Dear Mbed TLS Support Team,

I am currently working on a project using Mbed TLS version 3.6 on FreeRTOS, and I am encountering an issue with handling multiple CA certificates during the TLS handshake process. My device has a set of built-in certificates, and I need to try each certificate one by one to establish a successful connection to my server. However, I am facing difficulties in this process.

### System Information: - **Mbed TLS version**: 3.6 - **Operating System**: FreeRTOS - **Configuration**: Default - **Compiler and Options**: N/A (using default configuration)

### Expected Behavior: The first certificate (e.g., `cdotroot.cer`) cannot be verified by Mbed TLS, while the correct certificate should successfully establish a connection.

### Actual Behavior: Both the incorrect and correct CA certificates fail to establish a connection successfully.

### Steps to Reproduce: Here is a sample of my code: ```c static int load_and_verify_certificates(int conn_id, uint8_t *cert_buffer, size_t buffer_size) { int ret; bool connection_established = false; uint32_t cert_index = 0;

while (!connection_established && cert_index < MAX_CERT_COUNT) { size_t cert_size = buffer_size;

// Free and initialize the certificate context mbedtls_x509_crt_free(&cacert[conn_id]); mbedtls_x509_crt_init(&cacert[conn_id]);

// Load the built-in certificate ret = try_built_in_certificate(cert_buffer, &cert_size, cert_index); if (ret == CERT_ERR_INDEX_OUT_OF_RANGE) { break; } if (ret != CERT_SUCCESS) { cert_index++; continue; }

// Parse the certificate cert_buffer[cert_size] = '\0'; ret = mbedtls_x509_crt_parse(&cacert[conn_id], cert_buffer, cert_size + 1); if (ret < 0) { cert_index++; continue; }

// Set the certificate chain mbedtls_ssl_conf_ca_chain(&conf[conn_id], &cacert[conn_id], NULL);

// Perform the TLS handshake ret = mbedtls_ssl_handshake(&ssl[conn_id]); if (ret == 0) { uint32_t flags = mbedtls_ssl_get_verify_result(&ssl[conn_id]); if (flags == 0) { connection_established = true; // Cache the certificate cache_certificate(cert_buffer, cert_size, cert_index); break; } } else { LOGD("Failed to perform handshake with certificate index %d, error: -0x%x\n", cert_index, -ret); }

// Reset the SSL session ret = mbedtls_ssl_session_reset(&ssl[conn_id]); if (ret != 0) { LOGD("Failed to reset SSL session, error: -0x%x\n", -ret); return ret; }

cert_index++; }

return connection_established ? 0 : -1; } ```

### Additional Information: Here are the logs: ``` Reading certificate 'cdotroot.cer' at address 0x08100650, size: 1348 Failed to perform handshake with certificate index 0, error: -0x2700 Reading certificate 'digicertroot.cer' at address 0x08100B94, size: 1360 Failed to perform handshake with certificate index 1, error: -0x7300 ... ```

I suspect that the issue may be related to the limited RAM space available on my device. I am looking for guidance on how to properly iterate through and verify the built-in certificates within these constraints. Any suggestions or best practices for handling multiple certificates in such an environment would be greatly appreciated.

Thank you for your assistance.

Best regards,

[Tony]

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