diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 7d0f977..85e553a 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -760,7 +760,7 @@
  * \return          length of data written if successful, or a specific
  *                  error code
  */
-int mbedtls_pk_write_key_der( mbedtls_pk_context *ctx, unsigned char *buf, size_t size );
+int mbedtls_pk_write_key_der( const mbedtls_pk_context *ctx, unsigned char *buf, size_t size );
 
 /**
  * \brief           Write a public key to a SubjectPublicKeyInfo DER structure
@@ -775,7 +775,7 @@
  * \return          length of data written if successful, or a specific
  *                  error code
  */
-int mbedtls_pk_write_pubkey_der( mbedtls_pk_context *ctx, unsigned char *buf, size_t size );
+int mbedtls_pk_write_pubkey_der( const mbedtls_pk_context *ctx, unsigned char *buf, size_t size );
 
 #if defined(MBEDTLS_PEM_WRITE_C)
 /**
@@ -788,7 +788,7 @@
  *
  * \return          0 if successful, or a specific error code
  */
-int mbedtls_pk_write_pubkey_pem( mbedtls_pk_context *ctx, unsigned char *buf, size_t size );
+int mbedtls_pk_write_pubkey_pem( const mbedtls_pk_context *ctx, unsigned char *buf, size_t size );
 
 /**
  * \brief           Write a private key to a PKCS#1 or SEC1 PEM string
@@ -800,7 +800,7 @@
  *
  * \return          0 if successful, or a specific error code
  */
-int mbedtls_pk_write_key_pem( mbedtls_pk_context *ctx, unsigned char *buf, size_t size );
+int mbedtls_pk_write_key_pem( const mbedtls_pk_context *ctx, unsigned char *buf, size_t size );
 #endif /* MBEDTLS_PEM_WRITE_C */
 #endif /* MBEDTLS_PK_WRITE_C */