diff --git a/library/asn1write.c b/library/asn1write.c
new file mode 100644
index 0000000..0c1d18b
--- /dev/null
+++ b/library/asn1write.c
@@ -0,0 +1,218 @@
+/*
+ * ASN.1 buffer writing functionality
+ *
+ *  Copyright (C) 2006-2012, Brainspark B.V.
+ *
+ *  This file is part of PolarSSL (http://www.polarssl.org)
+ *  Lead Maintainer: Paul Bakker <polarssl_maintainer at polarssl.org>
+ *
+ *  All rights reserved.
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along
+ *  with this program; if not, write to the Free Software Foundation, Inc.,
+ *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "polarssl/config.h"
+
+#if defined(POLARSSL_ASN1_WRITE_C)
+
+#include "polarssl/asn1write.h"
+
+int asn1_write_len( unsigned char **p, unsigned char *start, size_t len )
+{
+    if( len < 0x80 )
+    {
+        if( *p - start < 1 )
+            return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = len;
+        return( 1 );
+    }
+
+    if( len <= 0xFF )
+    {
+        if( *p - start < 2 )
+            return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = len;
+        *--(*p) = 0x81;
+        return( 2 );
+    }
+
+    if( *p - start < 3 )
+        return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+    // We assume we never have lengths larger than 65535 bytes
+    //
+    *--(*p) = len % 256;
+    *--(*p) = ( len / 256 ) % 256;
+    *--(*p) = 0x82;
+
+    return( 3 );
+}
+
+int asn1_write_tag( unsigned char **p, unsigned char *start, unsigned char tag )
+{
+    if( *p - start < 1 )
+        return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+    *--(*p) = tag;
+
+    return( 1 );
+}
+
+int asn1_write_mpi( unsigned char **p, unsigned char *start, mpi *X )
+{
+    int ret;
+    size_t len = 0;
+
+    // Write the MPI
+    //
+    len = mpi_size( X );
+
+    if( *p - start < (int) len )
+        return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+    (*p) -= len;
+    mpi_write_binary( X, *p, len );
+
+    // DER format assumes 2s complement for numbers, so the leftmost bit
+    // should be 0 for positive numbers and 1 for negative numbers.
+    //
+    if ( X->s ==1 && **p & 0x80 )
+    {
+        if( *p - start < 1 )
+            return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = 0x00;
+        len += 1;
+    }
+
+    ASN1_CHK_ADD( len, asn1_write_len( p, start, len ) );
+    ASN1_CHK_ADD( len, asn1_write_tag( p, start, ASN1_INTEGER ) );
+
+    return( len );
+}
+    
+int asn1_write_null( unsigned char **p, unsigned char *start )
+{
+    int ret;
+    size_t len = 0;
+
+    // Write NULL
+    //
+    ASN1_CHK_ADD( len, asn1_write_len( p, start, 0) );
+    ASN1_CHK_ADD( len, asn1_write_tag( p, start, ASN1_NULL ) );
+
+    return( len );
+}
+
+int asn1_write_oid( unsigned char **p, unsigned char *start, char *oid )
+{
+    int ret;
+    size_t len = 0;
+
+    // Write OID
+    //
+    len = strlen( oid );
+
+    if( *p - start < (int) len )
+        return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+    (*p) -= len;
+    memcpy( *p, oid, len );
+
+    ASN1_CHK_ADD( len , asn1_write_len( p, start, len ) );
+    ASN1_CHK_ADD( len , asn1_write_tag( p, start, ASN1_OID ) );
+
+    return( len );
+}
+
+int asn1_write_algorithm_identifier( unsigned char **p, unsigned char *start,
+                                     char *algorithm_oid )
+{
+    int ret;
+    size_t null_len = 0;
+    size_t oid_len = 0;
+    size_t len = 0;
+
+    // Write NULL
+    //
+    ASN1_CHK_ADD( null_len, asn1_write_null( p, start ) );
+
+    // Write OID
+    //
+    ASN1_CHK_ADD( oid_len, asn1_write_oid( p, start, algorithm_oid ) );
+
+    len = oid_len + null_len;
+    ASN1_CHK_ADD( len, asn1_write_len( p, start, oid_len + null_len ) );
+    ASN1_CHK_ADD( len, asn1_write_tag( p, start,
+                                       ASN1_CONSTRUCTED | ASN1_SEQUENCE ) );
+
+    return( len );
+}
+
+int asn1_write_int( unsigned char **p, unsigned char *start, int val )
+{
+    int ret;
+    size_t len = 0;
+
+    // TODO negative values and values larger than 128
+    // DER format assumes 2s complement for numbers, so the leftmost bit
+    // should be 0 for positive numbers and 1 for negative numbers.
+    //
+    if( *p - start < 1 )
+        return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+    len += 1;
+    *--(*p) = val;
+
+    if ( val > 0 && **p & 0x80 )
+    {
+        if( *p - start < 1 )
+            return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = 0x00;
+        len += 1;
+    }
+
+    ASN1_CHK_ADD( len, asn1_write_len( p, start, len ) );
+    ASN1_CHK_ADD( len, asn1_write_tag( p, start, ASN1_INTEGER ) );
+
+    return( len );
+}
+
+int asn1_write_printable_string( unsigned char **p, unsigned char *start,
+                                 char *text )
+{
+    int ret;
+    size_t len = 0;
+
+    // Write string
+    //
+    len = strlen( text );
+
+    if( *p - start < (int) len )
+        return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
+    (*p) -= len;
+    memcpy( *p, text, len );
+
+    ASN1_CHK_ADD( len, asn1_write_len( p, start, len ) );
+    ASN1_CHK_ADD( len, asn1_write_tag( p, start, ASN1_PRINTABLE_STRING ) );
+
+    return( len );
+}
+    
+#endif