Add little endian import to Bignum

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

Used `echo xx | tac -rs .. | tr [a-z] [A-Z]` to transform the test data
to little endian and `echo "ibase=16;xx" | bc` to convert to decimal.

include/mbedtls/bignum.h

@@ -490,8 +490,24 @@ int mbedtls_mpi_read_binary( mbedtls_mpi *X, const unsigned char *buf,
                              size_t buflen );
 
 /**
- * \brief          Export an MPI into unsigned big endian binary data
- *                 of fixed size.
+ * \brief          Import X from unsigned binary data, little endian
+ *
+ * \param X        The destination MPI. This must point to an initialized MPI.
+ * \param buf      The input buffer. This must be a readable buffer of length
+ *                 \p buflen Bytes.
+ * \param buflen   The length of the input buffer \p p in Bytes.
+ *
+ * \return         \c 0 if successful.
+ * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if memory allocation failed.
+ * \return         Another negative error code on different kinds of failure.
+ */
+int mbedtls_mpi_read_binary_le( mbedtls_mpi *X,
+                                const unsigned char *buf, size_t buflen );
+
+/**
+ * \brief          Export X into unsigned binary data, big endian.
+ *                 Always fills the whole buffer, which will start 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

library/bignum.c

@@ -813,6 +813,34 @@ static void mpi_bigendian_to_host( mbedtls_mpi_uint * const p, size_t limbs )
     }
 }
 
+/*
+ * Import X from unsigned binary data, little endian
+ */
+int mbedtls_mpi_read_binary_le( mbedtls_mpi *X,
+                                const unsigned char *buf, size_t buflen )
+{
+    int ret;
+    size_t i;
+    size_t const limbs = CHARS_TO_LIMBS( buflen );
+
+    /* Ensure that target MPI has exactly the necessary number of limbs */
+    if( X->n != limbs )
+    {
+        mbedtls_mpi_free( X );
+        mbedtls_mpi_init( X );
+        MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, limbs ) );
+    }
+
+    MBEDTLS_MPI_CHK( mbedtls_mpi_lset( X, 0 ) );
+
+    for( i = 0; i < buflen; i++ )
+        X->p[i / ciL] |= ((mbedtls_mpi_uint) buf[i]) << ((i % ciL) << 3);
+
+cleanup:
+
+    return( ret );
+}
+
 /*
  * Import X from unsigned binary data, big endian
  */

tests/suites/test_suite_mpi.data

@@ -58,6 +58,9 @@ mpi_read_write_string:16:"-1":16:"":3:0:MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL
 Base test mbedtls_mpi_read_binary #1
 mbedtls_mpi_read_binary:"0941379d00fed1491fe15df284dfde4a142f68aa8d412023195cee66883e6290ffe703f4ea5963bf212713cee46b107c09182b5edcd955adac418bf4918e2889af48e1099d513830cec85c26ac1e158b52620e33ba8692f893efbb2f958b4424":10:"56125680981752282334141896320372489490613963693556392520816017892111350604111697682705498319512049040516698827829292076808006940873974979584527073481012636016353913462376755556720019831187364993587901952757307830896531678727717924"
 
+Base test mbedtls_mpi_read_binary_le #1
+mbedtls_mpi_read_binary_le:"0941379d00fed1491fe15df284dfde4a142f68aa8d412023195cee66883e6290ffe703f4ea5963bf212713cee46b107c09182b5edcd955adac418bf4918e2889af48e1099d513830cec85c26ac1e158b52620e33ba8692f893efbb2f958b4424":10:"219946662473865722255717126709915431768051735954189829340600976826409773245337023925691629251672268961177825243440202069039100741562168093042339401187848509859789949044607421190014088260008793380554914226244485299326152319899746569"
+
 Base test mbedtls_mpi_write_binary #1
 mbedtls_mpi_write_binary:10:"56125680981752282334141896320372489490613963693556392520816017892111350604111697682705498319512049040516698827829292076808006940873974979584527073481012636016353913462376755556720019831187364993587901952757307830896531678727717924":"0941379d00fed1491fe15df284dfde4a142f68aa8d412023195cee66883e6290ffe703f4ea5963bf212713cee46b107c09182b5edcd955adac418bf4918e2889af48e1099d513830cec85c26ac1e158b52620e33ba8692f893efbb2f958b4424":200:0
 

tests/suites/test_suite_mpi.function

@@ -328,6 +328,25 @@ exit:
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void mbedtls_mpi_read_binary_le( data_t * buf, int radix_A, char * input_A )
+{
+    mbedtls_mpi X;
+    unsigned char str[1000];
+    size_t len;
+
+    mbedtls_mpi_init( &X );
+
+
+    TEST_ASSERT( mbedtls_mpi_read_binary_le( &X, buf->x, buf->len ) == 0 );
+    TEST_ASSERT( mbedtls_mpi_write_string( &X, radix_A, (char *) str, sizeof( str ), &len ) == 0 );
+    TEST_ASSERT( strcmp( (char *) str, input_A ) == 0 );
+
+exit:
+    mbedtls_mpi_free( &X );
+}
+/* END_CASE */
+
 /* BEGIN_CASE */
 void mbedtls_mpi_write_binary( int radix_X, char * input_X,
                                data_t * input_A, int output_size,