fix!: remove padding from decryptedBytes if left in by the cipher

[andrejborstnik]

On (some?) Pixel devices (tested Pixel 6 and Pixel 7 on Android 14, AP2A.240705.004 build) for certain string lengths (e.g. 256) the encryption/decryption using CipherStorageKeystoreAesCbc doesn't erase the PKCS7 padding from the returned string.

To work around this issue I added a method that will detect & delete the padding if left in.

This technically a breaking change for any stored strings that end with a valid PKCS7 padding of characters up to with an ascii int value of IV.IV_LENGTH. Not sure how likely this use-case would be.

Opened this workaround as a discussion point, happy to discuss alternatives.

Also reported to Android https://issuetracker.google.com/issues/357896946, so they can look at the issue on their side

[andrejborstnik]

Based on my testing up to 2000 bytes the problematic strings lengths are

256 -> 271
528 -> 543
800 -> 815
1280 -> 1295
1552 -> 1567
1824 -> 1839

[jahead]

would this be the reason we are getting

• Retrieving password on android fails with Caused by: android.security.KeyStoreException: Unsupported key size #372
• E_CRYPTO_FAILED on Android 13 and Pixel phones. AES crashing out while decrypting. #567
• and a number of others?

[andrejborstnik]

would this be the reason we are getting

• Retrieving password on android fails with Caused by: android.security.KeyStoreException: Unsupported key size #372
• E_CRYPTO_FAILED on Android 13 and Pixel phones. AES crashing out while decrypting. #567
• and a number of others?

I have only seen the issue in the OP on Pixel devices and E_CRYPTO_FAILED happens on other devices as well. So I would be sceptical this is the only reason. But I would not exclude this being the reason for at least some of the decryption issues, e.g. this padding one seems related on the surface

[DorianMazur]

@andrejborstnik @jahead Consider using AES GCM #678