diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index da938021fb..beecd898ed 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -70,14 +70,15 @@
 #define POLARSSL_MPI_MAX_BITS                              ( 8 * POLARSSL_MPI_MAX_SIZE )    /**< Maximum number of bits for usable MPIs. */
 
 /*
- * When reading from files with mpi_read_file() the buffer should have space
+ * When reading from files with mpi_read_file() and writing to files with
+ * mpi_write_file() the buffer should have space
  * for a (short) label, the MPI (in the provided radix), the newline
  * characters and the '\0'.
  *
  * By default we assume at least a 10 char label, a minimum radix of 10
  * (decimal) and a maximum of 4096 bit numbers (1234 decimal chars).
  */
-#define POLARSSL_MPI_READ_BUFFER_SIZE                       1250   
+#define POLARSSL_MPI_RW_BUFFER_SIZE                       1250   
 
 /*
  * Define the base integer type, architecture-wise
diff --git a/library/bignum.c b/library/bignum.c
index 9af6980c6e..cbd25ab555 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -444,7 +444,7 @@ int mpi_read_file( mpi *X, int radix, FILE *fin )
      * Buffer should have space for (short) label and decimal formatted MPI,
      * newline characters and '\0'
      */
-    char s[ POLARSSL_MPI_READ_BUFFER_SIZE ];
+    char s[ POLARSSL_MPI_RW_BUFFER_SIZE ];
 
     memset( s, 0, sizeof( s ) );
     if( fgets( s, sizeof( s ) - 1, fin ) == NULL )
@@ -473,9 +473,10 @@ int mpi_write_file( const char *p, const mpi *X, int radix, FILE *fout )
     int ret;
     size_t n, slen, plen;
     /*
-     * Buffer should have space for minus sign, hexified MPI and '\0'
+     * Buffer should have space for (short) label and decimal formatted MPI,
+     * newline characters and '\0'
      */
-    char s[ 2 * POLARSSL_MPI_MAX_SIZE + 2 ];
+    char s[ POLARSSL_MPI_RW_BUFFER_SIZE ];
 
     n = sizeof( s );
     memset( s, 0, n );