diff --git a/src/circuit_builder.rs b/src/circuit_builder.rs
index a2c94f83..ccca4e2b 100644
--- a/src/circuit_builder.rs
+++ b/src/circuit_builder.rs
@@ -149,6 +149,11 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         self.copy_constraints.push((x, y));
     }
 
+    pub fn assert_zero(&mut self, x: Target) {
+        let zero = self.zero();
+        self.assert_equal(x, zero);
+    }
+
     pub fn add_generators(&mut self, generators: Vec<Box<dyn WitnessGenerator<F>>>) {
         self.generators.extend(generators);
     }
diff --git a/src/fri/recursive_verifier.rs b/src/fri/recursive_verifier.rs
index a64b02fe..6c245167 100644
--- a/src/fri/recursive_verifier.rs
+++ b/src/fri/recursive_verifier.rs
@@ -49,14 +49,7 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         inputs.push(proof.pow_witness);
 
         let hash = self.hash_n_to_m(inputs, 1, false)[0];
-        let purported_hash_bits = self.split_le_virtual(hash, 64);
-        // for &b in &purported_hash_bits {
-        //     self.generate_copy(b, self.zero());
-        // }
-        // ensure!(
-        //     hash.to_canonical_u64().trailing_zeros() >= config.proof_of_work_bits,
-        //     "Invalid proof of work witness."
-        // );
+        self.assert_trailing_zeros(hash, config.proof_of_work_bits);
 
         Ok(())
     }
diff --git a/src/gadgets/mod.rs b/src/gadgets/mod.rs
index 9a6a728e..22bef43b 100644
--- a/src/gadgets/mod.rs
+++ b/src/gadgets/mod.rs
@@ -1,4 +1,5 @@
 pub mod arithmetic;
 pub mod hash;
 pub mod polynomial;
+pub mod split_base;
 pub(crate) mod split_join;
diff --git a/src/gadgets/split_base.rs b/src/gadgets/split_base.rs
new file mode 100644
index 00000000..e622be75
--- /dev/null
+++ b/src/gadgets/split_base.rs
@@ -0,0 +1,33 @@
+use crate::circuit_builder::CircuitBuilder;
+use crate::field::extension_field::Extendable;
+use crate::field::field::Field;
+use crate::gates::base_sum::{BaseSplitGenerator, BaseSumGate};
+use crate::generator::{SimpleGenerator, WitnessGenerator};
+use crate::target::Target;
+use crate::util::ceil_div_usize;
+use crate::wire::Wire;
+use crate::witness::PartialWitness;
+
+impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
+    /// Split the given element into a list of 11 targets, where each one represents a
+    /// base-64 limb of the element, with little-endian ordering.
+    pub(crate) fn split_le_base64(&mut self, x: Target) -> Vec<Target> {
+        let gate = self.add_gate(BaseSumGate::<64>::new(11), vec![]);
+        let sum = Target::Wire(Wire {
+            gate,
+            input: BaseSumGate::<64>::WIRE_SUM,
+        });
+        self.route(x, sum);
+        (BaseSumGate::<64>::WIRE_LIMBS_START..BaseSumGate::<64>::WIRE_LIMBS_START + 11)
+            .map(|i| Target::Wire(Wire { gate, input: i }))
+            .collect()
+    }
+
+    /// Asserts that `x`'s bit representation has at least `trailing_zeros` trailing zeros.
+    pub(crate) fn assert_trailing_zeros(&mut self, x: Target, trailing_zeros: u32) {
+        let limbs = self.split_le_base64(x);
+        for i in 0..ceil_div_usize(trailing_zeros as usize, 6) {
+            self.assert_zero(limbs[i]);
+        }
+    }
+}
diff --git a/src/gates/base_sum.rs b/src/gates/base_sum.rs
index f7ebd3a0..9190e19c 100644
--- a/src/gates/base_sum.rs
+++ b/src/gates/base_sum.rs
@@ -19,10 +19,8 @@ pub struct BaseSumGate<const B: usize> {
 }
 
 impl<const B: usize> BaseSumGate<B> {
-    pub fn new<F: Extendable<D>, const D: usize>(config: &CircuitConfig) -> GateRef<F, D> {
-        GateRef::new(BaseSumGate::<B> {
-            num_limbs: config.num_routed_wires - 1,
-        })
+    pub fn new<F: Extendable<D>, const D: usize>(num_limbs: usize) -> GateRef<F, D> {
+        GateRef::new(BaseSumGate::<B> { num_limbs })
     }
 
     pub const WIRE_SUM: usize = 0;
@@ -36,7 +34,7 @@ impl<const B: usize> BaseSumGate<B> {
 
 impl<F: Extendable<D>, const D: usize, const B: usize> Gate<F, D> for BaseSumGate<B> {
     fn id(&self) -> String {
-        format!("{:?}", self)
+        format!("{:?} + Base: {}", self, B)
     }
 
     fn eval_unfiltered(&self, vars: EvaluationVars<F, D>) -> Vec<F::Extension> {
@@ -163,7 +161,6 @@ mod tests {
 
     #[test]
     fn low_degree() {
-        let config = CircuitConfig::default();
-        test_low_degree(BaseSumGate::<6>::new::<CrandallField, 4>(&config))
+        test_low_degree(BaseSumGate::<6>::new::<CrandallField, 4>(11))
     }
 }
diff --git a/src/gates/mod.rs b/src/gates/mod.rs
index b25337ff..49c37bae 100644
--- a/src/gates/mod.rs
+++ b/src/gates/mod.rs
@@ -1,10 +1,10 @@
 pub(crate) mod arithmetic;
+pub mod base_sum;
 pub mod constant;
 pub(crate) mod gate;
 pub mod gmimc;
 mod interpolation;
 pub(crate) mod noop;
 
-mod base_sum;
 #[cfg(test)]
 mod gate_testing;