diff --git a/src/arithmetic/bigint.rs b/src/arithmetic/bigint.rs
index 75ce9ad641..c4fc37ce82 100644
--- a/src/arithmetic/bigint.rs
+++ b/src/arithmetic/bigint.rs
@@ -170,6 +170,10 @@ impl<M> BoxedLimbs<M> {
             m: PhantomData,
         }
     }
+
+    fn into_limbs(self) -> Box<[Limb]> {
+        self.limbs
+    }
 }
 
 /// A modulus *s* that is smaller than another modulus *l* so every element of
@@ -534,7 +538,7 @@ pub(crate) fn elem_exp_vartime<M>(
 #[cfg(not(target_arch = "x86_64"))]
 pub fn elem_exp_consttime<M>(
     base: Elem<M, R>,
-    exponent: &PrivateExponent<M>,
+    exponent: &PrivateExponent,
     m: &Modulus<M>,
 ) -> Result<Elem<M, Unencoded>, error::Unspecified> {
     use crate::limb::Window;
@@ -629,7 +633,7 @@ pub fn elem_inverse_consttime<M: Prime>(
 #[cfg(target_arch = "x86_64")]
 pub fn elem_exp_consttime<M>(
     base: Elem<M, R>,
-    exponent: &PrivateExponent<M>,
+    exponent: &PrivateExponent,
     m: &Modulus<M>,
 ) -> Result<Elem<M, Unencoded>, error::Unspecified> {
     // Pretty much all the math here requires CPU feature detection to have
diff --git a/src/arithmetic/bigint/private_exponent.rs b/src/arithmetic/bigint/private_exponent.rs
index 7366f9ea4a..f0f5e34e9f 100644
--- a/src/arithmetic/bigint/private_exponent.rs
+++ b/src/arithmetic/bigint/private_exponent.rs
@@ -13,17 +13,16 @@
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
 use super::{elem_add, elem_sub, limb, BoxedLimbs, Limb, LimbMask, Modulus, Prime};
-
 use crate::error;
+use alloc::boxed::Box;
 
-// `M` represents the prime modulus for which the exponent is in the interval
-// [1, `m` - 1).
-pub struct PrivateExponent<M> {
-    limbs: BoxedLimbs<M>,
+pub struct PrivateExponent {
+    limbs: Box<[Limb]>,
 }
 
-impl<M> PrivateExponent<M> {
-    pub fn from_be_bytes_padded(
+impl PrivateExponent {
+    // `p` is the modulus for which the exponent is in the interval [1, `p` - 1).
+    pub fn from_be_bytes_padded<M>(
         input: untrusted::Input,
         p: &Modulus<M>,
     ) -> Result<Self, error::Unspecified> {
@@ -41,22 +40,22 @@ impl<M> PrivateExponent<M> {
             return Err(error::Unspecified);
         }
 
-        Ok(Self { limbs: dP })
+        Ok(Self {
+            limbs: dP.into_limbs(),
+        })
     }
 
     #[inline]
     pub(super) fn limbs(&self) -> &[Limb] {
         &self.limbs
     }
-}
 
-impl<M: Prime> PrivateExponent<M> {
     // Returns `p - 2`.
-    pub(super) fn for_flt(p: &Modulus<M>) -> Self {
+    pub(super) fn for_flt<P: Prime>(p: &Modulus<P>) -> Self {
         let two = elem_add(p.one(), p.one(), p);
         let p_minus_2 = elem_sub(p.zero(), &two, p);
         Self {
-            limbs: p_minus_2.limbs,
+            limbs: p_minus_2.limbs.into_limbs(),
         }
     }
 }
diff --git a/src/rsa/keypair.rs b/src/rsa/keypair.rs
index c351f157de..ef05960838 100644
--- a/src/rsa/keypair.rs
+++ b/src/rsa/keypair.rs
@@ -462,7 +462,7 @@ impl signature::KeyPair for KeyPair {
 
 struct PrivatePrime<M: Prime> {
     modulus: bigint::Modulus<M>,
-    exponent: bigint::PrivateExponent<M>,
+    exponent: bigint::PrivateExponent,
 }
 
 impl<M: Prime> PrivatePrime<M> {