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diff --git a/src/libknot/tsig-op.h b/src/libknot/tsig-op.h
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+/*  Copyright (C) 2017 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/*!
+ * \file
+ *
+ * \brief TSIG signing and validating.
+ *
+ * \addtogroup knot-tsig
+ * @{
+ */
+
+#pragma once
+
+#include <stdint.h>
+
+#include "libknot/rrtype/tsig.h"
+#include "libknot/rrset.h"
+
+/*!
+ * \brief Generate TSIG signature of a message.
+ *
+ * This function generates TSIG digest of the given message prepended with the
+ * given Request MAC (if any) and appended with TSIG Variables. It also appends
+ * the resulting TSIG RR to the message wire format and accordingly adjusts
+ * the message size.
+ *
+ * \note This function does not save the new digest to the 'digest' parameter
+ *       unless everything went OK. This allows sending the same buffer to
+ *       the 'request_mac' and 'digest' parameters.
+ *
+ * \param msg Message to be signed.
+ * \param msg_len Size of the message in bytes.
+ * \param msg_max_len Maximum size of the message in bytes.
+ * \param request_mac Request MAC. (may be NULL).
+ * \param request_mac_len Size of the request MAC in bytes.
+ * \param digest Buffer to save the digest in.
+ * \param digest_len In: size of the buffer. Out: real size of the digest saved.
+ * \param key TSIG used for signing.
+ * \param tsig_rcode RCODE of the TSIG.
+ * \param request_time_signed Clients time signed.
+ *
+ * \retval KNOT_EOK if everything went OK.
+ * \retval TODO
+ *
+ * \todo This function should return TSIG errors by their codes which are
+ *       positive values - this will be recognized by the caller.
+ */
+int knot_tsig_sign(uint8_t *msg, size_t *msg_len, size_t msg_max_len,
+                   const uint8_t *request_mac, size_t request_mac_len,
+                   uint8_t *digest, size_t *digest_len,
+                   const knot_tsig_key_t *key, uint16_t tsig_rcode,
+                   uint64_t request_time_signed);
+
+/*!
+ * \brief Generate TSIG signature of a 2nd or later message in a TCP session.
+ *
+ * This function generates TSIG digest of the given message prepended with the
+ * given Request MAC (if any) and appended with TSIG Variables. It also appends
+ * the resulting TSIG RR to the message wire format and accordingly adjusts
+ * the message size.
+ *
+ * \note This function does not save the new digest to the 'digest' parameter
+ *       unless everything went OK. This allows sending the same buffer to
+ *       the 'request_mac' and 'digest' parameters.
+ *
+ * \param msg Message to be signed.
+ * \param msg_len Size of the message in bytes.
+ * \param msg_max_len Maximum size of the message in bytes.
+ * \param prev_digest Previous digest sent by the server in the session.
+ * \param prev_digest_len Size of the previous digest in bytes.
+ * \param digest Buffer to save the digest in.
+ * \param digest_len In: size of the buffer. Out: real size of the digest saved.
+ * \param key TSIG key for signing.
+ * \param to_sign Data being signed.
+ * \param to_sign_len Size of the data being signed.
+ *
+ * \retval KNOT_EOK if successful.
+ * \retval TODO
+ *
+ * \todo This function should return TSIG errors by their codes which are
+ *       positive values - this will be recognized by the caller.
+ */
+int knot_tsig_sign_next(uint8_t *msg, size_t *msg_len, size_t msg_max_len,
+                        const uint8_t *prev_digest, size_t prev_digest_len,
+                        uint8_t *digest, size_t *digest_len,
+                        const knot_tsig_key_t *key, uint8_t *to_sign,
+                        size_t to_sign_len);
+
+/*!
+ * \brief Checks incoming request.
+ *
+ * \param tsig_rr TSIG extracted from the packet.
+ * \param wire Wire format of the packet (including the TSIG RR).
+ * \param size Size of the wire format of packet in bytes.
+ * \param tsig_key TSIG key.
+ *
+ * \retval KNOT_EOK If the signature is valid.
+ * \retval TODO
+ *
+ * \todo This function should return TSIG errors by their codes which are
+ *       positive values - this will be recognized by the caller.
+ */
+int knot_tsig_server_check(const knot_rrset_t *tsig_rr,
+                           const uint8_t *wire, size_t size,
+                           const knot_tsig_key_t *tsig_key);
+
+/*!
+ * \brief Checks incoming response.
+ *
+ * \param tsig_rr TSIG extracted from the packet.
+ * \param wire Wire format of the packet (including the TSIG RR).
+ * \param size Size of the wire format of packet in bytes.
+ * \param request_mac Request MAC. (may be NULL).
+ * \param request_mac_len Size of the request MAC in bytes.
+ * \param key TSIG key.
+ * \param prev_time_signed Time for TSIG period validity.
+ *
+ * \retval KNOT_EOK If the signature is valid.
+ * \retval TODO
+ *
+ * \todo This function should return TSIG errors by their codes which are
+ *       positive values - this will be recognized by the caller.
+ */
+int knot_tsig_client_check(const knot_rrset_t *tsig_rr,
+                           const uint8_t *wire, size_t size,
+                           const uint8_t *request_mac, size_t request_mac_len,
+                           const knot_tsig_key_t *key,
+                           uint64_t prev_time_signed);
+
+/*!
+ * \brief Checks signature of 2nd or next packet in a TCP session.
+ *
+ * \param tsig_rr TSIG extracted from the packet.
+ * \param wire Wire format of the packet (including the TSIG RR).
+ * \param size Size of the wire format of packet in bytes.
+ * \param prev_digest Previous digest sent by the server in the session.
+ * \param prev_digest_len Size of the previous digest in bytes.
+ * \param key TSIG key.
+ * \param prev_time_signed Time for TSIG period validity.
+ *
+ * \retval KNOT_EOK If the signature is valid.
+ * \retval TODO
+ *
+ * \todo This function should return TSIG errors by their codes which are
+ *       positive values - this will be recognized by the caller.
+ */
+int knot_tsig_client_check_next(const knot_rrset_t *tsig_rr,
+                                const uint8_t *wire, size_t size,
+                                const uint8_t *prev_digest,
+                                size_t prev_digest_len,
+                                const knot_tsig_key_t *key,
+                                uint64_t prev_time_signed);
+
+/*!
+ * \todo Documentation!
+ */
+int knot_tsig_add(uint8_t *msg, size_t *msg_len, size_t msg_max_len,
+                  uint16_t tsig_rcode, const knot_rrset_t *tsig_rr);
+
+/*! \brief Append TSIG RR to message.
+ *  \todo Proper documentation.
+ */
+int knot_tsig_append(uint8_t *msg, size_t *msg_len, size_t msg_max_len,
+                     const knot_rrset_t *tsig_rr);
+
+/*! \brief Return true if the TSIG RCODE allows signing the packet.
+ *  \todo Proper documentation.
+ */
+static inline bool knot_tsig_can_sign(uint16_t tsig_rcode) {
+	return tsig_rcode == KNOT_RCODE_NOERROR || tsig_rcode == KNOT_RCODE_BADTIME;
+}
+
+/*! @} */