Encrypt and decrypt with protocol actors

homomorpheR adds methods to openfhe.R's encrypt() and decrypt() generics, so the same two verbs serve both layers of the stack and the class of the first argument selects which layer answers. This page describes the actor-level methods; the key-level ones are documented in openfhe.R.

Encrypting

Encryption needs only public material, so a party handed that material at setup encrypts entirely on its own, with nothing to consult and no one to ask — which is what makes contribute() a purely local computation. Methods are registered on whatever holds the public material:

• a Site encrypts with the parameters it was given when it was wired, so encrypt(site, value) needs nothing besides the site itself;

• OpenFHEParams encrypts under a bundle held directly, which is what a party reads back from a site with site_params();

• the frozen PaillierParams and PaillierPublicKey encrypt under the Paillier scheme.

For the openfhe backends the encoding follows whatever the context was built for, read back from the context itself: packed reals under CKKS, packed integers under BFV and BGV. The exact schemes reject a value they cannot represent rather than round it; see OpenFHEParams.

There is deliberately no method on Master, and public_params() is deliberately not exported. An encryption entry point taking a master would advertise a privilege that does not exist, and would invite site-side code to reach back to a coordinator for something it was already given. A site is autonomous once configured.

Decrypting

Decryption is the asymmetric half of the pair, and that asymmetry is the point: it takes either secret material or the standing to convene every site, while encryption takes neither. Methods are registered on the decrypting party:

• CKKSMaster decrypts with the secret key it holds;

• ThresholdMaster holds no key material at all and recovers a value by asking each site for a partial decryption through partial_decrypt() and fusing the results, so no party — the master included — can decrypt alone;

• the frozen PaillierMaster and PaillierPrivateKey decrypt under the Paillier scheme.

The master methods take len, the number of packed slots to return, defaulting to 1.

Return semantics

The methods return deliberately different types, because the encodings differ:

• CKKSMaster and ThresholdMaster return a numeric of length len, decoded for whatever scheme the context was built for.

• PaillierMaster returns the scalar real its protocol accumulated.

• a PaillierCiphertext under a PaillierPrivateKey gives a gmp::bigz in [0, n). This preserves raw mod-n arithmetic; callers wanting signed integers should re-center themselves (if (m > n/2) m - n).

• a PaillierEncryptedReal gives a numeric in (-n/2, n/2). The method re-centers the raw mod-n residues so that negative real numbers and running totals that cross zero round-trip correctly. See PaillierEncryptedReal for the full convention.

Argument names

The generics belong to openfhe.R, and their argument names follow the OpenFHE C++ signatures: encrypt(key, pt, ...) for Encrypt(publicKey, plaintext), and decrypt(ct, key, ...) for Decrypt(ciphertext, privateKey). S7 requires every method to use the generic's names for the arguments it dispatches on, so those are the names here too, and in a call on a protocol actor they read by position: in encrypt(site, value), key is the site and pt the value; in decrypt(master, ct, len), ct is the master and key the encrypted value. Every call in this package and its vignettes is positional, so the names are never written out.

See also

openfhe.R::encrypt() and openfhe.R::decrypt() for the key-level methods; contribute(), which is how a Site encrypts its own data during a round; partial_decrypt() for the site side of a threshold decryption.