diff --git a/library/pkwrite.c b/library/pkwrite.c
index 0da3698..4e6a082 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -222,7 +222,7 @@
     return( (int) len );
 }
 
-int mbedtls_pk_write_pubkey_der( mbedtls_pk_context *key, unsigned char *buf, size_t size )
+int mbedtls_pk_write_pubkey_der( const mbedtls_pk_context *key, unsigned char *buf, size_t size )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     unsigned char *c;
@@ -310,7 +310,7 @@
     return( (int) len );
 }
 
-int mbedtls_pk_write_key_der( mbedtls_pk_context *key, unsigned char *buf, size_t size )
+int mbedtls_pk_write_key_der( const mbedtls_pk_context *key, unsigned char *buf, size_t size )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     unsigned char *c;
@@ -553,7 +553,7 @@
 #define PRV_DER_MAX_BYTES   RSA_PRV_DER_MAX_BYTES > ECP_PRV_DER_MAX_BYTES ? \
                             RSA_PRV_DER_MAX_BYTES : ECP_PRV_DER_MAX_BYTES
 
-int mbedtls_pk_write_pubkey_pem( mbedtls_pk_context *key, unsigned char *buf, size_t size )
+int mbedtls_pk_write_pubkey_pem( const mbedtls_pk_context *key, unsigned char *buf, size_t size )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     unsigned char output_buf[PUB_DER_MAX_BYTES];
@@ -578,7 +578,7 @@
     return( 0 );
 }
 
-int mbedtls_pk_write_key_pem( mbedtls_pk_context *key, unsigned char *buf, size_t size )
+int mbedtls_pk_write_key_pem( const mbedtls_pk_context *key, unsigned char *buf, size_t size )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     unsigned char output_buf[PRV_DER_MAX_BYTES];