diff --git a/library/mps_reader.h b/library/mps_reader.h
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+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+
+/**
+ * \file reader.h
+ *
+ * \brief This file defines reader objects, which together with their
+ *        sibling writer objects form the basis for the communication
+ *        between the various layers of the Mbed TLS messaging stack,
+ *        as well as the communication between the messaging stack and
+ *        the (D)TLS handshake protocol implementation.
+ *
+ * Readers provide a means of transferring incoming data from
+ * a 'producer' providing it in chunks of arbitrary size, to
+ * a 'consumer' which fetches and processes it in chunks of
+ * again arbitrary, and potentially different, size.
+ *
+ * Readers can be seen as datagram-to-stream converters,
+ * and they abstract away the following two tasks from the user:
+ * 1. The pointer arithmetic of stepping through a producer-
+ *    provided chunk in smaller chunks.
+ * 2. The merging of incoming data chunks in case the
+ *    consumer requests data in larger chunks than what the
+ *    producer provides.
+ *
+ * The basic abstract flow of operation is the following:
+ * - Initially, the reader is in 'producing mode'.
+ * - The producer hands an incoming data buffer to the reader,
+ *   moving it from 'producing' to 'consuming' mode.
+ * - The consumer subsequently fetches and processes the buffer
+ *   content. Once that's done -- or partially done and a consumer's
+ *   requests can't be fulfilled -- the producer revokes the reader's
+ *   access to the incoming data buffer, putting the reader back to
+ *   producing mode.
+ * - The producer subsequently gathers more incoming data and hands
+ *   it to reader until the latter switches back to consuming mode
+ *   if enough data is available for the last consumer request to
+ *   be satisfiable.
+ * - Repeat the above.
+ *
+ * From the perspective of the consumer, the state of the
+ * reader is a potentially empty list of input buffers that
+ * the reader has provided to the consumer.
+ * New buffers can be requested through calls to mbedtls_reader_get(),
+ * while previously obtained input buffers can be marked processed
+ * through calls to mbedtls_reader_consume(), emptying the list of
+ * input buffers and invalidating them from the consumer's perspective.
+ * The consumer need not be aware of the distinction between consumer
+ * and producer mode, because he only interfaces with the reader
+ * when the latter is in consuming mode.
+ *
+ * From the perspective of the producer, the state of the reader
+ * is one of the following:
+ * - Attached: An incoming data buffer is currently
+ *             being managed by the reader, and
+ * - Unset: No incoming data buffer is currently
+ *          managed by the reader, and all previously
+ *          handed incoming data buffers have been
+ *          fully processed.
+ * - Accumulating: No incoming data buffer is currently
+ *                 managed by the reader, but some data
+ *                 from the previous incoming data buffer
+ *                 hasn't been processed yet and is internally
+ *                 held back.
+ * The Unset and Accumulating states belong to producing mode,
+ * while the Attached state belongs to consuming mode.
+ *
+ * Transitioning from Unset or Accumulating to Attached is
+ * done via calls to mbedtls_reader_feed(), while transitioning
+ * from Consuming to either Unset or Accumulating (depending
+ * on what has been processed) is done via mbedtls_reader_reclaim().
+ *
+ * The following diagram depicts the producer-state progression:
+ *
+ *        +------------------+             reclaim
+ *        |      Unset       +<-------------------------------------+       get
+ *        +--------|---------+                                      |   +------+
+ *                 |                                                |   |      |
+ *                 |                                                |   |      |
+ *                 |                feed                  +---------+---+--+   |
+ *                 +-------------------------------------->    Attached    <---+
+ *                                                        |       /        |
+ *                 +-------------------------------------->    Consuming   <---+
+ *                 |     feed, enough data available      +---------+---+--+   |
+ *                 |     to serve previous consumer request         |   |      |
+ *                 |                                                |   |      |
+ *        +--------+---------+                                      |   +------+
+ *   +---->   Accumulating   |<-------------------------------------+    commit
+ *   |    +---+--------------+      reclaim, previous read request
+ *   |        |                        couldn't be fulfilled
+ *   |        |
+ *   +--------+
+ *     feed, need more data to serve
+ *     previous consumer request
+ *
+ */
+
+#ifndef MBEDTLS_READER_H
+#define MBEDTLS_READER_H
+
+#include <stdio.h>
+
+#include "mps_common.h"
+#include "mps_error.h"
+
+struct mbedtls_reader;
+typedef struct mbedtls_reader mbedtls_reader;
+
+/*
+ * Structure definitions
+ */
+
+struct mbedtls_reader
+{
+    unsigned char *frag;  /*!< The fragment of incoming data managed by
+                           *   the reader; it is provided to the reader
+                           *   through mbedtls_reader_feed(). The reader
+                           *   does not own the fragment and does not
+                           *   perform any allocation operations on it,
+                           *   but does have read and write access to it.   */
+    mbedtls_mps_stored_size_t frag_len;
+                          /*!< The length of the current fragment.
+                           *   Must be 0 if \c frag == \c NULL.             */
+    mbedtls_mps_stored_size_t commit;
+                          /*!< The offset of the last commit, relative
+                           *   to the first byte in the accumulator.
+                           *   This is only used when the reader is in
+                           *   consuming mode, i.e. frag != NULL;
+                           *   otherwise, its value is \c 0.                */
+    mbedtls_mps_stored_size_t end;
+                          /*!< The offset of the end of the last chunk
+                           *   passed to the user through a call to
+                           *   mbedtls_reader_get(), relative to the first
+                           *   byte in the accumulator.
+                           *   This is only used when the reader is in
+                           *   consuming mode, i.e. \c frag != \c NULL;
+                           *   otherwise, its value is \c 0.                */
+    mbedtls_mps_stored_size_t pending;
+                          /*!< The amount of incoming data missing on the
+                           *   last call to mbedtls_reader_get().
+                           *   In particular, it is \c 0 if the last call
+                           *   was successful.
+                           *   If a reader is reclaimed after an
+                           *   unsuccessful call to mbedtls_reader_get(),
+                           *   this variable is used to have the reader
+                           *   remember how much data should be accumulated
+                           *   before the reader can be passed back to
+                           *   the user again.
+                           *   This is only used when the reader is in
+                           *   consuming mode, i.e. \c frag != \c NULL;
+                           *   otherwise, its value is \c 0.                */
+
+    /* The accumulator is only needed if we need to be able to pause
+     * the reader. A few bytes could be saved by moving this to a
+     * separate struct and using a pointer here. */
+
+    unsigned char *acc;   /*!< The accumulator is used to gather incoming
+                           *   data if a read-request via mbedtls_reader_get()
+                           *   cannot be served from the current fragment.   */
+    mbedtls_mps_stored_size_t acc_len;
+                           /*!< The total size of the accumulator.           */
+    mbedtls_mps_stored_size_t acc_avail;
+                          /*!< The number of bytes currently gathered in
+                           *   the accumulator. This is both used in
+                           *   producing and in consuming mode:
+                           *   While producing, it is increased until
+                           *   it reaches the value of \c acc_remaining below.
+                           *   While consuming, it is used to judge if a
+                           *   read request can be served from the
+                           *   accumulator or not.
+                           *   Must not be larger than acc_len.              */
+    union
+    {
+        mbedtls_mps_stored_size_t acc_remaining;
+                              /*!< This indicates the amount of data still
+                               *   to be gathered in the accumulator. It is
+                               *   only used in producing mode.
+                               *   Must be at most acc_len - acc_available.  */
+        mbedtls_mps_stored_size_t frag_offset;
+                              /*!< This indicates the offset of the current
+                               *   fragment from the beginning of the
+                               *   accumulator.
+                               *   It is only used in consuming mode.
+                               *   Must not be larger than \c acc_avail.     */
+    } acc_share;
+};
+
+/*
+ * API organization:
+ * A reader object is usually prepared and maintained
+ * by some lower layer and passed for usage to an upper
+ * layer, and the API naturally splits according to which
+ * layer is supposed to use the respective functions.
+ */
+
+/*
+ * Maintenance API (Lower layer)
+ */
+
+/**
+ * \brief           Initialize a reader object
+ *
+ * \param reader    The reader to be initialized.
+ * \param acc       The buffer to be used as a temporary accumulator
+ *                  in case read requests through mbedtls_reader_get()
+ *                  exceed the buffer provided by mbedtls_reader_feed().
+ *                  This buffer is owned by the caller and exclusive use
+ *                  for reading and writing is given to the reade for the
+ *                  duration of the reader's lifetime. It is thus the caller's
+ *                  responsibility to maintain (and not touch) the buffer for
+ *                  the lifetime of the reader, and to properly zeroize and
+ *                  free the memory after the reader has been destroyed.
+ * \param acc_len   The size in Bytes of \p acc.
+ *
+ * \return          \c 0 on success.
+ * \return          A negative \c MBEDTLS_ERR_READER_XXX error code on failure.
+ */
+int mbedtls_reader_init( mbedtls_reader *reader,
+                         unsigned char *acc,
+                         mbedtls_mps_size_t acc_len );
+
+/**
+ * \brief           Free a reader object
+ *
+ * \param reader    The reader to be freed.
+ *
+ * \return          \c 0 on success.
+ * \return          A negative \c MBEDTLS_ERR_READER_XXX error code on failure.
+ */
+int mbedtls_reader_free( mbedtls_reader *reader );
+
+/**
+ * \brief           Pass chunk of data for the reader to manage.
+ *
+ * \param reader    The reader context to use. The reader must be
+ *                  in producing state.
+ * \param buf       The buffer to be managed by the reader.
+ * \param buflen    The size in Bytes of \p buffer.
+ *
+ * \return          \c 0 on success. In this case, the reader will be
+ *                  moved to consuming state, and ownership of \p buf
+ *                  will be passed to the reader until mbedtls_reader_reclaim()
+ *                  is called.
+ * \return          \c MBEDTLS_ERR_READER_NEED_MORE if more input data is
+ *                  required to fulfill a previous request to mbedtls_reader_get().
+ *                  In this case, the reader remains in producing state and
+ *                  takes no ownership of the provided buffer (an internal copy
+ *                  is made instead).
+ * \return          Another negative \c MBEDTLS_ERR_READER_XXX error code on
+ *                  different kinds of failures.
+ */
+int mbedtls_reader_feed( mbedtls_reader *reader,
+                         unsigned char *buf,
+                         mbedtls_mps_size_t buflen );
+
+/**
+ * \brief           Reclaim reader's access to the current input buffer.
+ *
+ * \param reader    The reader context to use. The reader must be
+ *                  in producing state.
+ * \param paused    If not \c NULL, the intger at address \p paused will be
+ *                  modified to indicate whether the reader has been paused
+ *                  (value \c 1) or not (value \c 0). Pausing happens if there
+ *                  is uncommitted data and a previous request to
+ *                  mbedtls_reader_get() has exceeded the bounds of the
+ *                  input buffer.
+ *
+ * \return          \c 0 on success.
+ * \return          A negative \c MBEDTLS_ERR_READER_XXX error code on failure.
+ */
+int mbedtls_reader_reclaim( mbedtls_reader     *reader,
+                            mbedtls_mps_size_t *paused );
+
+/*
+ * Usage API (Upper layer)
+ */
+
+/**
+ * \brief           Request data from the reader.
+ *
+ * \param reader    The reader context to use. The reader must
+ *                  in consuming state.
+ * \param desired   The desired amount of data to be read, in Bytes.
+ * \param buffer    The address to store the buffer pointer in.
+ *                  This must not be \c NULL.
+ * \param buflen    The address to store the actual buffer
+ *                  length in, or \c NULL.
+ *
+ * \return          \c 0 on success. In this case, \c *buf holds the
+ *                  address of a buffer of size \c *buflen
+ *                  (if \c buflen != \c NULL) or \c desired
+ *                  (if \c buflen == \c NULL). The user hass ownership
+ *                  of the buffer until the next call to mbedtls_reader_commit().
+ *                  or mbedtls_reader_reclaim().
+ * \return          #MBEDTLS_ERR_READER_OUT_OF_DATA if there is not enough
+ *                  data available to serve the read request. In this case,
+ *                  the reader remains intact, and additional data can be
+ *                  provided by reclaiming the current input buffer via
+ *                  mbedtls_reader_reclaim() and feeding a new one via
+ *                  mbedtls_reader_feed().
+ * \return          Another negative \c MBEDTLS_ERR_READER_XXX error
+ *                  code for different kinds of failure.
+ *
+ * \note            Passing \c NULL as \p buflen is a convenient way to
+ *                  indicate that fragmentation is not tolerated.
+ *                  It's functionally equivalent to passing a valid
+ *                  address as buflen and checking \c *buflen == \c desired
+ *                  afterwards.
+ */
+int mbedtls_reader_get( mbedtls_reader *reader,
+                        mbedtls_mps_size_t desired,
+                        unsigned char **buffer,
+                        mbedtls_mps_size_t *buflen );
+
+/**
+ * \brief           Signal that all input buffers previously obtained
+ *                  from mbedtls_writer_get() are fully processed.
+ *
+ *                  This function marks the previously fetched data as fully
+ *                  processed and invalidates their respective buffers.
+ *
+ * \param reader    The reader context to use.
+ *
+ * \return          \c 0 on success.
+ * \return          A negative \c MBEDTLS_ERR_READER_XXX error code on failure.
+ *
+ * \warning         Once this function is called, you must not use the
+ *                  pointers corresponding to the committed data anymore.
+ *
+ */
+int mbedtls_reader_commit( mbedtls_reader *reader );
+
+#endif /* MBEDTLS_READER_H */