Investigate possible encoding problem

[r-marques]

related to bigchaindb/bigchaindb#617

[r-marques]

This is issue seems to be related to week the use of weak keys in ed25519 see jedisct1/libsodium#112

This is possible if the attacker is able to control the public key, the signature and the message to be signed. For bigchaindb the clients can choose their public keys and do provide the signature, but the message is the transaction itself. I don't think it would be possible to forge a transaction such that a signature would verify in this case.

But just to be sure I think we should switch to [NaCL][https://nacl.cr.yp.to/] since it has a good reputation and libsodium is under active development and there are good python bindings for it.

libsodium does blacklist public keys and returns a bad signature error if a weak public key is being used https://github.com/jedisct1/libsodium/blob/22ab28be0a80878452836c08161a26005c460630/src/libsodium/crypto_sign/ed25519/ref10/open.c#L38