lox-extensions/src/lib.rs

/*! Implementation of a new style of bridge authority for Tor that
allows users to invite other users, while protecting the social graph
from the bridge authority itself.

We use uCMZ credentials (Orr`u, 2024 https://eprint.iacr.org/2024/1552.pdf) which improves issuer efficiency
over our original CMZ14 credential (GGM version, which is more efficient, but
makes a stronger security assumption) implementation: "Algebraic MACs and
Keyed-Verification Anonymous Credentials" (Chase, Meiklejohn, and
Zaverucha, CCS 2014)

The notation follows that of the paper "Hyphae: Social Secret Sharing"
(Lovecruft and de Valence, 2017), Section 4. */

// We want Scalars to be lowercase letters, and Points and credentials
// to be capital letters
#![allow(non_snake_case)]

use curve25519_dalek::scalar::Scalar;
#[cfg(feature = "bridgeauth")]
use ed25519_dalek::{Signature, SignatureError, SigningKey, Verifier, VerifyingKey};
use subtle::ConstantTimeEq;

#[cfg(feature = "bridgeauth")]
use chrono::{DateTime, Utc};
#[cfg(feature = "bridgeauth")]
use cmz::*;
#[cfg(feature = "bridgeauth")]
use curve25519_dalek::ristretto::RistrettoPoint as G;
use rand_core::OsRng;
#[cfg(feature = "bridgeauth")]
use sha2::Sha512;

pub mod bridge_table;
pub mod dup_filter;
pub mod lox_creds;
pub mod migration_table;
pub mod proto {
    pub mod blockage_migration;
    pub mod errors;
    pub mod issue_invite;
    pub mod level_up;
    pub mod migration;
    pub mod open_invite;
    pub mod redeem_invite;
    pub mod trust_promotion;
    pub mod update_cred;
    pub mod update_invite;
}

#[cfg(feature = "bridgeauth")]
use bridge_table::BridgeTable;
//    BridgeLine, EncryptedBucket, MAX_BRIDGES_PER_BUCKET, MIN_BUCKET_REACHABILITY,
//};
use lox_creds::*;
#[cfg(feature = "bridgeauth")]
use migration_table::{MigrationTable, MigrationType};
#[cfg(feature = "bridgeauth")]
use serde::{Deserialize, Serialize};
#[cfg(feature = "bridgeauth")]
use std::collections::HashSet;

/// Number of times a given invitation is ditributed
pub const OPENINV_K: u32 = 10;
/// TODO: Decide on maximum daily number of invitations to be distributed
pub const MAX_DAILY_BRIDGES: u32 = 100;
/// The BridgeDb. This will typically be a singleton object. The
/// BridgeDb's role is simply to issue signed "open invitations" to
/// people who are not yet part of the system.
#[derive(Debug, Serialize, Deserialize)]
#[cfg(feature = "bridgeauth")]
pub struct BridgeDb {
    /// The keypair for signing open invitations
    keypair: SigningKey,
    /// The public key for verifying open invitations
    pub pubkey: VerifyingKey,
    /// The set of open-invitation buckets
    openinv_buckets: HashSet<u32>,
    /// The set of open invitation buckets that have been distributed
    distributed_buckets: Vec<u32>,
    #[serde(skip)]
    today: DateTime<Utc>,
    pub current_k: u32,
    pub daily_bridges_distributed: u32,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
#[cfg(feature = "bridgeauth")]
pub struct OldKeyStore {
    //    /// Most recently outdated lox secret and private keys for verifying update_cred credentials
    priv_key: CMZPrivkey<G>,
    //    /// The public key for verifying update_cred credentials
    pub pub_key: CMZPubkey<G>,
}

#[derive(Debug, Default, Clone, Serialize, Deserialize)]
#[cfg(feature = "bridgeauth")]
pub struct OldKeys {
    /// Most recently outdated lox secret and private keys for verifying update_cred credentials
    lox_keys: Vec<OldKeyStore>,
    /// Most recently outdated open_invitation VerifyingKey for verifying update_openinv tokens
    bridgedb_key: Vec<VerifyingKey>,
    /// Most recently outdated invitation secret and private keys for verifying update_inv credentials
    invitation_keys: Vec<OldKeyStore>,
}

#[derive(Debug, Default, Clone, Serialize, Deserialize)]
#[cfg(feature = "bridgeauth")]
pub struct OldFilters {
    /// Most recently outdated lox id filter
    lox_filter: Vec<dup_filter::DupFilter<Scalar>>,
    /// Most recently outdated open invitation filter
    openinv_filter: Vec<dup_filter::DupFilter<Scalar>>,
    /// Most recently outdated invitation filter
    invitation_filter: Vec<dup_filter::DupFilter<Scalar>>,
}

/// An open invitation is a [u8; OPENINV_LENGTH] where the first 32
/// bytes are the serialization of a random Scalar (the invitation id),
/// the next 4 bytes are a little-endian bucket number, and the last
/// SIGNATURE_LENGTH bytes are the signature on the first 36 bytes.
pub const OPENINV_LENGTH: usize = 32 // the length of the random
                                     // invitation id (a Scalar)
    + 4 // the length of the u32 for the bucket number
    + ed25519_dalek::SIGNATURE_LENGTH; // the length of the signature

#[cfg(feature = "bridgeauth")]
impl BridgeDb {
    /// Create the BridgeDb.
    pub fn new() -> Self {
        let mut csprng = OsRng {};
        let keypair = SigningKey::generate(&mut csprng);
        let pubkey = keypair.verifying_key();
        Self {
            keypair,
            pubkey,
            openinv_buckets: Default::default(),
            distributed_buckets: Default::default(),
            today: Utc::now(),
            current_k: 0,
            daily_bridges_distributed: 0,
        }
    }
    /// Verify an open invitation. Returns the invitation id and the
    /// bucket number if the signature checked out. It is up to the
    /// caller to then check that the invitation id has not been used
    /// before.
    pub fn verify(
        invitation: [u8; OPENINV_LENGTH],
        pubkey: VerifyingKey,
    ) -> Result<(Scalar, u32), SignatureError> {
        // Pull out the signature and verify it
        let sig = Signature::try_from(&invitation[(32 + 4)..])?;
        pubkey.verify(&invitation[0..(32 + 4)], &sig)?;
        // The signature passed. Pull out the bucket number and then
        // the invitation id
        let bucket = u32::from_le_bytes(invitation[32..(32 + 4)].try_into().unwrap());
        let s = Scalar::from_canonical_bytes(invitation[0..32].try_into().unwrap());
        if s.is_some().into() {
            Ok((s.unwrap(), bucket))
        } else {
            // It should never happen that there's a valid signature on
            // an invalid serialization of a Scalar, but check anyway.
            Err(SignatureError::new())
        }
    }
}

/// The bridge authority. This will typically be a singleton object.
#[cfg(feature = "bridgeauth")]
#[derive(Debug, Serialize, Deserialize)]
pub struct BridgeAuth {
    /// The private key for the main Lox credential
    lox_priv: CMZPrivkey<G>,
    /// The public key for the main Lox credential
    pub lox_pub: CMZPubkey<G>,
    /// The private key for migration credentials
    migration_priv: CMZPrivkey<G>,
    /// The public key for migration credentials
    pub migration_pub: CMZPubkey<G>,
    /// The private key for migration key credentials
    migrationkey_priv: CMZPrivkey<G>,
    /// The public key for migration key credentials
    pub migrationkey_pub: CMZPubkey<G>,
    /// The private key for bucket reachability credentials
    reachability_priv: CMZPrivkey<G>,
    /// The public key for bucket reachability credentials
    pub reachability_pub: CMZPubkey<G>,
    /// The private key for invitation credentials
    invitation_priv: CMZPrivkey<G>,
    /// The public key for invitation credentials
    pub invitation_pub: CMZPubkey<G>,

    /// The public key of the BridgeDb issuing open invitations
    pub bridgedb_pub: VerifyingKey,

    /// The bridge table
    bridge_table: BridgeTable,

    // Map of bridge fingerprint to values needed to verify TP reports
    //pub tp_bridge_infos: HashMap<String, BridgeVerificationInfo>,
    /// The migration tables
    trustup_migration_table: MigrationTable,
    blockage_migration_table: MigrationTable,

    /// Duplicate filter for open invitations
    bridgedb_pub_filter: dup_filter::DupFilter<Scalar>,
    /// Duplicate filter for Lox credential ids
    id_filter: dup_filter::DupFilter<Scalar>,
    /// Duplicate filter for Invitation credential ids
    inv_id_filter: dup_filter::DupFilter<Scalar>,
    /// Duplicate filter for trust promotions (from untrusted level 0 to
    /// trusted level 1)
    trust_promotion_filter: dup_filter::DupFilter<Scalar>,
    // Outdated Lox Keys to be populated with the old Lox private and public keys
    // after a key rotation
    old_keys: OldKeys,
    old_filters: OldFilters,

    /// For testing only: offset of the true time to the simulated time
    #[serde(skip)]
    time_offset: time::Duration,
}

#[cfg(feature = "bridgeauth")]
impl BridgeAuth {
    pub fn new(bridgedb_pub: VerifyingKey) -> Self {
        // Initialization
        let mut rng = rand::thread_rng();
        cmz_group_init(G::hash_from_bytes::<Sha512>(b"CMZ Generator A"));
        // Create the private and public keys for each of the types of
        // credential with 'true' to indicate uCMZ
        let (lox_priv, lox_pub) = Lox::gen_keys(&mut rng, true);
        let (migration_priv, migration_pub) = Migration::gen_keys(&mut rng, true);
        let (migrationkey_priv, migrationkey_pub) = MigrationKey::gen_keys(&mut rng, true);
        let (reachability_priv, reachability_pub) = BucketReachability::gen_keys(&mut rng, true);
        let (invitation_priv, invitation_pub) = Invitation::gen_keys(&mut rng, true);
        Self {
            lox_priv,
            lox_pub,
            migration_priv,
            migration_pub,
            migrationkey_priv,
            migrationkey_pub,
            reachability_priv,
            reachability_pub,
            invitation_priv,
            invitation_pub,
            bridgedb_pub,
            bridge_table: Default::default(),
            //            tp_bridge_infos: HashMap::<String, BridgeVerificationInfo>::new(),
            trustup_migration_table: MigrationTable::new(MigrationType::TrustUpgrade),
            blockage_migration_table: MigrationTable::new(MigrationType::Blockage),
            bridgedb_pub_filter: Default::default(),
            id_filter: Default::default(),
            inv_id_filter: Default::default(),
            trust_promotion_filter: Default::default(),
            time_offset: time::Duration::ZERO,
            old_keys: Default::default(),
            old_filters: Default::default(),
        }
    }

    /// Get today's (real or simulated) date as u32
    pub fn today(&self) -> u32 {
        // We will not encounter negative Julian dates (~6700 years ago)
        // or ones larger than 32 bits
        (time::OffsetDateTime::now_utc().date() + self.time_offset)
            .to_julian_day()
            .try_into()
            .unwrap()
    }

    /// Get today's (real or simulated) date as a DateTime<Utc> value
    pub fn today_date(&self) -> DateTime<Utc> {
        Utc::now()
    }
}

// Try to extract a u32 from a Scalar
pub fn scalar_u32(s: &Scalar) -> Option<u32> {
    // Check that the top 28 bytes of the Scalar are 0
    let sbytes: &[u8; 32] = s.as_bytes();
    if sbytes[4..].ct_eq(&[0u8; 28]).unwrap_u8() == 0 {
        return None;
    }
    Some(u32::from_le_bytes(sbytes[..4].try_into().unwrap()))
}