Android devices come in a wide range of CPU power and memory configurations. Until recently, those on the lower end of the range weren't required to offer encrypted storage. The reason? The most common encryption scheme, AES256, requires too much from the CPU: The device's performance would have become unacceptably slow.
Now engineers at Google have developed a technique based on an encryption technique used in browser security and named for the maidenhair fern (denoting sincerity and discretion) to bring secure storage to these less expensive devices without bringing apps to a standstill.
The new encryption mode, called Adantium, uses the ChaCha stream cipher adapted from HTTPS encryption. The stream cipher is faster on lower-powered devices because its operation is based on the additions, rotations, and XORs available on every CPU; it doesn't require the built-in encryption primitives common on higher-powered processors.
ChaCha20 has been the basis for Google's HTTPs encryption since 2014, so its engineers have experience with the protocol and a high degree of certainty that using it for disk or file encryption will be both fast and secure. A paper on the implementation, Adiantum: length-preserving encryption for entry-level processors, will be presented at the Fast Software Encryption conference (FSE 2019) in March.
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