diff --git a/library/asn1write.c b/library/asn1write.c
index deb1a2f..0289e89 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -34,7 +34,7 @@
 #define mbedtls_free       free
 #endif
 
-int mbedtls_asn1_write_len( unsigned char **p, unsigned char *start, size_t len )
+int mbedtls_asn1_write_len( unsigned char **p, const unsigned char *start, size_t len )
 {
     if( len < 0x80 )
     {
@@ -98,7 +98,7 @@
 #endif
 }
 
-int mbedtls_asn1_write_tag( unsigned char **p, unsigned char *start, unsigned char tag )
+int mbedtls_asn1_write_tag( unsigned char **p, const unsigned char *start, unsigned char tag )
 {
     if( *p - start < 1 )
         return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
@@ -108,7 +108,7 @@
     return( 1 );
 }
 
-int mbedtls_asn1_write_raw_buffer( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_raw_buffer( unsigned char **p, const unsigned char *start,
                            const unsigned char *buf, size_t size )
 {
     size_t len = 0;
@@ -124,7 +124,7 @@
 }
 
 #if defined(MBEDTLS_BIGNUM_C)
-int mbedtls_asn1_write_mpi( unsigned char **p, unsigned char *start, const mbedtls_mpi *X )
+int mbedtls_asn1_write_mpi( unsigned char **p, const unsigned char *start, const mbedtls_mpi *X )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;
@@ -161,7 +161,7 @@
 }
 #endif /* MBEDTLS_BIGNUM_C */
 
-int mbedtls_asn1_write_null( unsigned char **p, unsigned char *start )
+int mbedtls_asn1_write_null( unsigned char **p, const unsigned char *start )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;
@@ -174,7 +174,7 @@
     return( (int) len );
 }
 
-int mbedtls_asn1_write_oid( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_oid( unsigned char **p, const unsigned char *start,
                     const char *oid, size_t oid_len )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
@@ -188,7 +188,7 @@
     return( (int) len );
 }
 
-int mbedtls_asn1_write_algorithm_identifier( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_algorithm_identifier( unsigned char **p, const unsigned char *start,
                                      const char *oid, size_t oid_len,
                                      size_t par_len )
 {
@@ -209,7 +209,7 @@
     return( (int) len );
 }
 
-int mbedtls_asn1_write_bool( unsigned char **p, unsigned char *start, int boolean )
+int mbedtls_asn1_write_bool( unsigned char **p, const unsigned char *start, int boolean )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;
@@ -226,7 +226,7 @@
     return( (int) len );
 }
 
-static int asn1_write_tagged_int( unsigned char **p, unsigned char *start, int val, int tag )
+static int asn1_write_tagged_int( unsigned char **p, const unsigned char *start, int val, int tag )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;
@@ -255,17 +255,17 @@
     return( (int) len );
 }
 
-int mbedtls_asn1_write_int( unsigned char **p, unsigned char *start, int val )
+int mbedtls_asn1_write_int( unsigned char **p, const unsigned char *start, int val )
 {
     return( asn1_write_tagged_int( p, start, val, MBEDTLS_ASN1_INTEGER ) );
 }
 
-int mbedtls_asn1_write_enum( unsigned char **p, unsigned char *start, int val )
+int mbedtls_asn1_write_enum( unsigned char **p, const unsigned char *start, int val )
 {
     return( asn1_write_tagged_int( p, start, val, MBEDTLS_ASN1_ENUMERATED ) );
 }
 
-int mbedtls_asn1_write_tagged_string( unsigned char **p, unsigned char *start, int tag,
+int mbedtls_asn1_write_tagged_string( unsigned char **p, const unsigned char *start, int tag,
     const char *text, size_t text_len )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
@@ -280,26 +280,26 @@
     return( (int) len );
 }
 
-int mbedtls_asn1_write_utf8_string( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_utf8_string( unsigned char **p, const unsigned char *start,
     const char *text, size_t text_len )
 {
     return( mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_UTF8_STRING, text, text_len) );
 }
 
-int mbedtls_asn1_write_printable_string( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_printable_string( unsigned char **p, const unsigned char *start,
                                  const char *text, size_t text_len )
 {
     return( mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_PRINTABLE_STRING, text, text_len) );
 }
 
-int mbedtls_asn1_write_ia5_string( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_ia5_string( unsigned char **p, const unsigned char *start,
                            const char *text, size_t text_len )
 {
     return( mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_IA5_STRING, text, text_len) );
 }
 
 int mbedtls_asn1_write_named_bitstring( unsigned char **p,
-                                        unsigned char *start,
+                                        const unsigned char *start,
                                         const unsigned char *buf,
                                         size_t bits )
 {
@@ -341,7 +341,7 @@
     return( mbedtls_asn1_write_bitstring( p, start, buf, bits ) );
 }
 
-int mbedtls_asn1_write_bitstring( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_bitstring( unsigned char **p, const unsigned char *start,
                           const unsigned char *buf, size_t bits )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
@@ -374,7 +374,7 @@
     return( (int) len );
 }
 
-int mbedtls_asn1_write_octet_string( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_octet_string( unsigned char **p, const unsigned char *start,
                              const unsigned char *buf, size_t size )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;