diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index da73759..3c5072c 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -164,6 +164,27 @@
                                             size_t par_len);
 
 /**
+ * \brief           Write an AlgorithmIdentifier sequence in ASN.1 format.
+ *
+ * \note            This function works backwards in data buffer.
+ *
+ * \param p         The reference to the current position pointer.
+ * \param start     The start of the buffer, for bounds-checking.
+ * \param oid       The OID of the algorithm to write.
+ * \param oid_len   The length of the algorithm's OID.
+ * \param par_len   The length of the parameters, which must be already written.
+ * \param has_par   If there are any parameters. If 0, par_len must be 0. If 1
+ *                  and \p par_len is 0, NULL parameters are added.
+ *
+ * \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_algorithm_identifier_ext(unsigned char **p,
+                                                const unsigned char *start,
+                                                const char *oid, size_t oid_len,
+                                                size_t par_len, int has_par);
+
+/**
  * \brief           Write a boolean tag (#MBEDTLS_ASN1_BOOLEAN) and value
  *                  in ASN.1 format.
  *