Virtual HSM

Virtual HSM (VHSM) is a software suite for storing and manipulating secret data outside the virtualized application environment. While HSM is a physical device connected to the computer, this software provides HSM functionality through the PKCS#11 API in virtual environment based on OpenVZ container technology.

Structure

Fig. 1 — VHSM overview

The architecture of the Virtual HSM is shown in the Fig. 1. The project consists of the following key components:

VHSM virtual environment

VHSM VE is the isolated environment that contains the VHSM server and secure storage. The server performs operations on secret data and storage keeps encrypted user data

Transport layer

transport exchanges data between client and server virtual environments; based on Netlink

Client virtual environment

client API and accompanying utilities for accessing the VHSM server from a client environment

Details

VHSM virtual environment

Fig. 2 — Data encryption in the VHSM

The VHSM VE contains the secure storage which is the database that stores the sensitive data in the encrypted form while other data (refer the table below) are stored unencrypted. The encryption key (master key) is generated from the user password using PBKDF2 and not stored in database. Utilizing PBKDF2 reduces the brute-force attack rate significantly if the database is compromised. The following table lists data that stored in the VHSM:

Data	Type	Encrypted	Comments
Key ID	string	no	keys in the VHSM can be accessed by their string-ID
Key data	blob	yes	encrypted key data (see the Fig. 2)
Key attributes	blob	no	data structure that holds PKCS#11 key attributes
Import date	date-time	no	key creation date
User name	string	no	VHSM user login
User auth key	blob	no	binary data for the user password check
User type	integer	no	CKU_USER or CKU_SO from the PKCS#11 standard
User flags	integer	no	Flags determine access rights (read, write, delete) for the user
Password derivation params	integer	no	PBKDF2 params used for master key derivation (refer the Fig. 3)
User VEIDs	integer	no	List of VEIDs where the user can get access to the VHSM

Fig. 3 — User registration

Fig. 4 — User authentication

The VHSM server is responsible for user authentication, interaction with the secure storage, and performing cryptographic operations. A client must be registered in the system to work with VHSM. VHSM admins (users with CKU_SO user type) can register new clients. These users can be both a real person or program. Anyway communication with the VHSM performed through the API. When the client is registered in the system a 256-bit authentication key is generated and encrypted in GCM mode using the master key. The GCM mode guarantees integrity and confidentiality of the user data and therefore makes it possible to authenticate the encryption key derived from the user password. The VHSM uses this feature for user authentication. Registration process is shown in the Fig. 3.

A user is authenticated (Fig. 4) using the login/password pair and the container ID (VEID) where authentication request is received from. When a user is registered it's bound to the set of containers where one can get access to the VHSM from. If the user attempts to access to the VHSM from a non-authorized container the request is refused.

Transport

The data exchanging protocol between clients and the VHSM is written in the Protocol Buffers language. Inter-container communication is based on Netlink, so the OS kernel contains the module that passes messages between virtual environments while VEs contain the transport API library which interacts with this module via Netlink sockets. The kernel module identifies containers by their VEIDs that are provided by OpenVZ. The VHSM environment ID is passed as a parameter to the module. Messages from other containers are routed to that VEID. The passed message contains serialized protobuf data and the header with the message type, container VEID and process PID (actually it's the unique socket ID generated by the kernel). Meaning of these parameters depends on the transfer direction. If the message comes from a client container then the module checks its VEID and forwards the message to the VHSM VE if the container is allowed to send requests to the VHSM. When the kernel module forwards the message it sets the VEID in the message header to the sender VEID and the PID to the sender process PID derived from the Netlink message header. So the VHSM can address the response to the client. If the message is transferred from the VHSM then the VEID and PID in the message header are set by the VHSM itself. It's possible because the VHSM sends responses to the client requests only and never initiates communication with a client.

Client virtual environment

Fig. 5 — PKCS#11 slots and VHSM users

The client part of the VHSM consists from the API-library and some auxiliary tools. The VHSM API library partially implements PKCS#11 v2.20 standard and provides some additional functions for the user management. Here the list of VHSM-specific features:

• Supported object classes

  Currently only CKO_SECRET_KEY with key types CKK_GENERIC_SECRET, CKK_AES is supported

• Supported mechanisms
  • encryption: CKM_AES_CTR (actually it's AES-GCM)
  • digest: CKM_SHA_1
  • signing: CKM_SHA_1_HMAC
  • key generation: CKM_GENERIC_SECRET_KEY_GEN
• Unsupported functions
  • slot management: C_InitToken, C_InitPin, C_SetPin, C_WaitForSlotEvent
  • session management: C_GetOperationState, C_SetOperationState
  • object management: C_CopyObject
  • signing: C_SignRecoverInit, C_SignRecover, C_VerifyRecoverInit, C_VerifyRecover
  • key management: C_GenerateKeyPair, C_WrapKey, C_UnwrapKey, C_DeriveKey
  • all dual functions
• Additional VHSM-specific functions
  • VHSM_GetUserSlot returns slotID for the specified user
  • VHSM_ImportSecretKey imports secret key with the default template
  • VHSM_GetObjectById returns object handle without use of C_FindObjects

  Following functions can be called only by CKU_SO user:

  • VHSM_CreateUser creates a new VHSM user using the given template
  • VHSM_ModifyUser allows to set user flags and VEIDs for the specified user
  • VHSM_DeleteUser removes user and all associated data by user name
• User flags

  VHSM users have associated flags that determine which functions can be called by the user

  • VF_READ — read-only functions (like encryption, signing, etc)
  • VF_CREATE — user can create objects
  • VF_MODIFY — user can change objects attributes
  • VF_DELETE — user can destroy objects
  • VF_CREATE_EXTENDED — user can create CKU_SO users. Only root user (created during VHSM installation) can has this flag
• PKCS#11 slot / token selection

  In the VHSM slots and tokens are abstract because users are authenticated by login/password pair. In fact, C_GetSlotList called from VE returns the list of usernames that can be used from this VE. Opening session with one of these slots means that in this session user wants to use username associated with this slot. Fig. 5 shows how it works.

• Multithreading

  Each thread must call C_Initialize to work with VHSM, as in PKCS#11, but if some thread logs into VHSM then other threads with open VHSM sessions do not automatically log into VHSM as it should be according to the standard;

OpenSSL can act as a layer between a client application and the VHSM API library – it's achieved by OpenSSL engines – and makes it possible to use a cryptographic device (or VHSM) without modifying the code of the client program. You may try pkcs#11 OpenSSL engines like engine_pkcs11, ibmpkcs11 with VHSM.

Building

Install the following packages to build the project on your system. On CentOS use EPEL repositories. In addition, VHSM uses some gcc-specific features, so gcc 4+ is required.

Package	Debian	CentOS
CMake 2.8	cmake	cmake28
OpenVZ kernel headers	linux-headers-2.6.32-5-openvz	vzkernel-headers
Google protobuf compiler	protobuf-compiler	protobuf-compiler
libprotobuf	libprotobuf-dev	protobuf-devel
crypto++ 5.6.2	libcrypto++-dev	cryptopp-devel
SQLite 3	libsqlite3-dev	sqlite-devel
cppunit (optional)	libcppunit-dev	cppunit-devel

To build the project just run cmake and then make. You may build binary packages with the command make pkg.

Installation

You may use binary packages or install manually:

• VHSM VE: vhsm
• Client VE: libvhsmapi.a, vhsm_admin
• Host: vhsm_transport.ko, vhsm_admin

Usage

To run VHSM perform the following steps:

1. Insert transport module: insmod vhsm_transport.ko vhsm_veid=[veid] where [veid] is VEID of the VHSM container
2. Start VHSM in the VHSM container: vhsm /path/to/storage. You can init secure storage and create root-user on the first run
3. Add VHSM users from host: vhsm_admin user create <admin login> <admin password> [options...]. Run vhsm_admin help for details.
4. Now you can use VHSM from client VEs