diff --git a/src/fields/fp.rs b/src/fields/fp.rs
index c475d98..45605cb 100644
--- a/src/fields/fp.rs
+++ b/src/fields/fp.rs
@@ -588,15 +588,26 @@ impl ff::Field for Fp {
     }
 
     fn pow_vartime<S: AsRef<[u64]>>(&self, exp: S) -> Self {
+        // PATCH (Aeneas compatibility): index-based loops instead of
+        // `iter().rev()` / `(0..64).rev()` (double-ended range iterators have
+        // no Aeneas model). Iteration order and arithmetic are IDENTICAL to
+        // upstream: limbs from most- to least-significant, bits from high to
+        // low, square-then-conditionally-multiply.
         let mut res = Self::one();
         let mut found_one = false;
-        for e in exp.as_ref().iter().rev() {
-            for i in (0..64).rev() {
+        let exp = exp.as_ref();
+        let mut j = exp.len();
+        while j > 0 {
+            j -= 1;
+            let e = exp[j];
+            let mut i = 64u32;
+            while i > 0 {
+                i -= 1;
                 if found_one {
                     res = res.square();
                 }
 
-                if ((*e >> i) & 1) == 1 {
+                if ((e >> i) & 1) == 1 {
                     found_one = true;
                     res *= self;
                 }