At the same time Sodium provides API for anonymous encryption without using ephemeral keys. Is it a bad practice to just use other user public key to encrypt a message?

  • $\begingroup$ I'm not really clear on the question. $\endgroup$ – Thomas M. DuBuisson Jun 5 '17 at 15:08
  • $\begingroup$ @ThomasM.DuBuisson, from NaCL docs: "NaCl's goal is to provide all of the core operations needed to build higher-level cryptographic tools". Isn't non-authenticated asymmetric encryption, the core operation to build cryptographic tools? AFAIK, sodium server the same purpose and provides this. $\endgroup$ – Andrey Kuznetsov Jun 5 '17 at 15:37
  • $\begingroup$ How is crypto_box with an ephemeral key not unauthenticated? Or do you mean not count hybrid operations, so you can't count most DH-only ECC schemes? $\endgroup$ – Thomas M. DuBuisson Jun 5 '17 at 15:40
  • $\begingroup$ Crypto_box with an ephemeral key is authenticated. But sodium provides int crypto_box_seal(unsigned char *c, const unsigned char *m, unsigned long long mlen, const unsigned char *pk) which does not require secret key for encryption. $\endgroup$ – Andrey Kuznetsov Jun 5 '17 at 15:49
  • $\begingroup$ You can't actually authenticate an ephemeral key sort of by definition, so I guess we aren't in agreement on some basic terminology. Notice crypto_box_seal is exactly an ephemeral key and crypto_box with a little extra trick to compute the nonce in a space-saving way. $\endgroup$ – Thomas M. DuBuisson Jun 5 '17 at 16:50

As I now understand the question: why doesn't NaCL provide a primitive for unauthenticated encryption, making ciphertext blobs that are only decrypt-able by a receiver identified by their public key? Note libsodium provides crypto_box_seal which does exactly this and NaCL claims to (try to) provide all necessary crypto primitives.

In answer, notice crypto_box_seal can be built using other primitives. Sodium says of crypto_box_seal:

ephemeral_pk ‖ box( m, recipient_pk, ephemeral_sk
                  , nonce=blake2b(ephemeral_pk ‖ recipient_pk))

So what we have in terms of NaCL prims are (pseudo-code):

sk = random();
ephemeral_pk = crypto_box_keypair(sk);
nonce = sha512(ephemeral_pk || recipient_pk); // NaCL uses SHA512 not Blake2b.
ct = ephemeral_pk || crypto_box(m, recipient_pk, sk, nonce);

This can be considered anonymous because the ephemeral key pairs used are not associated with any identity - the secret key should be destroyed immediately after computing the ciphertext message, ct.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.