Google adds Adiantum disk encryption to Android for budget smartphones

Google has developed an encryption method for smartphones with chips that do not have hardware encryption for AES, such as budget devices. The encryption is based on the ChaCha stream cipher.

Google explains the rationale for Adiantum’s development and the details behind the encryption on its Security Blog. Google has required full disk encryption since 2015 with the arrival of Android 6.0, but Android supports aes-256 for this and with slower system-on-chips this leads to poor performance. Google maintains a limit of 50MiB/s. If the transit is below this, the obligation does not apply.

This is not an issue for smartphones with chips based on ARMv8 because of the Cryptography Extensions, which provide hardware acceleration of AES encryption. In particular, budget smartphones and other devices with cheap chips have often been left out until now. Google calls the old ARM Cortex-A7 as a processor without hardware acceleration, which is too slow for AES.

Google has therefore designed an alternative in the form of Adiantum. The company uses a variant of the ChaCha stream cipher for this, in which aes-based techniques are integrated. Adiantum offers significant performance gains over aes-256 on a Cortex-A7, according to Google.

The company already used ChaCha20 in 2014 with a tls suite for the security of https connections. For Adiantum, the company chose ChaCha12, where the number stands for the number of rounds. The more rounds, the more secure the encryption. Although Google seems to be making significant concessions with this, it states that ChaCha12 can still keep up because round-8 has not yet been cracked, although the 7 variant was already broken in 2008.

Manufacturers can enable Adiantum with Android Pie smartphones that do not reach 50MiB/s with aes. Above that limit, Google recommends the use of aes. Adiantum becomes part of Android Q. Google has published its work in a paper titled Adiantum: length-preserving encryption for entry-level processors.