This is a feature request.
DeltaTouch removed key-export menu items around version 1.10, presumably following the Android client design. I’ve just found out that the Android GUI key export options were removed due to an Android-specific security problem. On mobile Linux, it would be possible to export public and private keys (including public keys of contacts) directly to a keyring, which really ought to be more secure than an Android “Downloads” folder. 
Importing the public keys of contacts from the keyring could also be much slicker than creating .asc files and loading them into a chat, especially for larger numbers of contacts.