OPTEE-OS+mbedTLS环境下，如何用ECC私钥二进制值创建CSR？

I'm building a CSR using OPTEE-OS and mbedTLS, and I've hit a roadblock with loading the private key. Here's my workflow so far:

1. Generated an EC key pair via GlobalPlatform API:

   res = TEE_AllocateTransientObject( TEE_TYPE_ECDSA_KEYPAIR, DSEC_ECDSA_SHA256_KEY_BITS, &key_pair);
   if (res != TEE_SUCCESS) { return res; }
   

2. Extracted the raw private key binary value:

   res = TEE_GetObjectBufferAttribute( key_pair, TEE_ATTR_ECC_PRIVATE_VALUE, buffer, &bufferlen);
   if (res != TEE_SUCCESS) { return res; }
   

3. Tried parsing this value with mbedTLS to set up a key context:

   mbedtls_pk_context priv_key;
   mbedtls_pk_init(&priv_key);
   ret = mbedtls_pk_parse_key( &priv_key, key, size, NULL , 0 );
   

The extracted value is a raw binary array (no PEM headers/footers or ASN.1 formatting), and I'm getting the error: -15616: PK - Invalid key tag or value.

Question: Is it possible to create an mbedtls_pk_context using just this raw private key binary?

Got it, let’s fix this. The problem is that mbedtls_pk_parse_key is designed to handle formatted keys—either PEM-encoded strings or DER-encoded ASN.1 structures. But what you’re pulling from OP-TEE is just the raw EC private key scalar (the big integer that’s the core of the private key), not a properly formatted key blob.

Luckily, you absolutely can create an mbedtls_pk_context from this raw value—you just need to build the EC key structure manually instead of relying on the parser. Here’s how to do it step by step:

Step 1: Set up the mbedTLS PK context for EC #

First, initialize the PK context and configure it to use an EC key:

mbedtls_pk_context priv_key;
mbedtls_ecp_keypair *ec_key;
int ret;

mbedtls_pk_init(&priv_key);
// Tell the PK context we're using an EC key
if ((ret = mbedtls_pk_setup(&priv_key, mbedtls_pk_info_from_type(MBEDTLS_PK_ECKEY))) != 0) {
    // Clean up and return error
    mbedtls_pk_free(&priv_key);
    return ret;
}

// Get a pointer to the underlying EC keypair structure
ec_key = mbedtls_pk_ec(priv_key);

Step 2: Load the correct EC curve parameters #

You used DSEC_ECDSA_SHA256_KEY_BITS in OP-TEE, which corresponds to the secp256r1 (NIST P-256) curve. Load that into the EC group:

if ((ret = mbedtls_ecp_group_load(&ec_key->grp, MBEDTLS_ECP_DP_SECP256R1)) != 0) {
    mbedtls_pk_free(&priv_key);
    return ret;
}

Step 3: Import the raw private key scalar #

Copy the raw binary value from your OP-TEE buffer into the EC key's private value (d). Make sure the buffer length matches the curve's private key size (32 bytes for secp256r1):

// Verify the private key length matches the curve's expected size
if (bufferlen != mbedtls_ecp_group_get_size(&ec_key->grp)) {
    mbedtls_pk_free(&priv_key);
    return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
}

// Read the raw binary into the mbedTLS multi-precision integer (MPI)
mbedtls_mpi_read_binary(&ec_key->d, buffer, bufferlen);

Step 4: Compute the public key (required for CSR) #

To generate a CSR, you need the public key too. Instead of extracting it from OP-TEE (though you could), you can compute it directly from the private key in mbedTLS:

if ((ret = mbedtls_ecp_mul(&ec_key->grp, &ec_key->Q, &ec_key->d, &ec_key->grp.G, NULL, NULL)) != 0) {
    mbedtls_pk_free(&priv_key);
    return ret;
}

Step 5: Generate your CSR #

Now your priv_key context is fully set up and ready to use with mbedTLS's CSR generation functions. You can use mbedtls_x509write_csr_init, mbedtls_x509write_csr_set_key, and the rest of the CSR writing API to build your certificate signing request.

Quick Notes: #

• Double-check that the EC curve in mbedTLS exactly matches what you used in OP-TEE. If you ever use a different curve (like secp384r1), you’ll need to swap out MBEDTLS_ECP_DP_SECP256R1 for the corresponding mbedTLS curve constant.
• Always validate the raw key length—mismatched lengths will cause hard-to-debug errors later.
• Don’t forget to call mbedtls_pk_free(&priv_key) when you’re done to avoid memory leaks.

内容的提问来源于stack exchange，提问作者Stoogy