From ec09e2525147cb7cf7c82df3a8e1aa69dd62fcf1 Mon Sep 17 00:00:00 2001
From: Minos Galanakis <minos.galanakis@arm.com>
Date: Thu, 20 Apr 2023 14:22:16 +0100
Subject: [PATCH] bignum_core: Aligned `xxx_core_shift_l` to `xxx_core_shift_r`

This patch modifies the left-shift implementation to closely
align in interface and behaviour to the existing right-shift
method.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
---
 library/bignum_core.c | 50 ++++++++++++++++++-------------------------
 library/bignum_core.h | 19 +++++++++-------
 2 files changed, 32 insertions(+), 37 deletions(-)

diff --git a/library/bignum_core.c b/library/bignum_core.c
index 26aff15f1..92a9d558a 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -353,52 +353,44 @@ void mbedtls_mpi_core_shift_r(mbedtls_mpi_uint *X, size_t limbs,
     }
 }
 
-int mbedtls_mpi_shift_l( mbedtls_mpi *X, size_t count )
+void mbedtls_mpi_core_shift_l(mbedtls_mpi_uint *X, size_t limbs,
+                              size_t count)
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    size_t i, v0, t1;
+    size_t i, v0, v1;
     mbedtls_mpi_uint r0 = 0, r1;
-    MPI_VALIDATE_RET( X != NULL );
 
-    v0 = count / (biL    );
-    t1 = count & (biL - 1);
+    v0 = count / (biL);
+    v1 = count & (biL - 1);
 
-    i = mbedtls_mpi_bitlen( X ) + count;
-
-    if( X->n * biL < i )
-        MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, BITS_TO_LIMBS( i ) ) );
-
-    ret = 0;
+    if (v0 > limbs || (v0 == limbs && v1 > 0)) {
+        memset(X, 0, limbs * ciL);
+        return;
+    }
 
     /*
      * shift by count / limb_size
      */
-    if( v0 > 0 )
-    {
-        for( i = X->n; i > v0; i-- )
-            X->p[i - 1] = X->p[i - v0 - 1];
+    if (v0 > 0) {
+        for (i = limbs; i > v0; i--) {
+            X[i - 1] = X[i - v0 - 1];
+        }
 
-        for( ; i > 0; i-- )
-            X->p[i - 1] = 0;
+        for (; i > 0; i--) {
+            X[i - 1] = 0;
+        }
     }
 
     /*
      * shift by count % limb_size
      */
-    if( t1 > 0 )
-    {
-        for( i = v0; i < X->n; i++ )
-        {
-            r1 = X->p[i] >> (biL - t1);
-            X->p[i] <<= t1;
-            X->p[i] |= r0;
+    if (v1 > 0) {
+        for (i = v0; i < limbs; i++) {
+            r1 = X[i] >> (biL - v1);
+            X[i] <<= v1;
+            X[i] |= r0;
             r0 = r1;
         }
     }
-
-cleanup:
-
-    return( ret );
 }
 
 mbedtls_mpi_uint mbedtls_mpi_core_add(mbedtls_mpi_uint *X,
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 25e7e1a6b..2b11ccaff 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -278,7 +278,7 @@ int mbedtls_mpi_core_write_be(const mbedtls_mpi_uint *A,
                               unsigned char *output,
                               size_t output_length);
 
-/** \brief              Shift an MPI right in place by a number of bits.
+/** \brief              Shift an MPI in-place right by a number of bits.
  *
  *                      Shifting by more bits than there are bit positions
  *                      in \p X is valid and results in setting \p X to 0.
@@ -294,16 +294,19 @@ void mbedtls_mpi_core_shift_r(mbedtls_mpi_uint *X, size_t limbs,
                               size_t count);
 
 /**
- * \brief          Perform a left-shift on an MPI: X <<= count
+ * \brief               Shift an MPI in-place left by a number of bits.
  *
- * \param X        The MPI to shift. This must point to an initialized MPI.
- * \param count    The number of bits to shift by.
+ *                      Shifting by more bits than there are bit positions
+ *                      in \p X is valid and results in setting \p X to 0.
  *
- * \return         \c 0 if successful.
- * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.
- * \return         Another negative error code on different kinds of failure.
+ *                      This function's execution time depends on the value
+ *                      of \p count (and of course \p limbs).
+ * \param[in,out] X     The number to shift.
+ * \param limbs         The number of limbs of \p X. This must be at least 1.
+ * \param count         The number of bits to shift by.
  */
-int mbedtls_mpi_shift_l( mbedtls_mpi *X, size_t count );
+void mbedtls_mpi_core_shift_l(mbedtls_mpi_uint *X, size_t limbs,
+                              size_t count);
 
 /**
  * \brief Add two fixed-size large unsigned integers, returning the carry.