[Fix][Android] Unsupported RSA key size for StrongBox

[frw]

Upon app startup, I'm noticing this error showing up

11-13 19:16:28.052   711   711 E android.hardware.keymaster@4.1-service.citadel: GenerateKey : device response error code: UNSUPPORTED_KEY_SIZE
11-13 19:16:28.052   749   779 E keystore2: keystore2::error: In generate_key.
11-13 19:16:28.052   749   779 E keystore2:
11-13 19:16:28.052   749   779 E keystore2: Caused by:
11-13 19:16:28.052   749   779 E keystore2:     0: While generating Key without explicit attestation key.
11-13 19:16:28.052   749   779 E keystore2:     1: Error::Km(ErrorCode(-6))
11-13 19:16:28.059 27168 27303 W CipherStorageBase: StrongBox security storage is not available.
11-13 19:16:28.059 27168 27303 W CipherStorageBase: java.security.ProviderException: Failed to generate key pair.
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at android.security.keystore2.AndroidKeyStoreKeyPairGeneratorSpi.generateKeyPairHelper(AndroidKeyStoreKeyPairGeneratorSpi.java:620)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at android.security.keystore2.AndroidKeyStoreKeyPairGeneratorSpi.generateKeyPair(AndroidKeyStoreKeyPairGeneratorSpi.java:545)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at java.security.KeyPairGenerator$Delegate.generateKeyPair(KeyPairGenerator.java:746)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at com.oblador.keychain.cipherStorage.CipherStorageKeystoreRsaEcb.generateKey(CipherStorageKeystoreRsaEcb.java:249)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at com.oblador.keychain.cipherStorage.CipherStorageBase.tryGenerateStrongBoxSecurityKey(CipherStorageBase.java:475)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at com.oblador.keychain.cipherStorage.CipherStorageBase.tryGenerateStrongBoxSecurityKey(CipherStorageBase.java:460)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at com.oblador.keychain.cipherStorage.CipherStorageBase.generateKeyAndStoreUnderAlias(CipherStorageBase.java:413)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at com.oblador.keychain.KeychainModule.internalWarmingBestCipher(KeychainModule.java:175)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at com.oblador.keychain.KeychainModule.$r8$lambda$DYujhqpjRgfFQ_gyuwMwyxxqDlk(Unknown Source:0)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at com.oblador.keychain.KeychainModule$$ExternalSyntheticLambda0.run(Unknown Source:2)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at java.lang.Thread.run(Thread.java:1012)
11-13 19:16:28.059 27168 27303 W CipherStorageBase: Caused by: android.security.KeyStoreException: Unsupported key size
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at android.security.KeyStore2.getKeyStoreException(KeyStore2.java:356)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at android.security.KeyStoreSecurityLevel.handleExceptions(KeyStoreSecurityLevel.java:57)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at android.security.KeyStoreSecurityLevel.generateKey(KeyStoreSecurityLevel.java:145)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     at android.security.keystore2.AndroidKeyStoreKeyPairGeneratorSpi.generateKeyPairHelper(AndroidKeyStoreKeyPairGeneratorSpi.java:587)
11-13 19:16:28.059 27168 27303 W CipherStorageBase:     ... 10 more

It seems like RSA 3072 is not supported for StrongBox but 2048 is (reference: https://developer.android.com/training/articles/keystore). Changing it to 2048 makes the error disappear.

[androideveloper]

This is indeed correct! I can confirm. What about the other scenarions with normal (non-strongbox) key generation? It will also change to 2048.

[androideveloper]

The strength of RSA encryption is based on the difficulty of factoring large numbers that are the product of two large prime numbers. A 2048-bit RSA key is believed to be secure against foreseeable cryptographic attacks, including brute force attacks, with the computational power available today and in the near future. The National Institute of Standards and Technology (NIST) and other cryptographic authorities have provided guidelines indicating that 2048-bit RSA keys are expected to be secure until at least 2030.

[androideveloper]

Although StrongBox is a little slower and resource constrained (meaning that it supports fewer concurrent operations) compared to TEE, StrongBox provides better security guarantees against physical and side-channel attacks. If you want to prioritize higher security guarantees over app resource efficiency, we recommend using StrongBox on the devices where it is available. Wherever StrongBox isn't available, your app can always fall back to TEE to store key materials.

[androideveloper]

let's go for 2048 key size and fix the strongbox issue