Arm Mbed and Pelion Device Management support forum

ECDH Shared Key Generation


I am trying to do the ECDH shared secret computation using the mbedTLS library. I am referring to multiple examples such as ecdh_curve25519.c and ecdh_main.c.

In my case, in my application firmware, I already have a device _priv key and I receive a server_public key; both generated using a curve ECP_DP_SECP256R1. I would like to do generate a shared secret from here on and preserve it for future use.

The following is the steps that I do:

  1. Create a new client context, entropy context, ctr_drbg context variables.

  2. use mbedtls_“respective”_init() to initalize all the three variables

  3. Seed a random number using mbedtls_ctr_drbg_seed() function.

  4. load the P256 elliptic curve in client context using mbedtls_ecp_group_load()

  5. Generate a keypair for client context using mbedtls_ecdh_gen_public(&ctx_cli.grp, &ctx_cli.d, &ctx_cli.Q, mbedtls_ctr_drbg_random, &ctrDrbg)

  6. Then use mbedtls_mpi_lset() to set Qp.Z =1

  7. Then read the server pub key using mbedtls_mpi_read_binary(&ctx_cli.Qp.X, server_pub, 65);

  8. Now the question is: Should I initialize the ctx_cli with my already generated device_priv key using
    mbedtls_mpi_read_binary(&ctx_cli.d, device_priv_key, 50) ?

  9. Then I plan to use mbedtls_ecdh_compute_shared(&ctx_cli.grp, &ctx_cli.z, &ctx_cli.Qp, &ctx_cli.d, mbedtls_ctr_drbg_random, &ctr_drbg); to compute the shared secret in z.

Also, if I am generating a hardware random number already in my platform, based on the HWTRNG, I should not be changing anything on the mbedtls_ecdh_compute_shared() function, correct? Instead I should be implementing the hardware poll along with HARDWARE_ALT define and the ctr_drbg_random will take care of the binding? Please confirm on this.

Please let me know if the flow is correct. In all the examples, they generate a key pair and just update the public key part (Qp.X) of the key. Thye do not touch the private key part (d) of the key. Please confirm if I can upload my private key directly in my case.

Also if my platform is a little endian, is there a recommended step before using mbedtls_mpi_read_binary functions?

Thanks so much for your help in advance!

Hi @ab90
As mentioned here , Mbed TLS is now maintained under open governance at .

The Mbed TLS support forum will now handle only issues encountered on Mbed OS and Pelion Device Management.

Please address all Mbed TLS core issues to the project’s mailing list at

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