For most Android app developers, using a trusted execution environment (TEE) means asking Android Keystore to create and use cryptographic keys—not installing code inside the TEE. Keystore keeps key material out of your app process and may use a TEE or StrongBox, depending on the device and the requested key configuration. Check the key’s reported security level, set only the authorizations your app needs, and treat custom TEE software as platform-level work.
What a TEE means for an Android app
A TEE is an isolated secure context intended to protect sensitive operations and data from the main Android environment. In ordinary app development, you generally do not communicate directly with that environment. You use public Android APIs—especially Android Keystore—to request cryptographic operations.
When a key is hardware-backed, Android’s app-facing AndroidKeyStore implementation forwards requests to the keystore daemon. The daemon manages key blobs created through KeyMint; the KeyMint hardware abstraction layer delegates sensitive operations to a trusted application in a secure environment, commonly TrustZone on ARM. Apps use the higher-level Java cryptography APIs; KeyMint itself is a low-level platform interface, not an app API. The Android Open Source Project explains this architecture in its hardware-backed Keystore documentation.
Android Keystore does not make every key hardware-backed. Whether secure hardware is available depends on the device and on whether it supports the requested algorithm, mode, digest, key size, and other parameters.
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Use Android Keystore for app-level keys
Create a Keystore key for the specific operations your app needs, then use it through Android’s cryptographic APIs. Define the key’s intended purposes and parameters when creating it: key authorizations cannot be changed afterward. Depending on the key configuration and device, Android can enforce allowed purposes, algorithms, modes, padding, digests, validity periods, and user-authentication requirements.
Key material does not enter your app process during cryptographic operations, as the Android Keystore system guide explains. That is protection against extraction, not a promise that a compromised app or operating system cannot ask the device to perform an operation the key permits. Also, secure hardware may not enforce every authorization—for example, temporal limits may depend on a secure clock that the device does not have.
Check where the key is protected
Do not infer hardware backing from the fact that a key is in Android Keystore. Inspect its KeyInfo security level:
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- For apps targeting Android 10 (API 29) or later, call
KeyInfo.getSecurityLevel(). A result ofTRUSTED_ENVIRONMENTorSTRONGBOXindicates secure hardware. - For apps targeting Android 9 (API 28) or lower, use
KeyInfo.isInsideSecurityHardware().
Use the result to make a decision consistent with your security policy. If a hardware-backed key is mandatory, do not silently treat a software-backed key as equivalent.
Choose Android Keystore or KeyChain
Use Android Keystore for credentials owned by your individual app. Use KeyChain when a credential may need to be shared system-wide under the user’s control and choice.
Decide whether to request StrongBox
StrongBox is an optional, more isolated secure-hardware implementation for Keystore keys. Devices running Android 9 (API 28) or later can include StrongBox KeyMint, but availability is not guaranteed. The Android Keystore guide characterizes StrongBox as slower, more resource-constrained, and capable of fewer concurrent operations than TEE-backed implementations; it says StrongBox is unnecessary for most apps and should be assessed against the threat model.
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| Choice | Availability and security level | Trade-offs |
|---|---|---|
| TEE-backed Keystore key | Depends on device support and the requested key configuration. Inspect KeyInfo; hardware backing is reported as TRUSTED_ENVIRONMENT. |
Typically a better fit than StrongBox when its protection meets the threat model and the app needs broader capability or more operational capacity. |
| StrongBox-backed Keystore key | Optional on devices running Android 9 (API 28) or later. Inspect KeyInfo; the reported level is STRONGBOX. |
Offers stronger isolation, but is slower, more constrained, supports fewer algorithms, and may support fewer concurrent operations. |
Before requesting StrongBox, check for PackageManager.FEATURE_STRONGBOX_KEYSTORE. StrongBox’s documented algorithm subset includes RSA 2048; AES 128/256; ECDSA and ECDH P-256; HMAC-SHA256 with 8–64 byte keys; Triple DES; and extended-length APDUs. Support can vary with the device and Android version, so verify the capabilities relevant to your app rather than assuming every request will work.
An unsupported algorithm or key size can result in StrongBoxUnavailableException. Handle that exception deliberately: fall back to a non-StrongBox Keystore key only if your application’s security policy permits it. If StrongBox is a hard requirement, fail the operation clearly rather than creating a less protected key without the user or system knowing.
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Writing a trusted app that runs inside a TEE is different from using Android Keystore. AOSP describes Trusty as one TEE implementation, comprising a secure OS, Android-kernel drivers, and libraries that let Android-side software communicate with trusted apps. The TEE processor may be a separate microprocessor or a virtualized instance of the main processor, isolated using hardware memory and I/O protections. Other TEE operating systems are possible; Trusty is not the only implementation.
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In the documented Trusty model, Android-side applications exchange messages with trusted apps through Trusty APIs, while the message format and meaning are defined by the application protocol. Trusty trusted apps are isolated processes, written in C or C++ (with limited C++ support in the documented version).
However, AOSP’s Trusty documentation states: “Third-party application development is not supported in this version of Trusty.” It explains that trusted apps are developed by one party and packaged with the Trusty kernel image, which is signed and verified at boot. Adding a trusted app also expands the trusted computing base and may expose device secrets. Custom TEE-side code is therefore a platform integration matter requiring the relevant OEM or platform authority; an ordinary Play-distributed app should not assume it can install code into Trusty.
Use app-level Keystore APIs when your goal is to protect an app’s keys or perform cryptographic operations. Consider TEE-side development only when you are working with the platform or device vendor and have a requirement that cannot be met through public Android APIs. AOSP’s Trusty TEE documentation provides the platform-level model and its limitations.
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Understand what the TEE does—and does not—guarantee
Android’s security overview describes several platform uses of trusted hardware and related protections: Gatekeeper performs device PIN, pattern, or password authentication in a TEE; hardware-backed keys can require user authentication; and Trusty is isolated from Android through hardware and software. The same overview discusses SELinux mandatory access controls and Verified Boot, whose chain extends from a hardware-protected root of trust through boot partitions. These platform features do not mean that an arbitrary app can call every protected service directly.
AOSP lists protected-content DRM, mobile payments, secure banking, full-disk encryption, multi-factor authentication, device-reset protection, replay-protected storage, protected wireless display, secure PIN or fingerprint processing, and malware detection as examples of TEE uses. These are examples of platform or device capabilities, not a menu of services that every third-party app can invoke.
For app design, distinguish the protection of key material from the security of the entire operation. Hardware-backed non-exportable keys can make extraction harder, but an attacker who controls a compromised app or operating system may still be able to invoke operations allowed by the key’s authorizations. Keep those permissions narrow and require user authentication when the threat model calls for it. For more detail on Android’s broader controls, see the Android security features overview.
Quick Recap
A practical decision checklist
- Use Android Keystore when an app needs to create or use its own cryptographic key without exposing key material to its process.
- Set key purposes, algorithms, modes, digests, validity, and authentication requirements at creation time; request only what the app needs.
- Inspect
KeyInfoand base any hardware-backing claim or policy on the reported security level. - Request StrongBox only when its additional isolation addresses a real threat and the device supports the necessary feature and key parameters.
- Define a fallback policy before handling an unavailable or unsupported StrongBox request.
- Use
KeyChainrather than app-owned Keystore credentials when system-wide credential sharing under user choice is the requirement. - Approach trusted-app code as OEM or platform integration, not as a deployable extension of a normal Android app.
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