diceware: add passphrase generator

passgen/diceware/api/diceware.api

@@ -1,3 +1,9 @@
+public final class dev/msfjarvis/aps/passgen/diceware/DicewarePassphraseGenerator {
+	public static final field DIGITS I
+	public fun <init> (Ldev/msfjarvis/aps/passgen/diceware/Die;Ljava/io/InputStream;)V
+	public final fun generatePassphrase (IC)Ljava/lang/String;
+}
+
 public final class dev/msfjarvis/aps/passgen/diceware/Die {
 	public fun <init> (ILdev/msfjarvis/aps/passgen/diceware/RandomIntGenerator;)V
 	public final fun roll ()I

passgen/diceware/src/main/kotlin/dev/msfjarvis/aps/passgen/diceware/DicewarePassphraseGenerator.kt

@@ -0,0 +1,42 @@
+/*
+ * Copyright © 2014-2021 The Android Password Store Authors. All Rights Reserved.
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package dev.msfjarvis.aps.passgen.diceware
+
+import java.io.InputStream
+import javax.inject.Inject
+
+/**
+ * Password generator implementing the Diceware passphrase generation mechanism. For detailed
+ * information on how this works, see https://theworld.com/~reinhold/diceware.html.
+ */
+public class DicewarePassphraseGenerator
+@Inject
+constructor(
+  private val die: Die,
+  wordList: InputStream,
+) {
+
+  private val wordMap = WordListParser.parse(wordList)
+
+  /** Generates a passphrase with [wordCount] words. */
+  public fun generatePassphrase(wordCount: Int, separator: Char): String {
+    return StringBuilder()
+      .apply {
+        repeat(wordCount) { idx ->
+          append(wordMap[die.rollMultiple(DIGITS)])
+          if (idx < wordCount - 1) append(separator)
+        }
+      }
+      .toString()
+      .trimEnd()
+  }
+
+  private companion object {
+
+    /** Number of digits used by indices in the default wordlist. */
+    const val DIGITS: Int = 5
+  }
+}

passgen/diceware/src/test/kotlin/dev/msfjarvis/aps/passgen/diceware/DicewarePassphraseGeneratorTest.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright © 2014-2021 The Android Password Store Authors. All Rights Reserved.
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package dev.msfjarvis.aps.passgen.diceware
+
+import kotlin.random.Random
+import kotlin.test.assertEquals
+import org.junit.Test
+
+class DicewarePassphraseGeneratorTest {
+  /** Pre-seeded [Random] instance to ensure tests are deterministic. */
+  private val random = Random(1_00_000)
+
+  private val intGenerator = RandomIntGenerator { it.random(random) }
+  @Test
+  fun generate_passphrase() {
+    val die = Die(6, intGenerator)
+
+    val generator =
+      DicewarePassphraseGenerator(
+        die,
+        WordListParserTest.getDefaultWordList(),
+      )
+
+    assertEquals("salvation_cozily_croon_trustee_fidgety", generator.generatePassphrase(5, '_'))
+  }
+}