Wednesday, September 2, 2015

Android Safe is no more... Long live AndSafe

Back in 2009, I got my first Android phone - the HTC Magic which ran Android 1.5.  There were only handful of apps available on Android Market (now Google Play).  I wanted something to replace my Palm Pilot MemoAES but couldn't find something that was simple to use and secure (e.g. no network permission).  So I decided to write my own and later I released the Android Safe as freeware.

Fast forward to 2014.  In Sep I received an email from Google saying that "The use of 'Android' in your title is not compliant with Android branding guidelines" and they would suspend my app unless I rename my app.

To be honest, I developed the app for my own use.  I put it on Android Market / Google Play just to share it in case someone out there wanted similar things.  I wasn't making any money out of it.  So I didn't do anything and eventually Google removed my app from Market.

In 2015, I finally got some time to sit down to see how I can improve Android Safe:

- it used 256-bit AES with ECB mode.  I picked ECB because I was lazy to implement a secure way to generate the IV.  Also, most of my memos are short (e.g. password, PIN etc) and without pattern.  So ECB was fine.  But if I am going to re-do it again, maybe I will use CBC instead

- 1024-round of PBKDF2 was used to generate the encryption key.  It was OK back then, but now I prefer to use something that is more CPU intensive and can stand against ASIC attacks.  After comparing bcrypt and scrypt, I decided to go with scrypt.  Theoretically scrypt is better than bcrypt, but scrypt is newer and this is usually a bad thing in cryptography as there are not enough cryptoanalysis done against it.  But anyway, I am using it as a key generator rather than cipher, so it is good enough for me.

- JNI library was used to accelerate the PBKDF2 key generation.  Back then, the Android NDK only supported ARM platform.  Now the app should compile for x86 and MIPS too

And so, AndSafe was born.  Besides changes on the encryption logic, the UI was redo to remove the outdated Gallery UI component.  

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