uOSCORE / uEDHOC – A Lightweight IoT Security Protocol Stack

uOSCORE / uEDHOC is a lightweight C implementation of the IETF protocols OSCORE (RFC8613) and EDHOC (RFC9528), specifically designed for microcontrollers. This implementation is independent of the operating system, cryptographic engine, and, in the case of uEDHOC, the transport protocol. Notably, uOSCORE / uEDHOC operates using only stack memory, avoiding heap allocation.

The OSCORE and EDHOC protocols were developed by the IETF to provide a lightweight alternative to (D)TLS for IoT deployments. Unlike (D)TLS, OSCORE and EDHOC function at the application layer, which is typically CoAP rather than at the transport layer. This allows for end-to-end authenticated and encrypted communication through CoAP proxies—a capability that transport layer security protocols like (D)TLS cannot achieve.

For detailed background information and performance evaluations in terms of speed, RAM, and flash memory requirements, refer to our paper, The Cost of OSCORE and EDHOC for Constrained Devices and the Benchmarks.

Enterprise Support

Eriptic Technologies has co-developed uOSCORE / uEDHOC and provides enterprise support services centered around it. Our offerings encompass:

• Integration of uOSCORE / uEDHOC to designated customer platforms.
• Tailored threat analysis for unique customer use cases.
• Designing security architecture for embedded systems.
• Developing custom embedded security solutions.

For more information visit our website eriptic.com or send us an email uoscore-uedhoc@eriptic.com

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
• Test coverage report uEDHOC

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 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:

/**
 * @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 data to be send
 */
enum err tx(void *sock, struct byte_array *data);

/**
 * @brief   The user should call inside this function its receive 
 *          function. The user should copy the received data in \p data.
 *          
 *          THE USER MUST MAKE SURE THAT HE/SHE IS NOT WRITING DATA OUTSIDE THE 
 *          RECEIVE BUFFER, I.E., THE LENGTH OF THE RECEIVED DATA IS SMALLER 
 *          THAN \p data->len. 
 * 
 *          After copying, the length of the received data should be written 
 *          in \p data->len.
 *
 * 
 * @param   sock a pointer used to identify the rx chanel, 
 *          e.g. a socket handler
 * @param   data the received message must be copied here
 */
enum err rx(void *sock, struct byte_array  *data);

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. 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.

/**
* @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

The build configuration can be adjusted in the makefile_config.mk.