- Added mpi_get_bit() and mpi_set_bit() individual bit setter/getter functions.

ChangeLog

@@ -9,6 +9,8 @@ Features
      by undefining POLARSSL_FS_IO
    * A error_strerror function() has been added to translate between
      error codes and their description.
+   * Added mpi_get_bit() and mpi_set_bit() individual bit setter/getter
+     functions.
 
 Changes
    * Major argument / variable rewrite. Introduced use of size_t

include/polarssl/bignum.h

@@ -146,6 +146,32 @@ void mpi_swap( mpi *X, mpi *Y );
  */
 int mpi_lset( mpi *X, t_sint z );
 
+/*
+ * \brief          Get a specific bit from X
+ *
+ * \param X        MPI to use
+ * \param pos      Zero-based index of the bit in X
+ *
+ * \return         Either a 0 or a 1
+ */
+int mpi_get_bit( mpi *X, size_t pos );
+
+/*
+ * \brief          Set a bit of X to a specific value of 0 or 1
+ *
+ * \note           Will grow X if necessary to set a bit to 1 in a not yet
+ *                 existing limb. Will not grow if bit should be set to 0
+ *
+ * \param X        MPI to use
+ * \param pos      Zero-based index of the bit in X
+ * \param val      The value to set the bit to (0 or 1)
+ *
+ * \return         0 if successful,
+ *                 1 if memory allocation failed,
+ *                 POLARSSL_ERR_MPI_BAD_INPUT_DATA if val is not 0 or 1
+ */
+int mpi_set_bit( mpi *X, size_t pos, unsigned char val );
+
 /**
  * \brief          Return the number of least significant bits
  *

library/bignum.c

@@ -170,6 +170,44 @@ cleanup:
     return( ret );
 }
 
+/*
+ * Get a specific bit
+ */
+int mpi_get_bit( mpi *X, size_t pos )
+{
+    if( X->n * biL <= pos )
+        return( 0 );
+
+    return ( X->p[pos / biL] >> ( pos % biL ) ) & 0x01;
+}
+
+/*
+ * Set a bit to a specific value of 0 or 1
+ */
+int mpi_set_bit( mpi *X, size_t pos, unsigned char val )
+{
+    int ret = 0;
+    size_t off = pos / biL;
+    size_t idx = pos % biL;
+
+    if( val != 0 && val != 1 )
+        return POLARSSL_ERR_MPI_BAD_INPUT_DATA;
+        
+    if( X->n * biL <= pos )
+    {
+        if( val == 0 )
+            return ( 0 );
+
+        MPI_CHK( mpi_grow( X, off + 1 ) );
+    }
+
+    X->p[off] = ( X->p[off] & ~( 0x01 << idx ) ) | ( val << idx );
+
+cleanup:
+    
+    return( ret );
+}
+
 /*
  * Return the number of least significant bits
  */

tests/suites/test_suite_mpi.data

@@ -518,6 +518,33 @@ mpi_is_prime:10:"32452867":0
 Test mpi_is_prime #20
 mpi_is_prime:10:"49979687":0
 
+Test bit getting (Value bit 25)
+mpi_get_bit:10:"49979687":25:1
+
+Test bit getting (Larger but same limb)
+mpi_get_bit:10:"49979687":26:0
+
+Test bit getting (Larger and non-existing limb)
+mpi_get_bit:10:"49979687":500:0
+
+Test bit getting (Value bit 24)
+mpi_get_bit:10:"49979687":24:0
+
+Test bit getting (Value bit 23)
+mpi_get_bit:10:"49979687":23:1
+
+Test bit set (Change existing value with a 1)
+mpi_set_bit:10:"49979687":24:1:10:"66756903"
+
+Test bit set (Change existing value with a 0)
+mpi_set_bit:10:"49979687":25:0:10:"16425255"
+
+Test bit set (Add above existing limbs with a 0)
+mpi_set_bit:10:"49979687":80:0:10:"49979687"
+
+Test bit set (Add above existing limbs with a 1)
+mpi_set_bit:10:"49979687":80:1:10:"1208925819614629224685863"
+
 MPI Selftest
 depends_on:POLARSSL_SELF_TEST
 mpi_selftest:

tests/suites/test_suite_mpi.function

@@ -142,6 +142,33 @@ mpi_write_file:radix_X:input_X:output_radix:output_file
 }
 END_CASE
 
+BEGIN_CASE
+mpi_get_bit:radix_X:input_X:pos:val
+{
+    mpi X;
+    mpi_init( &X );
+    TEST_ASSERT( mpi_read_string( &X, {radix_X}, {input_X} ) == 0 );
+    TEST_ASSERT( mpi_get_bit( &X, {pos} ) == {val} );
+
+    mpi_free( &X );
+}
+END_CASE
+
+BEGIN_CASE
+mpi_set_bit:radix_X:input_X:pos:val:radix_Y:output_Y
+{
+    mpi X, Y;
+    mpi_init( &X ); mpi_init( &Y );
+
+    TEST_ASSERT( mpi_read_string( &X, {radix_X}, {input_X} ) == 0 );
+    TEST_ASSERT( mpi_read_string( &Y, {radix_Y}, {output_Y} ) == 0 );
+    TEST_ASSERT( mpi_set_bit( &X, {pos}, {val} ) == 0 );
+    TEST_ASSERT( mpi_cmp_mpi( &X, &Y ) == 0 );
+
+    mpi_free( &X );
+}
+END_CASE
+
 BEGIN_CASE
 mpi_lsb:radix_X:input_X:nr_bits
 {