Refactor password generation (#860)

* Refactor password generation

* Update Extensions.kt

* Update app/src/main/java/com/zeapo/pwdstore/pwgen/PasswordGenerator.kt

Co-authored-by: Harsh Shandilya <me@msfjarvis.dev>

* Address review comments

Co-authored-by: Harsh Shandilya <me@msfjarvis.dev>

5 files changed, 314 insertions, 407 deletions

diff --git a/app/src/main/java/com/zeapo/pwdstore/pwgen/PasswordGenerator.kt b/app/src/main/java/com/zeapo/pwdstore/pwgen/PasswordGenerator.kt
index 9f7faa56..b4accec7 100644
--- a/app/src/main/java/com/zeapo/pwdstore/pwgen/PasswordGenerator.kt
+++ b/app/src/main/java/com/zeapo/pwdstore/pwgen/PasswordGenerator.kt
@@ -7,139 +7,131 @@ package com.zeapo.pwdstore.pwgen
 import android.content.Context
 import androidx.core.content.edit
 import com.zeapo.pwdstore.R
-import java.util.ArrayList
+import com.zeapo.pwdstore.utils.clearFlag
+import com.zeapo.pwdstore.utils.hasFlag
+
+enum class PasswordOption(val key: String) {
+    NoDigits("0"),
+    NoUppercaseLetters("A"),
+    NoAmbiguousCharacters("B"),
+    FullyRandom("s"),
+    AtLeastOneSymbol("y"),
+    NoLowercaseLetters("L")
+}
 
 object PasswordGenerator {
-    internal const val DIGITS = 0x0001
-    internal const val UPPERS = 0x0002
-    internal const val SYMBOLS = 0x0004
-    internal const val AMBIGUOUS = 0x0008
-    internal const val NO_VOWELS = 0x0010
-    internal const val LOWERS = 0x0020
+    const val DEFAULT_LENGTH = 16
 
-    internal const val DIGITS_STR = "0123456789"
-    internal const val UPPERS_STR = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-    internal const val LOWERS_STR = "abcdefghijklmnopqrstuvwxyz"
-    internal const val SYMBOLS_STR = "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"
-    internal const val AMBIGUOUS_STR = "B8G6I1l0OQDS5Z2"
-    internal const val VOWELS_STR = "01aeiouyAEIOUY"
+    const val DIGITS = 0x0001
+    const val UPPERS = 0x0002
+    const val SYMBOLS = 0x0004
+    const val NO_AMBIGUOUS = 0x0008
+    const val LOWERS = 0x0020
 
-    // No a, c, n, h, H, C, 1, N
-    private const val pwOptions = "0ABsvyL"
+    const val DIGITS_STR = "0123456789"
+    const val UPPERS_STR = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+    const val LOWERS_STR = "abcdefghijklmnopqrstuvwxyz"
+    const val SYMBOLS_STR = "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"
+    const val AMBIGUOUS_STR = "B8G6I1l0OQDS5Z2"
 
     /**
-     * Sets password generation preferences.
-     *
-     * @param ctx context from which to retrieve SharedPreferences from
-     * preferences file 'PasswordGenerator'
-     * @param argv options for password generation
-     * <table summary="options for password generation">
-     * <tr><td>Option</td><td>Description</td></tr>
-     * <tr><td>0</td><td>don't include numbers</td></tr>
-     * <tr><td>A</td><td>don't include uppercase letters</td></tr>
-     * <tr><td>B</td><td>don't include ambiguous charactersl</td></tr>
-     * <tr><td>s</td><td>generate completely random passwords</td></tr>
-     * <tr><td>v</td><td>don't include vowels</td></tr>
-     * <tr><td>y</td><td>include at least one symbol</td></tr>
-     * <tr><td>L</td><td>don't include lowercase letters</td></tr>
-    </table> *
-     * @param numArgv numerical options for password generation: length of
-     * generated passwords followed by number of passwords to
-     * generate
-     * @return `false` if a numerical options is invalid,
-     * `true` otherwise
+     * Enables the [PasswordOption]s in [options] and sets [targetLength] as the length for
+     * generated passwords.
      */
-    @JvmStatic
-    fun setPrefs(ctx: Context, argv: ArrayList<String>, vararg numArgv: Int): Boolean {
+    fun setPrefs(ctx: Context, options: List<PasswordOption>, targetLength: Int): Boolean {
         ctx.getSharedPreferences("PasswordGenerator", Context.MODE_PRIVATE).edit {
-            for (option in pwOptions.toCharArray()) {
-                if (argv.contains(option.toString())) {
-                    putBoolean(option.toString(), true)
-                    argv.remove(option.toString())
-                } else {
-                    putBoolean(option.toString(), false)
-                }
-            }
-            var i = 0
-            while (i < numArgv.size && i < 2) {
-                if (numArgv[i] <= 0) {
-                    // Invalid password length or number of passwords
-                    return false
-                }
-                val name = if (i == 0) "length" else "num"
-                putInt(name, numArgv[i])
-                i++
-            }
+            for (possibleOption in PasswordOption.values())
+                putBoolean(possibleOption.key, possibleOption in options)
+            putInt("length", targetLength)
         }
         return true
     }
 
+    fun isValidPassword(password: String, pwFlags: Int): Boolean {
+        if (pwFlags hasFlag DIGITS && password.none { it in DIGITS_STR })
+            return false
+        if (pwFlags hasFlag UPPERS && password.none { it in UPPERS_STR })
+            return false
+        if (pwFlags hasFlag LOWERS && password.none { it in LOWERS_STR })
+            return false
+        if (pwFlags hasFlag SYMBOLS && password.none { it in SYMBOLS_STR })
+            return false
+        if (pwFlags hasFlag NO_AMBIGUOUS && password.any { it in AMBIGUOUS_STR })
+            return false
+        return true
+    }
+
     /**
-     * Generates passwords using the preferences set by
-     * [.setPrefs].
-     *
-     * @param ctx context from which to retrieve SharedPreferences from
-     * preferences file 'PasswordGenerator'
-     * @return list of generated passwords
+     * Generates a password using the preferences set by [setPrefs].
      */
-    @JvmStatic
-    @Throws(PasswordGeneratorExeption::class)
-    fun generate(ctx: Context): ArrayList<String> {
+    @Throws(PasswordGeneratorException::class)
+    fun generate(ctx: Context): String {
         val prefs = ctx.getSharedPreferences("PasswordGenerator", Context.MODE_PRIVATE)
+        var numCharacterCategories = 0
 
         var phonemes = true
         var pwgenFlags = DIGITS or UPPERS or LOWERS
 
-        for (option in pwOptions.toCharArray()) {
-            if (prefs.getBoolean(option.toString(), false)) {
+        for (option in PasswordOption.values()) {
+            if (prefs.getBoolean(option.key, false)) {
                 when (option) {
-                    '0' -> pwgenFlags = pwgenFlags and DIGITS.inv()
-                    'A' -> pwgenFlags = pwgenFlags and UPPERS.inv()
-                    'L' -> pwgenFlags = pwgenFlags and LOWERS.inv()
-                    'B' -> pwgenFlags = pwgenFlags or AMBIGUOUS
-                    's' -> phonemes = false
-                    'y' -> pwgenFlags = pwgenFlags or SYMBOLS
-                    'v' -> {
-                        phonemes = false
-                        pwgenFlags = pwgenFlags or NO_VOWELS // | DIGITS | UPPERS;
+                    PasswordOption.NoDigits -> pwgenFlags = pwgenFlags.clearFlag(DIGITS)
+                    PasswordOption.NoUppercaseLetters -> pwgenFlags = pwgenFlags.clearFlag(UPPERS)
+                    PasswordOption.NoLowercaseLetters -> pwgenFlags = pwgenFlags.clearFlag(LOWERS)
+                    PasswordOption.NoAmbiguousCharacters -> pwgenFlags = pwgenFlags or NO_AMBIGUOUS
+                    PasswordOption.FullyRandom -> phonemes = false
+                    PasswordOption.AtLeastOneSymbol -> {
+                        numCharacterCategories++
+                        pwgenFlags = pwgenFlags or SYMBOLS
                     }
-                } // pwgenFlags = DIGITS | UPPERS;
+                }
+            } else {
+                // The No* options are false, so the respective character category will be included.
+                when (option) {
+                    PasswordOption.NoDigits,
+                    PasswordOption.NoUppercaseLetters,
+                    PasswordOption.NoLowercaseLetters -> {
+                        numCharacterCategories++
+                    }
+                    PasswordOption.NoAmbiguousCharacters,
+                    PasswordOption.FullyRandom,
+                        // Since AtLeastOneSymbol is not negated, it is counted in the if branch.
+                    PasswordOption.AtLeastOneSymbol -> {
+                    }
+                }
             }
         }
 
-        val length = prefs.getInt("length", 8)
-        var numCategories = 0
-        var categories = pwgenFlags and AMBIGUOUS.inv()
-
-        while (categories != 0) {
-            if (categories and 1 == 1)
-                numCategories++
-            categories = categories shr 1
-        }
-        if (numCategories == 0) {
-            throw PasswordGeneratorExeption(ctx.resources.getString(R.string.pwgen_no_chars_error))
+        val length = prefs.getInt("length", DEFAULT_LENGTH)
+        if (pwgenFlags.clearFlag(NO_AMBIGUOUS) == 0) {
+            throw PasswordGeneratorException(ctx.resources.getString(R.string.pwgen_no_chars_error))
         }
-        if (length < numCategories) {
-            throw PasswordGeneratorExeption(ctx.resources.getString(R.string.pwgen_length_too_short_error))
+        if (length < numCharacterCategories) {
+            throw PasswordGeneratorException(ctx.resources.getString(R.string.pwgen_length_too_short_error))
         }
-        if ((pwgenFlags and UPPERS) == 0 && (pwgenFlags and LOWERS) == 0) { // Only digits and/or symbols
+        if (!(pwgenFlags hasFlag UPPERS) && !(pwgenFlags hasFlag LOWERS)) {
             phonemes = false
-            pwgenFlags = pwgenFlags and AMBIGUOUS.inv()
-        } else if (length < 5) {
+            pwgenFlags = pwgenFlags.clearFlag(NO_AMBIGUOUS)
+        }
+        // Experiments show that phonemes may require more than 1000 iterations to generate a valid
+        // password if the length is not at least 6.
+        if (length < 6) {
             phonemes = false
         }
 
-        val passwords = ArrayList<String>()
-        val num = prefs.getInt("num", 1)
-        for (i in 0 until num) {
-            if (phonemes) {
-                passwords.add(Phonemes.phonemes(length, pwgenFlags))
+        var password: String?
+        var iterations = 0
+        do {
+            if (iterations++ > 1000)
+                throw PasswordGeneratorException(ctx.resources.getString(R.string.pwgen_max_iterations_exceeded))
+            password = if (phonemes) {
+                RandomPhonemesGenerator.generate(length, pwgenFlags)
             } else {
-                passwords.add(RandomPasswordGenerator.rand(length, pwgenFlags))
+                RandomPasswordGenerator.generate(length, pwgenFlags)
             }
-        }
-        return passwords
+        } while (password == null)
+        return password
     }
 
-    class PasswordGeneratorExeption(string: String) : Exception(string)
+    class PasswordGeneratorException(string: String) : Exception(string)
 }
diff --git a/app/src/main/java/com/zeapo/pwdstore/pwgen/Phonemes.kt b/app/src/main/java/com/zeapo/pwdstore/pwgen/Phonemes.kt
deleted file mode 100644
index c570ff1c..00000000
--- a/app/src/main/java/com/zeapo/pwdstore/pwgen/Phonemes.kt
+++ /dev/null
@@ -1,221 +0,0 @@
-/*
- * Copyright © 2014-2020 The Android Password Store Authors. All Rights Reserved.
- * SPDX-License-Identifier: GPL-3.0-only
- */
-package com.zeapo.pwdstore.pwgen
-
-internal object Phonemes {
-    private const val CONSONANT = 0x0001
-    private const val VOWEL = 0x0002
-    private const val DIPHTHONG = 0x0004
-    private const val NOT_FIRST = 0x0008
-
-    private val elements = arrayOf(
-        Element("a", VOWEL),
-        Element("ae", VOWEL or DIPHTHONG),
-        Element("ah", VOWEL or DIPHTHONG),
-        Element("ai", VOWEL or DIPHTHONG),
-        Element("b", CONSONANT),
-        Element("c", CONSONANT),
-        Element("ch", CONSONANT or DIPHTHONG),
-        Element("d", CONSONANT),
-        Element("e", VOWEL),
-        Element("ee", VOWEL or DIPHTHONG),
-        Element("ei", VOWEL or DIPHTHONG),
-        Element("f", CONSONANT),
-        Element("g", CONSONANT),
-        Element("gh", CONSONANT or DIPHTHONG or NOT_FIRST),
-        Element("h", CONSONANT),
-        Element("i", VOWEL),
-        Element("ie", VOWEL or DIPHTHONG),
-        Element("j", CONSONANT),
-        Element("k", CONSONANT),
-        Element("l", CONSONANT),
-        Element("m", CONSONANT),
-        Element("n", CONSONANT),
-        Element("ng", CONSONANT or DIPHTHONG or NOT_FIRST),
-        Element("o", VOWEL),
-        Element("oh", VOWEL or DIPHTHONG),
-        Element("oo", VOWEL or DIPHTHONG),
-        Element("p", CONSONANT),
-        Element("ph", CONSONANT or DIPHTHONG),
-        Element("qu", CONSONANT or DIPHTHONG),
-        Element("r", CONSONANT),
-        Element("s", CONSONANT),
-        Element("sh", CONSONANT or DIPHTHONG),
-        Element("t", CONSONANT),
-        Element("th", CONSONANT or DIPHTHONG),
-        Element("u", VOWEL),
-        Element("v", CONSONANT),
-        Element("w", CONSONANT),
-        Element("x", CONSONANT),
-        Element("y", CONSONANT),
-        Element("z", CONSONANT)
-    )
-
-    private val NUM_ELEMENTS = elements.size
-
-    private class Element internal constructor(internal var str: String, internal var flags: Int)
-
-    /**
-     * Generates a human-readable password.
-     *
-     * @param size length of password to generate
-     * @param pwFlags flag field where set bits indicate conditions the
-     * generated password must meet
-     * <table summary="bits of flag field">
-     * <tr><td>Bit</td><td>Condition</td></tr>
-     * <tr><td>0</td><td>include at least one number</td></tr>
-     * <tr><td>1</td><td>include at least one uppercase letter</td></tr>
-     * <tr><td>2</td><td>include at least one symbol</td></tr>
-     * <tr><td>3</td><td>don't include ambiguous characters</td></tr>
-     * <tr><td>5</td><td>include at least one lowercase letter</td></tr>
-    </table> *
-     * @return the generated password
-     */
-    fun phonemes(size: Int, pwFlags: Int): String {
-        var password: String
-        var curSize: Int
-        var i: Int
-        var length: Int
-        var flags: Int
-        var featureFlags: Int
-        var prev: Int
-        var shouldBe: Int
-        var first: Boolean
-        var str: String
-        var cha: Char
-
-        do {
-            password = ""
-            featureFlags = pwFlags
-            curSize = 0
-            prev = 0
-            first = true
-
-            shouldBe = if (RandomNumberGenerator.number(2) == 1) VOWEL else CONSONANT
-
-            while (curSize < size) {
-                i = RandomNumberGenerator.number(NUM_ELEMENTS)
-                str = elements[i].str
-                length = str.length
-                flags = elements[i].flags
-                // Filter on the basic type of the next Element
-                if (flags and shouldBe == 0) {
-                    continue
-                }
-                // Handle the NOT_FIRST flag
-                if (first && flags and NOT_FIRST > 0) {
-                    continue
-                }
-                // Don't allow VOWEL followed a Vowel/Diphthong pair
-                if (prev and VOWEL > 0 && flags and VOWEL > 0 &&
-                    flags and DIPHTHONG > 0
-                ) {
-                    continue
-                }
-                // Don't allow us to overflow the buffer
-                if (length > size - curSize) {
-                    continue
-                }
-                // OK, we found an Element which matches our criteria, let's do
-                // it
-                password += str
-
-                // Handle UPPERS
-                if (pwFlags and PasswordGenerator.UPPERS > 0) {
-                    if ((pwFlags and PasswordGenerator.LOWERS == 0) ||
-                        (first || flags and CONSONANT > 0) && RandomNumberGenerator.number(10) < 2) {
-                        val index = password.length - length
-                        password = password.substring(0, index) + str.toUpperCase()
-                        featureFlags = featureFlags and PasswordGenerator.UPPERS.inv()
-                    }
-                }
-
-                // Handle the AMBIGUOUS flag
-                if (pwFlags and PasswordGenerator.AMBIGUOUS > 0) {
-                    for (ambiguous in PasswordGenerator.AMBIGUOUS_STR.toCharArray()) {
-                        if (password.contains(ambiguous.toString())) {
-                            password = password.substring(0, curSize)
-
-                            // Still have upper letters
-                            if ((pwFlags and PasswordGenerator.UPPERS) > 0) {
-                                featureFlags = featureFlags or PasswordGenerator.UPPERS
-                                for (upper in PasswordGenerator.UPPERS_STR.toCharArray()) {
-                                    if (password.contains(upper.toString())) {
-                                        featureFlags = featureFlags and PasswordGenerator.UPPERS.inv()
-                                        break
-                                    }
-                                }
-                            }
-                            break
-                        }
-                    }
-                    if (password.length == curSize)
-                        continue
-                }
-
-                curSize += length
-
-                // Time to stop?
-                if (curSize >= size)
-                    break
-
-                // Handle DIGITS
-                if (pwFlags and PasswordGenerator.DIGITS > 0) {
-                    if (!first && RandomNumberGenerator.number(10) < 3) {
-                        var character: String
-                        do {
-                            cha = Character.forDigit(RandomNumberGenerator.number(10), 10)
-                            character = cha.toString()
-                        } while (pwFlags and PasswordGenerator.AMBIGUOUS > 0 &&
-                            PasswordGenerator.AMBIGUOUS_STR.contains(character))
-                        password += character
-                        curSize++
-
-                        featureFlags = featureFlags and PasswordGenerator.DIGITS.inv()
-
-                        first = true
-                        prev = 0
-                        shouldBe = if (RandomNumberGenerator.number(2) == 1) VOWEL else CONSONANT
-                        continue
-                    }
-                }
-
-                // Handle SYMBOLS
-                if (pwFlags and PasswordGenerator.SYMBOLS > 0) {
-                    if (!first && RandomNumberGenerator.number(10) < 2) {
-                        var character: String
-                        var num: Int
-                        do {
-                            num = RandomNumberGenerator.number(PasswordGenerator.SYMBOLS_STR.length)
-                            cha = PasswordGenerator.SYMBOLS_STR.toCharArray()[num]
-                            character = cha.toString()
-                        } while (pwFlags and PasswordGenerator.AMBIGUOUS > 0 &&
-                            PasswordGenerator.AMBIGUOUS_STR.contains(character))
-                        password += character
-                        curSize++
-
-                        featureFlags = featureFlags and PasswordGenerator.SYMBOLS.inv()
-                    }
-                }
-
-                // OK, figure out what the next Element should be
-                shouldBe = if (shouldBe == CONSONANT) {
-                    VOWEL
-                } else {
-                    if (prev and VOWEL > 0 || flags and DIPHTHONG > 0 ||
-                        RandomNumberGenerator.number(10) > 3
-                    ) {
-                        CONSONANT
-                    } else {
-                        VOWEL
-                    }
-                }
-                prev = flags
-                first = false
-            }
-        } while (featureFlags and (PasswordGenerator.UPPERS or PasswordGenerator.DIGITS or PasswordGenerator.SYMBOLS) > 0)
-        return password
-    }
-}
diff --git a/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomNumberGenerator.kt b/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomNumberGenerator.kt
index 698b3574..6c4c956e 100644
--- a/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomNumberGenerator.kt
+++ b/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomNumberGenerator.kt
@@ -4,27 +4,30 @@
  */
 package com.zeapo.pwdstore.pwgen
 
-import java.security.NoSuchAlgorithmException
 import java.security.SecureRandom
 
-internal object RandomNumberGenerator {
-    private var random: SecureRandom
+private val secureRandom = SecureRandom()
 
-    init {
-        try {
-            random = SecureRandom.getInstance("SHA1PRNG")
-        } catch (e: NoSuchAlgorithmException) {
-            throw SecurityException("SHA1PRNG not available", e)
-        }
-    }
+/**
+ * Returns a number between 0 (inclusive) and [exclusiveBound] (exclusive).
+ */
+fun secureRandomNumber(exclusiveBound: Int) = secureRandom.nextInt(exclusiveBound)
+
+/**
+ * Returns `true` and `false` with probablity 50% each.
+ */
+fun secureRandomBoolean() = secureRandom.nextBoolean()
 
-    /**
-     * Generate a random number n, where 0 &lt;= n &lt; maxNum.
-     *
-     * @param maxNum the bound on the random number to be returned
-     * @return the generated random number
-     */
-    fun number(maxNum: Int): Int {
-        return random.nextInt(maxNum)
-    }
+/**
+ * Returns `true` with probability [percentTrue]% and `false` with probability
+ * `(100 - [percentTrue])`%.
+ */
+fun secureRandomBiasedBoolean(percentTrue: Int): Boolean {
+    require(1 <= percentTrue) { "Probability for returning `true` must be at least 1%" }
+    require(percentTrue <= 99) { "Probability for returning `true` must be at most 99%" }
+    return secureRandomNumber(100) < percentTrue
 }
+
+fun <T> Array<T>.secureRandomElement() = this[secureRandomNumber(size)]
+fun <T> List<T>.secureRandomElement() = this[secureRandomNumber(size)]
+fun String.secureRandomCharacter() = this[secureRandomNumber(length)]
diff --git a/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomPasswordGenerator.kt b/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomPasswordGenerator.kt
index bd79f16a..e841ed76 100644
--- a/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomPasswordGenerator.kt
+++ b/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomPasswordGenerator.kt
@@ -4,77 +4,43 @@
  */
 package com.zeapo.pwdstore.pwgen
 
-internal object RandomPasswordGenerator {
+import com.zeapo.pwdstore.utils.hasFlag
+
+object RandomPasswordGenerator {
 
     /**
-     * Generates a completely random password.
+     * Generates a random password of length [targetLength], taking the following flags in [pwFlags]
+     * into account, or fails to do so and returns null:
      *
-     * @param size length of password to generate
-     * @param pwFlags flag field where set bits indicate conditions the
-     * generated password must meet
-     * <table summary ="bits of flag field">
-     * <tr><td>Bit</td><td>Condition</td></tr>
-     * <tr><td>0</td><td>include at least one number</td></tr>
-     * <tr><td>1</td><td>include at least one uppercase letter</td></tr>
-     * <tr><td>2</td><td>include at least one symbol</td></tr>
-     * <tr><td>3</td><td>don't include ambiguous characters</td></tr>
-     * <tr><td>4</td><td>don't include vowels</td></tr>
-     * <tr><td>5</td><td>include at least one lowercase</td></tr>
-    </table> *
-     * @return the generated password
+     * - [PasswordGenerator.DIGITS]: If set, the password will contain at least one digit; if not
+     *   set, the password will not contain any digits.
+     * - [PasswordGenerator.UPPERS]: If set, the password will contain at least one uppercase
+     *   letter; if not set, the password will not contain any uppercase letters.
+     * - [PasswordGenerator.LOWERS]: If set, the password will contain at least one lowercase
+     *   letter; if not set, the password will not contain any lowercase letters.
+     * - [PasswordGenerator.SYMBOLS]: If set, the password will contain at least one symbol; if not
+     *   set, the password will not contain any symbols.
+     * - [PasswordGenerator.NO_AMBIGUOUS]: If set, the password will not contain any ambiguous
+     *   characters.
+     * - [PasswordGenerator.NO_VOWELS]: If set, the password will not contain any vowels.
      */
-    fun rand(size: Int, pwFlags: Int): String {
-        var password: String
-        var cha: Char
-        var i: Int
-        var featureFlags: Int
-        var num: Int
-        var character: String
+    fun generate(targetLength: Int, pwFlags: Int): String? {
+        val bank = listOfNotNull(
+            PasswordGenerator.DIGITS_STR.takeIf { pwFlags hasFlag PasswordGenerator.DIGITS },
+            PasswordGenerator.UPPERS_STR.takeIf { pwFlags hasFlag PasswordGenerator.UPPERS },
+            PasswordGenerator.LOWERS_STR.takeIf { pwFlags hasFlag PasswordGenerator.LOWERS },
+            PasswordGenerator.SYMBOLS_STR.takeIf { pwFlags hasFlag PasswordGenerator.SYMBOLS }
+        ).joinToString("")
 
-        var bank = ""
-        if (pwFlags and PasswordGenerator.DIGITS > 0) {
-            bank += PasswordGenerator.DIGITS_STR
-        }
-        if (pwFlags and PasswordGenerator.UPPERS > 0) {
-            bank += PasswordGenerator.UPPERS_STR
-        }
-        if (pwFlags and PasswordGenerator.LOWERS > 0) {
-            bank += PasswordGenerator.LOWERS_STR
-        }
-        if (pwFlags and PasswordGenerator.SYMBOLS > 0) {
-            bank += PasswordGenerator.SYMBOLS_STR
-        }
-        do {
-            password = ""
-            featureFlags = pwFlags
-            i = 0
-            while (i < size) {
-                num = RandomNumberGenerator.number(bank.length)
-                cha = bank.toCharArray()[num]
-                character = cha.toString()
-                if (pwFlags and PasswordGenerator.AMBIGUOUS > 0 &&
-                    PasswordGenerator.AMBIGUOUS_STR.contains(character)) {
-                    continue
-                }
-                if (pwFlags and PasswordGenerator.NO_VOWELS > 0 && PasswordGenerator.VOWELS_STR.contains(character)) {
-                    continue
-                }
-                password += character
-                i++
-                if (PasswordGenerator.DIGITS_STR.contains(character)) {
-                    featureFlags = featureFlags and PasswordGenerator.DIGITS.inv()
-                }
-                if (PasswordGenerator.UPPERS_STR.contains(character)) {
-                    featureFlags = featureFlags and PasswordGenerator.UPPERS.inv()
-                }
-                if (PasswordGenerator.SYMBOLS_STR.contains(character)) {
-                    featureFlags = featureFlags and PasswordGenerator.SYMBOLS.inv()
-                }
-                if (PasswordGenerator.LOWERS_STR.contains(character)) {
-                    featureFlags = featureFlags and PasswordGenerator.LOWERS.inv()
-                }
+        var password = ""
+        while (password.length < targetLength) {
+            val candidate = bank.secureRandomCharacter()
+            if (pwFlags hasFlag PasswordGenerator.NO_AMBIGUOUS &&
+                candidate in PasswordGenerator.AMBIGUOUS_STR) {
+                continue
             }
-        } while (featureFlags and (PasswordGenerator.UPPERS or PasswordGenerator.DIGITS or PasswordGenerator.SYMBOLS or PasswordGenerator.LOWERS) > 0)
-        return password
+            password += candidate
+        }
+        return password.takeIf { PasswordGenerator.isValidPassword(it, pwFlags) }
     }
 }
diff --git a/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomPhonemesGenerator.kt b/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomPhonemesGenerator.kt
new file mode 100644
index 00000000..de375422
--- /dev/null
+++ b/app/src/main/java/com/zeapo/pwdstore/pwgen/RandomPhonemesGenerator.kt
@@ -0,0 +1,167 @@
+/*
+ * Copyright © 2014-2020 The Android Password Store Authors. All Rights Reserved.
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+package com.zeapo.pwdstore.pwgen
+
+import com.zeapo.pwdstore.utils.hasFlag
+import java.util.Locale
+
+object RandomPhonemesGenerator {
+    private const val CONSONANT = 0x0001
+    private const val VOWEL = 0x0002
+    private const val DIPHTHONG = 0x0004
+    private const val NOT_FIRST = 0x0008
+
+    private val elements = arrayOf(
+        Element("a", VOWEL),
+        Element("ae", VOWEL or DIPHTHONG),
+        Element("ah", VOWEL or DIPHTHONG),
+        Element("ai", VOWEL or DIPHTHONG),
+        Element("b", CONSONANT),
+        Element("c", CONSONANT),
+        Element("ch", CONSONANT or DIPHTHONG),
+        Element("d", CONSONANT),
+        Element("e", VOWEL),
+        Element("ee", VOWEL or DIPHTHONG),
+        Element("ei", VOWEL or DIPHTHONG),
+        Element("f", CONSONANT),
+        Element("g", CONSONANT),
+        Element("gh", CONSONANT or DIPHTHONG or NOT_FIRST),
+        Element("h", CONSONANT),
+        Element("i", VOWEL),
+        Element("ie", VOWEL or DIPHTHONG),
+        Element("j", CONSONANT),
+        Element("k", CONSONANT),
+        Element("l", CONSONANT),
+        Element("m", CONSONANT),
+        Element("n", CONSONANT),
+        Element("ng", CONSONANT or DIPHTHONG or NOT_FIRST),
+        Element("o", VOWEL),
+        Element("oh", VOWEL or DIPHTHONG),
+        Element("oo", VOWEL or DIPHTHONG),
+        Element("p", CONSONANT),
+        Element("ph", CONSONANT or DIPHTHONG),
+        Element("qu", CONSONANT or DIPHTHONG),
+        Element("r", CONSONANT),
+        Element("s", CONSONANT),
+        Element("sh", CONSONANT or DIPHTHONG),
+        Element("t", CONSONANT),
+        Element("th", CONSONANT or DIPHTHONG),
+        Element("u", VOWEL),
+        Element("v", CONSONANT),
+        Element("w", CONSONANT),
+        Element("x", CONSONANT),
+        Element("y", CONSONANT),
+        Element("z", CONSONANT)
+    )
+
+    private class Element(str: String, val flags: Int) {
+        val upperCase = str.toUpperCase(Locale.ROOT)
+        val lowerCase = str.toLowerCase(Locale.ROOT)
+        val length = str.length
+        val isAmbiguous = str.any { it in PasswordGenerator.AMBIGUOUS_STR }
+    }
+
+    /**
+     * Generates a random human-readable password of length [targetLength], taking the following
+     * flags in [pwFlags] into account, or fails to do so and returns null:
+     *
+     * - [PasswordGenerator.DIGITS]: If set, the password will contain at least one digit; if not
+     *   set, the password will not contain any digits.
+     * - [PasswordGenerator.UPPERS]: If set, the password will contain at least one uppercase
+     *   letter; if not set, the password will not contain any uppercase letters.
+     * - [PasswordGenerator.LOWERS]: If set, the password will contain at least one lowercase
+     *   letter; if not set and [PasswordGenerator.UPPERS] is set, the password will not contain any
+     *   lowercase characters; if both are not set, an exception is thrown.
+     * - [PasswordGenerator.SYMBOLS]: If set, the password will contain at least one symbol; if not
+     *   set, the password will not contain any symbols.
+     * - [PasswordGenerator.NO_AMBIGUOUS]: If set, the password will not contain any ambiguous
+     *   characters.
+     */
+    fun generate(targetLength: Int, pwFlags: Int): String? {
+        require(pwFlags hasFlag PasswordGenerator.UPPERS || pwFlags hasFlag PasswordGenerator.LOWERS)
+
+        var password = ""
+
+        var isStartOfPart = true
+        var nextBasicType = if (secureRandomBoolean()) VOWEL else CONSONANT
+        var previousFlags = 0
+
+        while (password.length < targetLength) {
+            // First part: Add a single letter or pronounceable pair of letters in varying case.
+
+            val candidate = elements.secureRandomElement()
+
+            // Reroll if the candidate does not fulfill the current requirements.
+            if (!candidate.flags.hasFlag(nextBasicType) ||
+                (isStartOfPart && candidate.flags hasFlag NOT_FIRST) ||
+                // Don't let a diphthong that starts with a vowel follow a vowel.
+                (previousFlags hasFlag VOWEL && candidate.flags hasFlag VOWEL && candidate.flags hasFlag DIPHTHONG) ||
+                // Don't add multi-character candidates if we would go over the targetLength.
+                (password.length + candidate.length > targetLength) ||
+                (pwFlags hasFlag PasswordGenerator.NO_AMBIGUOUS && candidate.isAmbiguous)) {
+                continue
+            }
+
+            // At this point the candidate could be appended to the password, but we still have
+            // to determine the case. If no upper case characters are required, we don't add
+            // any.
+            val useUpperIfBothCasesAllowed =
+                (isStartOfPart || candidate.flags hasFlag CONSONANT) && secureRandomBiasedBoolean(20)
+            password += if (pwFlags hasFlag PasswordGenerator.UPPERS &&
+                (!(pwFlags hasFlag PasswordGenerator.LOWERS) || useUpperIfBothCasesAllowed)) {
+                candidate.upperCase
+            } else {
+                candidate.lowerCase
+            }
+
+            // We ensured above that we will not go above the target length.
+            check(password.length <= targetLength)
+            if (password.length == targetLength)
+                break
+
+            // Second part: Add digits and symbols with a certain probability (if requested) if
+            // they would not directly follow the first character in a pronounceable part.
+
+            if (!isStartOfPart && pwFlags hasFlag PasswordGenerator.DIGITS &&
+                secureRandomBiasedBoolean(30)) {
+                var randomDigit: Char
+                do {
+                    randomDigit = secureRandomNumber(10).toString(10).first()
+                } while (pwFlags hasFlag PasswordGenerator.NO_AMBIGUOUS &&
+                    randomDigit in PasswordGenerator.AMBIGUOUS_STR)
+
+                password += randomDigit
+                // Begin a new pronounceable part after every digit.
+                isStartOfPart = true
+                nextBasicType = if (secureRandomBoolean()) VOWEL else CONSONANT
+                previousFlags = 0
+                continue
+            }
+
+            if (!isStartOfPart && pwFlags hasFlag PasswordGenerator.SYMBOLS &&
+                secureRandomBiasedBoolean(20)) {
+                var randomSymbol: Char
+                do {
+                    randomSymbol = PasswordGenerator.SYMBOLS_STR.secureRandomCharacter()
+                } while (pwFlags hasFlag PasswordGenerator.NO_AMBIGUOUS &&
+                    randomSymbol in PasswordGenerator.AMBIGUOUS_STR)
+                password += randomSymbol
+                // Continue the password generation as if nothing was added.
+            }
+
+            // Third part: Determine the basic type of the next character depending on the letter
+            // we just added.
+            nextBasicType = when {
+                candidate.flags.hasFlag(CONSONANT) -> VOWEL
+                previousFlags.hasFlag(VOWEL) || candidate.flags.hasFlag(DIPHTHONG) ||
+                    secureRandomBiasedBoolean(60) -> CONSONANT
+                else -> VOWEL
+            }
+            previousFlags = candidate.flags
+            isStartOfPart = false
+        }
+        return password.takeIf { PasswordGenerator.isValidPassword(it, pwFlags) }
+    }
+}