diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index 14884aa..513b7c0 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -848,6 +848,196 @@
 }
 
 /*
+ * STATE HANDLING: Output Certificate
+ */
+/* Check if a certificate should be written, and if yes,
+ * if it is available.
+ * Returns a negative error code on failure ( such as no certificate
+ * being available on the server ), and otherwise
+ * SSL_WRITE_CERTIFICATE_AVAILABLE or
+ * SSL_WRITE_CERTIFICATE_SKIP
+ * indicating that a Certificate message should be written based
+ * on the configured certificate, or whether it should be silently skipped.
+ */
+#define SSL_WRITE_CERTIFICATE_AVAILABLE  0
+#define SSL_WRITE_CERTIFICATE_SKIP       1
+
+
+static int ssl_tls13_write_certificate_coordinate( mbedtls_ssl_context* ssl )
+{
+
+    /* For PSK and ECDHE-PSK ciphersuites there is no certificate to exchange. */
+    if( mbedtls_ssl_tls13_some_psk_enabled( ssl ) )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= skip write certificate" ) );
+        return( SSL_WRITE_CERTIFICATE_SKIP );
+    }
+
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
+#if defined(MBEDTLS_SSL_CLI_C)
+    if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT )
+    {
+        /* The client MUST send a Certificate message if and only
+         * if the server has requested client authentication via a
+         * CertificateRequest message.
+         *
+         * client_auth indicates whether the server had requested
+         * client authentication.
+         */
+        if( ssl->handshake->client_auth == 0 )
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= skip write certificate" ) );
+            return( SSL_WRITE_CERTIFICATE_SKIP );
+        }
+    }
+#endif /* MBEDTLS_SSL_CLI_C */
+
+    return( SSL_WRITE_CERTIFICATE_AVAILABLE );
+#else /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
+    MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
+    return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
+#endif /* !MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
+}
+
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
+static int ssl_tls13_write_certificate( mbedtls_ssl_context *ssl,
+                                        unsigned char *buf,
+                                        size_t buflen,
+                                        size_t *olen )
+{
+    size_t i=0, n, total_len;
+    const mbedtls_x509_crt *crt = mbedtls_ssl_own_cert( ssl );
+    unsigned char *start;
+
+    /* TODO: Add bounds checks! Only then remove the next line. */
+    ((void) buflen );
+
+    /* empty certificate_request_context with length 0 */
+    buf[i] = 0;
+    /* Skip length of certificate_request_context and
+     * the length of CertificateEntry
+     */
+    i += 1;
+
+#if defined(MBEDTLS_SSL_CLI_C)
+    /* If the server requests client authentication but no suitable
+     * certificate is available, the client MUST send a
+     * Certificate message containing no certificates
+     * ( i.e., with the "certificate_list" field having length 0 ).
+     *
+     * authmode indicates whether the client configuration required authentication.
+     */
+    if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT &&
+        ( ( crt == NULL ) || ssl->conf->authmode == MBEDTLS_SSL_VERIFY_NONE ) )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= write empty client certificate" ) );
+        buf[i] = 0;
+        buf[i + 1] = 0;
+        buf[i + 2] = 0;
+        i += 3;
+        *olen = i;
+        return( 0 );
+    }
+#endif /* MBEDTLS_SSL_CLI_C */
+
+    start = &buf[i];
+    MBEDTLS_SSL_DEBUG_CRT( 3, "own certificate", crt );
+
+    i += 3;
+
+    while ( crt != NULL )
+    {
+        n = crt->raw.len;
+        if( n > buflen - 3 - i )
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "certificate too large, %" MBEDTLS_PRINTF_SIZET " > %d",
+                                        i + 3 + n, MBEDTLS_SSL_OUT_CONTENT_LEN ) );
+            return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );
+        }
+
+        buf[i] = (unsigned char)( n >> 16 );
+        buf[i + 1] = (unsigned char)( n >> 8 );
+        buf[i + 2] = (unsigned char)( n );
+
+        i += 3; memcpy( buf + i, crt->raw.p, n );
+        i += n; crt = crt->next;
+
+        /* Currently, we don't have any certificate extensions defined.
+         * Hence, we are sending an empty extension with length zero.
+         */
+        buf[i] = 0;
+        buf[i + 1] = 0;
+        i += 2;
+    }
+    total_len = &buf[i] - start - 3;
+    *start++ = (unsigned char)( ( total_len ) >> 16 );
+    *start++ = (unsigned char)( ( total_len ) >> 8 );
+    *start++ = (unsigned char)( ( total_len ) );
+
+    *olen = i;
+
+    return( 0 );
+}
+#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
+
+/* Update the state after handling the outgoing certificate message. */
+static int ssl_tls13_finalize_write_certificate( mbedtls_ssl_context* ssl )
+{
+#if defined(MBEDTLS_SSL_CLI_C)
+    if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT )
+    {
+        mbedtls_ssl_handshake_set_state( ssl, MBEDTLS_SSL_CLIENT_CERTIFICATE_VERIFY );
+        return( 0 );
+    }
+    else
+#endif /* MBEDTLS_SSL_CLI_C */
+
+    return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
+}
+
+int mbedtls_ssl_tls13_write_certificate( mbedtls_ssl_context* ssl )
+{
+    int ret;
+    MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> write certificate" ) );
+
+    /* Coordination: Check if we need to send a certificate. */
+    MBEDTLS_SSL_PROC_CHK_NEG( ssl_tls13_write_certificate_coordinate( ssl ) );
+
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
+    if( ret == SSL_WRITE_CERTIFICATE_AVAILABLE )
+    {
+        unsigned char *buf;
+        size_t buf_len, msg_len;
+
+        MBEDTLS_SSL_PROC_CHK( mbedtls_ssl_tls13_start_handshake_msg( ssl,
+                   MBEDTLS_SSL_HS_CERTIFICATE, &buf, &buf_len ) );
+
+        MBEDTLS_SSL_PROC_CHK( ssl_tls13_write_certificate( ssl, buf, buf_len,
+                                                           &msg_len ) );
+
+        mbedtls_ssl_tls13_add_hs_msg_to_checksum( ssl,
+                                                  MBEDTLS_SSL_HS_CERTIFICATE,
+                                                  buf,
+                                                  msg_len );
+
+        MBEDTLS_SSL_PROC_CHK( ssl_tls13_finalize_write_certificate( ssl ) );
+        MBEDTLS_SSL_PROC_CHK( mbedtls_ssl_tls13_finish_handshake_msg(
+                                  ssl, buf_len, msg_len ) );
+    }
+    else
+#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= skip write certificate" ) );
+        MBEDTLS_SSL_PROC_CHK( ssl_tls13_finalize_write_certificate( ssl ) );
+    }
+
+cleanup:
+
+    MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= write certificate" ) );
+    return( ret );
+}
+
+/*
  *
  * STATE HANDLING: Incoming Finished message.
  */