Autocompleting secrets sounds unsafe.
> The receiving side offers tab-completion on the codewords, so usually only a few characters must be typed.
replies(3):
Just think of each word as being one character from a large-ish alphabet.
Code-words are used so that the one time secret can be easily remembered or shared over a voice channel.
The secret can be any string you like, the protocol doesn't care, instead of "4-purple-sausages" it could be "4-65535" or "4-qtx", and have the same resistance to attack. The CLI encodes the secret as two words from the PGP word list, which was designed to be spoken and transcribed accurately even over a noisy voice channel (sort of like the Alpha/Bravo/Charlie/.. "military phonetic alphabet", except it's two alternating lists of 256 words each). In practice that pair of words is much easier to speak and listen and hold in your head for a minute or two than a random number, or the first two letters of each word divorced from the words themselves.
There are some provisions in the protocol (not yet implemented) to allow alternate word lists, so if the sender uses e.g. a French wordlist instead of the default English one, the receiving CLI learns about it early enough so that "wormhole rx" can auto-complete against the correct list. The server/attacker could learn which wordlist is in use, but still faces the same level of entropy about the PAKE secret itself.