diff --git a/src/fcircuit.rs b/src/fcircuit.rs
new file mode 100644
index 0000000..4942d9c
--- /dev/null
+++ b/src/fcircuit.rs
@@ -0,0 +1,155 @@
+use ark_crypto_primitives::sponge::{
+    constraints::CryptographicSpongeVar,
+    poseidon::{constraints::PoseidonSpongeVar, PoseidonConfig},
+    Absorb,
+};
+use ark_ec::CurveGroup;
+use ark_ff::{Field, PrimeField};
+use ark_r1cs_std::prelude::CurveVar;
+use ark_r1cs_std::{
+    boolean::Boolean,
+    eq::EqGadget,
+    fields::{fp::FpVar, FieldVar},
+};
+use ark_relations::r1cs::{ConstraintSystemRef, SynthesisError};
+use ark_std::marker::PhantomData;
+use std::fmt::Debug;
+
+use arkeddsa::constraints::verify;
+use folding_schemes::{frontend::FCircuit, Error};
+
+use crate::signature::{SigPk, SigPkVar};
+
+pub type CF<C> = <<C as CurveGroup>::BaseField as Field>::BasePrimeField;
+
+/// Test circuit to be folded
+#[derive(Clone, Debug)]
+pub struct EthDosCircuit<F: PrimeField, C: CurveGroup, GC: CurveVar<C, F>> {
+    _c: PhantomData<C>,
+    _gc: PhantomData<GC>,
+    config: PoseidonConfig<F>,
+}
+impl<F: PrimeField, C: CurveGroup, GC: CurveVar<C, F>> FCircuit<F> for EthDosCircuit<F, C, GC>
+where
+    F: Absorb,
+    C: CurveGroup<BaseField = F>,
+{
+    type Params = PoseidonConfig<F>;
+    type ExternalInputs = SigPk<C>;
+    type ExternalInputsVar = SigPkVar<C, GC>;
+
+    fn new(config: Self::Params) -> Result<Self, Error> {
+        Ok(Self {
+            _c: PhantomData,
+            _gc: PhantomData,
+            config,
+        })
+    }
+    fn state_len(&self) -> usize {
+        5
+    }
+    fn generate_step_constraints(
+        &self,
+        cs: ConstraintSystemRef<F>,
+        _i: usize,
+        z_i: Vec<FpVar<F>>,
+        external_inputs: Self::ExternalInputsVar,
+    ) -> Result<Vec<FpVar<F>>, SynthesisError> {
+        let pk_0_x = z_i[0].clone();
+        let pk_0_y = z_i[1].clone();
+        let pk_i_x = z_i[2].clone();
+        let pk_i_y = z_i[3].clone();
+        let mut degree = z_i[4].clone();
+
+        // get the 'msg' that has been signed, which is the hash of the previous-signer public key
+        let mut poseidon = PoseidonSpongeVar::new(cs.clone(), &self.config);
+        poseidon.absorb(&vec![pk_i_x, pk_i_y])?;
+        let h = poseidon.squeeze_field_elements(1)?;
+        let msg = h
+            .first()
+            .ok_or(ark_relations::r1cs::SynthesisError::Unsatisfiable)?;
+
+        // check that the last signer is signed by the new signer
+        let ei: SigPkVar<C, GC> = external_inputs.into();
+        let res = verify::<C, GC>(
+            cs.clone(),
+            self.config.clone(),
+            ei.pk.clone(),
+            (ei.sig_r, ei.sig_s),
+            msg.clone(),
+        )?;
+        res.enforce_equal(&Boolean::<F>::TRUE)?;
+
+        // increment the degree
+        degree = degree.clone() + FpVar::<F>::one();
+
+        let pk_i1_xy = ei.pk.to_constraint_field()?;
+        Ok(vec![vec![pk_0_x, pk_0_y], pk_i1_xy, vec![degree]].concat())
+    }
+}
+#[cfg(test)]
+pub mod tests {
+    use super::*;
+    use ark_bn254::Fr;
+    use ark_ec::AffineRepr;
+    use ark_r1cs_std::{alloc::AllocVar, R1CSVar};
+    use ark_relations::r1cs::ConstraintSystem;
+    use ark_std::Zero;
+    use rand::rngs::OsRng;
+
+    use crate::signature::{gen_signatures, hash_pk};
+    use arkeddsa::ed_on_bn254_twist::{constraints::EdwardsVar, EdwardsProjective};
+    use folding_schemes::transcript::poseidon::poseidon_canonical_config;
+
+    #[test]
+    fn test_sig() {
+        let mut rng = OsRng;
+        let poseidon_config = poseidon_canonical_config::<Fr>();
+
+        const N: usize = 1;
+        let ext_inps = gen_signatures::<OsRng, EdwardsProjective>(&mut rng, &poseidon_config, 1);
+        let e = ext_inps[0].clone();
+
+        let msg = hash_pk(&poseidon_config, e.pk);
+
+        e.pk.verify(&poseidon_config, &msg, &e.sig).unwrap();
+    }
+
+    fn ensure_fcircuit_trait<FC: FCircuit<Fr>>(params: FC::Params) {
+        let _ = FC::new(params);
+    }
+
+    // test to check that the Sha256FCircuit computes the same values inside and outside the circuit
+    #[test]
+    fn test_fcircuit() {
+        let mut rng = rand::rngs::OsRng;
+        let poseidon_config = poseidon_canonical_config::<Fr>();
+
+        let pks_sigs = gen_signatures::<OsRng, EdwardsProjective>(&mut rng, &poseidon_config, 1);
+
+        // here `Fr` is the BN254::G1::Fr = ed_on_bn254_twist::EdwardsProjective::Fq
+        let cs = ConstraintSystem::<Fr>::new_ref();
+
+        type FC = EthDosCircuit<Fr, EdwardsProjective, EdwardsVar>;
+        ensure_fcircuit_trait::<FC>(poseidon_config.clone());
+
+        let circuit = FC::new(poseidon_config).unwrap();
+        let xy: (Fr, Fr) = pks_sigs[0].pk.0.xy().unwrap();
+        let pk0 = vec![xy.0, xy.1];
+        let z_i: Vec<Fr> = vec![pk0.clone(), pk0, vec![Fr::zero()]].concat();
+
+        let external_inputs_var =
+            SigPkVar::<EdwardsProjective, EdwardsVar>::new_witness(cs.clone(), || Ok(pks_sigs[0]))
+                .unwrap();
+
+        let z_iVar = Vec::<FpVar<Fr>>::new_witness(cs.clone(), || Ok(z_i)).unwrap();
+        let computed_z_i1Var = circuit
+            .generate_step_constraints(cs.clone(), 0, z_iVar.clone(), external_inputs_var)
+            .unwrap();
+        // check that the degree (in the last state) is 1, the amount of signatures verified
+        assert_eq!(computed_z_i1Var.value().unwrap()[4], Fr::from(1_u32));
+        assert!(cs.is_satisfied().unwrap());
+        dbg!(cs.num_constraints());
+        dbg!(&computed_z_i1Var.value().unwrap());
+    }
+}
diff --git a/src/signature.rs b/src/signature.rs
new file mode 100644
index 0000000..5e234ca
--- /dev/null
+++ b/src/signature.rs
@@ -0,0 +1,157 @@
+use ark_crypto_primitives::sponge::{
+    poseidon::{PoseidonConfig, PoseidonSponge},
+    Absorb, CryptographicSponge,
+};
+use ark_ec::{AffineRepr, CurveGroup};
+use ark_ff::{BigInteger, PrimeField};
+use ark_r1cs_std::alloc::{AllocVar, AllocationMode};
+use ark_r1cs_std::boolean::Boolean;
+use ark_r1cs_std::prelude::CurveVar;
+use ark_relations::r1cs::{Namespace, SynthesisError};
+use ark_serialize::{CanonicalDeserialize, CanonicalSerialize};
+use ark_std::{rand::Rng, Zero};
+use core::borrow::Borrow;
+use rand_core::CryptoRngCore;
+use std::fmt::Debug;
+
+use arkeddsa::{signature::Signature, PublicKey, SigningKey};
+
+use crate::fcircuit::CF;
+
+// recall, here C = ed_on_bn254, so C::BaseField = BN254::ScalarField
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct SigPk<C: CurveGroup> {
+    pub pk: PublicKey<C>,
+    pub sig: Signature<C>,
+}
+impl<C: CurveGroup> Default for SigPk<C> {
+    fn default() -> Self {
+        Self {
+            pk: PublicKey(C::zero().into_affine()),
+            sig: Signature::new(C::zero().into_affine(), C::ScalarField::zero()),
+        }
+    }
+}
+impl<C: CurveGroup> SigPk<C> {
+    pub fn to_bytes(&self) -> Vec<u8> {
+        let sig_bytes = self.sig.to_bytes();
+        let pk_bytes = self.pk.to_bytes();
+        vec![sig_bytes, pk_bytes].concat()
+    }
+    pub fn from_bytes(b: Vec<u8>) -> Self {
+        let u_point_size = C::Affine::generator().serialized_size(ark_serialize::Compress::No);
+
+        let sig = Signature::<C>::from_bytes(&b[..32 + u_point_size]).unwrap();
+        let pk = PublicKey::<C>::from_bytes(&b[32 + u_point_size..]).unwrap();
+        Self { pk, sig }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct SigPkVar<C: CurveGroup, GC: CurveVar<C, CF<C>>> {
+    pub pk: GC,
+    pub sig_r: GC,
+    pub sig_s: Vec<Boolean<CF<C>>>,
+}
+impl<C: CurveGroup, GC: CurveVar<C, CF<C>>> Default for SigPkVar<C, GC> {
+    fn default() -> Self {
+        Self {
+            pk: GC::zero(),
+            sig_r: GC::zero(),
+            sig_s: vec![Boolean::<CF<C>>::FALSE; 253], // TODO 253-> fieldbitsize
+        }
+    }
+}
+
+impl<C, GC> AllocVar<SigPk<C>, CF<C>> for SigPkVar<C, GC>
+where
+    C: CurveGroup,
+    GC: CurveVar<C, CF<C>>,
+{
+    fn new_variable<T: Borrow<SigPk<C>>>(
+        cs: impl Into<Namespace<CF<C>>>,
+        f: impl FnOnce() -> Result<T, SynthesisError>,
+        mode: AllocationMode,
+    ) -> Result<Self, SynthesisError> {
+        f().and_then(|val| {
+            let cs = cs.into();
+
+            let e = val.borrow();
+            let pk = GC::new_variable(cs.clone(), || Ok(e.pk.0), mode)?;
+            let sig_r = GC::new_variable(cs.clone(), || Ok(e.sig.r), mode)?;
+            let sig_s = Vec::<Boolean<CF<C>>>::new_variable(
+                cs.clone(),
+                || Ok(e.sig.s.into_bigint().to_bits_le()),
+                mode,
+            )?;
+            let v = Self { pk, sig_r, sig_s };
+
+            Ok(v)
+        })
+    }
+}
+
+pub fn hash_pk<C: CurveGroup>(
+    poseidon_config: &PoseidonConfig<C::BaseField>,
+    pk: PublicKey<C>,
+) -> C::BaseField
+where
+    C::BaseField: PrimeField + Absorb,
+{
+    let mut poseidon = PoseidonSponge::new(poseidon_config);
+    let (pk_x, pk_y): (C::BaseField, C::BaseField) = pk.xy().unwrap();
+    poseidon.absorb(&vec![pk_x, pk_y]);
+    let k = poseidon.squeeze_field_elements::<C::BaseField>(1);
+    *k.first().unwrap()
+}
+
+// returns a vector of signatures & publickeys, where each signature is of the previous public key
+pub fn gen_signatures<R: Rng + CryptoRngCore, C: CurveGroup>(
+    rng: &mut R,
+    poseidon_config: &PoseidonConfig<C::BaseField>,
+    steps: usize,
+) -> Vec<SigPk<C>>
+where
+    C::BaseField: PrimeField + Absorb,
+{
+    let mut prev_pk = None;
+    let mut res: Vec<SigPk<C>> = Vec::new();
+    for _ in 0..steps {
+        let extinp = gen_sig(rng, poseidon_config, prev_pk);
+        res.push(extinp);
+        prev_pk = Some(extinp.pk);
+    }
+    res
+}
+
+// generates a new secret key, and signs the given `prev_pk` with it. If the `prev_pk==None`, it
+// will use the newly generated public key as the prev_pk.
+pub fn gen_sig<R: Rng + CryptoRngCore, C: CurveGroup>(
+    rng: &mut R,
+    poseidon_config: &PoseidonConfig<C::BaseField>,
+    prev_pk: Option<PublicKey<C>>,
+) -> SigPk<C>
+where
+    C::BaseField: PrimeField + Absorb,
+{
+    let sk = SigningKey::<C>::generate::<blake2::Blake2b512>(rng).unwrap();
+    let pk = sk.public_key();
+
+    // if prev_pk!=None, use it, else, set the new pk to it
+    let prev_pk = if prev_pk.is_some() {
+        prev_pk.unwrap()
+    } else {
+        *pk
+    };
+
+    let msg = hash_pk(poseidon_config, prev_pk);
+
+    let sig = sk
+        .sign::<blake2::Blake2b512>(&poseidon_config, &msg)
+        .unwrap();
+    pk.verify(&poseidon_config, &msg, &sig).unwrap();
+    SigPk {
+        pk: pk.clone(),
+        sig,
+    }
+}