//! Crypto primitives necessary for distributed shuffling

use num_bigint_dig::algorithms::mod_inverse;
use num_bigint_dig::prime::probably_prime;
use num_bigint_dig::traits::ModInverse;
use num_bigint_dig::{BigUint, IntoBigInt, IntoBigUint, RandPrime};
use num_integer::Integer;
use rand::prelude::*;
use serde::{Deserialize, Serialize};
use std::borrow::Cow;
use std::convert::{From, TryFrom, TryInto};

#[derive(Clone, Debug, PartialEq)]
pub struct Prime {
    prime: BigUint,
}

/// Type for serialization and sending over the network
#[repr(transparent)]
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct UncheckedPrime {
    p: BigUint,
}

impl TryFrom<BigUint> for Prime {
    type Error = String;

    fn try_from(value: BigUint) -> Result<Self, Self::Error> {
        if Prime::is_prime(&value) {
            Ok(Prime { prime: value })
        } else {
            Err("Given number is not a prime".to_owned())
        }
    }
}

impl TryFrom<UncheckedPrime> for Prime {
    type Error = String;

    fn try_from(value: UncheckedPrime) -> Result<Self, Self::Error> {
        value.p.try_into()
    }
}

impl From<Prime> for UncheckedPrime {
    fn from(value: Prime) -> Self {
        UncheckedPrime { p: value.prime }
    }
}

impl Prime {
    fn is_prime(i: &BigUint) -> bool {
        // TODO: better/safer prime checking function? check if it divides first n (~= 40 primes)
        // or https://crates.io/crates/glass_pumpkin
        probably_prime(i, 256)
    }

    pub fn random<Rng: CryptoRng + RngCore>(num_bits: usize, rng: &mut Rng) -> Self {
        Prime {
            prime: rng.gen_prime(num_bits),
        }
    }
}

/// Type for serialization and sending over the network
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub struct UncheckedRsa {
    e: BigUint,
    d: BigUint,
}

impl From<Rsa> for UncheckedRsa {
    fn from(value: Rsa) -> Self {
        UncheckedRsa {
            e: value.e,
            d: value.d,
        }
    }
}

impl TryFrom<(UncheckedRsa, &RsaParameter)> for Rsa {
    type Error = String;

    fn try_from(value: (UncheckedRsa, &RsaParameter)) -> Result<Self, Self::Error> {
        let (UncheckedRsa { e, d }, parameter) = value;
        if e == 1u32.into() || e.gcd(&parameter.lambda_n) != 1u32.into() {
            return Err("RSA encryption key not incorrect".to_owned());
        }
        if e.clone().mod_inverse(&parameter.lambda_n) != Some(d.clone().into_bigint().unwrap()) {
            return Err("RSA decryption key incorrect".to_owned());
        }
        Ok(Rsa {
            parameter: parameter.clone(),
            e,
            d,
        })
    }
}

#[derive(Clone, Debug, PartialEq)]
pub struct Rsa {
    parameter: RsaParameter,
    e: BigUint,
    d: BigUint,
}

#[derive(Clone, Debug, PartialEq)]
pub struct RsaParameter {
    n: BigUint,
    lambda_n: BigUint,
}

impl RsaParameter {
    pub fn from_primes(primes: &[Prime]) -> RsaParameter {
        let lambda_n = primes
            .iter()
            .map(|p| &p.prime - &BigUint::from(1u32))
            .fold(1u32.into(), |acc: BigUint, n: BigUint| acc.lcm(&n));
        let n = primes
            .iter()
            .fold(1u32.into(), |acc: BigUint, p| acc * &p.prime);
        RsaParameter { n, lambda_n }
    }
}

impl Rsa {
    /// encrypt given integer
    pub fn encrypt(&self, message: BigUint) -> BigUint {
        message.modpow(&self.e, &self.parameter.n)
    }

    /// decrypt given integer
    pub fn decrypt(&self, message: BigUint) -> BigUint {
        message.modpow(&self.d, &self.parameter.n)
    }

    pub fn gen_with_parameter<Rng: CryptoRng + RngCore>(
        parameter: RsaParameter,
        rng: &mut Rng,
    ) -> Rsa {
        let e = loop {
            let num_bytes = (parameter.lambda_n.bits() + 7) / 8;
            let mut number = vec![0u8; num_bytes as usize];
            rng.fill_bytes(&mut number);
            // ensure that e < lambda_n
            let number = BigUint::from_bytes_le(&number) % &parameter.lambda_n;
            if number.gcd(&parameter.lambda_n) == 1u32.into() {
                break number;
            }
        };
        // inverse exists, since gcd(e, lambda_n) == 1 (therefore e and lambda_n are coprime)
        let d = mod_inverse(Cow::Borrowed(&e), Cow::Borrowed(&parameter.lambda_n)).unwrap();
        Rsa {
            parameter,
            e,
            d: d.into_biguint().unwrap(),
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use std::convert::TryInto;

    #[test]
    fn encrypt_decrypt() {
        let rsa_parameter = RsaParameter {
            n: BigUint::from(3233u32),
            lambda_n: BigUint::from(780u32),
        };
        let key = Rsa {
            parameter: rsa_parameter,
            e: BigUint::from(17u32),
            d: BigUint::from(413u32),
        };
        let m = BigUint::from(65u8);
        let c = key.encrypt(m.clone());
        assert_eq!(c, BigUint::from(2790u32));
        let d = key.decrypt(c);
        assert_eq!(d, m);
    }

    #[test]
    fn generate_keys_1() {
        let mut rng = rand::thread_rng();
        let p = Prime::random(128, &mut rng);
        let rsa_parameter = RsaParameter::from_primes(&[p]);
        let key = Rsa::gen_with_parameter(rsa_parameter, &mut rng);
        let m = BigUint::from_bytes_be(&[65u8, 66, 67, 68]);
        let c = key.encrypt(m.clone());
        let d = key.decrypt(c);
        assert_eq!(d, m);
    }

    #[test]
    fn generate_keys_2() {
        let mut rng = rand::thread_rng();
        let p = Prime::random(128, &mut rng);
        let q = Prime::random(128, &mut rng);
        let rsa_parameter = RsaParameter::from_primes(&[p, q]);
        let key = Rsa::gen_with_parameter(rsa_parameter, &mut rng);
        let m = BigUint::from_bytes_be(&[65u8, 66, 67, 68]);
        let c = key.encrypt(m.clone());
        let d = key.decrypt(c);
        assert_eq!(d, m);
    }

    #[test]
    fn serde() {
        let mut rng = rand::thread_rng();
        let p: UncheckedPrime = Prime::random(128, &mut rng).into();
        let p_str = serde_json::to_string(&p).unwrap();
        assert_eq!(p, serde_json::from_str(&p_str).unwrap())
    }

    #[test]
    fn import() {
        let mut rng = rand::thread_rng();
        let ps = [Prime::random(128, &mut rng), Prime::random(128, &mut rng)];
        let rsa_parameter = RsaParameter::from_primes(&ps);
        let k = Rsa::gen_with_parameter(rsa_parameter.clone(), &mut rng);
        let send_rsa: UncheckedRsa = k.clone().into();
        assert_eq!(Ok(k), (send_rsa, &rsa_parameter).try_into())
    }
}