Hi Arun,

You can write it into a buffer with the `mbedtls_pk_write_key_der()` function.

Regards,

Janos

From: Arun Lal K M via mbed-tls <mbed-tls@lists.trustedfirmware.org>
Date: Friday, 7 June 2024 at 10:04
To: mbed-tls@lists.trustedfirmware.org <mbed-tls@lists.trustedfirmware.org>
Subject: [mbed-tls] How to read ecp private key

I am generating a ECP key in following way. And now how do I get the private key?

TEE_Result gen_ec_keys(mbedtls_pk_context* pk, mbedtls_entropy_f_source_ptr f_source,
    __maybe_unused TEE_Param params[TEE_NUM_PARAMS])
{
    int ret = 1;
    mbedtls_entropy_context entropy;
    mbedtls_ctr_drbg_context ctr_drbg;
    const char* pers = "gen_key";
    TEE_Result res = TEE_SUCCESS;

    unsigned char output_buf[16000];
    memset(output_buf, 0, 16000);

    mbedtls_entropy_init(&entropy);
    mbedtls_ctr_drbg_init(&ctr_drbg);

    if ((ret = mbedtls_entropy_add_source(&entropy, f_source,
        NULL, 48,
        MBEDTLS_ENTROPY_SOURCE_STRONG)) != 0) {
        params[2].value.a = 1;
        goto exit;
    }

    if ((ret = mbedtls_ctr_drbg_seed(&ctr_drbg, f_entropy, &entropy,
        (const unsigned char*)pers,
        strlen(pers))) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_ctr_drbg_seed returned -0x%04x\n",
            (unsigned int)-ret);
        params[2].value.a = 2;
        goto exit;
    }

    if ((ret = mbedtls_pk_setup(pk,
        mbedtls_pk_info_from_type(MBEDTLS_PK_ECKEY))) != 0) {
        EMSG(" failed\n  !  mbedtls_pk_setup returned -0x%04x", (unsigned int)-ret);
        params[2].value.a = 3;
        goto exit;
    }

    if ((ret = mbedtls_ecp_gen_key(MBEDTLS_ECP_DP_SECP384R1, mbedtls_pk_ec(*pk),
        mbedtls_ctr_drbg_random, &ctr_drbg)) != 0) {
        EMSG(" failed\n  !  mbedtls_ecp_gen_key returned -0x%04x",
            (unsigned int)-ret);
        params[2].value.a = 4;
        goto exit;
    }

exit:
    mbedtls_ctr_drbg_free(&ctr_drbg);
    mbedtls_entropy_free(&entropy);
    return res;
}
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