What would be the most appropriate way to store private keys for barcode signing in the Android app? Probably most experts here would say - just don't do it, cause it's insecure...

But I would like to understand the level of "insecurity" and risks to proposed solutions.

P.S. Commodity Smartphones to assume.

P.S. we can exchange keys not too often. App Backend API can be designed as needed. But permanent internet connection and variants of signing on the server-side is not possible.

  • 1
    "Most secure" ? Probably opinion-based. – Chenmunka Dec 21 '20 at 11:52
  • 2
    @Chenmunka sure.. But when arguements are providet it's makes the whole thing better understandable. Opinion-based is nearly everything in the IT. But i've removed "most". So what do you think? – aholbreich Dec 21 '20 at 12:44
  • 2
    Android keystore system - It is hardware backed (TEE). – defalt Dec 21 '20 at 12:49
  • 1
    @defalt: Write it as an answer, not as a comment. It will be helpful to many developers. – mentallurg Dec 21 '20 at 14:26
  • 3
    Does the private key belong to the user (one per user who will want to protect it) or to you (one key, shared by all users of the app some of whom will want to steal it) – Richard Tingle Dec 21 '20 at 22:27

All android 7+ devices are equipped with Trusted Execution Environment (TEE) as a mandatory requirement for Google apps licensing. It's a hardware backed keystore which provides isolated storage and data processing for cryptographic blobs. In Qualcomm Snapdragon and Samsung Exynos SoCs, TEE is based on ARM Trustzone. Some devices like in Pixel and iPhone have their own discrete TEE (Google's Titan M and Apple's T2 chip) which is called strongbox. Discrete TEEs are more isolated than ARM Trustzone and independent of the SoC used.

You can use android keystore provider APIs to

Every key stored within the keystore can have the following parameters set:

  • alias - used to identify the key.
  • key size (API 23).
  • purpose – encrypt/decrypt (API 23).
  • encryption mode, algorithm and padding (API 23).
  • should the key be authenticated with the keystore before usage? (API 23).
  • the time duration for which the key can be used after a successful authentication (API 23).
  • should a key be invalidated on new fingerprint enrolment? (API 24)
  • should a keystore require the screen to be unlocked before performing cryptographic operations? (API 28)
  • should a key be protected by a StrongBox hardware security module? (API 28)

You can also use it to encrypt authentication tokens for login, store passwords and encrypt the key which encrypts your app's large sensitive data.

"should a key be protected by a StrongBox hardware security module? (API 28)" seem to be very important in terms of overal security level.

For android 9+, apps can set preference to store keys in strongbox by calling setIsStrongBoxBacked(true). If it throws StrongBoxUnavailableException then apps should fallback to hardware backed keystore. Strongbox is immune from critical side channel vulnerabilities in SoC's CPU which may affect hardware backed keystore. The security of hardware backed keystore falls on the chipmaker of SoC: Gaping 'hole' in Qualcomm’s Secure World mobile vault leaked sensitive data.

Hardware Security Best Practices recommend StrongBox Keymaster. The module contains the following:

  • Its own CPU
  • Secure storage
  • A true random-number generator.
  • Additional mechanisms to resist package tampering and unauthorized sideloading of apps.

Android keystore system

How Secure is your Android Keystore Authentication? (Outdated, published before android 10 release)

  • Thank you very much. So if if i got you corect Hardware support for TEE is mandatory for all Android since (Let say API level 26). This is good news! Also the use of Android Keystore is what should be used. Thanks! – aholbreich Dec 21 '20 at 20:28
  • And i guess this would be IOS way? developer.apple.com/documentation/security/… ? – aholbreich Dec 21 '20 at 20:32
  • 1
    @aholbreich yes, or developer.apple.com/documentation/security/… to make sure that absolutely no software can read the private key—storing in the keychain means that the key is encrypted, but you can still decrypt and access it in software – lights0123 Dec 21 '20 at 21:05
  • @default the last point "should a key be protected by a StrongBox hardware security module? (API 28)" seem to be very important in terms of overal security level. Do you agree? Can you give your opinion on that? – aholbreich Dec 23 '20 at 13:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.