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Jun 21, 2016

Verification of Integrity and Consistency for Cloud Object Storage (VICOS)

What is VICOS?

VICOS is a system for verification of integrity and consistency of cloud object storage. It enables a group of mutually trusting clients to detect data-integrity and consistency violations when accessing cloud storage. In particular, it aims at services where multiple clients cooperate on data stored remotely on a potentially misbehaving service. VICOS enforces the consistency notion of fork-linearizability, supports wait-free client semantics for most operations, and has low computation and communication overhead.

The VICOS system protects data stored on cloud (object) storage services, such as OpenStack Swift or Amazon S3.

The implementation of VICOS follows the design presented and published in the paper:

  • Marcus Brandenburger, Christian Cachin, Nikola Knezevic: Don't trust the cloud, verify: integrity and consistency for cloud object stores. SYSTOR 2015. DOI 10.1145/2757667.2757681

Architecture and components

The following diagram shows the architecture of VICOS:

VICOS architecture

The VICOS system consists of three components:

  1. A cloud object store (COS) service, as offered by commercial providers. It maintains the object data (bulk data) stored by the clients using VICOS.

  2. The VICOS server that runs remotely as a cloud services accessed by the VICOS client; it stores integrity-specific metadata of the object data being outsourced to the cloud storage service. The metadata is protected through the AIP protocol for a simple kay-value store.

  3. The VICOS client enables clients to access the cloud storage service and transparently protect the integrity and consistency of their object data. It exposes the cloud object store interface to a client application. During each operation, the VICOS client consults the cloud object store (using a COS API) for the object data itself and the VICOS server for integrity-specific metadata (through an AIP client). The integrity-specific metadata consists of an unique key of an object in the COS and its cryptographic hash.

The cloud object store and the VICOS server are both in the untrusted domain; they may, in fact, collude together against the clients.

VICOS is written in Java, which is required to run the client and the server.

Getting started

The following steps guide you through the build phase and configuration, for deploying and running your own instance of VICOS (client and server) on your machine.


VICOS can be installed and run on any platform with at least Java 8 and Gradle version 2.13 or newer. You can verify your platform using the following command:

$ gradle -version

Gradle 2.14

Build time:   2016-06-14 07:16:37 UTC
Revision:     cba5fea19f1e0c6a00cc904828a6ec4e11739abc

Groovy:       2.4.4
Ant:          Apache Ant(TM) version 1.9.6 compiled on June 29 2015
JVM:          1.8.0_91 (Oracle Corporation 25.91-b14)
OS:           Linux 4.4.0-24-generic amd64

Furthermore, you need access to a cloud object store, such as Openstack Swift or an account for Amazon S3. For testing purpose we recommend to get a setup like Swift All in One.

Building VICOS

  1. Clone VICOS to your local environment from your terminal using

     $ git clone
  2. cd into this newly created directory

  3. Build VICOS by just calling the following command

     $ gradle installDist

This will build, test, and package the VICOS core library, the VICOS server, and the VICOS client CLI, a sample command-line interface application for demonstrating its functions . When the build has been successful, you find the outputs in the corresponding directories:

  • vicos/vicos-core/build/libs
  • vicos/vicos-server/build/install/vicos-server
  • vicos/vicos-client-cli/build/install/vicos-server

The developer guide section later explains how to integrate the VICOS core library into an application and how to extend it. The next section illustrates how to configure the sample client CLI application of VICOS included with the distribution.


NOTE -- Although support for RSA and DSA public-key signatures is in the protocol implementation, the corresponding key-management functions are not yet provided. The configuration therefore only explains how to use VICOS with HMAC keys.

The first step is to configure the VICOS server. Edit the file server.conf located in vicos-server/build/install/vicos-server/conf and change hostname and port in the server section to the IP address and port where the VICOS server will be reachable. Optionally, you can set the maximum number of pending operation at the server. Limiting the number of pending operations prevents the system from becoming unstable.

An example VICOS server configuration:

include "application"

akka {
  loglevel = "INFO"

vicos {
  server {
    hostname =
    port = 2775
  system {
    flowcontrol.pending-list-max-length = 128

Now configure and deploy the VICOS clients. For each client edit the file client.conf located in vicos-client-cli/build/install/vicos-client- cli/conf. An example client configuration is shown below. The parameters of the cos section are directly given to the jclouds library for configuring the BlobStore provider: here you need to define provider, endpoint, identity, and credential. For example, a local OpenStack Swift deployment (SAIO), one could have the parameters provider = "swift", endpoint = "", and identity = "account:user".

Additionally, specify the parameters of the VICOS server in the server section as defined earlier in the server.config file. This contains the hostname and a port where the server listens. Every client must have a unique name, which is specified here in identifier.

In order to run VICOS with the default signature implementation you need to generate an HMAC key that is used by all clients. This is a symmetric 128-bit key for HMAC-SHA1. The following command creates such a key and outputs it as a base64-formatted string:

$ openssl rand 16 -base64 

Insert the output in the hmac.key field of the client configuration.

An example VICOS client configuration:

include "application"
cos {
  provider = "PROVIDER"
  endpoint = "http://ADDR:PORT"
  identity = "ACCOUNT:USER"
  credential = "SECRET"

vicos {
  server {
    hostname = ""
    port = 2775

  client {
    identifier = "Client-001"

  system {
    signatures {
      type = "HMAC"
      // Shared key has to be created initially and distributed among all clients.
      // Create a key by running "openssl rand 16 -base64"
      hmac.key = "SECRET_AS_BASE64_STRING"

Running VICOS

You are now ready to start the VICOS server and use the VICOS client application to upload data to the cloud object store.

VICOS server

  • Start the VICOS server:

      $ cd vicos-server/build/install/vicos-server/
      $ ./bin/vicos-server
  • The server is now up and running, and waiting for initialization by a client (see below). The VICOS server console provides this set of commands:

    • help - List all commands usage
    • exit - Exits the shell and stops the server
    • status - Shows current server status
    • history - Shows history of all operations invoked so far
    • authenticators - Shows all authenticators
  • Moreover, the VICOS server provides also a set of commands to manipulate the data stored in the server. Those are for demonstration purpose only and simulate malicious behavior by the server. Note that any modification to the integrity-specific metadata are integrity violations and may be detected by the VICOS clients.

    Data manipulation commands:

    • get - Gets the value for a key
    • put - Stores a key-value pair
    • del - Deletes a value-key pair
    • list - Lists all keys

VICOS client

  • Start the VICOS client CLI:

      $ cd vicos-client-cli/build/install/vicos-client-cli/
      $ ./bin/vicos-client-cli
  • When the VICOS client starts, it tries to establish connections to the cloud storage server and the VICOS server. Before a group of clients may start using the system, a single user has to initialize the VICOS server using the init command. This command resets the VICOS server; all data stored previously will be lost.

  • You can now use the VICOS client CLI to store data in the VICOS-protected cloud storage service, using the following commands:


    • help - List all commands usage
    • exit - Exits the shell and stops the client

    Container operations:

    • createcontainer - Creates a container
    • deletecontainer - Deletes a container

    Blob operations:

    • putblob - Uploads a blob from source path
    • getblob - Downloads a blob to destination path
    • deleteblob - Deletes a blob
  • The following commands shows how to use the VICOS client; initializing the VICOS server, creating a container, uploading a file, and finally downloading the same file again.

    # init the VICOS server
    Server successfully initialized
    # create a container
    vicos-cli>createcontainer --container my_test_container
    Container successfully created
    # upload a file
    vicos-cli>putblob --container my_test_container --src /tmp/my_test_file
    Successfully uploaded my_test_container/my_test_file
    # download a file
    vicos-cli>getblob --container my_test_container --blob my_test_file --dest_path /tmp/download
    Successfully downloaded /tmp/download/my_test_file [35120 bytes]

Developer notes

Implementation details

The VICOS client uses the BlobStore interface of Apache jclouds for connecting to different cloud object stores (in the COS API). Jclouds offers a simplified API and supports different cloud providers such as Microsoft Azure Blob Storage, Amazon S3, or OpenStack Swift. Check the jclouds documentation for all supported cloud providers.

The VICOS server runs as a standalone web service, communicating with the VICOS client using the Akka framework. Akka is an event- driven framework which supports the actor model. The client as well as the VICOS code are implemented as actors within the framework. Actors are independent units which can only communicate by exchanging messages. Every actor has a mailbox that buffers all incoming messages. By default messages are processed in FIFO order by the actor. This allows the server protocol implementation to process all incoming messages sequentially and execute each protocol step atomically, that is, mutually exclusive with respect to all others. For more detailed information see the Akka documentation.

The VICOS server provides a simple key-value store (KVS) functionality, implemented by an authenticated dictionary (ADICT). The data objects themselves are outsourced to the cloud storage service, and the server maintains only integrity-specific metadata. More precisely, the KVS state is a map supporting GET, PUT, DEL, and LIST operations. The server stores the integrity-specific metadata of an object under the object's key in the cloud object store. The VICOS client translates object keys (the names of objects) before accessing the cloud object store. The translation appends the client identifier and a operation counter to the keys, and the client also stores them in the metadata of the object. This translation is needed in order to support two concurrent operations that access the same object and might interfere with each other. Translation creates distinct names for the object in the cloud object store and this prevents that the system gets into an inconsistent state.

The cryptographic signatures used in VICOS can be implemented in multiple ways. In the security model for VICOS, all clients trust each other, the server alone may act maliciously, and only clients issue digital signatures. Therefore, VICOS may use standard digital signatures, but also provides a simplified trust model with "signatures" provided by a message-authentication code (MAC). For many applications, where strong mutual trust exists among the clients, MACs suffice and will result in faster execution. On the other hand, this simplification renders the system more fragile and may expose it more easily to attacks by malicious clients. In particular, VICOS supports RSA and DSA signatures with 2048-bit keys and HMAC-SHA1 with 128-bit keys, all provided by the Java Cryptography Extension (JCE). The signature implementation can be selected in the configuration file, and the default choice is HMAC-SHA1.

Application integration

The VICOS client has been developed in a way that simplifies integration into existing applications to provide integrity protection. The VICOSBlobStore offers a simple object store API, adding Java Exceptions for signaling integrity and consistency violations as shown below:

public interface VICOSBlobStore {

    void init(Config config);

    InputStream getObject(String container, String name) throws KeyNotFoundException, IntegrityException;

    void createContainer(String container);

    void createObject(String container, String name, InputStream data, long length) throws IntegrityException;

    void deleteContainer(String container);

    void deleteBlob(String container, String name) throws KeyNotFoundException, IntegrityException;

    void dispose();

Note that the protocol implemented by VICOS is structured into an active and a passive phase. That is, once a method from VICOSBlobStore returns, the active phase ends and the passive phase continues asynchronously in the background. This allows client applications to continue without waiting for the passive phase to finish and therefore reduces latency for the clients.

Extending VICOS

VICOS uses a modular approach that allows developers to extend it with more functions, for example, to support more complex applications than a cloud object store. The developer only needs to implement the desired functionality by defining the state, a set of operations, and their compatibility relation. In VICOS there is already an implementation of a simple key-value store functionality, realized by the authenticated dictionary (ADICT).

There are two interfaces to support this: State and OperationProcessor. The State interface is a marker interface to model the protocol's state; its implementations must only provide an initialization function as follows:

public interface State {
  void init();

The OperationProcessor is an abstract class. Its instantiations must provide implementations of the methods query, authexec, refresh, isCompatible, and isUpdateOperation. They use parameters of type Operation and State:

public abstract class OperationProcessor<S extends State> {

  public abstract QueryResult query(S state, List<Operation> operation);

  public abstract AuthExecResult authexec(List<Operation> operation, Authenticator authenticator, Operations.Result result, AuxiliaryData auxiliaryData);

  public abstract S refresh(S state, Operation operation, AuxiliaryData auxiliaryData);

  public abstract boolean isCompatible(List<Operation> listOthers, Operation currentOperation);

  public abstract boolean isCompatible(Operation otherOperation, Operation currentOperation);

  public abstract boolean isUpdateOperation(Operation operation);

The KVS implementation in ADICT provides these methods with the KVSState class, which maintains a tree map for keys and values of type string, and with the KVSOperationProcessor class that executes KVS-specific operations on a KVSState.

Operations are described using Google's Protocol Buffers executed by the OperationProcessor.

This modular concept allows to extend the core of VICOS, the AIP protocol, and to tailor it to different applications.


  • Marcus Brandenburger, Christian Cachin, Nikola Knezevic: Don't trust the cloud, verify: integrity and consistency for cloud object stores. SYSTOR 2015. DOI 10.1145/2757667.2757681


The development has been supported in part by the European Commission in the ICT programme under contracts ICT-2009-257243 TCLOUDS, and through the Horizon 2020 Framework Programme (H2020-ICT-2014-1) under grant agreements number 644371 WITDOM and 644579 ESCUDO-CLOUD and in part by the Swiss State Secretariat for Education, Research and Innovation (SERI) under contracts number 15.0098 and 15.0087.


Copyright IBM Corp. 2016 All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


Verification of Integrity and Consistency for Cloud Object Storage




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