Context too short in crypto_kdf_derive_from_key

[goblin]

The "rxtx" used as ctx argument to crypto_kdf_derive_from_key is too short of a string. It should be exactly 8 bytes. I was using a code search on github and found it in 2 occurrences in your code:

leaf/src/drop/crypto/keyexchanger.cpp

Line 31 in 57aba45

crypto_kdf_derive_from_key((uint8_t *) key, secretbox :: key :: size, (this->_lesser ? 0 : 1), "rxtx", this->_bytes);

and

leaf/src/drop/crypto/keyexchanger.cpp

Line 38 in 57aba45

crypto_kdf_derive_from_key((uint8_t *) key, secretbox :: key :: size, (this->_lesser ? 1 : 0), "rxtx", this->_bytes);

I've raised an issue with libsodium that this interface is indeed very prone to errors.