[[["易于理解","easyToUnderstand","thumb-up"],["解决了我的问题","solvedMyProblem","thumb-up"],["其他","otherUp","thumb-up"]],[["没有我需要的信息","missingTheInformationINeed","thumb-down"],["太复杂/步骤太多","tooComplicatedTooManySteps","thumb-down"],["内容需要更新","outOfDate","thumb-down"],["翻译问题","translationIssue","thumb-down"],["示例/代码问题","samplesCodeIssue","thumb-down"],["其他","otherDown","thumb-down"]],["最后更新时间 (UTC)：2025-03-10。"],[],[],null,["# Authentication\n\nAndroid has the concept of user authenticators that are used to unlock the\ndevice and to gate access to cryptographic keys. This involves the following\ncomponents:\n\n- **Cryptographic key storage and service provider.** Stores cryptographic keys and provides standard crypto routines on top of those keys. Android supports a [hardware-backed\n Keystore](/docs/security/features/keystore) and KeyMint (previously Keymaster) for cryptographic services, including hardware-backed cryptography for key storage that might include a Trusted Execution Environment (TEE) or Secure Element (SE), such as StrongBox.\n- **User authenticators.** Attest to the user's presence and/or successful authentication. Android supports [Gatekeeper](/docs/security/features/authentication/gatekeeper) for PIN/pattern/password authentication and [Fingerprint](/docs/security/features/authentication/fingerprint-hal) for fingerprint authentication. Devices that ship with Android 9 and higher can use [`BiometricPrompt`](https://developer.android.com/reference/android/hardware/biometrics/BiometricPrompt) as a single integration point for fingerprint and additional biometrics. These components communicate their authentication state with the keystore service through an authenticated channel. (The [Android Keystore system](https://developer.android.com/training/articles/keystore.html) at the framework level is also backed by the keystore service.)\n\nEach of these components is vendor-specific, but the vendor implementation is required to satisfy\na [Hardware Abstraction Layer (HAL)](/docs/core/architecture/hal) interface\nspecification, and to pass the corresponding vendor test suite (VTS) tests.\n\nVendor implementations are typically also divided into two parts, connected\nby a vendor-specific communication mechanism :\n\n- A *HAL service* runs as an Android system process, receiving Binder requests from the Android system.\n- A *trusted application (TA)* runs in the secure environment, performing the actual secure operations.\n\nEnrollment\n----------\n\nOn first boot of the device after a factory reset, all authenticators are\nprepared to receive credential enrollments from the user. A user must initially\nenroll a PIN, pattern, or password with Gatekeeper (or Weaver, if available). This\ninitial enrollment creates a randomly generated, 64-bit user secure identifier\n(SID) that serves as an identifier for the user and as a binding token for the\nuser's cryptographic material. This user SID is cryptographically bound to the\nuser's password; successful authentications to Gatekeeper result in AuthTokens\nthat contain the user SID for that password.\n\nA user who wants to change an existing credential must present that\ncredential. If an existing credential is verified successfully, the user SID\nassociated with the existing credential is transferred to the new credential,\nenabling the user to keep accessing keys after changing a credential.\n\nIn some situations, a device administrator can perform an *untrusted\nenroll* to enroll a new credential without presenting an existing\ncredential. This allows the user to access the device, but keys created under\nthe old user SID are permanently lost.\n\nAuthentication\n--------------\n\nThis section describes a typical authentication flow, which involves\ninteractions between multiple components in both Android and the secure\nenvironment. Note that all secure components share a (per-boot) secret HMAC key\nthat they use to authenticate each other's messages.\n\nAfter a user has set up a credential and been assigned a user SID, they can\nstart authentication, which begins when a user provides a PIN, pattern,\npassword, fingerprint, or other strong biometric.\n\n\n**Figure 1.** Authentication flow\n\n1. A user provides an authentication method and the associated service makes a request to the HAL service.\n - For PIN, pattern, or password, `LockSettingsService` makes a request to `gatekeeperd`.\n - Biometrics-based authentication flows depend on the Android version. On devices running Android 8.x and lower, `FingerprintService` makes a request to `fingerprintd`). On devices running Android 9 and higher, `BiometricPrompt` makes a request to the appropriate biometric daemon (for example, `fingerprintd` for fingerprints or `faced` for face) using the appropriate \u003cvar translate=\"no\"\u003eBiometric\u003c/var\u003e`Manager` class, such as `FingerprintManager` or `FaceManager`. Regardless of version, biometric authentication occurs asynchronously after the request is sent.\n2. The HAL service sends data to its counterpart TA, which generates an AuthToken:\n - For PIN/pattern/password authentication, `gatekeeperd` sends the PIN, pattern, or password hash to the Gatekeeper TA in the TEE, via the Gatekeeper HAL service. If authentication in the TEE is successful, the Gatekeeper TA emits an AuthToken containing the corresponding user SID (signed with the shared HMAC key).\n - For fingerprint authentication, `fingerprintd` listens for fingerprint events and sends the data to the Fingerprint TA in the TEE, via the Fingerprint HAL. If authentication in the TEE is successful, the Fingerprint TA emits an AuthToken (signed with the AuthToken HMAC key).\n - For other biometric authentication, the appropriate biometric daemon listens for the biometric event and sends it to the appropriate biometric HAL service and TA.\n3. The daemon receives a signed AuthToken and passes it to the keystore service through an extension to the keystore service's Binder interface. (`gatekeeperd` also notifies the keystore service when the device is relocked and when the device password changes.)\n4. The Keystore service passes the AuthTokens to KeyMint and verifies them using the key shared with the Gatekeeper and supported biometric TEE component. KeyMint trusts the timestamp in the token as the last authentication time and bases a key release decision (to allow an app to use the key) on the timestamp.\n\nThe authentication flow does not require direct communication between TAs in\nthe secure environment: AuthTokens flow from the authenticator TA to the\n`keystore2` service in Android, which in turn passes them on to the KeyMint TA.\nThis also permits the `keystore2` service to quickly deny requests\nthat are bound to fail, as it has knowledge of the available AuthTokens in the\nsystem, saving a potentially costly IPC into the TEE.\n\nAuthToken format\n----------------\n\nThe format of the AuthToken is given by the AIDL specification in\n[`HardwareAuthToken.aidl`](https://android.googlesource.com/platform/hardware/interfaces/+/refs/heads/android16-release/security/keymint/aidl/android/hardware/security/keymint/HardwareAuthToken.aidl).\n\nDevice boot flow\n----------------\n\nOn every boot of a device, the AuthToken HMAC key must be generated and\nshared with all TEE components (Gatekeeper, KeyMint, and supported biometrics\ntrustlets). Thus, for added protection against replay attacks, the HMAC key\nmust be randomly generated every time the device reboots.\n\nThere are two common ways that TAs acquire access to this shared HMAC\nkey:\n\n- *Shared secret agreement:* The `keystore2` service performs a multi-way key agreement protocol at device startup, allowing secure derivation of the HMAC key between those TAs that participate. However, participating TAs must have access to a common preshared secret.\n- *Direct access:* TAs that reside within the same secure environment can use an internal interprocess communication mechanism (which is platform-dependent) to share the HMAC key.\n\n\u003cbr /\u003e\n\nIn either case, the HMAC key must **never** be made available\noutside the TEE.\n\nThe [Trusty](/docs/security/features/trusty) operating system,\nwhich runs next to Android, is an example of a TEE, but other TEEs can be used\ninstead. Trusty uses an internal IPC system to communicate directly between\nKeyMint and Gatekeeper or the appropriate biometric trustlet. The HMAC key is\nkept solely in KeyMint; Fingerprint and Gatekeeper request the key from\nKeyMint for each use and don't persist or cache the value.\n\nAs some TEEs lack an IPC infrastructure, no communication occurs between\napplets in the TEE. This also permits the keystore service to quickly deny\nrequests that are bound to fail as it has knowledge of the authentication table\nin the system, saving a potentially costly IPC into the TEE."]]
