Lubos Gaspar
email: lubos.gaspar@univ-st-etienne.fr

Viktor Fischer
email: fischer@univ-st-etienne.fr

Lilian Bossuet
email: lilian.bossuet@univ-st-etienne.fr

Robert Fouquet
email: fouquet@univ-st-etienne.fr

Secure extensions of FPGA soft core processors for symmetric key cryptography

Keywords: Hardware security, Crypto-processor, FPGA softcore, NIOS II, MicroBlaze, Cortex M1 IN OUT Instr

When used in cryptographic applications, generalpurpose processors are often completed by a cryptographic accelerator -crypto-coprocessor. Secret keys are usually stored in the internal registers of the processor, and are vulnerable to attacks on protocols, software/firmware or cache memory. The paper presents three ways of extending soft general purpose processors for cryptographic applications. The proposed extension is aimed at symmetric key cryptography and it guarantees secure key management. Three security zones are created and physically separated in each of three configurations: processor, cipher and key storage zones. In the three zones, the secret keys are manipulated in a different manner -in clear or enciphered, as common data or keys. The security zones are separated on the protocol, system, architectural and physical levels. The proposed principle is validated on Altera NIOS II, Xilinx MicroBlaze and Actel Cortex M1 soft core processor extensions. The NIOS II processor needs fewer clock cycles per data block encryption, because the security module is included in the processor's data path. The data path of the MicroBlaze is unchanged and thus shorter, but additional clock cycles are necessary for data transfers between the processor and the security module. The Cortex M1 processor is connected via AHB bus and the cryptographic extension is accessed as an ordinary peripheral -a coprocessor. Although the interfacing is different, the three processors with their extensions attain the required high security level.

I. INTRODUCTION

Data security systems implement very often both computationally extensive parallel cryptographic functions (such as multiplication in Galois fields, modular multiplication, matrix multiplication, etc.) and complex sequential algorithms (such as cipher modes, key management and cryptographic protocols). In order to fulfill contradictory speed/complexity requirements, sequential algorithms are mostly implemented by the use of general-purpose processors, while parallel functions are implemented in the coprocessor placed inside the same cryptographic module or logic device (when a monochip solution is required). This approach is very frequent in asymmetric key cryptography [START_REF] Sakiyama | Multicore curve-based cryptoprocessor with reconfigurable modular arithmetic logic units over GF (2 n )[END_REF], [START_REF] Machhout | Coupled FPGA/ASIC Implementation of Elliptic Curve Crypto-Processor[END_REF], and also in block ciphers (such as AES) hardware implementation [START_REF] Crowe | Single-chip FPGA implementation of a cryptographic co-processor[END_REF], [START_REF] Eslami | An area-efficient universal cryptography processor for smart cards[END_REF]. Some embedded systems using processor/coprocessor approach implement both symmetric and asymmetric key cryptography algorithms in the same device [START_REF] Hani | Design and implementation of a private and public key crypto processor for next-generation it security applications[END_REF].

The use of processors weakens security in all the above mentioned solutions. In order to face side-channel attacks [START_REF] Kocher | Differential power analysis[END_REF], [START_REF] Standaert | Power analysis of FPGAs: How practical is the attack?[END_REF], the keys must be changed/manipulated regularly using a key management protocol. When a general-purpose processor manipulates confidential keys, the keys are saved in clear in processor registers or cache memory and are exposed to potential software attacks. One such attack has been recently demonstrated by Bangerter et al. in [START_REF] Bangerter | Cache games-Bringing access-based cache attacks on AES to practice[END_REF]. A small malicious software monitored the processor's cache memory during encryption and the confidential key was recovered remotely from the captured temporary data within a few minutes.

In order to counter software attacks, the authors in [START_REF] Ashkenazi | Platform independent overall security architecture in multi-processor system-on-chip integrated circuits for use in mobile phones and handheld devices[END_REF] used two processors executing different tasks on different security levels. They created two virtual zones inside the physical memory: the protected memory zone was dedicated to private key storage and the unprotected one to public data memorization. The security processor had access to both zones and the general-purpose processor was allowed to access only the public virtual zone. Since both zones were located in the same physical memory, certain types of attacks, such as protocol attacks [START_REF] Anderson | Cryptographic processors -a survey[END_REF] or timing violation attacks, were still possible.

It is clear that software attacks targeting confidential keys can be countered only if enciphering is realized independently from the general-purpose processor (GPP), e.g. in a hardware cipher and if the keys are stored in a dedicated memory. However, storing the keys in an external memory is not sufficient when facing software attacks. If confidential keys pass to the cipher via the processor in clear, they are vulnerable to attacks. In the proposed novel solution, the processor can manipulate the keys only indirectly: the keys are read/written from/to the key memory via a cipher and the processor can never read them in clear.

The paper is organized as follows: Section II proposes and discusses creation of hardware security zones that enable secure key management in conjunction with GPPs. Section III compares three basic types of interfacing GPP with external modules. Section IV describes a novel principle of the security extension for soft GPPs. In Section V, the proposed security extension is evaluated and validated on three implementations based on NIOS II, MicroBlaze and Cortex M1 processors. Results are presented in Section VI and they are discussed in Section VII. Section VIII concludes the paper.

II. PRINCIPLE OF SEPARATION OF PROTECTED AND UNPROTECTED ZONES

We have explained in the previous section that for facing software attacks on embedded systems using GPPs, processor shouldn't have access to confidential keys in clear. It is thus necessary to isolate it from the key memory. In order to fulfill the highest security requirements, the separation should be realized on four levels:

• the protocol level,

• the system level,

• the architectural level,

• the physical level. Next, we will describe these levels and their principles in more detail.

A. Separation on the protocol level

Integrity and confidentiality of exchanged keys is one of the most discussed security issues. In order to achieve a high security level, a robust communication protocol is needed. During key exchange, encrypted keys are usually transfered to the processor in packets. When a packet arrives to the processor, the keys have to be decrypted before being used for data encryption or decryption. However, if the decryption of the keys is carried out by the processor, these keys are exposed to software attacks. Therefore, keys have to be decrypted outside the processor in a dedicated unit so that unencrypted key or part of it can never leave the unit. Moreover, keys have to be authenticated before being used for data enciphering/deciphering. Once keys are decrypted and authenticated, they can be used for data encryption/decryption and authentication, but still outside the processor so that it will not have access to them. However, encrypted/decrypted data blocks can be processed directly by the processor e.g. when performing cipher mode operations.

It is the protocol that has to clearly separate key management and data processing tasks and define how and by which system units these tasks are performed. In addition to the separation of tasks, the protocol defines multiple-level key hierarchy. Higher-level keys (i. e. master keys) are used to encipher lower-level keys (i.e. session keys). The highestlevel key has to be introduced to the system in unencrypted form through a separate entry by the trusted entity. All lowlevel keys have to be either generated inside the security module using a True Random Number Generator (TRNG) or received in a packet and decrypted. The low-level keys are used for data encryption, decryption and authentication.

B. Separation on the system level

The principle of the separation on the system level is illustrated in Fig. 1. This principle is based on the creation of three zones: a processor zone, a cipher zone and a key zone. The processor exchanges data with a cipher through the data bus (in black in Fig. 1). Encrypted session keys are also transported through this data bus when being exchanged with other communication counterparts. No other way for accessing the key memory from the GPP must exist.

Unencrypted keys are stored in a dedicated memory situated in a key storage zone. The key memory is separated from the GPP by the cipher. The keys are transfered between the cipher block and the key memory via the key bus (in grey in Fig. 1). This bus must be completely separated from the data bus interconnecting the GPP with the cipher. It is essential that paths allowing unencrypted secret keys to pass from the key bus to the data bus must not exist. This precondition is one of the most important from the security point of view because it enables separation of the key storage and the processor zones.

Before enciphering/deciphering data blocks, the cipher is initialized with the selected key via cipher key bus (in dashed grey in Fig. 1). Key selection is controlled by the processor through a control bus. The control path must be organized so that in case of an attack on the control bus, the selection of an incorrect key (e.g. by addressing an all-zeros key out of the key address range) must be prevented.

The principle of creation of security zones is independent from the type of encryption algorithm -any symmetric key block cipher with or without side channel attack countermeasures can be used. Furthermore, the two separation walls delimiting the cipher can be used during the dynamic cipher reconfiguration when changing cryptographic algorithm, while maintaining the key memory contents unchanged.

Generalpurpose processor

C. Separation on the architectural level

To achieve a high quality physical separation, the architecture of buses has to be organized in such a way that even after a physical attack, keys from the key zone cannot pass unprotected into the data zone. Bus multiplexers directing the flow of data to/from the cipher have to be placed so that even if their control is violated, no physical path for keys to escape from the key zone can be created.

D. Separation on the physical level

In order to achieve a higher level of physical separation, an isolated area of the chip has to be dedicated to each zone. On the border of the two neighboring zones, an empty area has to be used. This area represents an insulation wall between two neighboring zones (see Fig. 1). Only selected signals are allowed to cross this wall. This countermeasure minimizes the possibility of the loss or corruption of secret keys by residual electromagnetic radiation from the protected zone (i.e. key zone) to the unprotected zone (i.e. processor zone). Using this approach, a physical attack on one zone has a minimal impact on the other zones. This physical insulation principle is recommended by NSTISS in its Red/Black installation guidance [START_REF] Mcconnell | TEMPEST/2-95[END_REF].

III. COMMUNICATION INTERFACE

The data interface between the security module and the processor plays very important role in the system design. When considering the architecture, a trade-off has to be found between the performance, area and security criteria. Unfortunately, this can lead often to contradictory requirements.

A. Interface properties

Overall system performance depends on the interface type and parameters, such as bus width and latency. When managing keys, small data blocks are exchanged between the processor and the security module and pipelining is not efficient. In order to achieve higher performance, it is the best to suit the bus width to the cipher width. Unfortunately, this is not always possible.

From the security point of view, point-to-point communication is in general more secure than point-to-multipoint commnunication, because data are exchanged only between two units, and other peripherals are not physically connected. When using a point-to-multipoint interface, the bus is shared among all communication counterparts, and data exchanged between the security module and the processor can be potentially eavesdropped by other peripherals. In this case, some techniques, such as small firewalls protecting each peripheral on the bus [START_REF] Cotret | Distributed security for communications and memories in a multiprocessor architecture[END_REF], can be used.

B. Interface types

The security module can be interfaced with a GPP using the following types of connections:

1) The internal processor bus: The security module is included in the processor data path. Data pass to the module directly from registers as operands. Results are returned to the registers. The security module is controlled directly by the processor control unit through a dedicated control bus. The advantage of this solution is its high performance and minimal latency. Unfortunately, the module is a part of the processor's critical path and therefore it can slow the whole system down if not properly designed. A high security level is naturally achieved by the point-to-point connection.

2) The coprocessor-like bus: The security module is not included in the processor data path, but it is connected through a fast internal bus (often a coprocessor bus), running mostly at the same clock frequency as the processor core. This bus enables direct access to the processor registers thus minimizing communication latency, although the latency is higher when compared to the previous alternative. Considering the connection between a single security module and a processor, the connection has a point-to-point nature therefore high security level is maintained.

3) The peripheral bus: The security module is connected to the processor through a bus using a point-to-multipoint connection. In the best case, the module is connected directly to a high-performance system bus, otherwise data are transferred across one or several bus bridges increasing the latency. Furthermore, because bus is shared between all system units, performance is significantly decreased. From the security point of view, point-to-multipoint bus is less suitable for security applications because data transfers between security unit and processor can be potentionally eavesdropped by other units connected to the same bus.

IV. IMPLEMENTATION OF THE SECURITY MODULE

The implementation of the security module is illustrated in Fig. 2. Three zones (the processor interface, the cipher and the key storage zone) can be clearly distinguished. Three type of buses are used: data bus (in black), key transfer bus (in grey) and cipher key bus (in dashed grey). It is important that key buses never pass through the processor interface zone and data buses never pass through the key storage zone. The module is organized so that secret keys can never leave the key zone without passing through the cipher. This strict separation on the architectural and physical level guarantees a very high security level.

As presented in Sec. II, any encryption algorithm can be used in the security module. In order to validate the principle, we use a 128-bit Advanced Encryption Standard (AES). Input data, keys and output data are registered in cipher input/output registers. The cipher has a 128-bit folded data path and encryption is completed in 11 clock cycles.

Keys are organized in two hierarchical levels. High-level master keys for encryption and authentication are stored in the master key register. These keys are initialized via dedicated key input during system initialization by the trusted entity. Master keys are used solely for encryption, decryption and authentication of low-level session keys. Session keys are generated inside the module by a TRNG (any principle can be used) or received from the processor and decrypted and authenticated using master keys. When generated inside the module, keys are post-processed in the decipher core.

Session keys are used only for encryption/decryption (using cipher modes) and authentication of data (e. g. using CBC-MAC mode).

The authentication of keys is supported directly inside the module by including a comparator (CMP) that is responsible for comparison of digital fingerprints. The security module is controlled by a local control unit (CTRL) that interprets the fetched instructions. Comparison results or cipher status flags are saved in the status register, which can be read by the processor. This principle of demand-response dialog permits a secure and high-performance operation of the module.

A. Communication algorithm

The communication protocol between two devices denoted as side A and B is illustrated in Fig. 3 where tasks 1, 2, 3, 5, 9 and 10 are performed solely by the security module. On the other hand, tasks 6, 7, 8 are executed only by the GPP. Tasks 4 and 11 are performed partly by the GPP (xor, cmp) and partly by the security module (AES ciphering).

Before establishing the communication channel between sides A and B, master key registers of the communicating security modules have to be initialized with the same encryption (M K) and authentication keys (AM K) by a trusted entity T E.

Subsequently, the device that starts the communication (A) has to generate new session key SK. The session key CSK is generated by the TRNG inside the security module on the communication side A. The key CSK is then post-processed in the decipher, using a master key M K, and stored in the session key register. Afterwards, a digital fingerprint F P A is generated by enciphering the generated key using authentication master key AM K.

When both session key SK and its fingerprint F P A are generated, the data blocks (DAT A) can be encrypted using the session key SK and sent to the GPP. The GPP implements cipher modes by processing subsequent data blocks according to the selected algorithm. Finally, the processor creates a packet P . This packet contains generated session key CSK encrypted with the master key M K, its fingerprint F P A , and encrypted data CDAT A. At the end of the transaction, the packet P is sent to communication partner B.

Processor B receives the packet P , recognizes the packet header and extracts the encrypted session key CSK with its digital fingerprint F P A . The key is sent to the security module, where it is decrypted using master key M K and stored in the session key register. The fingerprint F P B of the session key SK is computed by enciphering it with the master authentication key AM K. The processor sends the received fingerprint F P A to the security module, where it is compared with the computed fingerprint F P B . If the fingerprints match, the receiving processor can decrypt data using the acquired session key SK by executing selected cipher mode operations.

V. EXTENSION OF SELECTED SOFT-CORE PROCESSORS

Proposed security extensions were implemented in three FPGA families using Altera NIOS II, Xilinx MicroBlaze and Actel version of Cortex M1 soft processors. Each processor system included the same security module that was connected to the processor using a wrapper, which was compatible with the processor interface. The wrapper is responsible for the translation of the control commands and the bus width conversion (128-bit security module data bus is transformed into 32-bit processor bus). Both communication interfaces use point-to-point communication increasing device security.

A. NIOS II with the security module extension

Implementation of the NIOS II processor with its security extension is illustrated in Fig. 4. All security module operations are implemented as custom instructions. Data are transfered from the processor register in 32-bit words via the wrapper. When the instruction execution is finished, data from the security module are sent back to the processor, again in 32-bit blocks.

Because of the use of custom instructions, the NIOS II control unit drives signals that control directly the operation of the security module. This direct connection of the control interface eliminates unwanted latency increase, thus accelerating the execution of custom instructions.

Since the security module is included in the NIOS II data path, the critical path of the processor is extended by the data path of the security module. This directly affects the processor maximal clock frequency.

Data registers

ALU

Control unit

A B ALU OUT

NIOS II

Master Key

B. MicroBlaze with the security module extension

Implementation of the MicroBlaze processor with its security extension is illustrated Fig. 5. In contrast to the NIOS II processor, custom instruction set implementation is not possible. However, MicroBlaze architecture supports a highperformance coprocessor-like Fast Simplex Link (FSL) bus, which allows interfacing external modules with processor registers. The FSL bus is 32-bit wide. Therefore 128-bit data blocks have to be divided into four 32-bit blocks before being transfered to the security module.

Unfortunately, FSL standard doesn't define any control interface, so that control signals cannot pass directly from the MicroBlaze control unit to the security module. Control instructions are transfered with data to the security module via the FSL data bus and FIFOs. Before each operation, one 32-bit instruction word has to be sent. However, this operation imposes an additional instruction on the program code, what slows down the code execution and data exchange between the processor and the security module. On the other hand, the security module is not a part of the processor's critical path, thus the clock frequency of the processor is not affected. Despite the fact that FIFOs insert additional latency, they enable separation of processor and security module clock domains, thus security module can run on a higher clock frequency than the complex processor.

C. Cortex M1 with the security module extension

Implementation of the Cortex M1 processor with its security module is illustrated Fig. 6. Unlike the previous two processors, Cortex M1 doesn't allow implementation of any point-to-point interface interface suitable for security module implementation. The only way to add custom blocks to the Cortex M1 system is to interface them through an AMBA bus. The AMBA specification describes two types of buses -AHB and APB bus. The APB bus is not considered in this work because of its lower performance. The AHB The three processors with their security extension modules presented in the previous section were described in VHDL and mapped to three FPGA families. The NIOS II system was implemented in Altera NIOS II evaluation board featuring Stratix II device EP2S60F672C5ES. The project was compiled and mapped to the selected device using Quartus II version 9.2. The MicroBlaze system and its extension were implemented in Xilinx ML605 evaluation kit featuring Virtex 6 device XC6VLX240TFF1156. For synthesis and mapping, ISE version 12.4 was used. The Cortex M1 system and its extension were implemented in Actel Fusion embedded development kit board featuring Actel Fusion device M1AFS1500-FGG484. The project was compiled and mapped to the selected device using Libero version 8.5 SP2. A small hardware module including the Cypress EZ USB interface device CY7C68013A was connected to all three evaluation boards for data transfers from/to the PC.

The implementation results concerning the logic area and memory requirements are presented in Tab. I and Tab. II. The area is expressed in number of occupied Adaptive Logic Modules (ALM) for Altera family, Slices for Xilinx family and Tiles for Actel family. For comparison, we recall that one ALM in Altera Stratix II family contains two 4-input Look-up tables (LUTs) and two flip-flops (FFs). One Slice in Xilinx Virtex 6 family contains four 6-input LUTs and eight FFs. One Tile in Actel Fusion family contains either one 3input combinatorial function or one FF. Therefore, the results cannot be directly compared. The memory requirements are given in kbits for all technologies. Besides the processor and its security extension, a small block containing 16-bit data interface to the external Cypress USB device was embedded in all systems. It was used only for testing purposes and it does not constitute an inherent part of the system. It is therefore not included in Tab. I and Tab. II. For clarity, we present the results for the processor and for its extension separately. The results are discussed in the next section.

In order to compare the achieved throughput fairly, the clock frequency of all three systems was set to 50 MHz. The throughput was evaluated by transferring packets from the PC to the FPGA (and vice versa) via USB interface. Each packet contained an encrypted session key, its digital fingerprint and five 128-bit payload blocks. Packets were analyzed in the processor, which then sent the session key and its fingerprint to the security module. Once the key was decrypted and authenticated, the processor sent data blocks to be decrypted. Subsequently, the processor recreated new packets containing received decrypted data and sent them back to the PC. When implementing this complete protocol, the NIOS II-based system achieved the overall throughput of 25,1 Mb/s, the MicroBlaze-based system achieved 18,4 Mb/s and the Cortex M1 system achieved 12,2 Mb/s.

The security of all three solutions depends on the type of interface between security module and processor. Since separation of the key and processor zones was achieved on the protocol and architectural level, all three implementations should be robust against software and timing violation attacks. In order to validate this assumption, software attacks were implemented in tests by reordering and/or replacing instructions. However, these tests have shown that during CBC decryption mode, replacement of GPP instructions led to a substitution of data by encrypted session keys previously stored in the GPP registers and their subsequent decryption inside the security module. It was clear that CBC mode can not be fully supported, otherwise security backdoor is created. Implementation of security module enabling the CBC decryption mode backdoor is illustrated in the Fig. 7. For the sake of security, CBC decryption mode is no longer supported in the implementation presented in this paper. However, CBC mode can be still used for authentication because CBC-MAC standard does not use the Decipher (encryption is safe). Furthermore, the timing violation attack was carried out by increasing the clock frequency over its maximum allowed value. This technique allowed to introduce faults to the control logic of the security module, however no key fragments were acquired from the module. The tests confirmed the robustness of the proposed techniques of separation that were shown to be secure-bydesign. Area requirements for NIOS II, MicroBlaze and Cortex M1 extensions from Tab. I and Tab. II seem to be different. This is due to differences between ALMs, slices and tiles in Altera, Xilinx and Actel families and also because of the size of the soft processors. In the Altera FPGA, the security module area is similar to that of the NIOS II processor (1327 versus 1204 ALMs giving 110%). However, since the security module included both AES cipher and decipher cores, we can conclude that the sole security extension cost due to zones separation is negligible. On the other hand, the MicroBlaze processor occupies bigger area and the security module overhead is only 45%. The separation cost remains negligible. In Cortex M1 system, the security module represents a 60% overhead therefore cost remains negligible.

As presented in Section VI, the MicroBlaze processor with its extension achieves approximately 73% of the throughput of the NIOS II implementation. This is caused by more complicated communication protocol (data and control words) across the FSL bus in MicroBlaze, compared to the straightforward custom instruction implementation in NIOS. This difference could be reduced if data were transferred to the security module using DMA transfers. This way, the FSL bus would serve only for transporting instructions to the security module. Moreover, the Cortex M1 processor with its security module extension achieves only 49% of the throughput of the NIOS II implementation. This is because of the nature of the AHB bus that is shared among all communicating units. Therefore access of the security module to the AHB bus is multiplexed with flash memory, RAM and I/O (USB). This fact limits significantly overall system performance.

Another useful aspect of comparing the three processor extensions is the system design complexity. When interfacing the NIOS II processor with the security module, several actions are needed: 1) the processor data path and control signals are output from the processor; 2) a wrapper containing simple control interface and data bus translation interface has to be implemented; 3) the wrapper with the security unit has to be interfaced with the NIOS II processor using SOPC builder software; 4) custom instructions have to be generated and described in the software implementation. We can conclude that the design is relatively complex, but as a result, the security module is closely tied to the processor and thus maintaining high performance. Connecting the security module to the MicroBlaze processor is simpler on hardware level, but more complex on protocol level: more complex state machine has to be implemented in the wrapper to pick up control instruction from the data stream transported via the Fast Simplex Link. Because of this fact, the overall speed of the system is significantly lower. Connecting security module via a peripheral bus is the most general solution. Once the data interface is designed, the module can be reused with any processor featuring a common peripheral bus. However, in this case, the speed of the system is the lowest.

Attacks that were carried out revealed a vulnerability in the CBC decryption mode and this backdoor was eliminated by disconnecting Decipher output from the output of the security module. Nevertheless, CBC decryption mode is often used nowadays and its secure implementation remains an open problem. Timing violation attacks that were carried out against the processor implementations were not successful. The implemented protocol and architectural separations of the key and processor zones were therefore shown to be efficient. Furthermore, higher security level could be achieved if the security zones were physically isolated on the chip. Although this physical isolation by insulation walls was not realized, it can be easily done in the future because of existing strict separation of modules.

The principle presented in this paper concerns three soft core GPPs. We believe that it can be extended to other GPPs. However, faster and perhaps more secure implementations can be obtained by creating a specific-purpose processor such as the one published in [START_REF] Gaspar | HCrypt: A Novel Concept of Crypto-processor with Secured Key Management[END_REF], which takes advantage of reduced instruction set oriented in cryptographic operations. Nevertheless, this custom processor has to comply with all the above mentioned separation techniques.

Since encryption standards are always evolving, it is important that the cipher implementation inside the security module could be easy to update. One of possible solutions could be to use a dynamic reconfiguration technique. This technique allows for creation of a special dynamically reconfigurable zone reserved for the complete Cipher zone from Fig. 1. Afterwards, the enciphering algorithm can be updated on the fly. It is very useful that the used physical separation techniques are similar to those used in dynamic reconfiguration. Thanks to this, only a small effort is necessary when implementing reconfigurability features in the device. Moreover, the zone that is dynamically reconfigurable can also be easily isolated from surrounding modules by creating an empty space that will serve as an insulation wall around the secured reconfigurable zone.

VIII. CONCLUSION

We have proposed a novel principle allowing generalpurpose processors to operate with secret keys in a highly secure way. The principle is based on the creation of separated processor, cipher and key zones. Separation is implemented on the protocol, system, architectural and physical levels, and it guarantees that unencrypted keys can never be transfered from protected key zone to unprotected processor zone. The only way to transfer the keys to the processor zone is to pass across the cipher zone: the keys are encrypted before entering the processor zone and must be decrypted when going in opposite direction (when entering the memory zone). The proposed solution enhances security substantially when compared with existing soft-core cryptographic extensions.

The separation principle was implemented in FPGAs and tested using NIOS II, MicroBlaze and Cortex M1 processors. The obtained throughput including the processing of packets, key management and data encryption/decryption and authentication was about 25, 18 and 12 Mb/s, respectively. This speed was limited mainly by processors and their data interfaces. The area of the system increased by 110% when compared with the smaller NIOS II processor, by 44% when the MicroBlaze processor was taken as a basis and by 60% when Cortex M1 was extended using the security module.

Figure 1 :

 1 Figure 1: Separation on the architectural level

Figure 2 :

 2 Figure 2: Security module implementation

Side B 1 . 1 ) 2 .Figure 3 :

 1123 Figure 3: Communication protocol between two devices

Figure 4 :

 4 Figure 4: Interfacing NIOS II with the security module

Figure 6 :

 6 Figure 6: Interfacing Cortex M1 with the security module

Figure 7 :

 7 Figure 7: Security module with CBC decryption mode backdoor

/ TRNG 1. Processor zone 2. Cipher zone 3. Key zone

		Cipher key
	Data Control bus bus	separation walls CIPHERPhysical	bus	bus Key data

KEY STORAGE Security module

Table I :

 I FPGA fine-grain resource utilization

		Extended	Extended	Extended
		NIOS II	MicoBlaze	Cortex M1
		(ALMs)	(Slices)	(Tiles)
	System total	2531	1954	15053
	→ Processor	1204	1350	9433
	→ Sec. module	1327	604	5620
	Ext. overhead	110.2%	44.7%	59.6%
	Table II: FPGA embedded RAM utilization
		Extended	Extended	Extended
		NIOS II	MicoBlaze	Cortex M1
		(RAM kb)	(RAM kb)	(RAM kb)
	System total	243.9	1206.0	216
	→ Processor	187.9	774.0	104
	→ Sec. module	56.0	432.0	112
	Ext. overhead	29.8%	55.8%	107.7%

ACKNOWLEDGMENT

The work presented in this paper was realized in the frame of the SecReSoC project number ANR-09-SEGI-013, supported by the French National Research Agency (ANR).


Arthur Zakinyan
email: zakinyan.a.r@mail.ru

Yury Dikansky

Drops deformation and magnetic permeability of a ferrofluid emulsion

Keywords: ferrofluid, emulsion, permeability, structural transformations

In the paper the novel soft magnetic composite system is investigated. A ferrofluid emulsion studied demonstrates the strong magnetic properties which are atypical for commonly known emulsions. Interaction of ferrofluid emulsions with a magnetic field is considered. Structural transformations in these media, such as deformation of emulsion microdroplets and emulsion inversion, are studied. The changes in the relative permeability of emulsion associated with structural transformations are investigated. The theory of the observed phenomena is developed, and the feasibility of effectively controlling the magnetic properties of ferrofluid emulsions by applying a magnetic field is demonstrated.

Introduction

The use of the materials which can become anisotropic under the external action is widely considered as an effective way to obtain desirable material properties. One of the approaches to achieve this goal is based on the use of composite material, whose properties strongly depend on its structural microgeometry. In several researches much attention has been paid to the use of external electric fields for the modification of properties of a composite material [START_REF] Volchek | Concentration dependence of droplet deformation in a phase separation process under an electric field[END_REF][START_REF] Park | Crystallization of poly(ethylene oxide) in a photopolymerizable monomer under an electric field[END_REF][START_REF] Wen | Structure-induced nonlinear dielectric properties in electrorheological fluids[END_REF]. The magnetic-field-induced structural transformations are also observed in different soft magnetic systems [START_REF] Bossis | Magnetorheology of a millimetric steel spheres suspension[END_REF][START_REF] Martinez-Pedrero | Aggregation of magnetic polystyrene particles: a light scattering study[END_REF]. As an example, in several recent works, attempts have been undertaken to create new liquid magnetizable composites based on ferrofluids that can more effectively interact with external magnetic fields [START_REF] Popplewell | Magnetorheological fluid composites[END_REF][START_REF] Lopez-Lopez | Magnetorheology for suspensions of solid particles dispersed in ferrofluids[END_REF][START_REF] Helgesen | Nanoparticle induced self-assembly[END_REF]. In the present work we study magnetic and structural properties of new composite system which is a ferrofluid emulsion.

A ferrofluid is a colloidal suspension of ultra-fine ferro-or ferri-magnetic nanoparticles suspended in a carrier fluid. Previously some properties of the emulsions with ferrofluid droplets dispersed in water have been studied in several works. The droplets of the studied emulsions hold the spherical shape and align in chain-like aggregates under the action of magnetic field. The structural [START_REF] Zhang | Field-induced forces in colloidal particle chains[END_REF][START_REF] Flores | Magnetic-field-induced nonequilibrium structures in a ferrofluid emulsion[END_REF], optical [START_REF] Ivey | Magnetic-field-induced structural transitions in a ferrofluid emulsion[END_REF], and magnetic [START_REF] Yu | Magnetization properties of magnetic emulsions[END_REF] properties of such emulsions have been investigated. In this paper we consider the ferrofluid emulsions with deformable droplets under the action of magnetic field. Furthermore, we will consider the emulsions comprising a ferrofluid as a dispersed phase and also as a dispersion medium.

As reported in [START_REF] Balabanov | Magnetic susceptibility of a magnetic emulsion[END_REF][START_REF] Blums | Magnetic Fluids[END_REF], ferrofluid with drop-like aggregates exhibits similar properties. Such system was obtained by diluting the initial kerosene-based ferrofluid with a coagulator mineral oil. Drop-like aggregates are easily deformable in magnetic fields, but the concentration of aggregates in ferrofluid cannot be controlled. In addition, it is difficult to determine the properties of aggregates and surrounding medium independently and precisely. The ferrofluid emulsions studied in the present work do not possess such disadvantages.

Experimental

In our experiments we used a kerosene-based ferrofluid with dispersed magnetite nanoparticles of about 10 nm diameter stabilized with oleic acid. The properties of the ferrofluid are: density is 1620 kg m -3 , dynamic viscosity is 29 mPa s, magnetite volume fraction is 19% and saturation magnetization is 55.4 kA/m. The ferrofluid used was made by OJSC "NIPIgaspererabotka" (Russia).

Ferrofluid emulsion may be produced by dispersing either a ferrofluid in a nonmagnetic liquid or a nonmagnetic liquid in ferrofluid. FH 51 aviation oil immiscible with the ferrofluid was selected as the nonmagnetic liquid. Its density is 776 kg m -3 and the dynamic viscosity is 14.5 mPa s. The main reason to use this oil is that the interfacial tension at the interface between it and the ferrofluid is quite low (= 10 -6 N/m). The radii of the emulsion droplets are varying from 1 to 5 m, so the ferrofluid can be considered as continuous liquid magnetizable medium. No stabilizing agents were used in the preparation of the emulsions.

The structural state of the emulsion was examined under an optical microscope. It was found that, when emulsion is subjected to a magnetic field, its drops can deform and take the shape of ellipsoid of revolution elongated along the direction of a magnetic field. Because of low interfacial tension, under the action of comparatively weak magnetic field significant deformation of droplets takes place. As an example, the layer of emulsion of oil drops in ferrofluid under the action of constant uniform magnetic field produced by the Helmholtz coils (H = 1.5 kA/m) is shown in Fig. 1. The volume fraction of oil drops is 10%. We have measured the semiaxial ratio a/b (a is the major semiaxis parallel to the field and b is the minor semiaxis) of the deformed oil drops in this emulsion as a function of the magnetic Bond number defined as Bm= 0 H 2 R/, where  0 is the magnetic constant, H is the magnetic field strength, and R is the radius of the drop in zero field. The results are shown in Fig. 2.

It was observed that, if an emulsion is concentrated to ~ 50 vol. %, it will invert when diluted with additional internal phase. These structural transformations must change the macroscopic properties of the emulsion, in particular its magnetic properties.

The relative permeability of the emulsion was determined by measuring the inductance of a solenoid with the emulsion inside. In our experiments we used the measuring bridge "LCR meter-817" to measure the inductance. The frequency of a measuring field was 1 kHz and its strength was sufficiently small (~ 50 A/m) to have no effect on the structural state of the emulsion. To study the influence of a magnetic field on the permeability of the ferrofluid emulsion, it was subjected to an external constant magnetic field directed in parallel with the alternating measuring field. To this end, the solenoid filled with the emulsion was placed in a cubic magnetizing system providing a uniform magnetic field at the place of the solenoid. Since the diameter of the solenoid (0.5 cm) is much less than its length (20 cm) we can assume that the magnetic field inside the solenoid is equal to the external magnetic field.

Fig. 3 shows the obtained dependences of the emulsion permeability on the constant magnetic field strength at different volume fractions of the dispersed phase. Squares in Fig. 3 correspond to the emulsion of oil droplets dispersed in the ferrofluid (hereafter, oil-in-ferrofluid emulsion); circles correspond to the emulsion of ferrofluid droplets dispersed in the oil (ferrofluid-in-oil emulsion). As is seen, these dependences have maxima, which are not observed for the initial pure ferrofluid and may serve as a characteristic peculiarity of ferrofluid emulsions. Similar field dependence of magnetic permeability of ferrofluid with drop-like aggregates was observed in [START_REF] Balabanov | Magnetic susceptibility of a magnetic emulsion[END_REF][START_REF] Blums | Magnetic Fluids[END_REF]. The field dependence of the permeability of the pure ferrofluid,  f , used in our experiments is shown in Fig. 4.

The appearance of a maximum can be explained in the following way. The influence of a magnetic field on the permeability of a ferrofluid emulsion shows up in the form of two competing factors. Upon stretching the microdrops in a constant field the demagnetizing factor diminishes for each drop and the magnetic permeability of an emulsion grows. On the other hand, the pure ferrofluid decreases in permeability when the biasing field is augmented, as follows from Fig. 4.

As is seen from Fig. 3, the run of the curves is nearly the same; however, the permeability of ferrofluid-in-oil emulsion rises more considerably in the low-field range. It may be assumed that the effect of drop elongation and the decrease in the demagnetizing factor are more pronounced for ferrofluid drops dispersed in the oil. It should be noted that weaker deformation of nonmagnetic drop in magnetic medium in comparison with the magnetic drop for the same values of the parameters was previously observed in [START_REF] Blums | Magnetic Fluids[END_REF].

The concentration dependence of permeability of an emulsion has been measured. The amount of the dispersed phase was increased by gradually raising the volume fraction of the ferrofluid in the emulsion. The obtained dependence of emulsion permeability on the volume fraction of the ferrofluid is presented in Fig. 5. In the absence of an additional magnetic field, the concentration dependence is seen to have an inflection point at concentration ~ 50 vol. %, which indicates a change in the phase state of the emulsion. At this concentration phase inversion from ferrofluid-in-oil emulsion to oil-in-ferrofluid takes place. The action of magnetic field changes the run of the concentration dependence in Fig. 5. In particular, the inflection point virtually disappears. Thus, it may be concluded that inversion of the emulsion shows up as a significant change in the character of its magnetization.

Each dependence of magnetic permeability has been measured three times, and the obtained results are within 5% of each other.

It should be noted that the sedimentation of droplets can take place in the studied emulsions. Microscopic observations and magnetic measurements have shown that marked changes of properties of the emulsion owing to sedimentation can be detected in 1 or 2 h after emulsion preparation. On the other hand, it takes several minutes to perform each measurement described above. Thus we can consider an emulsion stable during measurements and neglect the sedimentation in the further analysis.

Theory

Theoretical analysis of magnetic permeability of a ferrofluid with drop-like aggregates in effective medium approach is given in [START_REF] Blums | Magnetic Fluids[END_REF]. Wagner's formula valid in a limit of low concentrations was used for calculation of permeability. In the present work we extend this analysis to the case of high volume fractions by using Bruggeman's formula for the effective medium approximation. Also we take into account the effect of macroscopic magnetic anisotropy of the emulsion.

In consequence of droplets deformation under the action of magnetic field the emulsion becomes anisotropic. We will assume that emulsion droplets are identical and have a shape of a prolate ellipsoid of revolution with the semiaxial ratio  = a/b. As shown in [START_REF] Berthier | Anisotropic effective medium theories[END_REF], the magnetostatic problem for an ellipsoid characterized by a , embedded in an anisotropic medium, can be transformed into the problem for an ellipsoid with a renormalized ' = ( x / z ) 1/2 in an isotropic medium. Taking into account the macroscopic anisotropy of the medium, the components of  are determined by the system of nonlinear equations:

                   1 e i e i z N z z (1)                    1 e i e i x N x x (2)
where  i is the permeability of internal phase,  e is the permeability of external phase,  is the volume fraction of internal phase, and

                                 e e e e e N z 2 1 1 ln 2 1 3 2 (3) 2 1 z x N N   (4)
are the demagnetizing factors of particles of internal phase [START_REF] Landau | Electrodynamics of Continuous Media[END_REF], with e(')=(1-1/' 2 ) 1/2 being the eccentricity. The direction of a magnetic field corresponds to the z axis. The renormalized semiaxis ratio ' <  takes into account the effect of macroscopic magnetic anisotropy. The value of  z has been measured in the experiments described above. According to [START_REF] Volchek | Concentration dependence of droplet deformation in a phase separation process under an electric field[END_REF] and ( 2), the permeability of the emulsion nonlinearly depends on the volume fraction of the dispersed phase. The permeability also depends on the external magnetic field, since demagnetizing factor, N z , decreases with increasing field due to deformation of the droplets. To find the demagnetizing factor as a function of the field strength, we need to determine the relationship between the droplet deformation, , and the magnetic field strength. The problem of deformation of single ferrofluid drop under the action of magnetic field has been investigated experimentally and theoretically in several works; the review of these works is presented in [START_REF] Blums | Magnetic Fluids[END_REF].

The results of [START_REF] Blums | Magnetic Fluids[END_REF] are valid only in the limit of a small volume fraction, , of embedded droplets, when the magnetic interaction between them can be neglected. In this work we extend the results of [START_REF] Blums | Magnetic Fluids[END_REF] to the case of finite- composite materials by considering the magnetic interparticle interaction, which becomes significant on increase of . We will consider the competition between the magnetic energy and the interfacial energy. The equilibrium shape of each droplet corresponds to the minimum of the free energy of a unit volume of an emulsion. Within the effective medium approach the free energy, F, of a unit volume of monodisperse emulsion can be written as

  1 s 1 2 0 1 2 1 W V H F z        (5)
where V 1 =4/3R 3 is the volume of emulsion droplet,  0 =4 10 -7 H/m, W s1 is the surface energy of individual drop. We will neglect the magnetostriction effect and assume that V 1 is constant. Surface energy of an ellipsoidal drop is given by

                        2 1 2 3 1 2 2 1 s 1 arcsin 1 1 2      e e e e R W (6)
where is the interfacial tension and e()=(1-1/ 2 ) 1/2 . The equation determining the position of the free energy, F, extremum can be written as 0     F [START_REF] Lopez-Lopez | Magnetorheology for suspensions of solid particles dispersed in ferrofluids[END_REF] The relationship between the drop deformation and the field strength may be obtained from numerical analysis of [START_REF] Lopez-Lopez | Magnetorheology for suspensions of solid particles dispersed in ferrofluids[END_REF]. Fig. 6 shows the calculated dependence of the droplet deformation in a ferrofluid-in-oil emulsion on the dimensionless parameter Bm for different values of .

The magnetic Bond number, Bm, characterizes the relative strength of the magnetic and surface forces. The values  i = 6.4 and  e = 1 were used for calculations, presented in Fig. 6. As is seen, the deformation depends strongly on the volume fraction. A comparison of the calculated deformation of oil-in-ferrofluid emulsion droplets with the experimentally observed deformation is presented in Fig. 2.

Thus, the solution to the problem of magnetic permeability of a ferrofluid emulsion is presented by expressions (1) - [START_REF] Lopez-Lopez | Magnetorheology for suspensions of solid particles dispersed in ferrofluids[END_REF]. It is necessary to take into account that the permeability of one of the emulsion components (ferrofluid) is also field-dependent. We will employ the empiric field dependence of the ferrofluid permeability, shown in Fig. 4. The problem can be solved numerically. When describing the behavior of the oil-in-ferrofluid emulsion,  e denotes the permeability of the ferrofluid,  f , and  i = 1. For the ferrofluid-in-oil emulsion, on the contrary,  e = 1 and  i is the permeability of the ferrofluid. Drop mean radius R ≈ 3 m was used for calculations. The results of calculations are shown in Figs. 3 and5. As is seen, experimental results are in good agreement with theoretical calculations.

The essentially different responses of the ferrofluid-in-oil and oil-in-ferrofluid emulsions to application of magnetic field are observed in the concentration dependence of the relative change of the emulsion permeability (Fig. 7). In Fig. 7 curve 1 is the theoretical dependence calculated for the oil-in-ferrofluid emulsion; curve 2 is the theoretical dependence for the ferrofluid-in-oil emulsion. Both dependences are plotted for the case when the permeability changes under the action of field H = 750 A/m. This value of a magnetic field approximately corresponds to the maximum on the field dependence of emulsion permeability. For comparison, the data points are also shown. The ferrofluid-in-oil emulsion is seen to respond to the field more strongly. The smaller effect in the case of oil-in-ferrofluid emulsion is the direct consequence of weaker deformation of nonmagnetic drop in ferrofluid in comparison with the ferrofluid drop in nonmagnetic liquid. Fig. 7 also demonstrates the change of magnetic properties of an emulsion during the phase inversion process. From Figs. 5 and7, it follows that in the concentration region from 40 to 60 vol. % (approximately), the emulsion exhibits intermediate properties. In this concentration range the emulsion properties gradually changes from properties of ferrofluid-in-oil emulsion to properties of oil-in-ferrofluid emulsion. Magnetic measurements and microscopic observations made at different values of a magnetic field strength have shown that the magnetic field has no effect on the phase inversion process.

It should be noted that under the action of comparatively strong magnetic field (≥ 2.5 kA/m) the coalescence of emulsion droplets was observed. However, in the presented analysis we do not consider the coalescence influence on magnetic properties of an emulsion. Also we have neglected polydispersity of an emulsion.

Conclusion

Thus, in the present work we studied a soft magnetic system of new type. We have observed that the interaction of ferrofluid emulsions with a magnetic field is accompanied by deformation of emulsion droplets. The phase inversion in the ferrofluid emulsion also has been studied. The influence of these processes on magnetic properties of an emulsion has been investigated. The theory of the observed phenomena has been developed. The emulsions studied can be categorized as fluids with magnetic-field-controllable magnetic properties.

Figure Captions

Fig. 1 .Fig. 2 .Fig. 5 .

 125 Figure CaptionsFig. 1. Ferrofluid emulsion under the action of magnetic field (H=1.5 kA/m). Droplet volume fraction is 10%. Fig. 2. Semiaxis ratio of oil drops dispersed in ferrofluid as a function of magnetic Bond number, Bm. The volume fraction of oil drops is 10%. Dots are experiments; line is calculations. Fig. 3. Permeability of the ferrofluid emulsion vs. the magnetic field strength. Dots are experiments; lines are calculations. Squares correspond to the oil-in-ferrofluid emulsion; circles correspond to the ferrofluid-in-oil emulsion. Open symbols correspond to a droplet volume fraction of 20%, filled symbols to 35%. Fig. 4. Permeability of the ferrofluid vs. the magnetic field strength. Fig. 5. Permeability of the ferrofluid emulsion vs. the volume fraction of the ferrofluid. Dots are experiments; lines are calculations. Circles correspond to a constant external magnetic field strength H = 0; squares to H = 2.6 kA/m.

Fig. 6 .Fig. 7 .

 67 Fig.[START_REF] Popplewell | Magnetorheological fluid composites[END_REF]. Calculated dependence of the ferrofluid droplet semiaxis ratio, , on the magnetic Bond number, Bm, for different dispersed phase volume fractions of a ferrofluid-in-oil emulsion: 1, φ = 0; 2, φ = 0.2; 3, φ = 0.4. Fig.7. Ratio of the field-induced change in the permeability to the permeability in the absence of field vs. the volume fraction of the ferrofluid. Dots are experiments; lines are calculations. Line 1 correspond to oil-in-ferrofluid emulsion; line 2 to ferrofluid-in-oil emulsion.

  Figure 4.

Figure 5 .

 5 Figure 5.

  Figure 6.

Figure 7 .

 7 Figure 7.

Acknowledgments. This work was supported by Russian Foundation for Basic Research (project No. 10-02-90019-Bel_a) and also by Federal Education Agency of the Russian Federation in Scientific Program "Development of Scientific Potential of Higher School".


Yury Dikansky

Arthur Zakinyan
email: zakinyan.a.r@mail.ru

Marita Bedganian

Use of a magnetic fluid for particle size analysis by a sedimentation method

Keywords: sedimentation analysis, magnetic fluid, microparticle size measurement, size distribution, powdery substances

A new method of particle size analysis of micrometer-sized particles is discussed. The improved method of sedimentation analysis with magnetic fluids has the potential and versatility to characterize polydisperse systems.

The particle size measurement of a material is important in understanding its physical and chemical properties. In several industrial applications, knowledge of the product size distribution is critical for controlling and improving product quality and handling (e.g. in a food industry, pharmaceutical production, building industry, etc.). A sedimentation analysis is often applied for microparticle size measurements [START_REF] Syvitski | Principles, methods and application of particle size analysis[END_REF]. This method is based upon study of the terminal velocity acquired by particles sedimenting in a viscous liquid, i.e., larger particles sink faster than smaller particles when suspended in a liquid. In this paper, we report an improved method of sedimentation analysis of size distribution of microparticles of a powder and granular materials.

The basic difference of this method is the use of a magnetic fluid as medium, in which the motion (sedimentation) of particles is explored. A magnetic fluid (or ferrofluid) is a colloidal suspension of ultra-fine ferro-or ferri-magnetic nanoparticles suspended in a carrier fluid. If micrometer-sized non-ferromagnetic particles are added to a magnetic fluid, then the non-ferromagnetic inclusions form 'magnetic holes' in the fluid. It allows studying features of a motion of non-magnetic bodies in such media by measuring the change of magnetic properties at different points of the medium. In particular, the analysis of motion of particles and determination of their sizes can be carried out by measuring local changes of the magnetic permeability of the medium in which they move. Figure 1 shows the particle measurement experimental setup, which was used for these purposes. The experimental setup consists of a vertical glass tube (length ~ 1 m, diameter ~ 0.01 m) filled with magnetic fluid. We used a kerosene-based magnetic fluid with dispersed magnetite nanoparticles of about 10 nm diameter stabilized with oleic acid. The magnetic fluid used was made by OJSC "NIPIgaspererabotka" (Russia). The magnetic fluid had a magnetic permeability 7   . On the tube, the inductance coil of small length (5 mm) was located. The coil is connected to the measuring bridge (in our experiment "LCR meter-817"), which determines value of inductance of the coil. The magnetic measuring field produced by the coil is small enough (~ 50 A/m), so that its influence to any essential ad-hoc processes can be neglected.

A small quantity (~ 1 g) of the powder sample particles to be investigated is poured into the tube with the magnetic fluid. Timing with a stopwatch and measuring the inductance of the coil simultaneously starts. We can consider that all particles begin to move simultaneously and separately from each other. Thus, the particles are physically fractionated according to size before detection. When particles reach the place in the tube with magnetic fluid where the measuring coil is located, the inductance of the coil will change.

The sedimentation curve, L(t), can be constructed by results of measuring the change of inductance, L, of the coil caused by the sedimentation of particles and the time, t, relevant to this change. As an example, the sedimentation curves, measured for particles of a diamond powder (curve 1) and a sand (curve 2), are shown in Fig. 2. The inductance was measured accurately to H 01 . 0  , and the time was measured accurately to 0.1 s. Each powdered sample has been measured three times, and the obtained results are within 5% of each other.

The change of inductance must be proportional to the change of magnetic permeability of a magnetic fluid

    1 k L (1)
where 1

k is the constant of proportionality,  is the macroscopic magnetic permeability of a magnetic fluid with microparticles.

For observance of a requirement of independence of a motion of individual particles the concentration of particles must be low ( 1   ). In this case, the change of a magnetic permeability in comparison with initial value for a pure magnetic fluid is proportional to the volume fraction of studied particles, as it follows from the Maxwell equation

[2]   2 k   (2)
where 2

k is the constant of proportionality,  the particle volume fraction in the location of measuring coil. The optimal value of  is about 0.5%.

Let us present the whole population of measured particles as a set of monodisperse fractions. Then the particle volume fraction is given by

V V i   (3)
where i V is the volume of particles of some fraction, V is some constant volume. Combining Eq. ( 1) with ( 2) and (3) we find

i i kV L   (4)
where

V k k k 2 1  , i L
 is the change of inductance, happening when particles of some fraction reach the place of the tube with a magnetic fluid, where the measuring coil is located.

From Eq. ( 4) we can find each particle volume fraction or particle mass fraction:

      i i i i i L L m m V V ( 5
)
where i m is the mass of particles of some fraction, m the total mass of particles in a sample.

Using Eq. ( 5) we can construct the particle size distribution curve or histogram. We also can find a relative number of particles of some fraction, which is given by

    3 3 i i i i i r L r L N N (6)
where i r is the equivalent radius of particles of some fraction. The value of i r is given by the Stoke's equation

  2 1 2 9      i i gt h r (7) where 2 , 1
 is the density of the particle and the medium, g the acceleration of gravity,  the dynamic viscosity of the medium (magnetic fluid), i t the time of the particle motion, h the path length. We assume that the values of 1  , 2  ,  are known and the particles all have the same density. In our case, h is a distance from the top of magnetic fluid in the tube to the position of measuring coil. Measured change of inductance of the coil, i L  , corresponds to the measurement time, i t .

Thus, measuring the change of inductance of the coil and the time relevant to this change and using Eqs. ( 7) and ( 6) or (5), the particle size distribution of a powder may be obtained. As an example, the calculated particle size distribution curves of a diamond powder (curve 1) and a sand (curve 2), are shown in Fig. 3. These calculations are based on the sedimentation curves shown in Fig. 2.

The presented method allows to measure particle size distributions in the size range of 1-400 m. Sub-micrometer particles cannot be reliably measured due to the effects of Brownian motion. Particles greater than about 400 m cannot be measured because the time of acquiring the terminal velocity for these particles is too long. The method allows to measure weakly magnetic materials, and all particles of the sample should have the same density.

In several cases at late stages of sedimentation the analysis process can be sped up. For this purpose, the measuring coil is manually moved in small steps along the tube to scan and measure the finest particles of the sample.

It should be noted that the method of studying microparticles using magnetic fluids was previously offered in a publication [START_REF] Davies | [END_REF]. But that method completely differs from method offered in this work and consists in use of an optical microscopy for study of a monolayer of microparticles immersed in a magnetic fluid under the action of magnetic field.

The procedure presented in this paper is easier then classical sedimentation analysis.

The important feature of an offered method in comparison with a traditional sedimentation analysis is lack of the extremely inexact operations of double differentiation. This method combines the simplicity with the ability to measure not only the average particle size, but also the detailed particle size distribution. The technique of using magnetic fluids for measuring the size distribution of the particles could be useful in several areas of study such as soil science or the life sciences.

The authors have taken out a patent for this invention [4].

Figure Captions

Fig. 1 .

 1 Fig. 1. Experimental setup for particle size measurements.

Fig. 2 .Fig. 3 .Figur re 1 .Figur re 2 .

 2312 Fig. 2. Sedimentation curves: 1 -diamond powder, 2 -sand.Fig. 3. Particle size distribution curves: 1 -diamond powder, 2 -sand. Here i r  is the difference of radii of particles of adjacent fractions.

Acknowledgment: This work was supported by the Federal Education Agency of the Russian Federation in Scientific Program "Development of Scientific Potential of Higher School".


Nugroho D A 2010 Spinner

Philippe Borsa
email: philippe.borsa@ird.fr

Dharma Arif Nugroho

dolphin (Stenella longirostris) and other cetaceans in Raja Ampat waters, West Papua

Keywords: Halmahera Sea, Pacific Ocean, Coral Triangle, pelagic spinner dolphin, pilot whale, Risso's dolphin, bottlenose dolphin, false killer whale, sperm whale

Cetaceans were surveyed using ship transects across the seas of Raja Ampat (West Papua) in November and December 2007.

Sighting effort reached 120 hours, over a total distance of 1561 km. The most common cetacean sighted was the spinner dolphin, Stenella longirostris (~0.238 individuals/km), most frequently in pods of 15-20 individuals, in relatively-shallow to deep waters (82-2310 m). All those spinner dolphins that were positively identified from photographs were of the pelagic form (S. l. longirostris).

INTRODUCTION

The eastern half of the Indo-Malay-Papua archipelago lies within the "Coral Triangle", a biogeographic region where the marine life is highly diverse [START_REF] Roberts | Marine biodiversity hotspots and conservation priorities for tropical reefs[END_REF][START_REF] Hoeksema | Delineation of the Indo-Malayan centre of maximum marine biodiversity: the Coral Triangle[END_REF].

Yet the large-vertebrate fauna of these seas remains poorly known. Published research in the archipelago includes sightings and an investigation of cetacean catches in the Lamalera area, Sawu Sea [START_REF] Barnes | Indigeneous whaling and porpoise hunting in Indonesia[END_REF][START_REF] Barnes | Sea hunters of Indonesia, fishers and weavers of Lamalera[END_REF][START_REF] Rudolph | Preliminary checklist of Cetacea in the Indonesian archipelago and adjacent waters[END_REF], miscellaneous observations gathered throughout Indonesia [START_REF] Rudolph | Preliminary checklist of Cetacea in the Indonesian archipelago and adjacent waters[END_REF], opportunistic sightings off Komodo Island [START_REF] Hoffmann | First account of Bryde's whales Balaenoptera edeni (Anderson, 1878), and of spinner dolphins Stenella longirostris (Gray, 1828), from Komodo Island, Indonesia[END_REF], sightings from a transect through the Java Sea and the South China Sea [START_REF] Boer | A note on cetacean observations in the Indian Ocean Sanctuary and the South China Sea, Mauritius to the Philippines, April 1999[END_REF], a dedicated cetacean survey off eastern Kalimantan [START_REF] Kreb | Cetacean diversity and habitat preferences in tropical waters of East Kalimantan, Indonesia[END_REF], and a partial compilation of stranding records throughout Indonesia [START_REF] Mustika | Whale strandings in Indonesia, including the first record of a humpback whale (Megaptera novaeangliae) in the archipelago[END_REF]. [START_REF] Branch | Past and present distribution, densities and movements of blue whales in the Southern Hemisphere and adjacent waters[END_REF] reported opportunistic sightings and strandings of blue whales, Balaenoptera musculus [START_REF] Linnaeus | Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I, editio decima, reformata[END_REF]. Also, a series of reports has been produced from cetacean sighting surveys in the waters of Komodo National Park [START_REF] Kahn | Komodo National Park cetacean surveys: a rapid ecological assessment of cetacean diversity abundance and distribution[END_REF].

A four-week expedition at sea, on board the research vessel KR Baruna Jaya VIII (LIPI, Jakarta), provided a rare opportunity, the first to our knowledge, to take a census of marine mammals in the seas around Raja Ampat, West Papua, at the core of the Coral Triangle.

METHODS

The Raja Ampat archipelago at the western extremity of Bird's Head Peninsula (West Papua) is made up of the four large islands of Batanta, Misool, Salawati and Waigeo, plus a myriad of smaller islands scattered around these (Figure 1).

Observations of marine mammals were made during daytime, continuously (15-24 November 2007) or intermittently (27 November -05 December 2007), from the upper deck or from the wheelhouse of the 50-m long KR Baruna Jaya VIII (ca. 10 m above sea surface) by two observers equipped with binoculars and a reflex camera fitted with a 300-mm lens. The average speed of the vessel, whose multi-beam vertical sonar was in use during the whole survey, was 6.7 knots (12.4 km/hour). The total sighting effort is shown graphically in Figure 1 and quantitatively in Table 1. of each sighting event was recorded. For each sighting event we recorded the precise time and location, determined the species occurring, and estimated the group size of each species. Species names follow [START_REF] Rice | Marine mammals of the world, systematics and distribution[END_REF].

RESULTS AND DISCUSSION

Six cetacean species were identified during this survey. The precise location of sightings is given in Figure 1. Total encounter rate was 0.501 individual/km in the Halmahera Sea and 0.237 individual/km in the Pacific Ocean waters of Raja Ampat (Table 1). The most commonly encountered cetacean was the spinner dolphin, Stenella longirostris (Gray, 1828) (Table 1). Spinner dolphins occurred in compact pods of eight to over 50 individuals, generally of 15-20 individuals, and frequently exhibited conspicuous behaviour. In several instances, individuals with pink bellies were noted. Raja Ampat lies midway between the Philippines, where the subspecies reported is the pelagic spinner dolphin (S. l. longirostris), and northern Australia, where the dwarf form (S. l. roseiventris) occurs (Perrin, 2009a). Both inshore, reef, and deep-pelagic habitats exist in Raja Ampat, making eventually possible that the two subspecies co-occur there. Therefore, the question arises of whether the spinner dolphins sighted in Raja Ampat waters were of the dwarf or of the pelagic subspecies. Clues for the distinction of the two subspecies have been provided by [START_REF] Perrin | Spinner dolphins (Stenella longirostris) of the western Pacific and southeast Asia: pelagic and shallow-water forms[END_REF] and [START_REF] Perrin | External appearance of the dwarf spinner dolphin Stenella longirostris roseiventris[END_REF]: useful parameters are body length and the relative sizes of appendages. Measurements of total body length of the individuals were not available, but from the pictures taken, the dorsal fin height can be measured as a proportion of the distance from beak tip to dorsal fin tip. The dorsal fin height of all individuals recorded on photographs and on which measures could be taken (N=14, from four separate sighting events, i.e. 21% occurrences) was comprised between 14.5% and 19.1 % of the dorsal-fin/beak-tip length, indicating they were of the pelagic subspecies. Those individuals possessed the distinctive, tripartite color pattern characteristic of pelagic spinner dolphins (Figure 2). Spinner dolphins occurred in waters 134-560 m deep in the Halmahera Sea and 82-2310 m deep in the Pacific Ocean waters of Raja Ampat. Encounter rates were of the same order as those reported by [START_REF] Kreb | Cetacean diversity and habitat preferences in tropical waters of East Kalimantan, Indonesia[END_REF] off eastern Kalimantan, or by [START_REF] Ballance | Cetacean sightings around the Republic of the Maldives, April 1998[END_REF] in the Maldives archipelago, where spinner dolphin density was deemed to be high. Pelagic spinner dolphins feed on small mesopelagic fishes and squids, diving to 600 m or more [START_REF] Perrin | Spinner dolphins (Stenella longirostris) of the western Pacific and southeast Asia: pelagic and shallow-water forms[END_REF]Perrin, 2009b).

Hence, the relatively high abundance of pelagic spinner dolphins in the Halmahera Sea is an indication of abundant mesopelagic resources.

All five sperm whales (Physeter macrocephalus Linnaeus, 1758) sighted during the survey (Table 1) were encountered at the western extremity of the deeper part of Dampier Strait (22 November: 00°37-40'S, 130°47-56'E; depth 585-745 m). They were solitary, of large size, and appeared to be in foraging mode, i.e. alternating long dives and surface-resting with frequent blowing. A single pod of >18 short-finned pilot whales (Globicephala macrorhynchus [START_REF] Gray | On the cetaceous animals[END_REF] was observed, swimming parallel to a pod of spinner dolphins, also in the deep waters of the Dampier Strait (22 November: 00°39'S, 130°58'E; depth 820 m). Two small, separate groups of Risso's dolphins, Grampus griseus [START_REF] Cuvier | Recherches sur les ossemens fossiles, où l'on rétablit les caractères de plusieurs espèces d'animaux que les révolutions du globe paroissent avoir détruites[END_REF] totalling 15 individuals were sighted in the deep waters immediately north-west of Sorong ( 22November: 00°45-46'S; 131°11-13'E; depth 485-513 m). False killer whales, Pseudorca crassidens [START_REF] Owen | A history of British fossil mammals and birds[END_REF], were sighted twice, each time a pod of 5-6 individuals, in relatively shallow (100-190 m deep) waters of the sill at the western side of Dampier Strait, at the entrance of the Halmahera Sea (01°13'S, 130°25'E; 23 November); and off Salawati Island (00°45'S, 130°30'E; 04 December). Pilot whales and sperm whales mainly feed on deep-sea squid [START_REF] Soto | Cheetahs of the deep sea: deep foraging sprints in short-finned pilot whales off Tenerife (Canary Islands)[END_REF][START_REF] Olson | Pilot whales Globicephala melas and G. macrorhynchus[END_REF][START_REF] Whitehead | Sperm whale Physeter macrocephalus[END_REF]). Risso's dolphins are thought to feed on neritic and oceanic squid [START_REF] Baird | Risso's dolphin Grampus griseus[END_REF].

Although the overall encounter rate, all species combined, was higher in the Halmahera Sea (Table 1), species diversity was higher in Pacific Ocean waters, with 6 species positively identified (vs. only two, in the Halmahera Sea). The close-to-shore occurrence of deep-sea foraging cetaceans, such as the sperm whale and the short-finned pilot whale, is made possible by the steep transition from shore to deep-sea, as in Dampier Strait and elsewhere in the Raja Ampat archipelago. The present results indicate that Raja Ampat waters are a feeding area for various cetacean species, that occur in relatively high densities, at least during the period surveyed (late November -early December), thus providing indirect evidence of abundant and diverse resources, at least during that period.

ACKNOWLEDGEMENTS

We are grateful to Captain I. Daniel and to the crew of KR Baruna Jaya VIII for their excellent assistance. The research presented here was sponsored by LIPI (Ekspedisi Widya Nusantara) and by IRD. P.B.'s field work was conducted under Surat Izin Penelitihan no. 5936/SU/KS/2007 issued by LIPI, and MLD/OD no. 19750 issued by IRD Director general.