Sunscreen/seal_fhe/src/context.rs

use std::ffi::c_void;
use std::os::raw::c_int;
use std::ptr::null_mut;

use crate::bindgen;
use crate::error::*;
use crate::EncryptionParameters;
use crate::SecurityLevel;

/**
 * Performs sanity checks (validation) and pre-computations for a given set of encryption
 * parameters. While the EncryptionParameters class is intended to be a light-weight class
 * to store the encryption parameters, the SEALContext class is a heavy-weight class that
 * is constructed from a given set of encryption parameters. It validates the parameters
 * for correctness, evaluates their properties, and performs and stores the results of
 * several costly pre-computations.
 *
 * After the user has set at least the PolyModulus, CoeffModulus, and PlainModulus
 * parameters in a given EncryptionParameters instance, the parameters can be validated
 * for correctness and functionality by constructing an instance of SEALContext. The
 * constructor of SEALContext does all of its work automatically, and concludes by
 * constructing and storing an instance of the EncryptionParameterQualifiers class, with
 * its flags set according to the properties of the given parameters. If the created
 * instance of EncryptionParameterQualifiers has the ParametersSet flag set to true, the
 * given parameter set has been deemed valid and is ready to be used. If the parameters
 * were for some reason not appropriately set, the ParametersSet flag will be false,
 * and a new SEALContext will have to be created after the parameters are corrected.
 *
 * By default, SEALContext creates a chain of SEALContext.ContextData instances. The
 * first one in the chain corresponds to special encryption parameters that are reserved
 * to be used by the various key classes (PrivateKey, PublicKey, etc.). These are the
 * exact same encryption parameters that are created by the user and passed to the
 * constructor of SEALContext. The properties KeyContextData and KeyParmsId return the
 * ContextData and the ParmsId corresponding to these special parameters. The rest of the
 * ContextData instances in the chain correspond to encryption parameters that are derived
 * from the first encryption parameters by always removing the last one of the moduli in
 * the CoeffModulus, until the resulting parameters are no longer valid, e.g., there are
 * no more primes left. These derived encryption parameters are used by ciphertexts and
 * plaintexts and their respective ContextData can be accessed through the
 * GetContextData(ParmsId) function. The properties FirstContextData and LastContextData
 * return the ContextData corresponding to the first and the last set of parameters in
 * the "data" part of the chain, i.e., the second and the last element in the full chain.
 * The chain is a doubly linked list and is referred to as the modulus switching chain.
*/
pub struct Context {
    pub(crate) handle: *mut c_void,
}

unsafe impl Sync for Context {}
unsafe impl Send for Context {}

impl Context {
    /**
     * Creates an instance of SEALContext and performs several pre-computations
     * on the given EncryptionParameters.
     *
     * * `params` - The encryption parameters.
     * * `expand_mod_chain` - Determines whether the modulus switching chain
     * should be created.
     * * `security_level` - Determines whether a specific security level should be
     * enforced according to HomomorphicEncryption.org security standard.
     */
    pub fn new(
        params: &EncryptionParameters,
        expand_mod_chain: bool,
        security_level: SecurityLevel,
    ) -> Result<Self> {
        let mut handle: *mut c_void = null_mut();

        convert_seal_error(unsafe {
            bindgen::SEALContext_Create(
                params.get_handle(),
                expand_mod_chain,
                security_level as c_int,
                &mut handle,
            )
        })?;

        Ok(Context { handle })
    }

    /**
     * Returns handle to the underlying SEAL object.
     */
    pub fn get_handle(&self) -> *mut c_void {
        self.handle
    }
}

impl Drop for Context {
    fn drop(&mut self) {
        convert_seal_error(unsafe { bindgen::SEALContext_Destroy(self.handle) })
            .expect("Internal error in Context::drop().");
    }
}

#[cfg(test)]
mod tests {
    use crate::*;

    #[test]
    fn can_create_and_drop_context() {
        let params = BfvEncryptionParametersBuilder::new()
            .set_poly_modulus_degree(1024)
            .set_coefficient_modulus(
                CoefficientModulus::create(8192, &[50, 30, 30, 50, 50]).unwrap(),
            )
            .set_plain_modulus_u64(1234)
            .build()
            .unwrap();

        let ctx = Context::new(&params, false, SecurityLevel::TC128).unwrap();

        std::mem::drop(ctx);
    }
}