From 66414209519a912dc63d346916b52187982b9e73 Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Wed, 21 Sep 2022 15:36:16 +0200
Subject: [PATCH] Bignum core: Break shift_r function out of the classic
 shift_r

This commit contains the function prototype for mbedtls_mpi_core_shift_r,
and the implementation minimally modified from mbedtls_mpi_shift_r.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
---
 library/bignum.c      | 35 ++++++++++++++++++++++-------------
 library/bignum_core.h | 15 +++++++++++++++
 2 files changed, 37 insertions(+), 13 deletions(-)

diff --git a/library/bignum.c b/library/bignum.c
index 1c7f9197f..0787272fe 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -770,27 +770,38 @@ cleanup:
  * Right-shift: X >>= count
  */
 int mbedtls_mpi_shift_r( mbedtls_mpi *X, size_t count )
+{
+    MPI_VALIDATE_RET( X != NULL );
+    if( X->n != 0 )
+        mbedtls_mpi_core_shift_r( X->p, X->n, count );
+    return( 0 );
+}
+
+void mbedtls_mpi_core_shift_r( mbedtls_mpi_uint *X, size_t limbs,
+                               size_t count )
 {
     size_t i, v0, v1;
     mbedtls_mpi_uint r0 = 0, r1;
-    MPI_VALIDATE_RET( X != NULL );
 
     v0 = count /  biL;
     v1 = count & (biL - 1);
 
-    if( v0 > X->n || ( v0 == X->n && v1 > 0 ) )
-        return mbedtls_mpi_lset( X, 0 );
+    if( v0 > limbs || ( v0 == limbs && v1 > 0 ) )
+    {
+        memset( X, 0, limbs * ciL );
+        return;
+    }
 
     /*
      * shift by count / limb_size
      */
     if( v0 > 0 )
     {
-        for( i = 0; i < X->n - v0; i++ )
-            X->p[i] = X->p[i + v0];
+        for( i = 0; i < limbs - v0; i++ )
+            X[i] = X[i + v0];
 
-        for( ; i < X->n; i++ )
-            X->p[i] = 0;
+        for( ; i < limbs; i++ )
+            X[i] = 0;
     }
 
     /*
@@ -798,16 +809,14 @@ int mbedtls_mpi_shift_r( mbedtls_mpi *X, size_t count )
      */
     if( v1 > 0 )
     {
-        for( i = X->n; i > 0; i-- )
+        for( i = limbs; i > 0; i-- )
         {
-            r1 = X->p[i - 1] << (biL - v1);
-            X->p[i - 1] >>= v1;
-            X->p[i - 1] |= r0;
+            r1 = X[i - 1] << (biL - v1);
+            X[i - 1] >>= v1;
+            X[i - 1] |= r0;
             r0 = r1;
         }
     }
-
-    return( 0 );
 }
 
 /*
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 196736d05..4ba14331c 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -262,6 +262,21 @@ int mbedtls_mpi_core_write_be( const mbedtls_mpi_uint *A,
                                unsigned char *output,
                                size_t output_length );
 
+/** \brief              Shift a machine integer 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.
+ *
+ *                      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.
+ */
+void mbedtls_mpi_core_shift_r( mbedtls_mpi_uint *X, size_t limbs,
+                               size_t count );
+
 /**
  * \brief Conditional addition of two fixed-size large unsigned integers,
  *        returning the carry.