Add a password strength estimator wrapping the SDK zxcvbn API (#7186)

* Add a password strength estimator wrapping the SDK zxcvbn API

* Add tests for the password strength estimator SDK mapping

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Co-authored-by: ElementBot <android@element.io>

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/encryption/PasswordStrengthEstimator.kt

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.matrix.api.encryption
+
+/**
+ * Estimates the strength of a password or passphrase using the zxcvbn algorithm,
+ * as exposed by the Rust SDK. Backed by pattern matching (dictionary words,
+ * keyboard walks, repeats, dates, l33t speak, …) rather than naive per-character
+ * entropy, so it agrees with the iOS client for cross-platform parity.
+ *
+ * App-scoped: obtain an instance via dependency injection rather than constructing one.
+ */
+interface PasswordStrengthEstimator {
+    /**
+     * Estimate the strength of [password].
+     *
+     * @param password the password or passphrase to evaluate.
+     * @param userInputs optional context (e.g. user ID, display name, email) that the
+     *   estimator penalises when it appears inside the password, so personal information
+     *   does not inflate the score.
+     */
+    fun estimate(password: String, userInputs: List<String> = emptyList()): PasswordStrengthEstimate
+}
+
+/**
+ * The result of a [PasswordStrengthEstimator.estimate] call.
+ */
+data class PasswordStrengthEstimate(
+    /** Overall ranking, from [PasswordStrengthRanking.VeryWeak] to [PasswordStrengthRanking.VeryStrong]. */
+    val ranking: PasswordStrengthRanking,
+    /**
+     * `log10` of the estimated number of guesses needed to crack the password. Grows without an
+     * upper bound and is independent of any threshold configuration.
+     */
+    val score: Double,
+    /**
+     * [score] normalized to `0.0..1.0` against the estimator's `VeryStrong` threshold
+     * (`score / veryStrong`), suitable for driving a strength meter. Scores above the `VeryStrong`
+     * threshold can exceed `1.0`, so clamp before use.
+     */
+    val normalScore: Double,
+)
+
+/**
+ * A ranking of estimated password strength. Derived by the SDK from [PasswordStrengthEstimate.score]
+ * against a fixed set of thresholds tuned for modern hardware.
+ */
+enum class PasswordStrengthRanking {
+    VeryWeak,
+    Weak,
+    Fair,
+    Strong,
+    VeryStrong,
+}