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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.

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    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
    Commented Dec 21, 2020 at 22:27
  • Can you clarify whether the key is indeed specific to each phone (generated locally on the phone, and never leaves it), or is a key that is common to all devices, sent by a server? The latter would most probably be a very bad idea.
    – jcaron
    Commented Dec 22, 2020 at 10:58
  • @RichardTingle unfortunately it has to be shared key...
    – aholbreich
    Commented Dec 23, 2020 at 15:38
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    Distributing a private key seems like a very bad idea. Are you sure you can’t do otherwise? For instance generate a private key and CSR locally, send the CSR to a server to have it signed with a root certificate, get the certificate back? Intercepting data on a device you control is often trivial. Of course embedding a certificate in a QR code may be a challenge.
    – jcaron
    Commented Dec 23, 2020 at 16:24
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    If you are sharing the private key then no need to use keystore as someone can intercept and leak it and then you have to revoke it from every device. Instead, generate unique private key in keystore for every user and certify their public key using your self signed certificate.
    – defalt
    Commented Dec 23, 2020 at 19:20

1 Answer 1

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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)

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  • 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
    Commented Dec 21, 2020 at 20:28
  • And i guess this would be IOS way? developer.apple.com/documentation/security/… ?
    – aholbreich
    Commented Dec 21, 2020 at 20:32
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    @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
    Commented Dec 21, 2020 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
    Commented Dec 23, 2020 at 13:46

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