Support For Sign operation in KMS Enclave SDK Apis

[adp-nume]

Hi , We have modified AWS Nitro C SDK to Support Sign operations within Enclave. It works well without attestation document and gives the desired output. But when PCR value is added in KMS policy,
Like below,

        {
            "Sid": "Enable Enclave access",
            "Effect": "Allow",
            "Principal": "*",
            "Action": [
                "kms:Put*",
                "kms:Sign"
            ],
            "Resource": "*",
            "Condition": {
                "StringEqualsIgnoreCase": {
                    "kms:RecipientAttestation:ImageSha384": "700d541ae32c8b789a1f266aae221387c6910dc3bdc78cc55b02c2711ce0b08b54156ef45d6c733702d79ed35e59c6fb"
                }
            }
        }

It would end up giving following error.

Got non-200 answer from KMS: 400 - {"__type":"AccessDeniedException","Message":"User: arn:aws:sts::3598780XXXXX:assumed-role/nume-enclave-role/i-0b11f180a81f2fea2 is not authorized to perform: kms:Sign on resource: arn:aws:kms:us-east-1:3598780XXXXX:key/fb506b65-0192-4881-b1d2-8ce98aXXXXX because no resource-based policy allows the kms:Sign action"}

Could the team please add the support for Attestation document check verification for KMS Sign and PutKeyPolicy APIs (at KMS Backend)
We have a use case where we want to securely Sign the message as well as update update KMS policy from enclave.

[richardfan1126]

Are you running the enclave in debug mode?
If so, the image digest is always 000000000000000000000000000000000000000000000000

[adp-nume]

Hello Richard and thanks for your response.
We are trying it non Debug mode
nitro-cli run-enclave --eif-path merkle-tree-build.eif --cpu-count 12 --memory 15436

as visible in cloudtrail log, request is "AccessDenied"

{
    "eventVersion": "1.08",
    "userIdentity": {
        "type": "AssumedRole",
        "principalId": "AROAVHSTH55KT5XXXXXX:i-0b11f180a81fXXXXX",
        "arn": "arn:aws:sts::359878XXXXX:assumed-role/nume-enclave-role/i-0b11f180a81XXXXX",
        "accountId": "3598780XXXXX",
        "accessKeyId": "ASIAVHSTH55KWXXXXX",
        "sessionContext": {
            "sessionIssuer": {
                "type": "Role",
                "principalId": "AROAVHSTH55KT5DRN767X",
                "arn": "arn:aws:iam::3598780XXXXX:role/nume-enclave-role",
                "accountId": "3598780XXXXX",
                "userName": "nume-enclave-role"
            },
            "webIdFederationData": {},
            "attributes": {
                "creationDate": "2023-11-08T02:29:31Z",
                "mfaAuthenticated": "false"
            },
            "ec2RoleDelivery": "1.0"
        }
    },
    "eventTime": "2023-11-08T03:29:53Z",
    "eventSource": "kms.amazonaws.com",
    "eventName": "Sign",
    "awsRegion": "us-east-1",
    "sourceIPAddress": "52.207.XX.116",
    "userAgent": "aws-nitro_enclaves-sdk-c/v0.2.1-unknown",
    "errorCode": "AccessDenied",
    "errorMessage": "User: arn:aws:sts::3598780XXXXX:assumed-role/nume-enclave-role/i-0b11f180a81f2fea2 is not authorized to perform: kms:Sign on resource: arn:aws:kms:us-east-1:359878094677:key/fb506b65-0192-4881-b1d2-8ce98aXXXXX because no resource-based policy allows the kms:Sign action",
    "requestParameters": null,
    "responseElements": null,
    "requestID": "698bf67e-10c0-4894-8dce-18dbc2cce02b",
    "eventID": "f9e9986e-a01d-4cd4-8c11-522d701020a5",
    "readOnly": true,
    "eventType": "AwsApiCall",
    "managementEvent": true,
    "recipientAccountId": "3598780XXXXX",
    "eventCategory": "Management",
    "tlsDetails": {
        "tlsVersion": "TLSv1.3",
        "cipherSuite": "TLS_AES_256_GCM_SHA384",
        "clientProvidedHostHeader": "kms.us-east-1.amazonaws.com"
    }
}

[adp-nume]

Cloud trail logs of previously successful call when KMS policy doesn't contain Attestation value conditions

{
    "eventVersion": "1.08",
    "userIdentity": {
        "type": "AssumedRole",
        "principalId": "AROAVHSTH55KT5DXXXXX:i-0b11f180a81XXXXX",
        "arn": "arn:aws:sts::3598780XXXXX:assumed-role/nume-enclave-role/i-0b11f180a81XXXXX",
        "accountId": "3598780XXXXX",
        "accessKeyId": "ASIAVHSTH55KZNZTJMNW",
        "sessionContext": {
            "sessionIssuer": {
                "type": "Role",
                "principalId": "AROAVHSTH55KT5DRN767X",
                "arn": "arn:aws:iam::3598780XXXXX:role/nume-enclave-role",
                "accountId": "3598780XXXXX",
                "userName": "nume-enclave-admin"
            },
            "webIdFederationData": {},
            "attributes": {
                "creationDate": "2023-10-30T08:03:45Z",
                "mfaAuthenticated": "false"
            },
            "ec2RoleDelivery": "1.0"
        }
    },
    "eventTime": "2023-10-30T08:56:38Z",
    "eventSource": "kms.amazonaws.com",
    "eventName": "Sign",
    "awsRegion": "us-east-1",
    "sourceIPAddress": "52.207.XX.XXX",
    "userAgent": "aws-nitro_enclaves-sdk-c/v0.2.1-unknown",
    "requestParameters": {
        "messageType": "DIGEST",
        "keyId": "fb506b65-0192-4881-b1d2-8ce98aXXXXX",
        "signingAlgorithm": "ECDSA_SHA_256"
    },
    "responseElements": null,
    "requestID": "65b8599b-07b8-40d7-99e8-a791ef9c86cc",
    "eventID": "7f9f1342-6cfd-44af-b4aa-7fb3033ee51a",
    "readOnly": true,
    "resources": [
        {
            "accountId": "359878094677",
            "type": "AWS::KMS::Key",
            "ARN": "arn:aws:kms:us-east-1:359878XXXXX:key/fb506b65-0192-4881-b1d2-8ce98aXXXXX"
        }
    ],
    "eventType": "AwsApiCall",
    "managementEvent": true,
    "recipientAccountId": "359878XXXXX",
    "eventCategory": "Management",
    "tlsDetails": {
        "tlsVersion": "TLSv1.3",
        "cipherSuite": "TLS_AES_256_GCM_SHA384",
        "clientProvidedHostHeader": "kms.us-east-1.amazonaws.com"
    }
}

I am making sure that Attestation documents are getting passed in KMS C SDK Code.

int aws_kms_sign_blocking(
    struct aws_nitro_enclaves_kms_client *client,
    const struct aws_string *key_id,
    enum aws_signing_algorithm signing_algorithm,
    const struct aws_byte_buf *message,
    enum aws_message_type message_type,
    struct aws_string **signature /* TODO: err_reason */) {
    AWS_PRECONDITION(client != NULL);
    AWS_PRECONDITION(key_id != NULL);
    AWS_PRECONDITION(signing_algorithm != AWS_SA_UNINITIALIZED);
    AWS_PRECONDITION(message != NULL);
    AWS_PRECONDITION(signature != NULL);

    struct aws_string *response = NULL;
    struct aws_string *request = NULL;
    struct aws_kms_sign_response *response_structure = NULL;
    struct aws_kms_sign_request *request_structure = NULL;
    int rc = 0;

    request_structure = aws_kms_sign_request_new(client->allocator);
    if (request_structure == NULL) {
        return AWS_OP_ERR;
    }

    request_structure->recipient = aws_recipient_new(client->allocator);
    if (request_structure->recipient == NULL) {
        goto err_clean;
    }

    aws_byte_buf_init_copy(&request_structure->message, client->allocator, message);
    request_structure->key_id = aws_string_clone_or_reuse(client->allocator, key_id);
    request_structure->signing_algorithm = signing_algorithm;
    request_structure->message_type = message_type;

    rc = aws_attestation_request(
        client->allocator, client->keypair, &request_structure->recipient->attestation_document);
    if (rc != AWS_OP_SUCCESS) {
        goto err_clean;
    }

    request = aws_kms_sign_request_to_json(request_structure);
    const char *json_c_str = aws_string_c_str(request);
    //fprintf(stdout,"JSON string: %s\n", json_c_str);
    if (request == NULL) {
        goto err_clean;
    }

    rc = s_aws_nitro_enclaves_kms_client_call_blocking(client, kms_target_sign, request, &response);
    if (rc != 200) {
        fprintf(stderr, "Got non-200 answer from KMS: %d - %s\n", rc, aws_string_c_str(response));
        goto err_clean;
    }
    const char *json_c_str_2 = aws_string_c_str(response);
	rc = AWS_OP_SUCCESS;
    struct json_object *parsed_json = json_tokener_parse(json_c_str_2);
	struct json_object *sig_obj = json_object_object_get(parsed_json, "Signature");	
	const char *sig = json_object_get_string(sig_obj);
	*signature = aws_string_new_from_array(client->allocator,(const uint8_t *)sig, strlen(sig));
    aws_kms_sign_request_destroy(request_structure);
    aws_kms_sign_response_destroy(response_structure);
    aws_string_destroy(request);
    aws_string_destroy(response);

    return rc;
err_clean:
    aws_kms_sign_request_destroy(request_structure);
    aws_kms_sign_response_destroy(response_structure);
    aws_string_destroy(request);
    aws_string_destroy(response);
    return AWS_OP_ERR;
}

[cooli7wa]

I face the same issue. I think kms sign action don't support pcr config.

kms:RecipientAttestation:ImageSha384 only support:
Decrypt
DeriveSharedSecret
GenerateDataKey
GenerateDataKeyPair
GenerateRandom

Here is the doc:
https://docs.aws.amazon.com/kms/latest/developerguide/services-nitro-enclaves.html