Securing Embedded Smart Cameras with Trusted Computing

Abstract

Camera systems are used in many applications including video surveillance for crime prevention and investigation, traffic monitoring on highways or building monitoring and automation. With the shift from analog towards digital systems, the capabilities of cameras are constantly increasing. Today's smart camera systems come with considerable computing power, large memory, and wired or wireless communication interfaces. With onboard image processing and analysis capabilities, cameras not only open new possibilities but also raise new challenges. Often overlooked are potential security issues of the camera system. The increasing amount of software running on the cameras turns them into attractive targets for attackers. Therefore, the protection of camera devices and delivered data is of critical importance. In this work we present an embedded camera prototype that uses Trusted Computing to provide security guarantees for streamed videos. With a hardware-based security solution, we ensure integrity, authenticity, and confidentiality of videos. Furthermore, we incorporate image timestamping, detection of platform reboots, and reporting of the system status. This work is not limited to theoretical considerations but also describes the implementation of a prototype system. Extensive evaluation results illustrate the practical feasibility of the approach.

1. Introduction and Motivation

Video cameras are present in many parts of our daily lives. In surveillance applications they are used to monitor train stations, airports, or public places in cities [1]. Enforcement applications and traffic monitoring [2] are another application area where camera systems are frequently used. In all those applications multiple, spatially distributed cameras are used to cover large areas. But the deployment of cameras is no longer limited to public places. An example where cameras are installed in private environments is assisted living. Elderly people are monitored in their homes to detect unusual behavior such as the collapse of persons [3].

Technologically, camera systems have evolved from analog to fully digital and sometimes even smart systems. Modern cameras not only deliver videos in digital form, but are also capable of on-board processing and analysis of captured images. Together with increasing computing power, the amount of software running on cameras is also growing. Nowadays, many smart cameras are equipped with powerful embedded operating systems such as uClinux. These systems come with a variety of libraries and many applications and system services. The emerging field of visual sensor networks [4] aims to miniaturize cameras and turn them into truly pervasive sensors [5]. As part of these efforts, many cameras no longer use wired network connectivity but come with wireless interfaces which eases deployment significantly. It is expected that the wireless interfaces and the relatively large software stack will make smart cameras an attractive target for attackers. Considering the sensitivity of video data, appropriate countermeasures must be taken to provide security guarantees for information that is coming from a camera.

One way to ensure that sensitive data cannot be accessed by unauthorized parties, is to remove this data from the video stream before it leaves the camera. In the computer vision community, several approaches exist that, for example, detect and remove people's faces or vehicle license plates [6–8]. What is rarely discussed, is how such mechanisms can be integrated with established IT security techniques and the underlying platform. We, however, argue that any high-level security and privacy mechanism for visual sensor networks is meaningless without taking a holistic approach towards securing the entire camera device. To fill this gap, we apply Trusted Computing (TC) techniques and a dedicated microchip called Trusted Platform Module (TPM). Being a hardware-based security solution, TC is designed to achieve higher security than pure software solutions could do. Furthermore, TPMs are cheap, readily available, and implement a set of well defined and widely reviewed security primitives. Alternatives to TC would be hardware security solutions like ARM TrustZone or TI M-Shield which are integrated into several embedded processor systems. The disadvantage of these solutions is that they are proprietary and only little documentation is publicly available.

To our knowledge, this is the first work that applies and evaluates TC in embedded smart camera networks. A major challenge is the proper integration into a camera system and its computer vision task without considerably reducing overall system performance. In our previous work [9–11], we addressed the integrity, authenticity, and confidentiality of video data. This paper is based on our previous results and extends them in several ways. We contribute to the state of the art in at least the following three areas. (1) We discuss the establishment of the chain of trust on our TrustCAM [10] prototype. We evaluate the performance impact on system boot and discuss the resource tradeoff for different root of trust implementations. (2) We describe a timestamping mechanism for frame groups that ensures data freshness, guarantees correct frame order, and allows us to associate a world time interval with each frame. (3) Reboots of the camera system and the current system status are reliably reported with a periodic lifebeat.

The remainder of this paper is organized as follows. Section 2 discusses the goals and the underlying assumptions for our work. Since we base our work on Trusted Computing, Section 3 presents an overview of the fundamental concepts of this technology. Thereafter, in Section 4 we present our system architecture including our TrustCAM prototype platform. In Section 5 we discuss different aspects of the integration of TC into a camera system. This includes the establishment of a chain of trust, a trusted lifebeat as well as image encryption, signing, and timestamping. Implementation details and evaluation results are presented in Section 6. In Section 7, we summarize related work on security in camera systems and applications of Trusted Computing in embedded systems. Finally, we outline future work and conclude the article in Section 8.

2. Goals and Assumptions

The primary focus of this work lies on enhancing the security of an embedded smart camera system to provide certain guarantees for the delivered video and image data. This section outlines the goals and the assumptions we made.

2.1. Goals

For the design of our TrustCAM prototype system, we define the following goals.

 Camera Status Monitoring. Since cameras are often installed in remote locations that are not under full control of the operators, a mechanism is required that allows one to reliably check the current status of a camera. This should include a report about the executed software as well as the detection of unscheduled system reboots.

Authenticity of Videos. In many applications such as traffic monitoring and law enforcement, the origin of information is important. In visual surveillance, this is equivalent to knowing which camera captured a video stream. This can be achieved by explicitly authenticating the cameras of a network and embedding this information into the video streams.

Freshness of Videos. To prevent replay attacks where recorded videos are injected into the network to replace the live video stream, freshness of image data must be guaranteed. Even more, in many areas including enforcement applications, evidence is required when a video sequence was recorded. Applying timestamps to images delivered by a camera is a way to satisfy both of these requirements.

Integrity of Videos. Image data coming from a camera can be intentionally modified by an attacker during transmission or when stored in a database. Using checksums and digital signatures, data integrity can be ensured. An often overlooked issue is that integrity protection is not only important for single frames but also for sequences. Simple reordering of images can substantially change the meaning of a video.

Confidentiality of Videos. It must be assured that no third party can eavesdrop on sensitive information that is sent from the cameras to the control station. Confidentiality must not only be provided for image and video data transmitted over the network but also for videos that, for example, are stored on a camera to be transmitted at a later point in time.

Limited Access to Videos. Access to confidential video data must be limited to persons with adequate security clearance. For highly sensitive data, multiple system operators should be required to cooperate to reveal the data.

2.2. Assumptions and Scope

This work primarily deals with security issues related to the embedded camera system itself and video data delivered by it. We therefore make several assumptions about other aspects and system components.

Centralized Control. In our concept, cameras are assumed to be operated and controlled from a central facility. By definition, this control station is trusted. Physical and remote access to this facility is limited to authorized personnel. Appropriate guidelines for both, the personnel as well as the software components are established and frequent auditing is performed. For video streams that are not only viewed but also stored at the control station, we assume that this is done in the same format as they are delivered by the camera. Integrity and authenticity information as well as timestamps are not removed from the stream and confidential video data is not decrypted before being stored. This ensures that sensitive data is not only protected during transmission but also when archived.

Networking. We assume that all cameras that belong to the network can be accessed in one or more hops over a wireless connection. In a larger deployment, topology control and clustering would be required to ensure scalability. Moreover, we do not address network security issues including secure routing or the secure formation of camera clusters.

Physical Attacks. Our main concern are software attacks on the cameras and the delivered data. Attacks on camera hardware, including hardware manipulation as well as power and timing analysis [12] are beyond the scope of this work. We however assume that, for example, with specifically designed camera enclosures and circuit boards, a reasonable degree of resistance against tampering can be achieved. If a hardware attack involves the reboot of the camera, this should be detectable for camera operators.

Availability. In some cases, camera systems are considered as critical infrastructure and therefore guarantees about the availability of system services should be provided. Specifically, this also includes resistance against denial of service attacks. This would require to monitor and control resource usage and to validate incoming requests regarding, for example, their authenticity, integrity, and freshness. This is currently not addressed in our approach. Moreover, providing service and availability guarantees is inherently difficult when using a wireless communication channel that is easily jammed.

3. Trusted Computing Overview

Trusted Computing (TC) [13, 14] is an industry initiative headed by the Trusted Computing Group (TCG) [15]. The main output of the group is a set of specifications for a hardware chip—the Trusted Platform Module (TPM) [16]—and surrounding software infrastructure such as the TCG Software Stack (TSS) [17]. The TPM, as shown in Figure 1, is a purely passive device that cannot actively interfere with the boot process of the host system or prevent the execution of software. Internally, a TPM typically is implemented as a microcontroller (execution engine) with accelerators for RSA and SHA1. Additionally, the TPM provides a random number generator (RNG) as well as limited amount of volatile and non-volatile memory. With an opt-in procedure, users can choose if they want to make use of the TPM chip. Each TPM is uniquely identified via a special RSA key called Endorsement Key (EK). This EK is created either by the TPM manufacturer as part of the fabrication process or by the user when taking ownership of the TPM. Either way, the EK cannot be changed or removed throughout the entire lifetime of the TPM.

Figure 1

A Trusted Platform Module (TPM) consists of shielded locations (memory) and protected capabilities which are functions that operate on shielded locations.

RSA keys can be generated for different purposes such as data encryption or signing. Upon creation, keys can be declared migratable or not. While migratable keys can be transferred to a different TPM, non-migratable keys cannot. A password called usage secret can be specified upon key creation. If specified, this password has to be provided every time the key is used. Likewise, a migration secret can be specified that must be supplied if the key is to be migrated to another TPM. Regardless of key type and migratability, a private TPM key can never be extracted from the chip as plain text but only in encrypted form. By definition, every key is required to have a parent key that is used to encrypt the private key when it has to be swapped out of the TPM due to limited internal memory. At the root of this key hierarchy is the Storage Root Key () which never leaves the TPM.

Aside from basic cryptographic functionality, TC and the TPM provide the following three roots of trust.

Root of Trust for Measurement (RTM). In TC, measuring is the process of computing the SHA1 hash of an application binary before it is executed. Since the TPM is a purely passive device, it cannot initiate measurements or interfere with the system boot process. Another trusted building block is required to perform the initial measurement. On a PC system, this typically is an immutable part of the BIOS which measures the next software component before it passes control to it. Assuming that all subsequent components proceed the same way, a sequence of measurements—called chain of trust—is created going from the BIOS up to the application level. For Linux systems, the Integrity Measurement Architecture [18] allows to measure every driver, library, and application that is loaded or executed on a system. The measurement values are stored inside the TPM in secure memory regions called Platform Configuration Registers (PCRs). As the amount of memory inside the TPM is limited, a special operation called TPM_Extend is used when writing to PCRs:

(1)

With the TPM_Extend operation, the current PCR value is not overwritten but the new measurement is accumulated with the current PCR value. PCRs are only reset upon platform reboot. Using only the accumulated PCR values, it is difficult to assert the state of a system. For a verifier, the individual measurements representing the software components executed on the system might be of greater interest. To facilitate that, the TCG measurement concepts propose the use of a PCR log that is stored outside the TPM. This log contains one entry for each measurement that was extended into a PCR. Using these log entries, a verifier can reproduce the current PCR values step by step and thereby get knowledge about the executed software components. Note that even though the log is stored outside the TPM, any manipulation can be easily detected since the reproduced PCR values would not match those securely stored inside the TPM.

Root of Trust for Reporting (RTR). Reporting of the state of a platform is called attestation and is done with the TPM_Quote command. As part of that, PCR values are signed inside the TPM using a key unique to that TPM. In theory, this key could be the EK of the TPM. In practice this is not done due to privacy reasons. If the EKs were always used for signing the PCRs, all these signatures could be tracked to a single TPM and hence a group of persons that use the machine with this TPM for several different purposes. Consequently, not directly the EK but alias keys are used to sign the PCRs. They are called Attestation Identity Keys (AIKs) and are generated with the help of an external trusted third party called PrivacyCA. Details on AIK creation can be found, for example, in work by Pirker et al. [19]. With version 1.2 of the TPM specification an additional mechanism was added to create AIKs. It is called Direct Anonymous Attestation (DAA) and is based on group signatures.

Root of Trust for Storage (RTS). The RTS allows one to use the TPM to securely store data. Binding of data refers to encrypting data with a TPM key and hence guaranteeing that the data only is accessible by this specific TPM instance. Sealing of data allows one to specify a set of PCR values the data is sealed to. As with binding, the unsealing can only be done by the specific TPM instance that holds the private sealing key. Additionally, the plain text is only released if the current PCR values match those specified upon sealing.

Functionality that was added to the TPM specification in version 1.2 is timestamping and non-volatile (NV) storage inside the TPM which can be used for custom applications. One usage of the NV storage is for certificates shipped by the TPM or platform manufacturers. The remaining available space can be used for custom applications. Access to NV storage can be defined to require authorization or be limited to a certain platform state represented by a set of PCR values.

Timestamping is an important functionality in many applications. For simplicity and cost reasons, the TPM does not contain a realtime clock. Instead, a tick counter is included that is reset to zero upon system bootup. The TPM specification recommends that the tick counter value is incremented at least every millisecond. The actual increment rate is vendor specific and can be queried from the TPM as the tick rate . To be able to distinguish different tick counter sessions resulting from platform reboots, a random tick session nonce is generated every time the is reset. Associating the pairs with world time is left to the application.

4. System Architecture

In our visual sensor network architecture, cameras are assumed to be spatially distributed to cover a large area. Network connectivity is provided by wireless communication technologies. Cameras are controlled and operated from a central facility subsequently called Control Station (CS). Each camera can be reached from the CS in one or more hops. As described in Section 2, we assume that the CS is a secure and trustworthy facility.

Figure 2 shows a network of camera nodes and one central control station. Every camera is equipped with a TPM chip called . Likewise, the computing infrastructure of the CS contains a TPM subsequently referred to as . In addition to , the CS also hosts a database where cryptographic keys generated during camera setup, and data received from the cameras as part of periodic lifebeats, are stored. Moreover, we assume that the CS has a reliable and accurate time source which is required to associate lifebeat events and timestamps with world time.

Figure 2

A network of TPM-equipped cameras managed by a central control station.

4.1. Camera Setup and Deployment

Before a camera is deployed, it has to be set up. It is assumed that this setup is done when the camera is under full control of the operating personnel. The main part of the setup involves the generation of TPM keys on the camera and at the control station. All keys are generated as 2048 bit RSA keys. The following setup steps and the key generation are done for every camera of the network.

TPM Ownership. Calling the TPM_TakeOwnership operation of the cameras sets an owner secret and generates the Storage Root Key . The owner secret is not required during normal operation of the camera and is set to a random value unique to every camera. For maintenance operations, the camera's owner secret is stored in the database of the control station.

Identity Key Creation. An Attestation Identity Key serves as an alias for the Endorsement Key () and is used during platform attestation. Contrary to a conventional PC, there are not multiple human users on a smart camera. The system software running on the camera takes the role of a single system user. Moreover, all cameras in the network are uniquely identified and well known by the operators. Consequently, there is no need for the anonymity gained by using multiple AIKs in conjunction with a PrivacyCA. Therefore, only a single Attestation Identity Key is generated during setup that serves for platform attestation. The public part is stored in the CS database together with .

Signature Key Creation. For signing data such as events or images delivered by a camera, a nonmigratable singing key is created with as its parent. Being non-migratable ensures that the private key cannot leave the camera's . This provides assurance that data signed with this particular key really originates from this specific camera.

Binding Key Creation. To ensure confidentiality of sensitive data, images sent by the camera to the CS have to be encrypted. This encryption can be done for full images or special regions of interest where, for example, motion or faces have been detected.

To ensure confidentiality, at least one non-migratable binding key is created by the control station's . The public part of this key, , is exported from and stored on the camera. Note that the private part of cannot be exported from and therefore data encrypted with can only be decrypted at the CS and not by an intermediate attacker who interferes with the transmission. To decrypt data bound with , the usage secret of the key has to be supplied by the system operator. To avoid that a single operator who has access to the control station and knowledge of this usage secret can decrypt data, additional binding keys to can be generated. Privacy sensitive data can then be encrypted with multiple binding keys. Assuming that no single operator knows all the usage secrets for the binding keys, two or more operators have to cooperate to decrypt the data. The binding keys can also be used to realize different security levels. Data at different abstraction levels (e.g., full images versus images where people's faces have been removed versus textual event descriptions) can be encrypted with different binding keys. Depending on security clearance, only certain abstraction levels can be accessed by an operator.

Table 1 summarizes the cryptographic keys generated as part of the camera setup procedure.

Table 1 The cryptographic keys generated during setup of a single camera. The Control Station and Camera columns denote the storage location of the keys. Binding keys are generated by while all other keys are generated by . All keys are non-migratable, 2048 bit RSA keys. The subscript denotes the public RSA key.

4.2. Key Management Considerations

Regarding key management, our primary assumption is that keys are distributed during setup where the system is under full control of the operating personnel. The proposed system currently supports no mechanisms for key distribution during runtime. Considering our application domain we believe that this is a reasonable assumption. Cameras of a visual surveillance network are intentionally placed and installed by experts. In such a relatively static environment there is little need for dynamic key exchange.

For economic reasons, camera operators nevertheless might wish to perform initial setup and configuration of cameras remotely. This can be realized if the camera manufacturer separately provides an EK certificate for the camera's TPM. The required protocols and public key infrastructure for such an approach, however, are not considered in this work.

Aside from distribution, the management of keys needs to be considered in case a component of the system has to be upgraded or exchanged. Our concept proposes to use only non-migratable TPM keys which means that private keys cannot be transferred to another TPM. For the cameras this is clearly a desirable property since it ensures that data signed with the TPM key actually comes from the camera the TPM is part of. In cases where a camera is replaced, we do not see any need to migrate from the old to the new camera. Instead, a new is created for the new camera. The key of the old camera however should be deleted by clearing the TPM to ensure that it cannot be used after the camera has been taken out of service.

For the control station the situation is different. All data that was encrypted by the cameras of the network with their public binding keys to is lost if the hardware infrastructure of the control station is upgraded and this upgrade also includes . To allow such maintenance, the binding keys to could be made migratable. This would allow to transfer the binding keys to the updated control station hardware. Key migration can only be performed if the migration password specified upon key creation is supplied. Clearly, it is critical that appropriate policies for management of these migration secrets are applied. These policies must also ensure that the old is properly cleared and all its keys are invalidated.

The public binding keys to generated by have to be stored on the camera. Since they are public, no special protection is required because all an attacker can do with these keys is to encrypt data that only can be decrypted at the control station. The question remains where to store these keys on the camera. If the keys have to be placed in the camera's file system, this means that the file system has to be specific for every deployed camera. To avoid this, we make use of the non-volatile storage of to store the public binding keys to . Additionally, the NV space can be used to store small amounts of camera-specific configuration data. Access to the NV space with the binding keys and configuration data can be limited to a specific system configuration.

4.3. TrustCAM Hard- and Software Prototype

Our custom TrustCAM prototype system is largely built from commercially available components. TrustCAM is based on the BeagleBoard [20] which has a dual-core processor with an ARM Cortex A8 CPU clocked at 480 MHz and a TMS320C64x+ digital signal processor running at 360 MHz. The system is equipped with 256 MB RAM and 256 MB NAND flash. Via USB, we connect a color SVGA CMOS sensor (Logitech QuickCam Pro 9000) and an RA-Link RA-2571 802.11b/g WiFi adapter. An XBee radio provides a second, low-performance communication channel. Finally, an Atmel AT97SC3203S—the only commercial TPM designed for embedded devices—is connected to the mainboard via the I2C bus. Figure 3 shows a picture of the prototype system.

Figure 3

The TrustCAM prototype with the image sensor, the XBee radio, and the Atmel I2C TPM at the top level. Behind that are the processing board and WiFi radio.

As operating system we use an ARM Linux system together with a customized, OMAP-specific kernel. For TPM access, we use a modified version of the TrouSerS [21] TCG software stack where we have replaced the trusted device driver library (TDDL). Our fully custom TDDL implementation manages access to the TPM via the I2C bus.

To simplify application development and to allow reuse of components, we designed a software framework that supports composition of applications from individual blocks which are instantiated and interconnected. This approach follows the concept of modeling the dataflow between the individual components. Conceptually, every block has an output memory where its results can be accessed by subsequent blocks. To maintain consistency of stored data, access to shared memory is guarded by a lock that is passed between the producing and consuming block. Blocks can form chains of arbitrary length where each pair of blocks is connected by shared memory and a lock. In our implementation, a processing block is realized as an individual process expecting well-defined input data and generating output consumable by subsequent blocks. The shared memories are implemented as POSIX shared memory synchronized by an interprocess locking mechanism.

Using separate processes instead of threads for the processing blocks offers a number of benefits. Blocks can potentially be implemented in any programming language as long as there exists shared memory and locking support. Moreover, separate processes allow to easily implement watchdog functionality that monitors individual parts of the processing chain and restarts blocks as required. As shown in Figure 4, a central entity called NodeManager, is running on every camera node. The NodeManager is the only entity that starts processing blocks and forms processing chains by connecting the individual blocks. A script is used to specify which blocks have to be started, how they are interconnected, and what their parameters are. This design allows the NodeManager to monitor the status of processing blocks and keep track of consumed and available system resources to decide if additional applications can be executed. Furthermore, the NodeManger is responsible for managing the locks that guard the shared memory regions. Additional details and performance evaluations for the camera software framework are provided in [22].

Figure 4

The NodeManager is responsible for creation of processing chains and management of inter-process communication. The output of individual blocks is stored in shared memory that can be accessed by one or more consumers.

5. Trusted Computing Integration

The lifecycle of a smart camera starts with its setup and deployment which we described in Section 4.1. When the camera boots, the chain of trust has to be established starting at a root of trust for measurement. In the following Section 5.1, we present the realization of this boot procedure for our TrustCAM prototype system. Once the system is booted, the computer vision tasks are executed. To check the status of the system and to detect unscheduled system reboots, the control station sends a periodic lifebeat request which is described in Section 5.2. If the control station requests a video stream from the camera, data integrity, authenticity, and freshness must be ensured. This is discussed in Section 5.3. Considering the sensitivity of video data, also the confidentiality of images must be preserved. Our approach to achieve this, is described in Section 5.4. Additionally, we demonstrate the realization of two security levels.

5.1. Trusted Boot and Chain of Trust

As described in Section 3, on PC systems the Root of Trust for Measurement (RTM) is typically implemented as an immutable part of the BIOS. Recent CPUs and chipsets from AMD and Intel provide an instruction set extension that allows one to establish a chain of trust after the system has already booted. This is achieved via a so-called dynamic root of trust. Since both of these mechanisms are not available on today's embedded systems, we discuss how the chain of trust can be established on an existing embedded device. Our approach is based on the concepts of a static RTM.

The OMAP 3530 CPU of our system uses a multistage boot process [23]. On power-up, the system executes the first bootloader stage located in an internal 32 kB ROM. After performing basic hardware configuration, the ROM code creates a list of boot devices. This list is based on six hardware pins of the CPU called SYS_BOOT pins. Upon board design, a certain boot sequence can be defined by hardwiring these pins accordingly. By default, the BeagleBoard boot order is NAND, USB, UART 3, and MMC. With only minor modifications of the board design, this boot sequence can be hardwired to, for example, always boot from UART 3.

After the ROM code has prepared the boot device list based on the SYS_BOOT pins, the next bootloader stage is copied into SRAM. This second bootloader stage is called X-Loader and it has to be small enough to fit into the 64 kB of the SRAM. The X-Loader then initializes additional peripherals including the SDRAM controller and then loads the U-Boot bootloader as the third stage into SDRAM. U-Boot finally loads and executes the Linux kernel. Figure 5(a) gives an overview of this default OMAP boot procedure.

Figure 5

The boot procedures of the unmodified BeagleBoard, a TPM-enabled ideal system, and the TrustCAM prototype. Hardware components are drawn as gray boxes. Dashed lines represent the measuring of software components and extending these measurements into the TPM's PCRs. This is always done before the next component is executed.Unmodified BeagleBoard/OMAP boot procedure without TPM integrationIdeal boot sequence with TPM-enabled ROM code that measures the first stage of the bootloader and extends it into the TPMTrustCAM prototype boot procedure using UART booting to load X-Loader from a microcontroller that acts as trusted building block

To integrate the TPM into the boot process and establish the chain of trust, modifications to the system are required. Ideally, the internal ROM of the OMAP should measure the second bootloader stage (X-Loader) and extend it into one of the PCRs (Figure 5(b)). To be able to measure X-Loader, code for the SHA1 hash algorithm needs to be integrated into the ROM code. This however can be avoided if the SHA1 engine of the TPM is used. This keeps modifications of the ROM code at a minimum and should allow to integrate the RTM functionality into the ROM code without exceeding the 32 kB ROM size. The downside of this approach is that measuring of X-Loader would take significantly longer compared to a software SHA1 implementation running on the OMAP CPU. This is primarily due to the low performance of the TPM and the relatively slow I2C communication. In Section 6.3 we provide comparison measurements and a discussion of the performance of the two approaches. Note that the "ideal" integration of an RTM into our TrustCAM prototype—or any other OMAP-based embedded system—would require the cooperation of the CPU manufacturer to integrate the TPM-enabled ROM code during production.

For the implementation of an RTM for the TrustCAM prototype we therefore chose a different approach that is shown in Figure 5(c). The SYS_BOOT pins of the OMAP allow to force the ROM code to request the second bootloader stage from UART 3 as a first boot device. This pin configuration can easily be hardwired in a custom PCB design. In our design we use a trusted building block which is connected to the OMAP's UART 3 and answers the download request. This could be a one-time programmable memory together with minimal, additional logic. For our prototype, we realized this component with a microcontroller that downloads the second stage (X-Loader) bootloader. Once X-Loader has been downloaded, the application on the microcontroller terminates and no further interaction between the OMAP CPU and the microcontroller is possible until the next reboot of the system. Allowing no further communication between the two systems is important since it ensures that a potential attacker who gains access to the system that runs on the OMAP CPU, cannot access or modify the X-Loader located on the microcontroller. Compared to modifying the ROM code, our prototype approach provides no resistance against hardware attacks. With full physical access to a camera, it is easy to change the boot procedure and prevent the correct establishment of the chain of trust. As stated in Section 2, hardware attacks are not in the focus of our current work. We nevertheless believe that the proposed mechanism to establish the RTM can still be valuable for legacy devices especially when combined with the Trusted Lifebeat described in Section 5.2. Hardware attacks often cannot be performed on a running system or require a reboot to become effective. The lifebeat allows operators to detect such unexpected events and initiate further actions like retrieval and inspection of the camera.

In the ideal case, the X-Loader code is measured by the ROM code into PCR 1. For the TrustCAM prototype, the X-Loader supplied by the microcontroller is not measured. Once X-Loader is in control, the remaining boot sequence for the ideal case and the TrustCAM prototype is identical. If not already done by the ROM code, X-Loader ensures that the TPM is properly started up. Next, it measures the U-Boot bootloader into PCR 2 before passing control to it. U-Boot measures the Linux kernel (PCR 3), its parameters (PCR 4), and the compressed root filesystem (PCR 5). Once control is passed to Linux, the root filesystem is mounted read-only and system startup continues. Note that contrary to a PC system, it is feasible to measure the entire root file system at once since typical sizes range from a few to a few dozens of MB. Keeping the number of measurements small, considerably simplifies verification of the system state. A verifier can easily check the overall status without complex evaluations of PCR logs. To be able to attest which computer vision applications actually are executed, we extend the NodeManager introduced in Section 4.3. Being responsible for starting the computer vision processing blocks, the NodeManager measures the configuration script and every block into PCR 6 before they are started. For typical scenarios, a processing chain is expected to be composed of no more than ten processing blocks. This keeps the number of measurements in PCR 6 relatively small. A log of the individual values that are extended into PCR 6 is kept on a partition separate from the root filesystem. The full chain of trust of the TrustCAM prototype, including the measurements done by the NodeManager, is shown in Figure 5(c). By measuring the vision block separately from the root filesystem, a verifier cannot only get general information about the system firmware but also gain insight which image processing tasks are executed by the camera. Table 2 summarizes PCR usage of the TrustCAM prototype.

Table 2 TrustCAM PCR usage. Each of the PCRs 1 to 5 only stores the measurement of a single software component. PCR 6 contains the accumulated measurements of the computer vision blocks started by the NodeManager.

5.2. Trusted Lifebeat

The main purpose of a lifebeat is to determine the state of a system based on the periodic transmission of status messages. If a status message is not received for a predefined amount of time, then it can be concluded that the system is no longer operational. The proposed trusted lifebeat mechanism extends this basic concept by supporting the following properties.

Platform Attestation. Based on TC attestation techniques, the status of the platform is reported to the system operator. This not only allows one to reliably check which firmware is running on a camera but also which computer vision applications are executed. This is especially important if the NodeManager is capable of reconfiguring the system dynamically at runtime.

Reboot Detection. It is important to reliably detect unintended reboots of a system as these are often an indicator for attacks. The trusted lifebeat allows one to securely detect and report reboots of a camera system. If such a reboot is detected, the camera should be retrieved for inspection.

World Time Mapping. We use the internal tick counter of the TPM for secure timestamping of images delivered by a camera (see Section 5.3 for details). For that purpose, the tick counter has to be associated with world time. The trusted lifebeat is used to realize this mapping of tick counter values to world time.

Contrary to a conventional lifebeat, in our architecture the lifebeat is not automatically sent by a camera but is periodically requested by the control station. This is done to supply a randomly generated nonce to ensure freshness of the platform attestation information contained in the lifebeat. The lifebeat response not only includes the attestation result but also the current TPM tick counter value (), the tick session nonce (), and the tick rate (). In detail, the trusted lifebeat protocol works as follows.

(1)The control station sends a random nonce and the list of requested PCRs to a camera. Additionally, the control station records the current UTC time .

If the camera does not respond within a predefined amount of time, it is considered to be out of service and should be retrieved for inspection.

(2)The camera performs a TPM_TickStampBlob operation resulting in:

(2)

is the current tick value, identifies the tick session with a unique nonce, and is the number of microseconds per tick.

(3)Then, the camera performs a TPM_Quote operation and generates that

(3)

Note that is included in the signature instead of the nonce supplied by the control station. however is implicitly included as it is part of . Including associates tick count and platform state. This provides the verifier with information about the platform state at the time the TickStamp operation was performed.

(4), , the requested PCR values, the timer values (, , ), and the stored measurement log for the processing blocks started by the NodeManager are returned to the control station.

(5)When the response from the camera is received, the control station stores the current UTC time as .

(6)The control station verifies the provided data as follows:

aRetrieve of the intended camera from the CS database and verify the signature of ,

(4)

If the signature verification succeeds and the contained nonce matches the supplied nonce , one has assurance that the tick values are authentic, unmodified, and fresh.

(b)If and are not identical, this means that the camera was rebooted and the TPM has been reset since the last lifebeat event. If this reboot was not intentionally triggered by a system operator, it might be an indication for an attack on the camera. In such a case, the camera should be retrieved for inspection.

(c)Verify the signature of using from the CS database. If verification succeeds, one knows that the provided system state information is authentic and unmodified. Freshness of the attestation data is ensured implicitly via nonce included in .

(d)Check the returned PCR values for the bootloader(s), the kernel, and the root filesystem against "known good" values stored in the CS database. Evaluate the PCR values that represents the processing blocks started by the NodeManager together with the supplied PCR log. Checks include if all processing blocks, for example, are known and categorized as uncritical. Due to the limited number of PCR values that need to be evaluated, the overall system status verification is considerably simplified compared to a general purpose PC system.

(7)If any of the aforementioned checks fail, the camera should be taken out of service and retrieved for inspection.

(8)The time values and and the tick counter values , , and are stored as a single record in the database of the CS. This associates the tick value of the tick session with the UTC time interval to .

The described, remotely triggered trusted lifebeat procedure does not necessarily have to be executed at a fixed time interval but the control station can send requests at random intervals. It however should be ensured that these intervals do not exceed a previously configured maximum time.

5.3. Image Signing and Timestamping

In applications such as law enforcement or traffic monitoring, it is important to provide evidence where (i.e., by which camera) and when an image was taken. Having TPMs on the cameras provides basic functionality required for this task. Authenticity and integrity checking of images is realized by signing image data delivered by a camera using the non-migratable TPM signing key . Because this key cannot be used outside the TPM, the signature proves that an image actually originates from the camera the TPM belongs to. To answer the question when an image was taken, we do not perform simple signing but make use of the TPM_TickStampBlob function. This function not only signs the image data provided by the video streaming application, but also includes the current TPM tick counter information in the signature. Image signing and timestamping is done as follows.

Acquire image data from the camera sensor.

Call the TPM_TickStampBlob function that signs the current TPM tick session nonce, tick counter value and the image:

(5)

, , , as well as are transferred to the control station or alternatively are stored on the camera for later use.

At the control station, belonging to the expected camera is retrieved from the database.

Verify the timestamp data:

(6)

If verification succeeds, integrity and authenticity of the image data is ensured.

From the CS database, retrieve the most recent lifebeat that took place before the timestamping of the image. This data includes , , , , and as described in Section 5.2. The database query is performed using as the key such that and . In case of archived video data, also a subsequent lifebeat might be available in the database. In that case, it additionally has to be checked that and .

Associate the time value with UTC time:

(7)

with

(8)

Knowing the duration the TPM requires for TPM_Quote, one can narrow down the world time interval the image capture time lies in:

(9)

If none of the aforementioned steps failed, one now knows that the image data (1) was not modified, (2) comes from a specific camera (i.e., the camera with the TPM that protects ), and (3) was taken within a certain, narrow timeframe.

Figure 6 provides a graphical overview of the relationship between the last lifebeat event prior to an image timestamping operation and the image timestamp. The image timestamp represented by () has to be associated with a UTC timestamp . This UTC time actually lies in an interval with a size determined by the trusted lifebeat. The size of this interval depends on the time that is required for transmitting the lifebeat data over the network and the time that is required to executed the lifebeat request on the camera. Overall execution time is dominated by the times the TPM requires for TPM_TickStampBlob () and TPM_Quote (). For a given TPM model, those times are relatively constant and can be measured in advance to be taken into account. This has been done in the presented formula by subtracting the runtime for TPM_Quote from . Network latency, especially in wireless networks, cannot easily be predicted and therefore cannot be treated in the same way. Practical considerations and evaluations of image timestamping and the trusted lifebeat are presented in Section 6.

Figure 6

Timeline showing the lifebeat and image timestamping on a camera. and denote the points in UTC time recorded by the CS when the lifebeat request is sent and the response is received. denotes the duration for transmitting the request/response. and are the time durations for the TPM_TickStampBlob and TPM_Quote operations. Using that information, a world time interval can be associated with the image timestamp represented by .

5.4. Confidentiality of Sensitive Image Data

Image data that is either stored on a camera for later use or directly delivered to the control station, requires special protection to maintain confidentiality. To ensure that image data can only be accessed at the control station, the binding keys to are used for encryption. Since the private keys required for decryption can only be used inside , this ensures that access to the control station is an absolute requirement for accessing the data. The purpose of using more than one binding key is to support multiple levels of protection for different abstractions of image data. For example, by on-board processing a smart camera could identify sensitive regions such as people's faces. In a second processing step the sensitive regions are extracted from the original images. The remaining background images provide sufficient information for a many surveillance tasks without revealing critical personal information. Nevertheless, both the sensitive regions () and the blanked background images, () need to be protected against unauthorized access.

For that purpose, two 256 bit AES session keys, and , are created at application startup. Once the application is running, new AES session keys can be generated at a configurable time interval. The AES session keys are bound to with and :

(10)

(11)

The sensitive regions and background images are JPEG compressed. The sensitive parts are encrypted with :

(12)

The resulting ciphertext and the bound session key are embedded into the background JPEG image as custom EXIF data. This combined image is then encrypted with :

(13)

The encrypted image data and the bound session key are sent to the control station. Control station staff with low security clearance is only in possession of the usage secret for . This allows access to the background images which are sufficient for monitoring the actions of persons. In a special event where, for example, a law was violated, a supervisor with higher security clearance and knowledge of the usage secret of can decrypt the embedded sensitive regions. In both cases, access to the control station is an absolute requirement to be able to access confidential data.

6. Implementation and Evaluation

In this section we discuss selected implementation aspects together with evaluation results for the individual system components. After outlining the evaluation setup, we discuss which performance can be expected from commercially available TPM implementations. Thereafter, we present several performance measurements we did for our TrustCAM prototype implementation.

6.1. Evaluation Setup

For the evaluation of the proposed security concepts, we use our TrustCAM prototype described in Section 4 and a laptop running Linux that acts as control station. Figure 7 presents an overview of the setup. Communication between TrustCAM and the laptop is performed via WiFi. For the TrustCAM prototype, Figure 7 shows all relevant hard- and software components. At the application level, the trusted lifebeat and the secure video streaming application are shown. While the lifebeat is implemented as a single processing block, the secure video streaming application consists of five blocks that are interconnected by shared memory. Even though not explicitly drawn, access to these shared memory regions is managed by the NodeManager.

Figure 7

The evaluation setup with the TrustCAM prototype and a laptop acting as control station. For the TrustCAM prototype hard- (gray boxes) and software (white boxes) components are shown. The TPM is attached via the I2C bus. A microcontroller used for trusted boot is connected to UART 3. The image sensor and WiFi radio are connected via USB. For low-power wireless networking, an XBee radio is used. The software layers consist of a custom Linux kernel, standard system libraries, a modified TrouSerS TSS, and the TrustCAM software framework. On top of that, two running applications are shown: the trusted lifebeat and secure video streaming. To ensure prioritization, all access to the TPM is done via the TPM Manager.

An important component of the TrustCAM framework is the TPM Manager. It enables us to prioritize access to the TPM by supporting multiple request queues. Every time a TPM command is completed, the TPM Manager retrieves the next request to be processed from the queue with the highest priority. One request can contain more than one TPM command to be executed. This ensures that commands that logically belong together such as the TPM_TickStampBlob and TPM_Quote of the trusted lifebeat are executed in sequence and are not interleaved with other commands. Internally, this can be realized using exclusive TPM transport sessions. Queue shown in Figure 7 has the highest priority and is used to handle incoming lifebeat requests. The lifebeat is given the highest priority because it not only reports the system status but also the current tick counter values. As described in Section 5.3, these values are used to associate the timestamp of an image with a world time interval. To keep this interval as small as possible, lifebeat events get higher priority than other TPM operations. For our evaluation setup, we use a second queue with lower priority than to handle the timestamping of outgoing image data. The TPM Manager currently does not provide any mechanisms to prevent starvation or guarantee fair TPM access. For the presented application scenario this however is not an issue since lifebeat requests only occur with low frequency.

An additional feature of the TPM manager is that, contrary to the TSS, requests do not have to be blocking. Once a request was placed in the appropriate queue, the call returns and the processing block can continue with other work. When the TPM completes the request, the processing block is notified via a callback. For TPM access the TPM Manager relies on the TrouSerS software stack with a custom TDDL layer for access to the I2C TPM. In a fully optimized implementation, the TSS could be removed and the TPM could be accessed directly by the TPM Manager.

6.2. TPM Performance

The TPM specification does not state any minimal performance requirements for implementations. Manufacturers therefore are free to find a tradeoff between performance and costs. Since TPM chips are intended to be used not only in business but also consumer products, most manufacturers focus on low price. Table 3 shows the performance of selected TPM commands that are relevant for the evaluation scenarios. The results are based on our previous work [10] and are extended with measurements for the Atmel I2C TPM. The TPM_OIAP command is used to establish authorization sessions. TPM_Quote signs the current PCR values while TPM_Sign and TPM_TickStampBlob are used for signing and timestamping. TPM_Seal, TPM_Unseal, and TPM_Unbind allow to encrypt and decrypt data using TPM keys. Note that binding is a pure public key operation that does not require the TPM and therefore is performed in software. All performance measurements have been done at the parameter block generation layer of the TSS which means that no TSS overhead is included in these results. On TrustCAM, the overhead for the TrouSerS TSS typically is between 5 and 15 ms depending on the actual command. For completeness, we measured the performance for both, 2048 and 1024 bit RSA keys where applicable. For our implementation we only use 2048 bit keys.

Table 3 Runtime measurements for selected TPM commands on different TPM 1.2 chips and the TPM Emulator on TrustCAM. Where applicable, RSA key sizes of 2048 and 1024 bits were evaluated. Values are averaged over 10 runs.

The Infineon TPM implementation has the lowest runtimes followed by the Intel TPM which is integrated into recent chipsets. To illustrate the performance gap between TPM chips and current embedded computing systems, Table 3 also includes performance measurements of a pure software TPM implementation running on the TrustCAM prototype. These measurements show that the emulator running on TrustCAM is more than 4 times faster than the fastest TPM from Infineon. The only available TPM that can be used in an embedded system (Atmel TPM AT97SC3203S with I2C bus interface) is about 10 times slower than the software TPM on TrustCAM. Clearly, a software TPM solution would be attractive from a performance point of view. It is, however, an open research question if software TPMs, for example, based on CPU security extensions, can provide security guarantees similar to those of hardware TPMs [24]. Despite their performance issues, we therefore rely on commercially available and well-tested hardware TPMs.

6.3. Trusted Boot

For the evaluation of trusted boot, two aspects are of primary interest. The first is the impact on the total boot time that is introduced by measuring the relevant system components. The second aspect is the increase in code size by the addition of TPM support and the SHA1 hash algorithm that is required for doing measurements. To answer these questions, we prototypically implemented the trusted boot procedure for our TrustCAM platform.

For X-Loader and U-Boot we use versions from the BeagleBoard project. We extend both with I2C TPM support to be able to write the measurements into the PCRs. As our kernel we use a modified version of Linux 2.6.34 with custom support for the Atmel I2C TPM chip. The kernel is compiled as a monolithic binary without loadable modules. Table 4 summarizes the binary sizes of the system components that are measured during boot. For the implementation of the "ideal" trusted boot procedures, the ROM code of the OMAP processor would have to be modified. Even though this is not possible without support from the manufacturer, we still provide figures that show the expected increase in code size for this component. For adding basic TPM support and a SHA1 software implementation, the expected growth of the binary would be around 8 kB. This consists of about 1 kB for the TPM_Extend command and about 7 kB for the SHA1 algorithm compiled for the OMAP processor. As a reference we used the SHA1 implementation of Polar SSL [25] which is designed for embedded systems. With additional effort, this size could be reduced further. The TinyECC project [26], for example, reports a code size of less than 4 kB for SHA1 implementations on certain sensor motes. The alternative to using a software SHA1 implementation is to use the SHA1 engine of the TPM. In this case, the ROM code would only grow by 1.1 kB. While code size is significantly smaller, performance of the TPM's SHA1 engine is a lot lower compared to the SHA1 implementation running on OMAP. With the TPM, hashing of the next bootloader stage (X-Loader) takes 5.9 s while the software SHA1 implementation only takes 1.6 ms. The reason for this huge difference is not only the slow TPM but, to a large part, also the slow I2C communication link. The TPM can only operate in I2C standard mode which is limited to 100 kbit/s. The 28.5 kB of X-Loader would need about 2.3 s to transmit. Due to implementation limitations, our I2C bus is only running at half speed which means that transmission time doubles to 4.6 s. The resulting difference of 1.3 s to the measured total runtime of 5.9 s, is consumed by the TPM for the actual SHA1 hashing and I2C overheads (e.g., acknowledge messages).

Table 4 Total binary sizes of the system components together with measurement runtimes (average over 100 runs) using the SHA1 hash algorithm. The increase of binary size over for the additional measurement and TPM code is given in brackets. The OMAP internal ROM code is less than 32 kB but the exact size is unknown. To measure the second bootloader stage (X-Loader), the ROM code could be extended with a software SHA1 implementation running on the OMAP CPU or it could use the TPM's SHA1 engine. Following the boot sequence outlined in Section 5.1, components are measured by the previous bootloader stage before control is passed on. The OMAP ROM code acts as root of trust and is not measured.

For X-Loader, that has to fit into the 64 kB SRAM size, limitations are not that critical. By adding TPM and software SHA1 support, its size increases by 7.8 kB to a total of 28.5 kB. X-Loader then measures the next bootloader stage (U-Boot) which takes 8.3 ms. U-Boot only requires extensions for TPM support since it already comes with a SHA1 implementation. U-Boot measures the Linux kernel, the kernel parameters, and the root filesystem. This takes 96 ms, 0.3 ms, and 1157.4 ms, respectively. Assuming that an SHA1 software implementation can be fitted into the ROM code, the total increase in boot time would be 1.3 s. If however the TPM has to be used for SHA1 hashing, boot time would increase by 7.1 s. If the maximum I2C communication speed of 100 kbit/s can be achieved, the impact on boot time still would be about 4.8 s. It must be noted that other TPM commands require a lot less data transmission over the I2C bus. For most TPM commands, only a few hundred bytes are exchanged and therefore transmission times are not critical.

Contrary to the ideal case where ROM code can be modified, for the TrustCAM prototype we use a slightly different boot procedure. As described in Section 5.1, we rely on a microcontroller that acts as a trusted building block. It downloads the X-Loader bootloader via UART3 to the main system. This download is done with 115200 bps which results in a download time of 2.4 s for the 28.5 kB of the X-Loader binary. Compared to the ideal boot process, the X-Loader binary is not measured and extended into PCR 1. For the prototype system with X-Loader downloaded via the serial connection, the total impact of trusted boot on system boot time of the prototype system is 3.7 s.

Clearly it should be the goal to prolong system bootup as little as possible. An overhead of a few seconds however is acceptable for a camera system that is not intended to be frequently rebooted.

6.4. Trusted Lifebeat

The TPM commands that are involved in the trusted lifebeat are TPM_Quote and TPM_TickStampBlob. Assuming 2048 bit RSA keys, this results in a combined runtime of about 1.7 s on the TrustCAM prototype. Shorter runtimes, as achieved by TPMs of other manufacturers, would be an advantage since this would allow to keep the world time intervals associated with the TPM tick counter smaller. For the same reason, it should be ensured that lifebeat requests are processed with the smallest delay possible. This is achieved via the priority queues of the TPM manager described in Section 5.2.

World time that is associated with a lifebeat event falls between recorded at the control station when lifebeat request is sent and that is recorded when the response is received. Since the amount of data transmitted in the request and response is very small (a few hundred bytes), the lifebeat runtime to is dominated by the TPM operations. The runtimes for the TPM commands TPM TickStampBlob () and TPM Quote () are relatively constant. As discussed in Section 5.3, this allows us to subtract the runtime for TPM_Quote from the total lifebeat runtime and to estimate the minimal achievable time between and . For our prototype, this is . Even though the lifebeat is given priority over other TPM commands, it is still possible that an image timestamping request was sent to the TPM immediately before the lifebeat request was received. In such a case, the world time interval associated with the lifebeat would extend to . Including all overheads, typical time intervals between and for our prototype are between 1.5 and 2 s.

It is noteworthy that the trusted lifebeat is a vital system component which has to be operational even in situations where no video data is streamed and the WiFi radio is turned off to conserve power. To facilitate that, our prototype allows to exchange the lifebeat messages via the low-power XBee radio.

6.5. Secure Video Streaming

The secure video streaming application is designed to ensure authenticity, integrity, freshness, and confidentiality for streamed images and videos. The implementation of the encryption block shown in Figure 7, follows the concepts presented in Section 5.4. For evaluation purposes we use sensitive regions of fixed size ( pixels) that are extracted from the images. Such a JPEG compressed sensitive region typically consumes around 2.5 kB. As shown in Table 5, AES 256 encryption of this data takes less than 1 ms. Depending on resolution and color depth, the remaining, JPEG compressed background typically consumes between 12 kB (, grayscale) and 40 kB (, RGB). Encryption of the background image together with the already encrypted sensitive region that is embedded as custom EXIF data takes between 3 ms and 8 ms. Table 5 provides additional performance figures for AES256 on TrustCAM. Binding of the AES session keys using the public binding keys of takes about 5 ms and has to be done only at startup or when new session keys are created. These figures show that the overhead introduced by data encryption is relatively small and should be acceptable for most applications. Table 6 shows the overall performance of the camera system in terms of achieved frames per second. Specifically, the encryption columns shows the achieved framerates when streaming encrypted images. Comparing these figures with the plain streaming case where images are compressed and directly streamed, the impact on the framerate is between 0.3 and 1.5 frames.

Table 5 Average runtimes (100 runs) for SHA1 and AES 256 on the TrustCAM prototype. For comparison, measurements on a laptop (Core2 Duo, 1.6 GHz) are given.

Table 6 Framerates (avg. over 1000 frames) for different types of video streaming from TrustCAM to the control station via WiFi. The sensor delivers YUYV images at either or pixels. Optionally, they are converted to grayscale. Plain streaming shows the performance when images are JPEG compressed and streamed. For timestamping, image groups are timestamped by the TPM. The encryption column shows the achieved framerates if sensitive regions ( pixels) and the remaining background are encrypted before streaming. The last column presents the frames rates when doing both, encryption and timestamping.

This picture changes however, when considering image signing and timestamping as described in Section 5.3. Table 3 shows the runtime for the TPM_TickStampBlob operation on the Atmel I2C TPM is about 800 ms without overheads. This runtime clearly makes it impossible to sign and timestamp every single image delivered by the camera since the effective framerate would be reduced to little more than one frame per second.

We consequently adapt the image timestamping and signing procedure such that sequences of images instead of individual images are timestamped. We accumulate the hashes for a group of frames in a way similar to the TPM's PCR_Extend operation:

(14)

The accumulated hash for frames is signed by :

(15)

As shown in Table 5, SHA1 hashing of typical image sizes takes less than 2 ms. However, TPM timestamping of the hash takes a significant amount of time. We therefore do not wait for the TPM operation to complete but continue with the processing and streaming of video data. Specifically, the accumulation of the hash sum for the next group of images (beginning at frame ) is started. This continuous operation is possible since TPM commands are executed in parallel to the main processor. Once the TPM completes the timestamping command, the processing block is notified by the TPM Manager and the timestamp is retrieved. Together with the start and end indices of the group which are also included in the signature, the timestamp is attached to the next streamed frame. At this point, the accumulated hash sum of the next image group is sent to the TPM Manager for timestamping. The size of the image groups is automatically adapted to the current load of the system.

The presented approach allows us to overcome the problem of low TPM performance. By timestamping groups of frames instead of individual images, we can deliver a video stream without interruptions from TPM operations. At the same time, the security of the system is not reduced. For videos that are stored and viewed at a later point in time, integrity of images can easily be verified. For live video, integrity guarantees for the currently displayed frame cannot be given since the signature for the current image group is delivered at a later point in time. If there are no pending lifebeat requests, this delay typically is less than 1 s. Providing integrity and authenticity information for a live video stream with a delay of 1 s should be sufficient for most surveillance applications.

Note that our approach of signing groups of images also solves a problem that is introduced when using advanced video codecs such as MPEG2 or H.264. Contrary to the currently used Motion-JPEG, these codecs do not deliver full images for every video frame but differential information relative to one or more base frames. To provide meaningful signatures for such video data, it is necessary to sign groups of pictures including the base frames and the differential information instead of single frames. This means that our approach is directly applicable for such advanced video codecs.

Another issue is that not every frame is associated with a TPM tick counter value at the time it is captured. The tick counter value of the group signature effectively corresponds to the capture time of the last frame of the group. This can be compensated, if the framerate at which the camera captures images is known. It is then possible to assign a world time interval not only to the last frame of a group but again to every frame. The order of frames within a group is guaranteed via the accumulated, signed hash sum. Correct group ordering is achieved via the group timestamps.

Aside from the already mentioned properties, the proposed approach of signing image groups also has an obvious disadvantage. If a frame of a group is lost, damaged, or manipulated, the integrity and authenticity of the entire group cannot be verified and the recording time cannot be determined.

As can be seen from Table 6, the performance impact of timestamping on the achieved framerates is very small. We also evaluated the framerates when performing both, image encryption and timestamping. The reduction of framerates compared to simple streaming is between 0.5 and 2.3 fps. For larger image sizes a considerable performance reduction can be seen. This does not result from low encryption performance but from JPEG compression. JPEG encoding of a color image takes about 130 ms on TrustCAM. Further performance details are provided in [9].

For the verification of the image signature, the control station computes the hash sum for every incoming frame and stores it in a local cache. Additionally, incoming frames are checked for an attached image group signature . As the attached data also contains the start and end indices of the image group, the control station now computes the expected accumulated hash sum for the frames indicated by the start and end indices. It then loads the public signing key that belongs to the expected camera from its local database and verifies the signature of the image group:

(16)

If verification is successful, one has assurance that (1) the images of the group were not modified and (2) the images of the group come from the expected camera. The image capture time can be computed based on the lifebeat timestamps as previously described.

7. Related Work

In this section we discuss related work that addresses security questions in the context of camera systems. Additionally, we cover related research on Trusted Computing and its applications in embedded systems.

7.1. Security in Camera Systems

Serpanos and Papalambrou [27] provide an extensive discussion of security- and privacy-related issues in smart camera networks. They discuss the need for confidentiality, integrity, and freshness of data transmitted between nodes. In cases where images are transmitted, privacy of observed persons is a critical issue as it not only involves protection of sensitive information against external attackers but also against legitimate system operators. To achieve this goal, relevant parts of the images need to be recognized and encrypted.

Senior et al. [28] discuss critical aspects of a secure surveillance system including what data is available and in what form (e.g., raw images versus metadata), who has access to data and in what form (e.g., plain versus encrypted), and how long it is stored. User privacy is a major concern that is addressed in the proposed system concept. Incoming videos are analyzed and sensitive information is extracted. The extracted data is rerendered and multiple streams with different levels of data abstraction are created. By encryption of streams, multilevel access authorization is realized. The authors suggest that video analysis, processing, and encryption could either be done by a dedicated privacy console or directly by the cameras.

The work of Friedman [29] targets the questions of authenticity and integrity of images taken with a digital still image camera. Specifically, he aims at restoring credibility of photographic images by extending the microprocessor embedded in a digital camera with a unique, private signature key. This key is used to sign images before they are stored on mass storage. The public key required for verification is assumed to be made available by the camera manufacturer. Friedman suggests that the software required for signature verification should be made publicly available. Even though there exists no known implementation of the proposed concepts, this work can be seen as one of the earliest approaches towards a trustworthy, digital camera system.

Cavallaro [1] argues that digitalization of video surveillance introduces new privacy threats. Therefore, personal and behavioral data should be separated directly on the camera. While system operators only get access to behavioral data, a separate stream containing personal data is made available to law enforcement authorities. A benefit of this strict separation is prevention of operator misuse. Possible implementation approaches are not discussed in this work. To preserve privacy of monitored people, the system by Schiff et al. [7] called Respectful Cameras, automatically detects and blanks people's faces in captured images. Dufaux and Ebrahimi [30] propose a slightly different approach which does not remove but scrambles sensitive image regions. After detection of relevant areas, images are transformed using DCT. The signs of the coefficient of sensitive regions are then flipped pseudorandomly. The seed for the pseudorandom number generator is encrypted. Decryption is only possible for persons who are in possession of the corresponding decryption key. According to the authors, main benefits are minimal performance impact and that video streams with scrambled regions can still be viewed with standard players.

Boult [31] argues that many existing approaches are targeted at removing privacy sensitive image data without providing mechanisms to reconstruct the original image. Based on this observation, he proposes a system concept called PICO that relies on cryptography to protect selected image regions such as faces. This enables the monitoring of person's actions without revealing his/her identity. The faces are only decrypted if, for example, a crime was committed by the person. Encryption is supposed to be done as part of image compression and uses a combination of symmetric and asymmetric cryptography. Additionally, it is suggested to compute checksums of frames or subsequences to ensure data integrity. In related work, Chattopadhyay and Boult present PrivacyCam [8], a camera system based on a Blackfin DSP clocked at 400 MHz, 32 MB of SDRAM and an Omnivision OV7660 color CMOS sensor. uClinux is used as operating system. Regions of interest are identified based on a background subtraction model and resulting regions are encrypted using an AES session key.

Quisquater et al. [32] propose an approach for integrity protection and authentication for digital video stored on tape in the DV format. They use SHA1 to compute the hash of the image. To be less sensitive to transmission or tape errors, the authors suggest to divide images into blocks that are hashed separately. Authenticity is ensured by signing the hash values of images. The hash of the previous image is also included in the signature to maintain correct ordering of video frames.

Digital watermarks are another technique to secure digital media content. A watermark is a signal that is embedded into digital data that can later be detected, extracted, and analyzed by a verifier. According to Memon and Wong [33], a watermark can serve several different purposes. This can be proof of ownership where a private key is used to generate the watermark. Other applications are authentication and integrity protection, usage control, and content protection. Depending on the application domain, watermarks can be visible or invisible. An example where watermarking is used as part of a digital rights management system for a secure, embedded camera is presented by Mohanty [34]. He describes a secure digital camera system that is able to provide integrity, authenticity, and ownership guarantees for digital video content. This is achieved using a combination of watermarking and encryption techniques. Due to the high computational effort, a custom hardware prototype based on an FPGA is used to meet the realtime requirements.

7.2. Embedded Trusted Computing

The Low Pin Count (LPC) bus that is used to connect TPMs to the PC platform, typically is not available on embedded systems. Some manufacturers additionally equip their TPMs with a serial, two-wire interface making them suitable for embedded systems. Grossmann et al. [35] demonstrate the use of an Atmel AT97SC3203S TPM together with an MSP430 microcontroller from Texas Instruments in the context of a teletherapeutic application. In this scenario, the software state of the embedded device is attested using the TPM before sensitive information is transmitted. The authors also give measurement results for runtime and energy consumption for selected TPM commands. For example, the TPM_Quote operation was measured to take about 800 ms during which a current of 39 mA (@ 3.3 V) is drawn.

For secFleck, Hu et al. [36] mount an Atmel I2C TPM on an extension board for the Fleck mote platform which is powered by an Atmel Atmega128 running at 8 MHz. Apparently, the TPM is not used for platform attestation but only as random number generator, for RSA en- and decryption and signature creation and verification. secFleck is also used by Dua et al. [37] to enhance security of a participatory sensing application where users sense their local environment and make these measurements available to other users. The TPM is used to attest the integrity of the users' platforms. In the proposed protocol, the PCRs are signed directly using the EK which violates the TPM specification and therefore should not be possible with a compliant TPM chip. In another work by the same authors [38], a similar approach for trustworthy sensing is discussed. A peripheral platform equipped with a TPM, sensors, and bluetooth is used for sensing. A second platform, for example, a mobile phone, can query the device via bluetooth and in turn receives sensed data signed by the peripherals TPM. The authors again claim to use the EK for attestation which is not permitted by the TPM specification.

Aaraj et al. [39] evaluate the performance of a pure software TPM on an embedded platform (Xscale PXA-250 at 400 MHz with 32 MB RAM). They present runtime measurements for TPM commands including TPM Quote (1239 ms with a 2048 bit RSA key) and TPM Sign (902 ms, 2048 bit RSA key). Based on these results, the authors replaced RSA with elliptic curve cryptography (ECC) which reduced the time for TPM Quote to 381 ms (224 bit key) and TPM Sign to 191 ms (224 bit key). On average, execution time was reduced by a factor of 6.5. ECC is not supported by the current TPM specification but may be adopted in future versions. On another system (Xtensa CPU @320 Mhz) with partially customizable hardware, the authors implemented dedicated CPU instructions to accelerate ECC. With these hardware optimizations, runtimes for TPM_Quote could be reduced to 84.154 ms on a unicore and 30.70 ms on a hexa-core system.

Dietrich and Winter [24, 40] also investigate the possibility of using software-based TPM implementations for embedded systems. Many embedded systems already come with integrated security functionality such as ARM TrustZone that can be used to develop software TPM solutions. The same authors explore the use of Smart Cards or SIM cards as found in mobile phones to implement TPM functionality. Research on software TPM implementations is still in early stages but preliminary results suggest that they might be able to provide security levels similar to those of hardware TPMs.

Reconfigurable hardware such as FPGAs is commonly used in embedded systems. In such a system, not only the software but also the hardware needs to be included in platform attestation. Glas et al. [41, 42] integrate a TPM with an FPGA system. They introduce a component called Trust-Block that is responsible for securely booting the system. The FPGA itself is split into a user-programmable part and a static section. All reconfiguration of the FPGA has to be performed via functionality provided by this static section. It is also responsible for measuring the new system configuration into the TPM. Eisenbarth et al. [43] also integrate TC into an FPGA system but they do not use a dedicated TPM chip but integrate the TPM functionality as custom logic into the FPGA. Using a so-called Bitstream Trust Engine, they realize authenticity and integrity guarantees. Additionally, they also measure the TPM's implementation netlist. The main advantages of this approach are that the TPM itself becomes part of the chain of trust, TPM functionality can be easily updated and extended and TPM performance can be increased. Schelleckens et al. [44] also aim to integrate TPM functionality into FPGA-based systems. Contrary to other approaches, they do not rely on additional hardware or hardware modifications to realizes TPM functionality but use physical unclonable functions. These are unique characteristics inherent to all ICs including SRAM-based FPGAs. These functions can be used to realize secure key storage which in turn provides the foundation for a TPM implementation.

8. Conclusions and Future Work

In this paper we presented the prototype of an embedded, trustworthy smart camera system that is based on Trusted Computing technology. Using the capabilities of the Trusted Platform Module, we have realized several security properties for image and video data delivered by a camera system. Specifically, we ensure integrity, authenticity, freshness, and confidentiality. Furthermore we discussed the implementation of a chain of trust for our TrustCAM prototype and a trusted lifebeat that allows one to check the platform status and detect system reboots.

As shown in our evaluations, the performance of commercial TPMs falls behind the requirements of an embedded camera system with its realtime image processing and streaming capabilities. We, however, demonstrated that these limitations not necessarily have to impact overall system performance if the TPM functions are properly integrated into the computer vision tasks.

We believe that security of video surveillance infrastructure is a critical issue that has not been adequately addressed so far. We see this work as a first step towards filling the gap between approaches from the computer vision and the IT security communities. Trusted Computing can become a major building block in a joint effort towards a holistic security concept for trustworthy camera systems.

Nevertheless, a number of open issues remain to be addressed in future work. For trusted boot we use an addition trusted building block as initial root of trust for measurement. Many ARM processors come with on-board security extensions such as ARM TrustZone or TI M-Shield. Potentially, they can be used to implement such a root of trust but both specifications are proprietary and not publicly available. Another direction for future investigations is the Mobile Trusted Module (MTM) [45]. The MTM is specified by the TCG and is a variant of the standard TPM. It adds secure boot functionality which, contrary to trusted boot, not only allows one to measure the system status but also can prevent a system from booting if the software stack was modified.

Currently, our system is relatively static. We do not include features such as over-the-air updates of camera software. Furthermore, the assumption of a single, centralized control station might not be realistic for larger deployments. Especially the trusted lifebeat is expected to suffer from scalability issues. To reduce the load on the CS, cameras could be clustered. The status of cameras acting as cluster heads is checked directly by the control station. The cluster heads then check the status of their cluster members. The aggregated results for a cluster are reported to the CS.

In our assumptions in Section 2 we postulate that the control station is secure and trustworthy. Future work will have to elaborate the security requirements for the CS. These include topics such as checking CS requests sent to the cameras for authenticity, integrity and freshness, and reliable safeguards against operator misuse.