Use AttachThingPrincipal with an AWS SDK or CLI

    /// <summary>
    /// Attaches a certificate to an IoT Thing.
    /// </summary>
    /// <param name="thingName">The name of the Thing.</param>
    /// <param name="certificateArn">The ARN of the certificate to attach.</param>
    /// <returns>True if successful, false otherwise.</returns>
    public async Task<bool> AttachThingPrincipalAsync(string thingName, string certificateArn)
    {
        try
        {
            var request = new AttachThingPrincipalRequest
            {
                ThingName = thingName,
                Principal = certificateArn
            };

            await _amazonIoT.AttachThingPrincipalAsync(request);
            _logger.LogInformation($"Attached certificate {certificateArn} to Thing {thingName}");
            return true;
        }
        catch (Amazon.IoT.Model.ResourceNotFoundException ex)
        {
            _logger.LogError($"Cannot attach certificate - resource not found: {ex.Message}");
            return false;
        }
        catch (Exception ex)
        {
            _logger.LogError($"Couldn't attach certificate to Thing. Here's why: {ex.Message}");
            return false;
        }
    }

//! Attach a principal to an AWS IoT thing.
/*!
  \param principal: A principal to attach.
  \param thingName: The name for the thing.
  \param clientConfiguration: AWS client configuration.
  \return bool: Function succeeded.
 */
bool AwsDoc::IoT::attachThingPrincipal(const Aws::String &principal,
                                       const Aws::String &thingName,
                                       const Aws::Client::ClientConfiguration &clientConfiguration) {
    Aws::IoT::IoTClient client(clientConfiguration);
    Aws::IoT::Model::AttachThingPrincipalRequest request;
    request.SetPrincipal(principal);
    request.SetThingName(thingName);
    Aws::IoT::Model::AttachThingPrincipalOutcome outcome = client.AttachThingPrincipal(
            request);
    if (outcome.IsSuccess()) {
        std::cout << "Successfully attached principal to thing." << std::endl;
    }
    else {
        std::cerr << "Failed to attach principal to thing." <<
                  outcome.GetError().GetMessage() << std::endl;
    }

    return outcome.IsSuccess();
}

To attach a certificate to your thing

The following attach-thing-principal example attaches a certificate to the MyTemperatureSensor thing. The certificate is identified by an ARN. You can find the ARN for a certificate in the AWS IoT console.

aws iot attach-thing-principal \
    --thing-name MyTemperatureSensor \
    --principal arn:aws:iot:us-west-2:123456789012:cert/2e1eb273792174ec2b9bf4e9b37e6c6c692345499506002a35159767055278e8

This command produces no output.

For more information, see How to Manage Things with the Registry in the AWS IoT Developers Guide.

    /**
     * Attaches a certificate to an IoT Thing asynchronously.
     *
     * @param thingName The name of the IoT Thing.
     * @param certificateArn The ARN of the certificate to attach.
     *
     * This method initiates an asynchronous request to attach a certificate to an IoT Thing.
     * If the request is successful, it prints a confirmation message and additional information about the Thing.
     * If an exception occurs, it prints the error message.
     */
    public void attachCertificateToThing(String thingName, String certificateArn) {
        AttachThingPrincipalRequest principalRequest = AttachThingPrincipalRequest.builder()
            .thingName(thingName)
            .principal(certificateArn)
            .build();

        CompletableFuture<AttachThingPrincipalResponse> future = getAsyncClient().attachThingPrincipal(principalRequest);
        future.whenComplete((attachResponse, ex) -> {
            if (attachResponse != null && attachResponse.sdkHttpResponse().isSuccessful()) {
                System.out.println("Certificate attached to Thing successfully.");

                // Print additional information about the Thing.
                describeThing(thingName);
            } else {
                Throwable cause = ex != null ? ex.getCause() : null;
                if (cause instanceof IotException) {
                    System.err.println(((IotException) cause).awsErrorDetails().errorMessage());
                } else if (cause != null) {
                    System.err.println("Unexpected error: " + cause.getMessage());
                } else {
                    System.err.println("Failed to attach certificate to Thing. HTTP Status Code: " +
                        attachResponse.sdkHttpResponse().statusCode());
                }
            }
        });

        future.join();
    }

suspend fun attachCertificateToThing(
    thingNameVal: String?,
    certificateArn: String?,
) {
    val principalRequest =
        AttachThingPrincipalRequest {
            thingName = thingNameVal
            principal = certificateArn
        }

    IotClient.fromEnvironment { region = "us-east-1" }.use { iotClient ->
        iotClient.attachThingPrincipal(principalRequest)
        println("Certificate attached to $thingNameVal successfully.")
    }
}

class IoTWrapper:
    """Encapsulates AWS IoT actions."""

    def __init__(self, iot_client, iot_data_client=None):
        """
        :param iot_client: A Boto3 AWS IoT client.
        :param iot_data_client: A Boto3 AWS IoT Data Plane client.
        """
        self.iot_client = iot_client
        self.iot_data_client = iot_data_client

    @classmethod
    def from_client(cls):
        iot_client = boto3.client("iot")
        iot_data_client = boto3.client("iot-data")
        return cls(iot_client, iot_data_client)

    def attach_thing_principal(self, thing_name, principal):
        """
        Attaches a certificate to an AWS IoT thing.

        :param thing_name: The name of the thing.
        :param principal: The ARN of the certificate.
        """
        try:
            self.iot_client.attach_thing_principal(
                thingName=thing_name, principal=principal
            )
            logger.info("Attached principal %s to thing %s.", principal, thing_name)
        except ClientError as err:
            if err.response["Error"]["Code"] == "ResourceNotFoundException":
                logger.error("Cannot attach principal. Resource not found.")
                return
            logger.error(
                "Couldn't attach principal to thing. Here's why: %s: %s",
                err.response["Error"]["Code"],
                err.response["Error"]["Message"],
            )
            raise