djcs_t_share_decrypt needs private_key

[robinhundt]

Hi,
we'd like to use this library to implement a secure multi party computation protocol for a paper. Looking at the source code, I stumbled over the djcs_t_share_decrypt method which needs the private key.

libhcs/include/libhcs/djcs_t.h

Lines 216 to 228 in 0e1deea

	/**
	* For a given ciphertext @p cipher1, compute the server @p au's share and store
	* the result in the variable @p rop. These shares can be managed, and then
	* combined when sufficient shares have been accumulated using the
	* djcs_t_share_combine function.
	*
	* @param vk A pointer to an initialised djcs_t_private_key
	* @param au A pointer to an initialised djcs_t_auth_server
	* @param rop mpz_t where the calculated share is stored
	* @param cipher1 mpz_t which stores the ciphertext to decrypt
	*/
	void djcs_t_share_decrypt(djcs_t_private_key *vk, djcs_t_auth_server *au,
	mpz_t rop, mpz_t cipher1);

This is in contrast to the equivalent method for the standard Paillier implementation here:

libhcs/include/libhcs/pcs_t.h

Lines 362 to 374 in 0e1deea

	/**
	* For a given ciphertext @p cipher1, compute the server @p au's share and store
	* the result in the variable @p rop. These shares can be managed, and then
	* combined when sufficient shares have been accumulated using the
	* pcs_t_share_combine function.
	*
	* @param vk A pointer to an initialised pcs_t_private_key
	* @param au A pointer to an initialised pcs_t_auth_server
	* @param rop mpz_t where the calculated share is stored
	* @param cipher1 mpz_t which stores the ciphertext to decrypt
	*/
	void pcs_t_share_decrypt(pcs_t_public_key *vk, pcs_t_auth_server *au,
	mpz_t rop, mpz_t cipher1);

which says private key in the doc string, but actually takes a public key.

I've looked at the local_ecample.c and there paillier is used with the share_decrypt method and the public key as a parameter. Intuitively this seems to be the correct way to me, since share_decrypt is intended to be used to 'partially' decrypt a ciphertext with the secret key share which the auth server holds, right?

Is my understanding of the intended functionality wrong and there is a reason why djcs_t_share_decrypt needs a private key, while for the Paillier equivalent the public key suffices?

Thanks for the library :)

[robinhundt]

If anyone stumbles over this: the fork at https://github.com/lemonviv/libhcs seemingly contains a fixed version of djcs.
I've also started my own fork where I've mainly fixed up some of the C++ interfaces https://github.com/robinhundt/libhcs and will likely also incorporate the fix for djcs.