BLUE TOOTH SECURITY IMPLEMENTED USING VHDL

BLUE TOOTH SECURITY IMPLEMENTED

USING VHDL

http://www.eforu.page.tl/

ABSTRACT: Blue tooth is a short-range radio link intended to be a cable replacement between

portable and/or fixed electronic devices. A frequency hop transceiver is applied to

combat interference and fading. It also has a built-in security at the physical layer. Blue

tooth employs several layers of data encryption and user authentication measures. This

paper deals with the base band layer of Blue tooth and its security. The general format of

packet transmission is access code followed by the payload and the header. The datas

transmitted in this technology may be corrupted or accessed by the public users.

Although there are some security measures in Bluetooth, they is a need for improved

security of datas. Hence the Bluetooth is modified with additional security measures by

employing the RSA algorithm. This RSA algorithm is applied in the payload of the

packet. We implement this by the method of cryptography where the payload is

encrypted at the transmitter and decrypted at the recevier using RSA cryptography.

When compared to the builtin security this additional security allows secure exchange of

datas. This was simulated in VHDL software.

INTRODUCTION:


Blue tooth operates in the unlicensed ISM band at 2.4 GHz. The key features are

robustness, low complexity, low power, and low cost. On the channel, information is

exchanged through packets. Each packet is transmitted on a different hop frequency. A

packet nominally covers a single slot, but can be extended to cover up to five slots. The

Blue tooth protocol uses a combination of circuit and packet switching. Slots can be

reserved for synchronous packets. The Blue tooth system consists of a radio unit, a link

control unit, and a support unit for link management and host terminal interface

functions. This paper describes the specifications of the Blue tooth link controller which

carries out the base band protocols and lower level link routines. The Blue tooth system

provides a point-to-point connection or a point-to-multipoint connection. Two or more

units sharing the same channel form a Pico net. One Blue tooth unit acts as the master of

the Pico net, whereas the other units acts as slaves. Multiple Pico nets with overlapping

coverage areas forms a scatter net. Each Pico net can only have a single master. The Pico

nets shall not be time or frequency synchronized. Each Pico net has its own hopping

channel. The channel is represented by a pseudo-random hopping sequence hopping

through the 79 or 23 RF channels. The channel is divided into time slots each 625 ms in

length. A TDD scheme is used where master and slave transmit alternatively. The data

transmitted has a symbol rate of 1 Ms/s. Two different types of links that can be

established between master and slaves are Synchronous Connection-Oriented (SCO) link

and Asynchronous Connection-Less (ACL) link.


The SCO link is a point-to-point link between a master and a single slave in the

Pico net. The ACL link is a point-to-multipoint link between the master and all the slaves

participating on the Pico net. There are many different packet types, which can be

transmitted. In the packet the payload can have either data field or voice field or even

both. The 2/3 rates FEC scheme is used for error correction. Before transmission, both

the header and the payload are scrambled with a data whitening word in order to

randomize the data from highly redundant patterns and to minimize DC bias in the

packet. Before the user information is sent over the air interface, several bit

manipulations are performed in the transmitter to increase reliability and security. Low

power operations are ensured in blue tooth. After implementing a blue tooth packet with

base band specifications we employ RSA cryptography for secure data transmissions.

2. PACKETS

The above figure shows the standard packet format.


ACCESS CODE HEADER PAYLAOD

LSB

72 54 0-2745

MSB

2.1. ACCESS CODE

Each packet, with the exception of the FHS packet starts with a 72-bit access

code. This access code is used for synchronization DC offset compensation and

identification. The access code identifies all packets exchanged on the channel of the Pico

net. All packets sent in the same Pico net are preceded by the same channel access

code. The access code is also used in paging and inquiry procedures. In this case, the

access code itself is used as a signaling message and neither a header nor a payload is

present. The access code consists of a preamble, a sync word, and possibly a trailer.

There are three different types of access codes defined. They are the Channel Access

Code (CAC), the Device Access Code (DAC) and the Inquiry Access Code (IAC).

2.2. PACKET HEADER

The header contains link control (LC) information and consists of 6 fields. They

are the AM_ADDR(3- bit active member address), TYPE(4-bit type code), FLOW(1-bit

flow control), ARQN(1-bit acknowledge indication), SEQN(1-bit sequence number),

HEC (8-bit header error check).

2.2.1. AM_ADDR

The AM_ADDR represents a member address and is used to distinguish between

the active members participating on the Pico net. In a Pico net, one or more slaves are

connected to a single master. To identify each slave separately, each slave is assigned a

temporary 3-bit address to be used when it is active. The all-zero address is reserved for

broadcasting packets from the master to the slaves.

2.2.2. TYPE

Sixteen different types of packets can be distinguished. The 4-bit TYPE code

specifies which packet type is used. The interpretation of the TYPE code depends on the

physical link type associated with the packet. First, it shall be determined whether the

packet is sent on an SCO link or an ACL link. Then it can be determined which type of

SCO packet or ACL packet has been received. The TYPE code also reveals how many

slots the current packet will occupy.

2.2.3. FLOW


This bit is used for flow control of packets over the ACL link. When the RX

buffer for the ACL link in the recipient is full and is not emptied by the link support unit,

a STOP indication (FLOW=0) is returned to stop the transmission of data temporarily.

When the RX buffer is empty, a GO indication (FLOW=1) is returned. When no packet is

received or the received header is in error, a GO is assumed implicitly.

2.2.4. ARQN

The 1-bit acknowledgment indication ARQN is used to inform the source of a

successful transfer of payload data with CRC and can be positive acknowledge ACK or

negative acknowledge NAK. If the reception was successful, an ACK (ARQN=1) is

returned, otherwise a NAK (ARQN=0) is returned. When no return message regarding

acknowledge is received, a NAK is assumed implicitly. NAK is also the default return

information.

2.2.5. SEQN

The SEQN bit provides a sequential numbering scheme to order the data packet

stream. For each new transmitted packet that contains data with CRC, the SEQN bit is

inverted. This is required to filter out retransmissions in the destination. If a

retransmission occurs due to a failing ACK, the destination receives the same packet

twice. By comparing the SEQN of consecutive packets, correctly received

retransmissions can be discarded. The SEQN has to be added due to a lack of packet

numbering in the unnumbered ARQ scheme.

2.2.6. HEC

Each header has a header-error-check to check the header integrity. The HEC

consists of an 8-bit word generated by the polynomial 647 (octal representation). Before

generating the HEC, the HEC generator is initialized with an 8-bit value. After the

initialization, a HEC is calculated for the 10 header bits. Before checking the HEC, the

receiver must initialize the HEC check circuitry with the proper 8-bit UAP (or DCI). If

the HEC does not check, the entire packet is disregarded.

2.3. PACKET TYPES


The packets used on the Pico net are related to the physical links they are used in.

There are two physical links defined. They are the SCO link and the ACL link. For each

of these links, 12 different packet types can be defined. Four control packets will be

common to all link types.

To indicate the different packets on a link, the 4-bit TYPE code is used. The

common packet types are ID, NULL, POLL, FHS and DM1. The SCO packets defined so

far are HV1, HV2, HV3 and DV. The ACL packets are DM1, DH1, DM3, DH3, DM5,

DH5 and AUX1.

2.4. PAYLOAD

The two fields in the payload are the (synchronous) voice field and the

(asynchronous) data field. The ACL packets only have the data field and the SCO packets

only have the voice field with the exception of the DV packets, which has both.

2.4.1. Voice field

The voice field has a fixed length. For the HV packets, the voice field length is

240 bits and for the DV packet the voice field length is 80 bits. No payload header is

present.

2.4.2. Data field

The data field consists of three segments. They are the payload header, the

payload body and the CRC code.

2.4.2.1. Payload header

Only data fields have a payload header. The payload header is one or two bytes

long. The payload header specifies the logical channel (2-bit L_CH indication), controls

the flow on the logical channels (1-bit FLOW indication), and has a payload length

indicator (5 bits and 9 bits for 1-byte and 2-bytes payload header, respectively).

2.4.2.2. Payload body

The payload body includes the user host information and determines the effective

user throughput. The length of the payload body is indicated in the length field of the

payload header. The optional processes are indicated in dashed boxes.


TX payload

CRC generation

Encryption Whitening Encoding

2.4.2.3. CRC code generation


RF interface

Rx payload

CRC checking Decryption De-Whitening Decoding

PALOAD BIT PROCESS

The 16-bit cyclic redundancy check code in the payload is generated by

the CRC-CCITT polynomial 210041 (octal representation). It is generated in a similar

fashion as the HEC. Before determining the CRC code, an 8-bit value is used to initialize

the CRC generator. The 8 bits are loaded into the 8 least significant (left-most) positions

of the LFSR circuit. The other 8 bits are at the same time reset to zero. Subsequently, the

CRC code is calculated over the information. Then the CRC code is appended to the

information. At the receiver side, the CRC circuitry is in the same way initialized with

the 8-bit UAP (DCI) before the received information is checked.

3. ERROR CORRECTIONThere are three error correction schemes defined for Blue tooth to reduce the

number of retransmissions. They are 1/3 rate FEC, 2/3 rates FEC, and ARQ scheme for

the data. The scheme followed in this paper is 2/3 rates FEC. The purpose of the FEC

scheme on the data payload is to reduce the number of retransmissions.

4. ERROR CHECKINGThe packets can be checked for errors or wrong delivery using the channel access

code, the HEC in the header, and the CRC in the payload. At packet reception, first the

access code is checked. Since the 64-bit sync word in the channel access code is derived

from the 24-bit master LAP, this checks if the LAP is correct, and prevents the receiver

from accepting a packet of another Pico net. The HEC and CRC are used to check both

on errors and on a wrong address.

5. DATA WHITENINGBefore transmission, both the header and the payload are scrambled with a data

whitening word in order to randomize the data from highly redundant patterns and to

minimize DC bias in the packet. The scrambling is performed prior to the FEC encoding.

At the receiver, the received data is described using the same whitening word generated

in the recipient. The describing is performed after FEC decoding.


6. BLUETOOTH SECURITYThe Blue tooth technology provides peer-to-peer communications over short

distances. In order to provide usage protection and information confidentiality, the

system has to provide security measures both at the application layer and the link layer.

These measures shall be appropriate for a peer environment. Four different entities are

used for maintaining security at the link layer: a public address which is unique for each

user, two secret keys, and a random number, which is different for each new transaction.

7. RSA ALGORITHMRSA cryptosystem is followed here protecting sensitive information. The RSA

encryption is used in the transmission of payload and the RSA decryption is used in

reception of payload. The RSA cipher is a public key cryptosystem. The knowledge of

the encoding key does not reveal the knowledge of the decoding key. The public key is

open to the public and the private key is kept as a secret.

The steps involved in the RSA cipher are as follows

1.Preparation

a) Choose two primes p and q so that their product n=p*q is greater than the used

alphabet length M.

b) Compute p (n)= (p-1)*(q-1).

2. Encryption

a) Choose a public encoding key e that has to be relatively prime to p (n).

b) Encrypt each plain letter P by computing C= p ^ e MOD n. Here the public key is

(n, e).

3. Decryption

a) The private decoding key d is chosen as the inverse of e MOD p (n): e*d=1 MOD

p(n).

Mathematically, find integers d and k that fulfill: e*d=1+k*p (n) via the extended

Euclidean Algorithm.


b) Decrypt by computing P= C ^ d MOD n. Here the private key is (d, n).

8. CONCLUSION

Blue tooth employs frequency hopping (1600 hops/sec), which adds some

protection against eavesdropping. The datas transmitted in this technology may be

corrupted or accessed by the public users. Although there are some security measures in

Bluetooth, we require full security of datas. Hence additional security measures is

included in this project by implementing the RSA algorithm. The current status of

security is limited for the higher capacity of data’s. When the capacity of data’s increase

the division of hopping will not be satisfactory. The Blue tooth’s security seemed to be

adequate only for small ad hoc networks. It seems that the security of Blue tooth is still

inadequate for any serious, security sensitive work. So there is a possibility of leakage of

data’s to the public. By implementing the RSA algorithm in the Blue tooth base band

controller the message will be more secure. This has been tested and implemented with

the front end of FPGA kit and the back end of WARP.

References

1. R. L. Rivest, A. Shamir, and L. Adleman, “ A method for obtaining digital signatures and public key cryptosystems,” Communication, ACM. Vol. 21, pp. 120- 126, 1978

2. J. Bhasker, A VHDL synthesis primer, Star Galaxy Publishing, New York, 19963. Zoran Salcic, Asim Smailagic, Digital System Design and Prototyping using Field

Programmabel Logic, Kluwer Academic Publishers, 19974. Peter J. Ashenden, The Designers’ guide to VHDL, Morgan Kaufmann

Publishers, Inc. pp. 351, Netherlands, 1996


BLUE TOOTH SECURITY IMPLEMENTED USING VHDL

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