diff --git a/error.rs b/error.rs
new file mode 100644
index 0000000..96f2427
--- /dev/null
+++ b/error.rs
@@ -0,0 +1,10 @@
+error_chain! {
+    errors {
+        NotEnoughShares {
+            description("not enough signature shares")
+        }
+        DuplicateEntry {
+            description("signature shares contain a duplicated index")
+        }
+    }
+}
diff --git a/mod.rs b/mod.rs
new file mode 100644
index 0000000..9579d98
--- /dev/null
+++ b/mod.rs
@@ -0,0 +1,293 @@
+mod error;
+
+use pairing::{CurveAffine, CurveProjective, Engine, Field, PrimeField};
+use rand::{ChaChaRng, Rand, Rng, SeedableRng};
+use ring::digest;
+
+use self::error::{ErrorKind, Result};
+
+/// Returns a hash of the given message in `G2`.
+pub fn hash_g2<E, M>(msg: M) -> E::G2
+where
+    E: Engine,
+    <E as Engine>::G2: Rand,
+    M: AsRef<[u8]>,
+{
+    let digest = digest::digest(&digest::SHA512, msg.as_ref());
+    // The `pairing` crate's `G2` implements `Rand`. We initialize a seedable RNG with the SHA512
+    // digest, and use it to generate the element.
+    let mut msg_u32: Vec<u32> = Vec::with_capacity((digest.as_ref().len() + 3) / 4);
+    for chunk in digest.as_ref().chunks(4) {
+        let mut x = u32::from(chunk[0]);
+        for b in chunk.into_iter().skip(1) {
+            x <<= 8;
+            x |= u32::from(*b);
+        }
+        msg_u32.push(x);
+    }
+    let mut rng = ChaChaRng::from_seed(&msg_u32);
+    rng.gen()
+}
+
+/// A public key, or a public key share.
+#[derive(Debug)]
+pub struct PublicKey<E: Engine>(E::G1);
+
+impl<E: Engine> PartialEq for PublicKey<E>
+where
+    E::G2: PartialEq,
+{
+    fn eq(&self, other: &PublicKey<E>) -> bool {
+        self.0 == other.0
+    }
+}
+
+impl<E: Engine> PublicKey<E> {
+    /// Returns `true` if the signature matches the element of `E::G2`.
+    pub fn verify_g2<H: Into<E::G2Affine>>(&self, sig: &Signature<E>, hash: H) -> bool {
+        E::pairing(self.0, hash) == E::pairing(E::G1::one(), sig.0)
+    }
+
+    /// Returns `true` if the signature matches the message.
+    pub fn verify<M: AsRef<[u8]>>(&self, sig: &Signature<E>, msg: M) -> bool {
+        self.verify_g2(sig, hash_g2::<E, M>(msg))
+    }
+}
+
+/// A signature, or a signature share.
+#[derive(Debug)]
+pub struct Signature<E: Engine>(E::G2);
+
+impl<E: Engine> PartialEq for Signature<E>
+where
+    E::G2: PartialEq,
+{
+    fn eq(&self, other: &Signature<E>) -> bool {
+        self.0 == other.0
+    }
+}
+
+/// A secret key, or a secret key share.
+#[derive(Debug)]
+pub struct SecretKey<E: Engine>(E::Fr);
+
+impl<E: Engine> PartialEq for SecretKey<E>
+where
+    E::G2: PartialEq,
+{
+    fn eq(&self, other: &SecretKey<E>) -> bool {
+        self.0 == other.0
+    }
+}
+
+impl<E: Engine> SecretKey<E> {
+    /// Creates a new secret key.
+    pub fn new<R: Rng>(rng: &mut R) -> Self {
+        SecretKey(rng.gen())
+    }
+
+    /// Returns the matching public key.
+    pub fn public_key(&self) -> PublicKey<E> {
+        PublicKey(E::G1Affine::one().mul(self.0))
+    }
+
+    /// Signs the given element of `E::G2`.
+    pub fn sign_g2<H: Into<E::G2Affine>>(&self, hash: H) -> Signature<E> {
+        Signature(hash.into().mul(self.0))
+    }
+
+    /// Signs the given message.
+    pub fn sign<M: AsRef<[u8]>>(&self, msg: M) -> Signature<E> {
+        self.sign_g2(hash_g2::<E, M>(msg))
+    }
+}
+
+/// A public key and an associated set of public key shares.
+pub struct PublicKeySet<E: Engine> {
+    /// The coefficients of a polynomial whose value at `0` is the "master key", and value at
+    /// `i + 1` is key share number `i`.
+    coeff: Vec<E::G1>,
+}
+
+impl<E: Engine> PublicKeySet<E> {
+    /// Returns the threshold `t`: any set of `t + 1` signature shares can be combined into a full
+    /// signature.
+    pub fn threshold(&self) -> usize {
+        self.coeff.len() - 1
+    }
+
+    /// Returns the public key.
+    pub fn public_key(&self) -> PublicKey<E> {
+        PublicKey(self.coeff[0])
+    }
+
+    /// Returns the `i`-th public key share.
+    pub fn public_key_share<T>(&self, i: T) -> PublicKey<E>
+    where
+        T: Into<<E::Fr as PrimeField>::Repr>,
+    {
+        let mut x = E::Fr::one();
+        x.add_assign(&E::Fr::from_repr(i.into()).expect("invalid index"));
+        let mut pk = *self.coeff.last().expect("at least one coefficient");
+        for c in self.coeff.iter().rev().skip(1) {
+            pk.mul_assign(x);
+            pk.add_assign(c);
+        }
+        PublicKey(pk)
+    }
+
+    /// Verifies that the given signatures are correct and combines them into a signature that can
+    /// be verified with the main public key.
+    pub fn combine_signatures<'a, ITR, IND>(&self, items: ITR) -> Result<Signature<E>>
+    where
+        ITR: IntoIterator<Item = (&'a IND, &'a Signature<E>)>,
+        IND: Into<<E::Fr as PrimeField>::Repr> + Clone + 'a,
+    {
+        let sigs: Vec<_> = items
+            .into_iter()
+            .map(|(i, sig)| {
+                let mut x = E::Fr::one();
+                x.add_assign(&E::Fr::from_repr(i.clone().into()).expect("invalid index"));
+                (x, sig)
+            })
+            .collect();
+        if sigs.len() < self.coeff.len() {
+            return Err(ErrorKind::NotEnoughShares.into());
+        }
+        let mut result = E::G2::zero();
+        let mut indexes = Vec::new();
+        for (x, sig) in sigs.iter().take(self.coeff.len()) {
+            if indexes.contains(x) {
+                return Err(ErrorKind::DuplicateEntry.into());
+            }
+            indexes.push(x.clone());
+            // Compute the value at 0 of the Lagrange polynomial that is `0` at the other data
+            // points but `1` at `x`.
+            let mut l0 = E::Fr::one();
+            for (x0, _) in sigs.iter().take(self.coeff.len()).filter(|(x0, _)| x0 != x) {
+                let mut denom = *x0;
+                denom.sub_assign(x);
+                l0.mul_assign(x0);
+                l0.mul_assign(&denom.inverse().expect("indices are different"));
+            }
+            let mut summand = sig.0;
+            summand.mul_assign(l0);
+            result.add_assign(&summand);
+        }
+        Ok(Signature(result))
+    }
+}
+
+/// A secret key and an associated set of secret key shares.
+pub struct SecretKeySet<E: Engine> {
+    /// The coefficients of a polynomial whose value at `0` is the "master key", and value at
+    /// `i + 1` is key share number `i`.
+    coeff: Vec<E::Fr>,
+}
+
+impl<E: Engine> SecretKeySet<E> {
+    /// Creates a set of secret key shares, where any `threshold + 1` of them can collaboratively
+    /// sign and decrypt.
+    pub fn new<R: Rng>(threshold: usize, rng: &mut R) -> Self {
+        SecretKeySet {
+            coeff: (0..(threshold + 1)).map(|_| rng.gen()).collect(),
+        }
+    }
+
+    /// Returns the threshold `t`: any set of `t + 1` signature shares can be combined into a full
+    /// signature.
+    pub fn threshold(&self) -> usize {
+        self.coeff.len() - 1
+    }
+
+    /// Returns the `i`-th secret key share.
+    pub fn secret_key_share<T>(&self, i: T) -> SecretKey<E>
+    where
+        T: Into<<E::Fr as PrimeField>::Repr>,
+    {
+        let mut x = E::Fr::one();
+        x.add_assign(&E::Fr::from_repr(i.into()).expect("invalid index"));
+        let mut pk = *self.coeff.last().expect("at least one coefficient");
+        for c in self.coeff.iter().rev().skip(1) {
+            pk.mul_assign(&x);
+            pk.add_assign(c);
+        }
+        SecretKey(pk)
+    }
+
+    /// Returns the corresponding public key set. That information can be shared publicly.
+    pub fn public_keys(&self) -> PublicKeySet<E> {
+        let to_pub = |c: &E::Fr| E::G1Affine::one().mul(*c);
+        PublicKeySet {
+            coeff: self.coeff.iter().map(to_pub).collect(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use std::collections::BTreeMap;
+
+    use pairing::bls12_381::Bls12;
+    use rand;
+
+    #[test]
+    fn test_simple_sig() {
+        let mut rng = rand::thread_rng();
+        let sk0 = SecretKey::<Bls12>::new(&mut rng);
+        let sk1 = SecretKey::<Bls12>::new(&mut rng);
+        let pk0 = sk0.public_key();
+        let msg0 = b"Real news";
+        let msg1 = b"Fake news";
+        assert!(pk0.verify(&sk0.sign(msg0), msg0));
+        assert!(!pk0.verify(&sk1.sign(msg0), msg0)); // Wrong key.
+        assert!(!pk0.verify(&sk0.sign(msg1), msg0)); // Wrong message.
+    }
+
+    #[test]
+    fn test_threshold_sig() {
+        let mut rng = rand::thread_rng();
+        let sk_set = SecretKeySet::<Bls12>::new(3, &mut rng);
+        let pk_set = sk_set.public_keys();
+        let msg = "Totally real news";
+
+        // The threshold is 3, so 4 signature shares will suffice to recreate the share.
+        let sigs: BTreeMap<_, _> = [5, 8, 7, 10]
+            .into_iter()
+            .map(|i| (*i, sk_set.secret_key_share(*i).sign(msg)))
+            .collect();
+
+        // Each of the shares is a valid signature matching its public key share.
+        for (i, sig) in &sigs {
+            pk_set.public_key_share(*i).verify(sig, msg);
+        }
+
+        // Combined, they produce a signature matching the main public key.
+        let sig = pk_set.combine_signatures(&sigs).expect("signatures match");
+        assert!(pk_set.public_key().verify(&sig, msg));
+
+        // A different set of signatories produces the same signature.
+        let sigs2: BTreeMap<_, _> = [42, 43, 44, 45]
+            .into_iter()
+            .map(|i| (*i, sk_set.secret_key_share(*i).sign(msg)))
+            .collect();
+        let sig2 = pk_set.combine_signatures(&sigs2).expect("signatures match");
+        assert_eq!(sig, sig2);
+    }
+
+    /// Some basic sanity checks for the hash function.
+    #[test]
+    fn test_hash_g2() {
+        let mut rng = rand::thread_rng();
+        let msg: Vec<u8> = (0..1000).map(|_| rng.gen()).collect();
+        let msg_end0: Vec<u8> = msg.iter().chain(b"end0").cloned().collect();
+        let msg_end1: Vec<u8> = msg.iter().chain(b"end1").cloned().collect();
+
+        let hash = hash_g2::<Bls12, _>;
+        assert_eq!(hash(&msg), hash(&msg));
+        assert_ne!(hash(&msg), hash(&msg_end0));
+        assert_ne!(hash(&msg_end0), hash(&msg_end1));
+    }
+}