Is it safe to reuse the same nonce for decryption an indefinite amount of times in this context?
Specifically, is it safe to re-use the same nonce for decryption an indefinite amount of times, if you only use it once for encryption?


Indeed, all good security definitions (under which ciphers are proven secure) will place no restrictions on the input of the decryption algorithm. The intution behind this is that the the input to the encryption algorithm will be send over the network at which point anything can happen to it.

Security only breaks down if you encryption two different messages using the same key-nonce pair.


  Now, the question is, is this procedure safe?


Yes. In fact the encryption scheme is essentially a reinvention and instantiation of PKCS#5's PBES2 which is provably secure1 (even if you were to use a fixed nonce). Your parameter choice in this case is to use randomized-nonce ChaCha-Poly for encryption and scrypt for key-derivation.



1: Our local pedants may note that the security definition of LORX from that paper only implies CPA-security, but an extension to CCA-security definitions should be straightforward as the proof essentially relies on hiding the key from the adversary and then "inheriting" the encryption scheme's security properties.