# uOSCORE / uEDHOC
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This repository contains C implementations for constrained (and non-constrained) devices of the IETF protocols:
* [OSOCRE (RFC8613)](https://tools.ietf.org/html/rfc8613) and
* [EDHOC (draft-ietf-lake-edhoc-15 )](https://datatracker.ietf.org/doc/html/draft-ietf-lake-edhoc-15).
Main features of uOSCORE and uEDHOC are their independence from the OS, cryptographic engine and in the case of uEDHOC transport protocol. Additionally, uOSCORE and uEDHOC use only stack memory (no heap).
For more background and evaluation in terms of speed, RAM and flash requirements see our paper [The Cost of OSCORE and EDHOC for Constrained Devices](https://arxiv.org/pdf/2103.13832.pdf).
## How to Build and Link
* check the configurations in `makefile_config.mk` and adjust them if necessary
* run `make`
* link the static library `build/libuoscore-uedhoc.a` in your project
## Test coverage
* [Test coverage report uOSCORE](https://eriptic.github.io/uoscore-uedhoc/src/oscore/index.html)
* [Test coverage report uEDHOC](https://eriptic.github.io/uoscore-uedhoc/src/edhoc/index.html)
## Project (Folder) Structure
```
.
|---cddl_models/
|---externals/
|---inc/
|---samples/
|---scripts/
|---src/
|---test/
|---test_vectors/
```
* The folder `cddl_models` contains CDDL models for all CBOR structures.
* The folder `externals` contains the external libraries and tools as git submodules.
* The folder `inc` contains all header file.
* The folder `samples` contains some usage examples.
* The folder `scripts` contains scripts for generatinc C code from CDDL models and converting the json formatted EDHOC test vectors to a C header
* The folder `src` contains all source file.
* The folder `test` contains automated tests.
* The folder `test_vectors` contains tests vectors.
## API and Usage Model
#### uOSCORE
The API of uOSCORE consists of three functions:
* `oscore_context_init()`,
* `coap2oscore()` and
* `oscore2coap()`.
`coap2oscore()` and `oscore2coap()` convert CoAP to OSCORE packets and vice versa. `oscore_context_init()` initializes the OSCORE security context.
First, `oscore_context_init()` function needs to be called on the client and server side, then `coap2oscore()` and `oscore2coap()` are called just before sending or receiving packets over the network.
#### uEDHOC
The API of uEDHOC consists of four functions:
* `ephemeral_dh_key_gen()`
* `edhoc_initiator_run()`,
* `edhoc_responder_run()`,
* `edhoc_exporter()`,
`ephemeral_dh_key_gen()` is used to generate fresh ephemeral DH keys before running the protocol. This function requires a random seed suable for cryptographic purposes. `edhoc_initiator_run()` and `edhoc_responder_run() ` has to be called on the initiator and responder side respectively. They return the External Authorization data `EAD_x`, the derived shared secret `PRK_out`. `PRK_out` is used as input for `edhoc_exporter()` to derive application specific keys, e.g., OSCORE master secret and OSCORE master salt.
The EDHOC protocol requires the exchange of three messages (and an optional message 4) which is independent of the underlying message transport protocol. For example [appendix-A.2 in the EDHOC specification](https://datatracker.ietf.org/doc/html/draft-ietf-lake-edhoc-15#appendix-A.2) describes how EDHOC can be transferred over CoAP, however CoAP is not mandatory. In order to be independent of the transport protocol uEDHOC uses two callback functions which need to be implemented by the user for handling the sending and receiving of messages. These functions are:
```c
/**
* @brief The user should call inside this function its send function.
*
*
* @param sock a pointer used to identify the rx chanel,
* e.g. a socket handler
* @param data pointer to the data to be send
* @param data_len length of the data
*/
enum err tx(void *sock, uint8_t *data, uint32_t data_len);
/**
* @brief The user should call inside this function its receive
* function and copy the received data in the buffer .
* The length of the buffer must be
* checked before copying into it by using .
* After copying the length of the received data should be written
* in .
*
*
* @param sock a pointer used to identify the rx chanel,
* e.g. a socket handler
* @param data pointer to a buffer where the edhoc message must be copied
* @param data_len length of the received data. When this function is
* called inside EDHOC is initialized with the actual
* available length of the .
*/
enum err rx(void *sock, uint8_t *data, uint32_t *data_len);
```
Note that uEDHOC does not provide correlation of messages. Correlation may be handled on the transport layer completely or partially. In cases when the correlation cannot be handled by the transport protocol the edhoc message needs to be prepended with a connection identifier, that is used on the other side to determine to which session a given message belongs. In order to remain conform with the specification in the cases where the transport cannot handle correlation a connection identifier needs to be prepended in `tx()` function and removed in the `rx()` function.
## Supported Cipher Suites
##### uOSCORE
| Algorithms |
| --------------------------- |
| AES-CCM-16-64-128, SHA-256 |
##### uEDHOC
| Suit | Algorithms |
| ---- | -------------------------------------------------------------------------- |
| 0 | AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA, AES-CCM-16-64-128, SHA-256 |
| 1 | AES-CCM-16-128-128, SHA-256, 16, X25519, EdDSA, AES-CCM-16-64-128, SHA-256 |
| 2 | AES-CCM-16-64-128, SHA-256, 8, P-256, ES256, AES-CCM-16-64-128, SHA-256 |
| 3 | AES-CCM-16-128-128, SHA-256, 16, P-256, ES256, AES-CCM-16-64-128, SHA-256 |
## Using Different Cryptographic Libraries or Hardware Accelerators
The logic of uOSCORE and uEDHOC is independent form the cryptographic library, i.e., the cryptographic library can easily be exchanged by the user. For that the user needs to provide implementations for the functions specified in `crypto_wrapper.c`.
## Preventing Nonce Reuse Attacks in OSCORE
AES keys should never be used more than once with a given nonce, see [RFC5084](https://datatracker.ietf.org/doc/html/rfc5084). In order to avoid this situation, the user has 2 options while creating context structure:
- setting `fresh_master_secret_salt = true`, when given context is new (freshly obtained e.g. with EDHOC)
- setting `fresh_master_secret_salt = false`, when the same context is used between reboots/reconnections. In this case, the user must enable Non-volatile Memory support (see `OSCORE_NVM_SUPPORT` in `makefile_config.mk`) and implement two functions that require access to NVM (see below).
Note that using NVM support is independent of the parameter above. Although it is required for using the same context multiple times, it will also be utilized (if enabled) to store context obtained with EDHOC, enabling the user to reuse it after the reboot. This behaviour is useful in situations where multiple sessions need to be stored on a device, while at the same time being able to start a completely new session with EDHOC. When such feature is not needed, `OSCORE_NVM_SUPPORT` can be disabled so only fresh sessions are acceptable.
```c
/**
* @brief When the same OSCORE master secret and salt are reused through
* several reboots of the device, e.g., no fresh shared secret is
* derived through EDHOC (or some other method) the Sender Sequence
* Number MUST be stored periodically in NVM.
* @param nvm_key part of the context that is permitted to be used for identifying the right store slot in NVM.
* @param ssn SSN to be written in NVM.
* @retval ok or error code if storing the SSN was not possible.
*/
enum err nvm_write_ssn(const struct nvm_key_t *nvm_key, uint64_t ssn);
/**
* @brief When the same OSCORE master secret and salt are reused through
* several reboots of the device, e.g., no fresh shared secret is
* derived through EDHOC (or some other method) the Sender Sequence
* Number MUST be restored from NVM at each reboot.
* @param nvm_key part of the context that is permitted to be used for identifying the right store slot in NVM.
* @param ssn SSN to be read out from NVM.
* @retval ok or error code if the retrieving the SSN was not possible.
*/
enum err nvm_read_ssn(const struct nvm_key_t *nvm_key, uint64_t *ssn);
```
## Additional configuration options
Following preprocessor directives allow for better memory usage adjustments:
* **edhoc.h**
* EDHOC_BUF_SIZES_RPK - select when authentication can be based on raw public keys, or
* EDHOC_BUF_SIZES_C509_CERT - select when authentication can be based on CBOR encoded certificates, or
* EDHOC_BUF_SIZES_X509_CERT - select when authentication can be based on X.509 certificates
* EDHOC_MESSAGE_4_SUPPORTED - select when generation of message 4 should be supported
* **oscore.h**
* OSCORE_MAX_PLAINTEXT_LEN - expected maximal length of OSCORE packet
* E_OPTIONS_BUFF_MAX_LEN - expected maximal length of buffer with all encrypted CoAP options
* I_OPTIONS_BUFF_MAX_LEN - expected maximal length of buffer with all not encrypted CoAP options