diff --git a/library/bignum_internal.h b/library/bignum_internal.h
new file mode 100644
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+++ b/library/bignum_internal.h
@@ -0,0 +1,50 @@
+/**
+ * \file bignum_internal.h
+ *
+ * \brief Internal-only bignum public-key cryptosystem API.
+ *
+ * This file declares bignum-related functions that are to be used
+ * only from within the Mbed TLS library itself.
+ *
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+#ifndef MBEDTLS_BIGNUM_INTERNAL_H
+#define MBEDTLS_BIGNUM_INTERNAL_H
+
+/**
+ * \brief          Perform a modular exponentiation: X = A^E mod N
+ *
+ * \warning        This function is not constant time with respect to \p E (the exponent).
+ *
+ * \param X        The destination MPI. This must point to an initialized MPI.
+ *                 This must not alias E or N.
+ * \param A        The base of the exponentiation.
+ *                 This must point to an initialized MPI.
+ * \param E        The exponent MPI. This must point to an initialized MPI.
+ * \param N        The base for the modular reduction. This must point to an
+ *                 initialized MPI.
+ * \param prec_RR  A helper MPI depending solely on \p N which can be used to
+ *                 speed-up multiple modular exponentiations for the same value
+ *                 of \p N. This may be \c NULL. If it is not \c NULL, it must
+ *                 point to an initialized MPI. If it hasn't been used after
+ *                 the call to mbedtls_mpi_init(), this function will compute
+ *                 the helper value and store it in \p prec_RR for reuse on
+ *                 subsequent calls to this function. Otherwise, the function
+ *                 will assume that \p prec_RR holds the helper value set by a
+ *                 previous call to mbedtls_mpi_exp_mod(), and reuse it.
+ *
+ * \return         \c 0 if successful.
+ * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.
+ * \return         #MBEDTLS_ERR_MPI_BAD_INPUT_DATA if \c N is negative or
+ *                 even, or if \c E is negative.
+ * \return         Another negative error code on different kinds of failures.
+ *
+ */
+int mbedtls_mpi_exp_mod_unsafe(mbedtls_mpi *X, const mbedtls_mpi *A,
+                               const mbedtls_mpi *E, const mbedtls_mpi *N,
+                               mbedtls_mpi *prec_RR);
+
+#endif /* bignum_internal.h */