/*! The encrypted table of bridges.  The table consists of a number of
 * buckets, each holding some number (currently up to 3) of bridges.
 * Each bucket is individually encrypted with a bucket key.  Users will
 * have a credential containing a bucket (number, key) combination, and
 * so will be able to read one of the buckets.  Users will either
 * download the whole encrypted bucket list or use PIR to download a
 * piece of it, so that the bridge authority does not learn which bucket
 * the user has access to. */

use aes_gcm::aead;
use aes_gcm::aead::{generic_array::GenericArray, Aead, NewAead};
use aes_gcm::Aes128Gcm;
use rand::RngCore;
use std::convert::TryInto;

/// Each bridge information line is serialized into this many bytes
pub const BRIDGE_BYTES: usize = 128;

/// The max number of bridges per bucket
pub const MAX_BRIDGES_PER_BUCKET: usize = 3;

/// The size of a plaintext bucket
pub const BUCKET_BYTES: usize = BRIDGE_BYTES * MAX_BRIDGES_PER_BUCKET;

/// The size of an encrypted bucket
pub const ENC_BUCKET_BYTES: usize = BUCKET_BYTES + 12 + 16;

/// A bridge information line
#[derive(Debug)]
pub struct BridgeLine {
    /// IPv4 or IPv6 address
    pub addr: [u8; 16],
    /// port
    pub port: u16,
    /// other protocol information, including pluggable trasport, public
    /// key, etc.
    pub info: [u8; BRIDGE_BYTES - 18],
}

impl Default for BridgeLine {
    /// An "empty" BridgeLine is represented by all zeros
    fn default() -> Self {
        Self {
            addr: [0; 16],
            port: 0,
            info: [0; BRIDGE_BYTES - 18],
        }
    }
}

impl BridgeLine {
    /// Encode a BridgeLine to a byte array
    pub fn encode(&self) -> [u8; BRIDGE_BYTES] {
        let mut res: [u8; BRIDGE_BYTES] = [0; BRIDGE_BYTES];
        res[0..16].copy_from_slice(&self.addr);
        res[16..18].copy_from_slice(&self.port.to_be_bytes());
        res[18..].copy_from_slice(&self.info);
        res
    }
    /// Decode a BridgeLine from a byte array
    pub fn decode(data: &[u8; BRIDGE_BYTES]) -> Self {
        let mut res: Self = Default::default();
        res.addr.copy_from_slice(&data[0..16]);
        res.port = u16::from_be_bytes(data[16..18].try_into().unwrap());
        res.info.copy_from_slice(&data[18..]);
        res
    }
    /// Encode a bucket to a byte array
    pub fn bucket_encode(bucket: &[BridgeLine; MAX_BRIDGES_PER_BUCKET]) -> [u8; BUCKET_BYTES] {
        let mut res: [u8; BUCKET_BYTES] = [0; BUCKET_BYTES];
        let mut pos: usize = 0;
        for bridge in bucket {
            res[pos..pos + BRIDGE_BYTES].copy_from_slice(&bridge.encode());
            pos += BRIDGE_BYTES;
        }
        res
    }
    /// Decode a bucket from a byte array
    pub fn bucket_decode(data: &[u8; BUCKET_BYTES]) -> [BridgeLine; MAX_BRIDGES_PER_BUCKET] {
        let mut pos: usize = 0;
        let mut res: [BridgeLine; MAX_BRIDGES_PER_BUCKET] = Default::default();
        for bridge in res.iter_mut().take(MAX_BRIDGES_PER_BUCKET) {
            *bridge = BridgeLine::decode(data[pos..pos + BRIDGE_BYTES].try_into().unwrap());
            pos += BRIDGE_BYTES;
        }
        res
    }
}

/// A BridgeTable is the internal structure holding the buckets
/// containing the bridges, the keys used to encrypt the buckets, and
/// the encrypted buckets.  The encrypted buckets will be exposed to the
/// users of the system, and each user credential will contain the
/// decryption key for one bucket.
#[derive(Debug, Default)]
pub struct BridgeTable {
    keys: Vec<[u8; 16]>,
    buckets: Vec<[BridgeLine; MAX_BRIDGES_PER_BUCKET]>,
    encbuckets: Vec<[u8; ENC_BUCKET_BYTES]>,
}

// Invariant: the lengths of the keys and buckets vectors are the same.
// The encbuckets vector only gets updated when encrypt_table is called.

impl BridgeTable {
    /// Append a new bucket to the bridge table
    pub fn new_bucket(&mut self, bucket: [BridgeLine; MAX_BRIDGES_PER_BUCKET]) {
        // Pick a random key to encrypt this bucket
        let mut rng = rand::thread_rng();
        let mut key: [u8; 16] = [0; 16];
        rng.fill_bytes(&mut key);
        self.keys.push(key);
        self.buckets.push(bucket);
    }

    /// Create the vector of encrypted buckets from the keys and buckets
    /// in the BridgeTable.  All of the entries will be (randomly)
    /// re-encrypted, so it will be hidden whether any individual bucket
    /// has changed (except for entirely new buckets, of course).
    pub fn encrypt_table(&mut self) {
        let mut rng = rand::thread_rng();
        self.encbuckets.clear();
        for (key, bucket) in self.keys.iter().zip(self.buckets.iter()) {
            let mut encbucket: [u8; ENC_BUCKET_BYTES] = [0; ENC_BUCKET_BYTES];
            let plainbucket: [u8; BUCKET_BYTES] = BridgeLine::bucket_encode(bucket);
            // Set the AES key
            let aeskey = GenericArray::from_slice(key);
            // Pick a random nonce
            let mut noncebytes: [u8; 12] = [0; 12];
            rng.fill_bytes(&mut noncebytes);
            let nonce = GenericArray::from_slice(&noncebytes);
            // Encrypt
            let cipher = Aes128Gcm::new(aeskey);
            let ciphertext: Vec<u8> = cipher.encrypt(&nonce, plainbucket.as_ref()).unwrap();
            encbucket[0..12].copy_from_slice(&noncebytes);
            encbucket[12..].copy_from_slice(ciphertext.as_slice());
            self.encbuckets.push(encbucket);
        }
    }

    /// Decrypt an individual encrypted bucket, given its key
    pub fn decrypt_bucket(
        key: &[u8; 16],
        encbucket: &[u8; ENC_BUCKET_BYTES],
    ) -> Result<[BridgeLine; MAX_BRIDGES_PER_BUCKET], aead::Error> {
        // Set the nonce and the key
        let nonce = GenericArray::from_slice(&encbucket[0..12]);
        let aeskey = GenericArray::from_slice(key);
        // Decrypt
        let cipher = Aes128Gcm::new(aeskey);
        let plaintext: Vec<u8> = cipher.decrypt(&nonce, encbucket[12..].as_ref())?;
        // Convert the plaintext bytes to an array of BridgeLines
        Ok(BridgeLine::bucket_decode(
            plaintext.as_slice().try_into().unwrap(),
        ))
    }
}