Enable integration with C projects

[geonnave]

The approach is as follows:

• modify Cargo.toml to also generate a static library (libedhoc_rs.a)
• build a FFI-friendly wrapper, so that the relevant functions can be called with C types
• annotate functions with #[no_mangle] pub extern "C" and structs with #[repr(C)]
• configure cbindgen so that it automatically generates C headers for the FFI-friendly wrapper
  • it runs automatically on every build (cbindgen is configured in build.rs)

Now every build will also build the static library and the appropriate headers.

A script is provided to facilitate the generation of static library and the headers. Example usage:

./build_for_c.sh rust-cryptocell310

Here is the basic usage form C:

static const uint8_t ID_CRED_I[] = "a104412b";
static const uint8_t ID_CRED_R[] = "a104410a";
static const uint8_t CRED_I[] = "A2027734322D35302D33312D46462D45462D33372D33322D333908A101A5010202412B2001215820AC75E9ECE3E50BFC8ED60399889522405C47BF16DF96660A41298CB4307F7EB62258206E5DE611388A4B8A8211334AC7D37ECB52A387D257E6DB3C2A93DF21FF3AFFC8";
static const uint8_t G_I[] = "ac75e9ece3e50bfc8ed60399889522405c47bf16df96660a41298cb4307f7eb6";
static const uint8_t CRED_R[] = "A2026008A101A5010202410A2001215820BBC34960526EA4D32E940CAD2A234148DDC21791A12AFBCBAC93622046DD44F02258204519E257236B2A0CE2023F0931F1F386CA7AFDA64FCDE0108C224C51EABF6072";
static const uint8_t R[] = "72cc4761dbd4c78f758931aa589d348d1ef874a7e303ede2f140dcf3e6aa4aac";
static const uint8_t I[] = "fb13adeb6518cee5f88417660841142e830a81fe334380a953406a1305e8706b";
static const uint8_t G_R[] = "bbc34960526ea4d32e940cad2a234148ddc21791a12afbcbac93622046dd44f0";

RustEdhocInitiatorC initiator = initiator_new(I, 32*2, G_R, 32*2, ID_CRED_I, 4*2, CRED_I, 107*2, ID_CRED_R, 4*2, CRED_R, 84*2);
RustEdhocResponderC responder = responder_new(R, 32*2, G_I, 32*2, ID_CRED_I, 4*2, CRED_I, 107*2, ID_CRED_R, 4*2, CRED_R, 84*2);

EdhocMessageBuffer message_1;
initiator_prepare_message_1(&initiator, &message_1);
responder_process_message_1(&responder, &message_1);
EdhocMessageBuffer message_2;
uint8_t c_r_sent;
responder_prepare_message_2(&responder, &message_2, &c_r_sent);
uint8_t c_r_received;
initiator_process_message_2(&initiator, &message_2, &c_r_received);
EdhocMessageBuffer message_3;
uint8_t prk_out_initiator[SHA256_DIGEST_LEN];
initiator_prepare_message_3(&initiator, &message_3, prk_out_initiator);
uint8_t prk_out_responder[SHA256_DIGEST_LEN];
responder_process_message_3(&responder, &message_3, prk_out_responder);

[geonnave]

Some demo code is available in the DotBot-firmware repository, in the integrate-edhoc-rs branch.