKNOWN_WORD =
+      "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon" + " typo";
+
+  private static final String WORD_NOT_IN_LIST_WITH_VALID_WORD_COUNT =
+      "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon"
+          + " abandon typo";
+
+  @Test
+  void validateMnemonicPrioritizesFormatBeforeAnyDeeperChecks() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    assertEquals(
+        Bip39ErrorCode.ERR_INVALID_MNEMONIC_FORMAT,
+        service.validateMnemonic("abandon	abandon typo").errorCode());
+  }
+
+  @Test
+  void validateMnemonicPrioritizesWordCountBeforeWordListChecks() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    assertEquals(
+        Bip39ErrorCode.ERR_INVALID_WORD_COUNT,
+        service.validateMnemonic(INVALID_WORD_COUNT_WITH_UNKNOWN_WORD).errorCode());
+  }
+
+  @Test
+  void validateMnemonicPrioritizesWordListBeforeChecksumMismatch() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    assertEquals(
+        Bip39ErrorCode.ERR_WORD_NOT_IN_LIST,
+        service.validateMnemonic(WORD_NOT_IN_LIST_WITH_VALID_WORD_COUNT).errorCode());
+  }
+
+  @Test
+  void mnemonicToEntropyUsesSamePriorityOrder() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    Bip39Exception invalidWordCount =
+        assertThrows(
+            Bip39Exception.class,
+            () -> service.mnemonicToEntropy(INVALID_WORD_COUNT_WITH_UNKNOWN_WORD));
+    assertEquals(Bip39ErrorCode.ERR_INVALID_WORD_COUNT, invalidWordCount.getErrorCode());
+
+    Bip39Exception wordNotInList =
+        assertThrows(
+            Bip39Exception.class,
+            () -> service.mnemonicToEntropy(WORD_NOT_IN_LIST_WITH_VALID_WORD_COUNT));
+    assertEquals(Bip39ErrorCode.ERR_WORD_NOT_IN_LIST, wordNotInList.getErrorCode());
+  }
+}
diff --git a/src/test/java/com/example/bip39/api/FixedFailureCasesTest.java b/src/test/java/com/example/bip39/api/FixedFailureCasesTest.java
new file mode 100644
index 0000000..1cf40f1
--- /dev/null
+++ b/src/test/java/com/example/bip39/api/FixedFailureCasesTest.java
@@ -0,0 +1,102 @@
+package com.example.bip39.api;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import com.example.bip39.error.Bip39ErrorCode;
+import com.example.bip39.error.Bip39Exception;
+import com.example.bip39.model.ValidationResult;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class FixedFailureCasesTest {
+
+  private static final String INVALID_WORD_COUNT_MNEMONIC =
+      "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon";
+
+  private static final String CHECKSUM_MISMATCH_MNEMONIC =
+      "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon"
+          + " abandon";
+
+  private static final String WORD_NOT_IN_LIST_MNEMONIC =
+      "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon"
+          + " typo";
+
+  private static final String INVALID_FORMAT_MNEMONIC =
+      "abandon	abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon"
+          + " about";
+
+  @Test
+  void appendixCCase1RejectsInvalidEntropyLength() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    Bip39Exception exception =
+        assertThrows(Bip39Exception.class, () -> service.entropyToMnemonic(new byte[15]));
+
+    assertEquals(Bip39ErrorCode.ERR_ENTROPY_LENGTH, exception.getErrorCode());
+  }
+
+  @Test
+  void appendixCCase2RejectsInvalidWordCount() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    Bip39Exception exception =
+        assertThrows(
+            Bip39Exception.class, () -> service.mnemonicToEntropy(INVALID_WORD_COUNT_MNEMONIC));
+
+    assertEquals(Bip39ErrorCode.ERR_INVALID_WORD_COUNT, exception.getErrorCode());
+  }
+
+  @Test
+  void appendixCCase3RejectsChecksumMismatch() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    Bip39Exception exception =
+        assertThrows(
+            Bip39Exception.class, () -> service.mnemonicToEntropy(CHECKSUM_MISMATCH_MNEMONIC));
+
+    assertEquals(Bip39ErrorCode.ERR_CHECKSUM_MISMATCH, exception.getErrorCode());
+  }
+
+  @Test
+  void appendixCCase4RejectsWordNotInList() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    Bip39Exception exception =
+        assertThrows(
+            Bip39Exception.class, () -> service.mnemonicToEntropy(WORD_NOT_IN_LIST_MNEMONIC));
+
+    assertEquals(Bip39ErrorCode.ERR_WORD_NOT_IN_LIST, exception.getErrorCode());
+  }
+
+  @Test
+  void appendixCCase5RejectsInvalidFormatInCoreApis() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    Bip39Exception entropyException =
+        assertThrows(
+            Bip39Exception.class, () -> service.mnemonicToEntropy(INVALID_FORMAT_MNEMONIC));
+    assertEquals(Bip39ErrorCode.ERR_INVALID_MNEMONIC_FORMAT, entropyException.getErrorCode());
+
+    ValidationResult validationResult = service.validateMnemonic(INVALID_FORMAT_MNEMONIC);
+    assertFalse(validationResult.ok());
+    assertEquals(Bip39ErrorCode.ERR_INVALID_MNEMONIC_FORMAT, validationResult.errorCode());
+    assertNull(validationResult.normalizedMnemonic());
+    assertNull(validationResult.wordCount());
+    assertNull(validationResult.invalidWord());
+  }
+
+  @Test
+  void appendixCCase6RejectsInvalidSeedWordListStructure() {
+    DefaultBip39Service service = new DefaultBip39Service();
+
+    Bip39Exception exception =
+        assertThrows(
+            Bip39Exception.class,
+            () -> service.mnemonicToSeed(List.of("abandon", "", "abandon"), ""));
+
+    assertEquals(Bip39ErrorCode.ERR_INVALID_MNEMONIC_FORMAT, exception.getErrorCode());
+  }
+}
diff --git a/src/test/java/com/example/bip39/entropy/SecureEntropyGeneratorTest.java b/src/test/java/com/example/bip39/entropy/SecureEntropyGeneratorTest.java
new file mode 100644
index 0000000..aa1f802
--- /dev/null
+++ b/src/test/java/com/example/bip39/entropy/SecureEntropyGeneratorTest.java
@@ -0,0 +1,62 @@
+package com.example.bip39.entropy;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import com.example.bip39.api.DefaultBip39Service;
+import com.example.bip39.error.Bip39ErrorCode;
+import com.example.bip39.error.Bip39Exception;
+import com.example.bip39.util.Bip39Constants;
+import java.util.Arrays;
+import org.junit.jupiter.api.Test;
+
+class SecureEntropyGeneratorTest {
+
+  private static final String ZERO_ENTROPY_MNEMONIC =
+      "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon"
+          + " about";
+
+  @Test
+  void rejectsUnsupportedEntropyLengths() {
+    SecureEntropyGenerator generator =
+        new SecureEntropyGenerator(bytes -> Arrays.fill(bytes, (byte) 0));
+
+    Bip39Exception exception =
+        assertThrows(Bip39Exception.class, () -> generator.generateEntropy(12));
+
+    assertEquals(Bip39ErrorCode.ERR_ENTROPY_LENGTH, exception.getErrorCode());
+  }
+
+  @Test
+  void generatesAllAllowedEntropyLengths() {
+    SecureEntropyGenerator generator =
+        new SecureEntropyGenerator(bytes -> Arrays.fill(bytes, (byte) 0));
+
+    for (int entropyLength : Bip39Constants.ALLOWED_ENTROPY_LENGTHS_BYTES) {
+      assertEquals(entropyLength, generator.generateEntropy(entropyLength).length);
+    }
+  }
+
+  @Test
+  void generatesDeterministicEntropyWhenSourceIsInjected() {
+    byte[] expectedEntropy = new byte[16];
+    for (int index = 0; index < expectedEntropy.length; index++) {
+      expectedEntropy[index] = (byte) (index + 1);
+    }
+
+    SecureEntropyGenerator generator =
+        new SecureEntropyGenerator(
+            bytes -> System.arraycopy(expectedEntropy, 0, bytes, 0, bytes.length));
+
+    assertArrayEquals(expectedEntropy, generator.generateEntropy(16));
+  }
+
+  @Test
+  void convertsGeneratedEntropyThroughBip39ServiceUsingThinAdapter() {
+    SecureEntropyGenerator generator =
+        new SecureEntropyGenerator(bytes -> Arrays.fill(bytes, (byte) 0));
+
+    assertEquals(ZERO_ENTROPY_MNEMONIC, generator.generateMnemonic(16, new DefaultBip39Service()));
+  }
+}
diff --git a/src/test/java/com/example/bip39/normalize/MnemonicInputNormalizerTest.java b/src/test/java/com/example/bip39/normalize/MnemonicInputNormalizerTest.java
new file mode 100644
index 0000000..7c2ebd6
--- /dev/null
+++ b/src/test/java/com/example/bip39/normalize/MnemonicInputNormalizerTest.java
@@ -0,0 +1,60 @@
+package com.example.bip39.normalize;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.example.bip39.api.DefaultBip39Service;
+import com.example.bip39.error.Bip39ErrorCode;
+import com.example.bip39.error.Bip39Exception;
+import com.example.bip39.model.ValidationResult;
+import org.junit.jupiter.api.Test;
+
+class MnemonicInputNormalizerTest {
+
+  private static final String ZERO_ENTROPY_MNEMONIC =
+      "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon"
+          + " about";
+
+  @Test
+  void normalizesWhitespaceAndCaseForEnglishProfile() {
+    String rawMnemonic =
+        "  ABANDON\tabandon\nABANDON\rabandon  abandon\tabandon\nabandon\rabandon"
+            + "  abandon\tabandon\nABANDON\rABOUT  ";
+
+    assertEquals(
+        ZERO_ENTROPY_MNEMONIC, MnemonicInputNormalizer.normalizeMnemonicInput(rawMnemonic));
+  }
+
+  @Test
+  void appliesNfkdBeforeLowercasing() {
+    String rawMnemonic = "\tＡＢＡＮＤＯＮ\nＡＢＯＵＴ\r";
+
+    assertEquals("abandon about", MnemonicInputNormalizer.normalizeMnemonicInput(rawMnemonic));
+  }
+
+  @Test
+  void letsUiLayerExplicitlyNormalizeBeforeCallingStrictCoreApis() {
+    DefaultBip39Service service = new DefaultBip39Service();
+    String rawMnemonic =
+        "  ABANDON\tabandon\nABANDON\rabandon  abandon\tabandon\nabandon\rabandon"
+            + "  abandon\tabandon\nABANDON\rABOUT  ";
+
+    Bip39Exception exception =
+        assertThrows(Bip39Exception.class, () -> service.mnemonicToEntropy(rawMnemonic));
+    assertEquals(Bip39ErrorCode.ERR_INVALID_MNEMONIC_FORMAT, exception.getErrorCode());
+
+    ValidationResult rawValidation = service.validateMnemonic(rawMnemonic);
+    assertFalse(rawValidation.ok());
+    assertEquals(Bip39ErrorCode.ERR_INVALID_MNEMONIC_FORMAT, rawValidation.errorCode());
+
+    String normalizedMnemonic = MnemonicInputNormalizer.normalizeMnemonicInput(rawMnemonic);
+    assertArrayEquals(new byte[16], service.mnemonicToEntropy(normalizedMnemonic));
+
+    ValidationResult normalizedValidation = service.validateMnemonic(normalizedMnemonic);
+    assertTrue(normalizedValidation.ok());
+    assertEquals(ZERO_ENTROPY_MNEMONIC, normalizedValidation.normalizedMnemonic());
+  }
+}