Merge pull request #6230 from tom-cosgrove-arm/issue-6223-core-add

Bignum: extract core_add from the prototype

library/bignum.c

@@ -867,8 +867,7 @@ int mbedtls_mpi_cmp_int( const mbedtls_mpi *X, mbedtls_mpi_sint z )
 int mbedtls_mpi_add_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi *B )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    size_t i, j;
-    mbedtls_mpi_uint *o, *p, c, tmp;
+    size_t j;
     MPI_VALIDATE_RET( X != NULL );
     MPI_VALIDATE_RET( A != NULL );
     MPI_VALIDATE_RET( B != NULL );
@@ -882,7 +881,7 @@ int mbedtls_mpi_add_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
         MBEDTLS_MPI_CHK( mbedtls_mpi_copy( X, A ) );
 
     /*
-     * X should always be positive as a result of unsigned additions.
+     * X must always be positive as a result of unsigned additions.
      */
     X->s = 1;
 
@@ -892,27 +891,25 @@ int mbedtls_mpi_add_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
 
     MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, j ) );
 
-    o = B->p; p = X->p; c = 0;
+    /* j is the number of non-zero limbs of B. Add those to X. */
 
-    /*
-     * tmp is used because it might happen that p == o
-     */
-    for( i = 0; i < j; i++, o++, p++ )
-    {
-        tmp= *o;
-        *p +=  c; c  = ( *p <  c );
-        *p += tmp; c += ( *p < tmp );
-    }
+    mbedtls_mpi_uint *p = X->p;
+
+    mbedtls_mpi_uint c = mbedtls_mpi_core_add( p, p, B->p, j );
+
+    p += j;
+
+    /* Now propagate any carry */
 
     while( c != 0 )
     {
-        if( i >= X->n )
+        if( j >= X->n )
         {
-            MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, i + 1 ) );
-            p = X->p + i;
+            MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, j + 1 ) );
+            p = X->p + j;
         }
 
-        *p += c; c = ( *p < c ); i++; p++;
+        *p += c; c = ( *p < c ); j++; p++;
     }
 
 cleanup:

library/bignum_core.c

@@ -316,8 +316,6 @@ int mbedtls_mpi_core_write_be( const mbedtls_mpi_uint *X,
     return( 0 );
 }
 
-
-
 void mbedtls_mpi_core_shift_r( mbedtls_mpi_uint *X, size_t limbs,
                                size_t count )
 {
@@ -360,7 +358,24 @@ void mbedtls_mpi_core_shift_r( mbedtls_mpi_uint *X, size_t limbs,
     }
 }
 
+mbedtls_mpi_uint mbedtls_mpi_core_add( mbedtls_mpi_uint *X,
+                                       const mbedtls_mpi_uint *A,
+                                       const mbedtls_mpi_uint *B,
+                                       size_t limbs )
+{
+    mbedtls_mpi_uint c = 0;
 
+    for( size_t i = 0; i < limbs; i++ )
+    {
+        mbedtls_mpi_uint t = c + A[i];
+        c = ( t < A[i] );
+        t += B[i];
+        c += ( t < B[i] );
+        X[i] = t;
+    }
+
+    return( c );
+}
 
 mbedtls_mpi_uint mbedtls_mpi_core_add_if( mbedtls_mpi_uint *X,
                                           const mbedtls_mpi_uint *A,

library/bignum_core.h

@@ -277,6 +277,28 @@ int mbedtls_mpi_core_write_be( const mbedtls_mpi_uint *A,
 void mbedtls_mpi_core_shift_r( mbedtls_mpi_uint *X, size_t limbs,
                                size_t count );
 
+/**
+ * \brief Add two fixed-size large unsigned integers, returning the carry.
+ *
+ * Calculates `A + B` where `A` and `B` have the same size.
+ *
+ * This function operates modulo `2^(biL*limbs)` and returns the carry
+ * (1 if there was a wraparound, and 0 otherwise).
+ *
+ * \p X may be aliased to \p A or \p B.
+ *
+ * \param[out] X    The result of the addition.
+ * \param[in] A     Little-endian presentation of the left operand.
+ * \param[in] B     Little-endian presentation of the right operand.
+ * \param limbs     Number of limbs of \p X, \p A and \p B.
+ *
+ * \return          1 if `A + B >= 2^(biL*limbs)`, 0 otherwise.
+ */
+mbedtls_mpi_uint mbedtls_mpi_core_add( mbedtls_mpi_uint *X,
+                                       const mbedtls_mpi_uint *A,
+                                       const mbedtls_mpi_uint *B,
+                                       size_t limbs );
+
 /**
  * \brief Conditional addition of two fixed-size large unsigned integers,
  *        returning the carry.