diff --git a/library/bignum_mod_raw.c b/library/bignum_mod_raw.c
index 22e56b7..03924d2 100644
--- a/library/bignum_mod_raw.c
+++ b/library/bignum_mod_raw.c
@@ -124,6 +124,34 @@
 
 /* BEGIN MERGE SLOT 3 */
 
+size_t mbedtls_mpi_mod_raw_inv_prime_working_limbs( size_t AN_limbs )
+{
+    /* mbedtls_mpi_mod_raw_inv_prime() needs a temporary for the exponent,
+     * which will be the same size as the modulus and input (AN_limbs),
+     * and additional space to pass to mbedtls_mpi_core_exp_mod(). */
+    return( AN_limbs +
+            mbedtls_mpi_core_exp_mod_working_limbs( AN_limbs, AN_limbs ) );
+}
+
+void mbedtls_mpi_mod_raw_inv_prime( mbedtls_mpi_uint *X,
+                                    const mbedtls_mpi_uint *A,
+                                    const mbedtls_mpi_uint *N,
+                                    size_t AN_limbs,
+                                    const mbedtls_mpi_uint *RR,
+                                    mbedtls_mpi_uint *T )
+{
+    /* Inversion by power: g^|G| = 1 => g^(-1) = g^(|G|-1), and
+     *                       |G| = N - 1, so we want
+     *                 g^(|G|-1) = g^(N - 2)
+     */
+    mbedtls_mpi_uint *Nminus2 = T;
+    (void) mbedtls_mpi_core_sub_int( Nminus2, N, 2, AN_limbs );
+
+    mbedtls_mpi_core_exp_mod( X,
+                              A, N, AN_limbs, Nminus2, AN_limbs,
+                              RR, T + AN_limbs );
+}
+
 /* END MERGE SLOT 3 */
 
 /* BEGIN MERGE SLOT 4 */
diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index d7b6dd1..698119e 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -174,6 +174,51 @@
 
 /* BEGIN MERGE SLOT 3 */
 
+/**
+ * \brief          Returns the number of limbs of working memory required for
+ *                 a call to `mbedtls_mpi_mod_raw_inv_prime()`.
+ *
+ * \param AN_limbs The number of limbs in the input `A` and the modulus `N`
+ *                 (they must be the same size) that will be given to
+ *                 `mbedtls_mpi_mod_raw_inv_prime()`.
+ *
+ * \return         The number of limbs of working memory required by
+ *                 `mbedtls_mpi_mod_raw_inv_prime()`.
+ */
+size_t mbedtls_mpi_mod_raw_inv_prime_working_limbs( size_t AN_limbs );
+
+/**
+ * \brief Perform fixed-width modular inversion of a Montgomery-form MPI with
+ *        respect to a modulus \p N that must be prime.
+ *
+ * \p X may be aliased to \p A, but not to \p N or \p RR.
+ *
+ * \param[out] X     The modular inverse of \p A with respect to \p N.
+ *                   Will be in Montgomery form.
+ * \param[in] A      The number to calculate the modular inverse of.
+ *                   Must be in Montgomery form. Must not be 0.
+ * \param[in] N      The modulus, as a little-endian array of length \p AN_limbs.
+ *                   Must be prime.
+ * \param AN_limbs   The number of limbs in \p A, \p N and \p RR.
+ * \param[in] RR     The precomputed residue of 2^{2*biL} modulo N, as a little-
+ *                   endian array of length \p AN_limbs.
+ * \param[in,out] T  Temporary storage of at least the number of limbs returned
+ *                   by `mbedtls_mpi_mod_raw_inv_prime_working_limbs()`.
+ *                   Its initial content is unused and its final content is
+ *                   indeterminate.
+ *                   It must not alias or otherwise overlap any of the other
+ *                   parameters.
+ *                   It is up to the caller to zeroize \p T when it is no
+ *                   longer needed, and before freeing it if it was dynamically
+ *                   allocated.
+ */
+void mbedtls_mpi_mod_raw_inv_prime( mbedtls_mpi_uint *X,
+                                    const mbedtls_mpi_uint *A,
+                                    const mbedtls_mpi_uint *N,
+                                    size_t AN_limbs,
+                                    const mbedtls_mpi_uint *RR,
+                                    mbedtls_mpi_uint *T );
+
 /* END MERGE SLOT 3 */
 
 /* BEGIN MERGE SLOT 4 */