diff --git a/ChangeLog b/ChangeLog
index 539256b86..13975519a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -26,6 +26,8 @@ Changes
      (Credits go to Marco Lizza)
    * Added POLARSSL_MPI_WINDOW_SIZE definition to allow easier time to memory
      trade-off
+   * Introduced POLARSSL_MPI_MAX_SIZE and POLARSSL_MPI_MAX_BITS for MPI size
+     management (Closes ticket #44)
 
 Bugfix
    * Fixed faulty HMAC-MD2 implementation. Found by dibac. (Closes
diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index 1e46d126b..2b8e82314 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -56,6 +56,16 @@
  */
 #define POLARSSL_MPI_WINDOW_SIZE                           6        /**< Maximum windows size used. */
 
+/*
+ * Maximum size of MPIs allowed in bits and bytes for user-MPIs.
+ * ( Default: 512 bytes => 4096 bits )
+ *
+ * Note: Calculations can results temporarily in larger MPIs. So the number
+ * of limbs required (POLARSSL_MPI_MAX_LIMBS) is higher.
+ */
+#define POLARSSL_MPI_MAX_SIZE                              512      /**< Maximum number of bytes for usable MPIs. */
+#define POLARSSL_MPI_MAX_BITS                              ( 8 * POLARSSL_MPI_MAX_SIZE )    /**< Maximum number of bits for usable MPIs. */
+
 /*
  * Define the base integer type, architecture-wise
  */
@@ -572,7 +582,7 @@ int mpi_is_prime( mpi *X, int (*f_rng)(void *), void *p_rng );
  * \brief          Prime number generation
  *
  * \param X        Destination MPI
- * \param nbits    Required size of X in bits ( 3 <= nbits <= 4096 )
+ * \param nbits    Required size of X in bits ( 3 <= nbits <= POLARSSL_MPI_MAX_BITS )
  * \param dh_flag  If 1, then (X-1)/2 will be prime too
  * \param f_rng    RNG function
  * \param p_rng    RNG parameter
diff --git a/library/bignum.c b/library/bignum.c
index d4035b62b..36e78e165 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -440,7 +440,10 @@ int mpi_read_file( mpi *X, int radix, FILE *fin )
     t_uint d;
     size_t slen;
     char *p;
-    char s[1024];
+    /*
+     * Buffer should have space for (short) label and hexified MPI and '\0'
+     */
+    char s[ 2 * POLARSSL_MPI_MAX_SIZE + 10 ];
 
     memset( s, 0, sizeof( s ) );
     if( fgets( s, sizeof( s ) - 1, fin ) == NULL )
@@ -465,7 +468,10 @@ int mpi_write_file( const char *p, const mpi *X, int radix, FILE *fout )
 {
     int ret;
     size_t n, slen, plen;
-    char s[2048];
+    /*
+     * Buffer should have space for minus sign, hexified MPI and '\0'
+     */
+    char s[ 2 * POLARSSL_MPI_MAX_SIZE + 2 ];
 
     n = sizeof( s );
     memset( s, 0, n );
@@ -1867,7 +1873,7 @@ int mpi_gen_prime( mpi *X, size_t nbits, int dh_flag,
     size_t k, n;
     mpi Y;
 
-    if( nbits < 3 || nbits > 4096 )
+    if( nbits < 3 || nbits > POLARSSL_MPI_MAX_BITS )
         return( POLARSSL_ERR_MPI_BAD_INPUT_DATA );
 
     mpi_init( &Y );
diff --git a/library/rsa.c b/library/rsa.c
index de8e76596..8cadcadd9 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -142,7 +142,7 @@ int rsa_check_pubkey( const rsa_context *ctx )
         return( POLARSSL_ERR_RSA_KEY_CHECK_FAILED );
 
     if( mpi_msb( &ctx->N ) < 128 ||
-        mpi_msb( &ctx->N ) > 4096 )
+        mpi_msb( &ctx->N ) > POLARSSL_MPI_MAX_BITS )
         return( POLARSSL_ERR_RSA_KEY_CHECK_FAILED );
 
     if( mpi_msb( &ctx->E ) < 2 ||