V-Apps can use persistent storage on the device, using the storage module in the V-App SDK.
Storage is organized in 32-byte slots, and it is currently limited to at most 4 slots per application. Therefore, V-Apps can store a total of 128 bytes.
V-Apps that want to use storage must declare it using the n_storage_slots in the manifest in their Cargo.toml.
A V-App that needs more than the small amount of supported persistent storage might achieve a similar effect by outsourcing the (encrypted) storage to the client, and only storing a commitment in the actual V-App storage.
⚠️ Important:
Like for Vanadium's code and RAM outsourcing, depending on the implementation, this might leak the storage access pattern, therefore developers need to be careful to make sure that this does not have security implications. See security.md for a discussion on this topic.
Most real-world usages of persistent storage are not related to low-level cryptography, and are therefore not expected to be impacted by this potential side channel.