Reorg documentation layout. Rename manual to handbook.

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+PSK Database
+======================
+
+.. versionadded:: 2.4.0
+
+Many applications need to store pre-shared keys (hereafter PSKs) for
+authentication purposes.
+
+An abstract interface to PSK stores is provided in ``psk_db.h``
+
+.. cpp:class:: PSK_Database
+
+   .. cpp:function:: bool is_encrypted() const
+
+      Returns true if (at least) the PSKs themselves are encrypted. Returns
+      false if PSKs are stored in plaintext.
+
+   .. cpp:function:: std::set<std::string> list_names() const
+
+      Return the set of valid names stored in the database, ie values for which
+      ``get`` will return a value.
+
+   .. cpp:function:: void set(const std::string& name, const uint8_t psk[], size_t psk_len)
+
+      Save a PSK. If ``name`` already exists, the current value will be
+      overwritten.
+
+   .. cpp:function:: secure_vector<uint8_t> get(const std::string& name) const
+
+      Return a value saved with ``set``. Throws an exception if ``name`` doesn't
+      exist.
+
+   .. cpp:function:: void remove(const std::string& name)
+
+      Remove ``name`` from the database. If ``name`` doesn't exist, ignores the request.
+
+   .. cpp::function:: std::string get_str(const std::string& name) const
+
+      Like ``get`` but casts the return value to a string.
+
+   .. cpp:function:: void set_str(const std::string& name, const std::string& psk)
+
+      Like ``set`` but accepts the psk as a string (eg for a password).
+
+   .. cpp:function:: template<typename Alloc> void set_vec(const std::string& name, \
+                                              const std::vector<uint8_t, Alloc>& psk)
+
+      Like ``set`` but accepting a vector.
+
+The same header also provides a specific instantiation of ``PSK_Database`` which
+encrypts both names and PSKs. It must be subclassed to provide the storage.
+
+.. cpp:class:: Encrypted_PSK_Database : public PSK_Database
+
+   .. cpp:function:: Encrypted_PSK_Database(const secure_vector<uint8_t>& master_key)
+
+      Initializes or opens a PSK database. The master key is used the secure the
+      contents. It may be of any length. If encrypting PSKs under a passphrase,
+      use a suitable key derivation scheme (such as PBKDF2) to derive the secret
+      key. If the master key is lost, all PSKs stored are unrecoverable.
+
+      Both names and values are encrypted using NIST key wrapping (see NIST
+      SP800-38F) with AES-256. First the master key is used with HMAC(SHA-256)
+      to derive two 256-bit keys, one for encrypting all names and the other to
+      key an instance of HMAC(SHA-256). Values are each encrypted under an
+      individual key created by hashing the encrypted name with HMAC. This
+      associates the encrypted key with the name, and prevents an attacker with
+      write access to the data store from taking an encrypted key associated
+      with one entity and copying it to another entity.
+
+      Names and PSKs are both padded to the next multiple of 8 bytes, providing
+      some obfuscation of the length.
+
+      One artifact of the names being encrypted is that is is possible to use
+      multiple different master keys with the same underlying storage. Each
+      master key will be responsible for a subset of the keys. An attacker who
+      knows one of the keys will be able to tell there are other values
+      encrypted under another key.
+
+   .. cpp:function:: virtual void kv_set(const std::string& index, const std::string& value) = 0
+
+      Save an encrypted value. Both ``index`` and ``value`` will be non-empty
+      base64 encoded strings.
+
+   .. cpp:function:: virtual std::string kv_get(const std::string& index) const = 0
+
+      Return a value saved with ``kv_set``, or return the empty string.
+
+   .. cpp:function:: virtual void kv_del(const std::string& index) = 0
+
+      Remove a value saved with ``kv_set``.
+
+   .. cpp:function:: virtual std::set<std::string> kv_get_all() const = 0
+
+      Return all active names (ie values for which ``kv_get`` will return a
+      non-empty string).
+
+A subclass of ``Encrypted_PSK_Database`` which stores data in a SQL database
+is also available. This class is declared in ``psk_db_sql.h``:
+
+.. cpp:class:: Encrypted_PSK_Database_SQL : public Encrypted_PSK_Database
+
+  .. cpp:function:: Encrypted_PSK_Database_SQL(const secure_vector<uint8_t>& master_key, \
+                                 std::shared_ptr<SQL_Database> db, \
+                                 const std::string& table_name)
+
+     Creates or uses the named table in ``db``. The SQL schema of the table is
+     ``(psk_name TEXT PRIMARY KEY, psk_value TEXT)``.