p≡p JSON adapter
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p≡p JSON Server Adapter


The p≡p JSON Server Adapter provides a REST-like jQuery-compatible API to connect with the p≡p engine. It is language-independent and can be used by any client.


In order to use the p≡p JSON Server Adapter, you need to build and run it. Currently, Linux (Debian 9, Ubuntu 16.04) and MacOS (10.11, 10.12) are supported, Windows is about to follow. Newer versions should also work (file a bug report if not) but are not in our main focus, yet.


  • C++ compiler: tested with g++ 4.8 and 4.9, and clang++ 2.8. Newer versions should work, too.
  • GNU make
  • libboost-thread-dev (tested with 1.58)
  • libboost-program-options-dev (tested with 1.58)
  • libboost-filesystem-dev (tested with 1.58)
  • libevent-dev 2.0.21 or 2.0.22 (or build from source, see below)
  • p≡p Engine (which needs gpgme-thread, a patched libetpan, libboost-system-dev)
  • OSSP libuuid


Install the dependencies

Debian 9:

apt install -y build-essential libboost1.62-dev libboost-system1.62-dev libboost-filesystem1.62-dev libboost-program-options1.62-dev libboost-thread1.62-dev libgpgme-dev uuid-dev

macOS 10.12:

Use homebrew or macports to install the required libraries.

For more explicit instructions on how to do this with macports, see the section below.

Build and install the pEp Engine. Instructions can be found here: https://cacert.pep.foundation/dev/repos/pEpEngine/file/ef23982e4744/README.md.

Build and install libevent

mkdir ~/code/json-ad
hg clone https://cacert.pep.foundation/dev/repos/pEpJSONServerAdapter/ ~/code/json-ad
cd ~/code/json-ad
./configure --prefix="$HOME/code/json-ad/libevent-2.0.22-stable/build/" --disable-openssl
make install

Build and install the JSON server

cd cd ~/code/json-ad/server

Edit the build configuration to your needs in ./Makefile.conf, or create a ./local.conf that sets any of the make variables documented in ./Makefile.conf.

If a dependency is not found in your system's default include or library paths, you will have to specify the according paths in a make variable. Typically, this has to be done at least for the pEp Engine, libetpan and libevent.

Below are two sample ./local.conf files, for orientation.

macOS 10.12:








Debian 9:





Now, build and install the server:

make all
make install

With make test you can execute the server's tests.


Install MacPorts for your version of macOS.

If MacPorts is already installed on your machine, but was installed by a different user, make sure your PATH variable is set as follows in ~/.profile:

export PATH="/opt/local/bin:/opt/local/sbin:$PATH"

Install dependencies packaged with MacPorts as follows.

sudo port install gpgme boost ossp-uuid

Running the pEp JSON Adapter

You can use make run to start the server.

  1. Run ./pep-json-server. This creates a file that is readable only by the current user (/tmp/pEp-json-token-${USER}) and contains the address and port the JSON adapter is listening on, normally and a "security-token" that must be given in each function call to authenticate you as the valid user.

  2. Visit that address (normally in your JavaScript-enabled web browser to see the "JavaScript test client".

  3. Call any function ("version()" or "get_gpg_path()" should work just fine) with the correct security token.

Using the p≡p JSON Adapter

In the following section, you'll find background information on how to use the adapter and its functions.

Session handling

When using the p≡p engine, a session is needed to which any adapter can connect. The p≡p JSON Server Adapter automatically creates one session per HTTP client connection (and also closes that session automatically when the client connections is closed). Therefore, the client does not need to take care of the session management. However, the client has to set up a HTTP persistent connection.

API Principles

All C data types are mapped the same way, so some day the JSON wrapper can be generated from the p≡p Engine header files (or the JSON wrapper and the p≡p engine header are both generated from a common interface description file).

C type JSON mapping
bool JSON boolean
int JSON decimal number
size_t JSON decimal number
char* (representing a UTF-8-encoded NULL-terminated string JSON string
char* (representing a binary string base64-encoded JSON string
enum either JSON decimal number or JSON object containing one decimal number as member
struct JSON object
linked lists (e.g. bloblist_t, stringlist_t, identity_list etc.) JSON array of their member data type (without the next pointer)

The parameter type PEP_SESSION is handled automatically by the JSON Server Adapter and the PEP_SESSION parameter is omitted from the JSON API.

enum types

Enum types are represented as JSON objects with one member, whose name is derived from the enum type name, holding the numeric value of the enum.

Some enum types are still represented directly as JSON decimal number. It shall be changed in a future version of the JSON Adapter.

String types

The JSON Server Adapter does automatic memory management for string parameters. The current p≡p Engine's API distinguish between const char* parameters and char* parameters. const char* normally means: the "ownership" of the string remains at the caller, so the JSON Adapter frees the string automatically after the call. char* normally means: the "ownership" of the string goes to the Engine, so the JSON Adapter does not free string.

If there are functions that have a different semantics the behavior of the JSON wrapper has to be changed.

Parameter (value) restrictions

Some API functions have restrictions on their parameter values. The JSON Adapter does not know these restrictions (because it does not know the semantics of the wrapped functions at all). So it is the client's responsibility to fulfill these parameter restrictions! Especially when there are restrictions that are checked with assert() within the p≡p Engine, it is impossible for the JSON Adapter to catch failed assertions - the Engine and the Adapter process will be terminated immediatetely when the Engine is compiled in debug mode (= with enabled assert() checking).

Currently there are no range checks for numerical parameter types (e.g. a JSON decimal number can hold a bigger value than the int parameter type of a certain C function).

API Reference

An complete overview with all functions that are callable from the client can be found in the [API Reference](pEp JSON Server Adapter/API Reference).

That API reference is a generated file that shows the current API briefly. There is also a (currently manually written) file that holts a copy of the documentation from the Engine's header files: [API reference detail.md]

Most of the callable functions are functions from the C API of the p≡p Engine. They are described in detail, incl. pre- and post-conditions in the appropriate C header files of the Engine.

Extending / customizing

If you want to extend or customize the p≡p JSON Adapter, there are several rules and definitions to take into account.


  • The FunctionMap function in json-adapter.cc defines which functions are callable via the JSON-RPC interface. The existing entries show the syntax of that map.

  • For each type there must exist specializations of the template classes "In" (for input parameters) and "Out" (for output parameters). The linker will tell you, which specializations are needed.

  • The specializations for "generic types" are in function_map.cc.

  • The specializations for "p≡p-specific types" are in pep-types.cc.


The following issues are planned but not yet implemented.

  • Windows build:

    • implement get_token_filename() for MS Windows (security-token.cc line 43)
    • do the Windows-specific stuff to build the software on Windows
  • Add unit tests

  • Fix the bugs that are found by the Unit tests, if any.

  • Generate all the tedious boiler plate code

    • the content of pep-types.cc
    • perhaps the FunctionMap 'function' in mt-server.cc
    • perhaps the JavaScript side of the HTML test page to ensure to be consistent with the server side in pep-types.cc
  • Adapt the "p≡p Transport API", when it is final. (either manually or by code generator, if ready)