FPGA realizes safe and reliable Bluetooth communication

Bluetooth technology is destined to become a universal low-cost wireless technology that can be applied to a wide range of data communication applications. But there are still two main aspects that need further consideration, namely, the data security and data integrity issues in Bluetooth communications. These two aspects will limit the scope of Bluetooth technology. When designing wireless products, by using programmable logic, Bluetooth technology can simultaneously meet the requirements of data security and integrity.

Bluetooth data security

The Bluetooth standard defines a series of security mechanisms that require each Bluetooth device to implement key management, authentication, and encryption functions to provide basic protection for short-range wireless communications. In addition, the frequency-hopping communication method adopted by the Bluetooth technology itself is also an effective security means to prevent eavesdropping.

Key management

Bluetooth devices use several different keys in high-level software to ensure the secure transmission of data.

encryption

The encryption method is used in the Bluetooth device to ensure the confidentiality of the connection. But before the data is encrypted, there needs to be an authenticated established connection. Bluetooth encryption uses an 8-bit to 128-bit key to encrypt the data payload in the Bluetooth packet, but does not encrypt the Bluetooth access code and packet header. The specific details of payload encryption depend on the required encryption strength and the regional regulations of the country where the product will ultimately be applied.

The Bluetooth system uses a serial encryption algorithm called E0 to encrypt the data. The E0 algorithm is resynchronized for each data payload (payload). E0 serial encryption consists of three parts:

● Initial part, generate load key;

● Keystream bit generator;

● Encryption and decryption hardware circuit.

There are three encryption modes:

● Encryption mode 1: No data is encrypted;

● Encryption mode 2: point-to-multipoint (broadcast) data stream is not encrypted, point-to-point data stream is encrypted;

● Encryption mode 3: All data streams are encrypted.

Physical layer data security-frequency hopping spread spectrum

In addition to other security measures adopted by the Bluetooth standard, the mechanism of frequency hopping communication adopted by Bluetooth communication also makes eavesdropping extremely difficult.

As mentioned earlier, Bluetooth radios work in the 2.4GHz band. In most of North America and Europe, Bluetooth devices operate in the frequency band from 2.402 to 2.480 GHz, and the entire frequency band is divided into 79 sub-channels with a bandwidth of 1 MHz. In frequency hopping communication, the data signal is modulated by a narrow-band carrier signal, and these narrow-band carrier signals continuously hop from one frequency to another as a function of time. The Bluetooth standard uses a frequency hopping sequence that hops 1600 times per second.

The frequency hopping code known by both the sending and receiving parties determines the frequency of the RF carrier and the order of frequency hopping. In order to receive signals correctly, the receiver must be set to the same frequency hopping code as the sender and monitor the carrier signal at the correct time and the correct frequency. A logical channel can be maintained only when synchronized correctly. The FHSS signal seen by other receivers is only impulse noise of very short duration.

FHSS relies on changes in frequency to combat interference. If the radio frequency unit encounters interference at a certain frequency, it will retransmit the interfered signal when jumping to another frequency point in the next step. Therefore the total interference can become very low, with little or no bit errors.

Bluetooth data integrity

Forward Error Correction (FEC)

There are three types of error correction used by Bluetooth:

● 1/3 coding rate FEC

● 2/3 code rate FEC

● Automatic data retransmission request (ARQ) scheme

The purpose of FEC (Forward Error Correction) is to reduce the number of data load retransmissions. However, the disadvantage of using FEC is that it will significantly reduce the achievable actual data transmission rate.

Data whitening

All packet headers and payload information must be whitened using data whitening bits before transmission. This is mainly to avoid excessively long continuous 0 or 1 bit stream patterns during transmission. The baseband processor needs to determine whether the data is 0 or 1 from the received analog data signal, but excessively long continuous 0 or 1 bit streams will cause problems. Because there is no reference point like the DC signal in the received analog data signal, it must rely on the last few transmission signals received for correction. Any continuous 0 or 1 long sequence bit stream string may cause the calibration to fail. Therefore, it is necessary to use data whitening technology to scramble the signal to greatly reduce the possibility of a long sequence of 0 or 1 bit stream strings.

Programmable solutions for advanced data security

For most applications that require privacy to be considered first, the data security provided by Bluetooth is insufficient. In general, there are still some problems to be solved in terms of Bluetooth security. The data security measures provided by Bluetooth seem to be sufficient for small applications, but any sensitive data or data that will cause problems should not be transmitted directly via Bluetooth. For example, the encryption scheme used by Bluetooth itself has certain weaknesses. The E0 sequence encryption with 128-bit key length can be cracked by O (2 ^ 64) in some cases.

Imagine a possible situation where an attacker manages to obtain the encryption key used to ensure communication between two devices, and then can eavesdrop on the messages sent between the two devices. Moreover, an attacker can also impersonate one of the devices and insert the wrong information. Lucent believes that one way to avoid this problem is for users to use a longer personal identification code (PIN code) instead of a short PIN code, thereby increasing the difficulty for attackers to obtain encryption keys. This, in turn, means entering the PIN number manually. This is really inconvenient because you need to enter a PIN code every time you establish a secure connection.

Another way to overcome this security problem is to use a more robust encryption algorithm, such as Digital Encryption Standard (DES), or even three times DES to replace the E0 sequence encryption algorithm. DES is a block encryption method, which means that the encryption process is performed block by block. In the DES algorithm, the original information is divided into 64-bit fixed-length data blocks, and then the 56-bit encryption key is used to generate 64-bit encrypted information through replacement and combination methods.

Unlike the Bluetooth serial encryption algorithm, it can be proved mathematically that the block encryption algorithm is completely safe. The DES block cipher is highly random and nonlinear, and the generated encrypted ciphertext is related to the plaintext and each bit of the key. The number of available encryption keys for DES is up to 72 x 1015. The key applied to each plaintext message is randomly generated from this huge number of keys. The DES algorithm has been widely adopted and is considered to be very reliable, and now there is a more secure variant of the DES algorithm-called three times DES (TDES), which uses different keys to encrypt the information three times in succession deal with.

All of these encryption algorithms can be implemented using low-cost programmable logic devices and ready-to-use intellectual property (IP) products for advanced encryption processing. For example, the DES function can be realized by only requiring chip logic resources at a cost of $ 2. At present, the programmable logic device of 100,000 system gates can be purchased for only 10 dollars in large quantities, and it is in stock and can be used immediately. These devices also allow other features to be added to the design, such as advanced error correction. Therefore, programmable logic devices can significantly reduce system-level costs.

Software-based encryption solutions have excellent flexibility but low performance. On the other hand, the hardware-based encryption solution has high performance, but once the design is completed, the flexibility is very poor. An encryption solution based on programmable logic can simultaneously provide the advantages of the aforementioned two solutions, that is, having a high level of flexibility and high performance.

The use of a stronger key encryption algorithm allows Bluetooth technology to be safely applied to a wide range of security applications that have the most important status. These applications include: financial electronic transactions: ATM, smart cards; secure e-commerce transactions; secure office communications; secure video surveillance systems; digital set-top boxes; high-definition television (HDTV) and other consumer electronic devices.

Programmable solutions for advanced data integrity

The Bluetooth standard defines a method that can effectively prevent random errors, that is, data whitening and error checking methods for data transmission. However, there is still such a specific data stream in theory, which may continue to cause Bluetooth transmission errors, although this situation is difficult to encounter in the usual Bluetooth transmission.

And in more severe environments, such as industrial environments, office buildings, airports, and urban public transportation factories, there is still the risk of errors during transmission. In these places, noise and interference from various other Bluetooth devices, wireless networks, mobile phone systems, or other electronic devices operating in the same frequency band can cause problems. For these situations, programmable logic and advanced error correction IP can be used again to achieve error-free communication. For harsh communication environments, a more powerful forward error correction technique, such as Turbo convolutional coding, can be used.

urbo coding is an advanced forward error correction algorithm, which is the standard error correction algorithm of the third generation wireless communication system (as implemented in WCDMA). The principle of Turbo coding is that the encoder generates a data stream containing two independently coded bit sequences and one uncoded bit sequence. Due to the interleaving process, the two sets of check bit sequences are weakly correlated. In the Turbo decoder, two sets of check bit sequences are decoded using soft decision outputs called external information. The efficiency of Turbo decoding comes from the sharing of external information during a series of decoding cycles. External information is passed from one check decoding step to another, from one loop to another. Off-the-shelf IP for such Turbo convolutional codes is already available and can be easily implemented with low-cost programmable logic. This solution can ensure Bluetooth data integrity in harsh and error-prone environments.

Programmable logic solutions in Bluetooth applications

At present, the challenges facing realizing this bright prospect are focused on how to apply Bluetooth to the next generation of products. This can be a difficult task, especially when facing existing product structures. How to integrate Bluetooth subsystem? How to minimize software development overhead and impact on the system? How to create a device configuration for specific applications?

A huge advantage of programmable logic is that the system configuration can be adjusted according to special requirements. For example, the maximum performance of the currently provided Bluetooth devices is only 721Kbps, and can only support several concurrent networks (piconet). For access points in public factories (such as airport lounges or corporate meeting rooms, etc.), a large number of such devices may need to be tandem to support high-bandwidth connections for large numbers of users.

Programmable logic can provide flexibility to the design in many ways, whether it is to connect a different RF section to the selected baseband controller, or to integrate more user interface peripherals as in the example above, or to implement data as shown in Figure 3. Value-added functions such as security and integrity. In all cases, you will find that the hardware design will be faster, and you can make design corrections multiple times to ensure that the system can work normally in a shorter time, and all of this does not have the NRE fee when using ASIC.

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