Add little endian export to Bignum

The function `mbedtls_mpi_write_binary()` writes big endian byte order,
but we need to be able to write little endian in some caseses. (For
example when handling keys corresponding to Montgomery curves.)

Used `echo xx | tac -rs ..` to transform the test data to little endian.

include/mbedtls/bignum.h

@@ -522,6 +522,24 @@ int mbedtls_mpi_read_binary_le( mbedtls_mpi *X,
 int mbedtls_mpi_write_binary( const mbedtls_mpi *X, unsigned char *buf,
                               size_t buflen );
 
+/**
+ * \brief          Export X into unsigned binary data, little endian.
+ *                 Always fills the whole buffer, which will end with zeros
+ *                 if the number is smaller.
+ *
+ * \param X        The source MPI. This must point to an initialized MPI.
+ * \param buf      The output buffer. This must be a writable buffer of length
+ *                 \p buflen Bytes.
+ * \param buflen   The size of the output buffer \p buf in Bytes.
+ *
+ * \return         \c 0 if successful.
+ * \return         #MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL if \p buf isn't
+ *                 large enough to hold the value of \p X.
+ * \return         Another negative error code on different kinds of failure.
+ */
+int mbedtls_mpi_write_binary_le( const mbedtls_mpi *X,
+                                 unsigned char *buf, size_t buflen );
+
 /**
  * \brief          Perform a left-shift on an MPI: X <<= count
  *

library/bignum.c

@@ -888,6 +888,45 @@ cleanup:
     return( ret );
 }
 
+/*
+ * Export X into unsigned binary data, little endian
+ */
+int mbedtls_mpi_write_binary_le( const mbedtls_mpi *X,
+                                 unsigned char *buf, size_t buflen )
+{
+    size_t stored_bytes = X->n * ciL;
+    size_t bytes_to_copy;
+    size_t i;
+
+    if( stored_bytes < buflen )
+    {
+        bytes_to_copy = stored_bytes;
+    }
+    else
+    {
+        bytes_to_copy = buflen;
+
+        /* The output buffer is smaller than the allocated size of X.
+         * However X may fit if its leading bytes are zero. */
+        for( i = bytes_to_copy; i < stored_bytes; i++ )
+        {
+            if( GET_BYTE( X, i ) != 0 )
+                return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
+        }
+    }
+
+    for( i = 0; i < bytes_to_copy; i++ )
+        buf[i] = GET_BYTE( X, i );
+
+    if( stored_bytes < buflen )
+    {
+        /* Write trailing 0 bytes */
+        memset( buf + stored_bytes, 0, buflen - stored_bytes );
+    }
+
+    return( 0 );
+}
+
 /*
  * Export X into unsigned binary data, big endian
  */

tests/suites/test_suite_mpi.data

@@ -70,6 +70,15 @@ mbedtls_mpi_write_binary:16:"123123123123123123123123123":"012312312312312312312
 Test mbedtls_mpi_write_binary #2 (Buffer too small)
 mbedtls_mpi_write_binary:16:"123123123123123123123123123":"23123123123123123123123123":13:MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL
 
+Base test mbedtls_mpi_write_binary_le #1
+mbedtls_mpi_write_binary_le:10:"56125680981752282334141896320372489490613963693556392520816017892111350604111697682705498319512049040516698827829292076808006940873974979584527073481012636016353913462376755556720019831187364993587901952757307830896531678727717924":"24448b952fbbef93f89286ba330e62528b151eac265cc8ce3038519d09e148af89288e91f48b41acad55d9dc5e2b18097c106be4ce132721bf6359eaf403e7ff90623e8866ee5c192320418daa682f144adedf84f25de11f49d1fe009d374109":200:0
+
+Test mbedtls_mpi_write_binary_le #1 (Buffer just fits)
+mbedtls_mpi_write_binary_le:16:"123123123123123123123123123":"2331122331122331122331122301":14:0
+
+Test mbedtls_mpi_write_binary_le #2 (Buffer too small)
+mbedtls_mpi_write_binary_le:16:"123123123123123123123123123":"23311223311223311223311223":13:MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL
+
 Base test mbedtls_mpi_read_file #1
 mbedtls_mpi_read_file:10:"data_files/mpi_10":"01f55332c3a48b910f9942f6c914e58bef37a47ee45cb164a5b6b8d1006bf59a059c21449939ebebfdf517d2e1dbac88010d7b1f141e997bd6801ddaec9d05910f4f2de2b2c4d714e2c14a72fc7f17aa428d59c531627f09":0
 

tests/suites/test_suite_mpi.function

@@ -378,6 +378,37 @@ exit:
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void mbedtls_mpi_write_binary_le( int radix_X, char * input_X,
+                                  data_t * input_A, int output_size,
+                                  int result )
+{
+    mbedtls_mpi X;
+    unsigned char buf[1000];
+    size_t buflen;
+
+    memset( buf, 0x00, 1000 );
+
+    mbedtls_mpi_init( &X );
+
+    TEST_ASSERT( mbedtls_mpi_read_string( &X, radix_X, input_X ) == 0 );
+
+    buflen = mbedtls_mpi_size( &X );
+    if( buflen > (size_t) output_size )
+        buflen = (size_t) output_size;
+
+    TEST_ASSERT( mbedtls_mpi_write_binary_le( &X, buf, buflen ) == result );
+    if( result == 0)
+    {
+
+        TEST_ASSERT( hexcmp( buf, input_A->x, buflen, input_A->len ) == 0 );
+    }
+
+exit:
+    mbedtls_mpi_free( &X );
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_FS_IO */
 void mbedtls_mpi_read_file( int radix_X, char * input_file,
                             data_t * input_A, int result )