diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index 44afae0..fb111ca 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -55,7 +55,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-int mbedtls_asn1_write_len( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_len( unsigned char **p, const unsigned char *start,
                             size_t len );
 /**
  * \brief           Write an ASN.1 tag in ASN.1 format.
@@ -69,7 +69,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-int mbedtls_asn1_write_tag( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_tag( unsigned char **p, const unsigned char *start,
                             unsigned char tag );
 
 /**
@@ -85,7 +85,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-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 );
 
 #if defined(MBEDTLS_BIGNUM_C)
@@ -103,7 +103,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-int mbedtls_asn1_write_mpi( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_mpi( unsigned char **p, const unsigned char *start,
                             const mbedtls_mpi *X );
 #endif /* MBEDTLS_BIGNUM_C */
 
@@ -119,7 +119,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-int mbedtls_asn1_write_null( unsigned char **p, unsigned char *start );
+int mbedtls_asn1_write_null( unsigned char **p, const unsigned char *start );
 
 /**
  * \brief           Write an OID tag (#MBEDTLS_ASN1_OID) and data
@@ -135,7 +135,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-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 );
 
 /**
@@ -154,7 +154,7 @@
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
 int mbedtls_asn1_write_algorithm_identifier( unsigned char **p,
-                                             unsigned char *start,
+                                             const unsigned char *start,
                                              const char *oid, size_t oid_len,
                                              size_t par_len );
 
@@ -171,7 +171,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-int mbedtls_asn1_write_bool( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_bool( unsigned char **p, const unsigned char *start,
                              int boolean );
 
 /**
@@ -188,7 +188,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-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 );
 
 /**
  * \brief           Write an enum tag (#MBEDTLS_ASN1_ENUMERATED) and value
@@ -203,7 +203,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative \c MBEDTLS_ERR_ASN1_XXX error code on failure.
  */
-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 );
 
 /**
  * \brief           Write a string in ASN.1 format using a specific
@@ -222,7 +222,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative error code on failure.
  */
-int mbedtls_asn1_write_tagged_string( unsigned char **p, unsigned char *start,
+int mbedtls_asn1_write_tagged_string( unsigned char **p, const unsigned char *start,
                                       int tag, const char *text,
                                       size_t text_len );
 
@@ -242,7 +242,7 @@
  * \return          A negative error code on failure.
  */
 int mbedtls_asn1_write_printable_string( unsigned char **p,
-                                         unsigned char *start,
+                                         const unsigned char *start,
                                          const char *text, size_t text_len );
 
 /**
@@ -260,7 +260,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative error code on failure.
  */
-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 );
 
 /**
@@ -278,7 +278,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative error code on failure.
  */
-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 );
 
 /**
@@ -295,7 +295,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative error code on failure.
  */
-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 );
 
 /**
@@ -316,7 +316,7 @@
  * \return          A negative error code on failure.
  */
 int mbedtls_asn1_write_named_bitstring( unsigned char **p,
-                                        unsigned char *start,
+                                        const unsigned char *start,
                                         const unsigned char *buf,
                                         size_t bits );
 
@@ -334,7 +334,7 @@
  * \return          The number of bytes written to \p p on success.
  * \return          A negative error code on failure.
  */
-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 );
 
 /**