4. SECURITY ASPECTS IN EMBEDDED SYSTEMS

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1 4. SECURITY ASPECTS IN EMBEDDED SYSTEMS 4.0 Introduction Now a day embedded systems and other wireless devices are increasingly being connected to each other and are very much involved in network communications. These devices are very much capable enough to run and execute network or internet based complicated applications that run usually run in a personal computer. Since the mentioned equipments are very much get themselves into the transfer of data secured in a networks and internets they demand full protection from unauthorized access thus escalating the need for embedded systems security very critical and important. The data that need to be secure can be put in various categories that demand different security levels.[40] Thus based on the user s classification for whom the data need to be protected the data type can be divided into two the first one personal or private data that belongs to the particular user and restricted or protected data of that user. The data of the users that for which its security is compromised will have a straight and direct effect upon the user are usually called as personal or private data. A user s internet banking password hacking can be viewed as a simple example of compromising on such security. If we compromise on the data or content s security then rather than the end user the author or the data provider fatally suffers the loss. Multimedia content that are digitalized such as digitalized images, copyrighted audio and video contents can be taken as examples of such data. The security of data is not only taken into account during data transfer through public network but also at most security concerns need to be stressed at the end user devices. The encryption and decryption secret or private key of data if at all goes into wrong hand such security breaches happens then the entire security arrangements can go in vein.

2 Same techniques are being followed by the protocols that take part in the communication of the data safely and secured manner along an open network which is public in nature. At the data source side the data that are restricted by the user involves high priority towards the care to make sure that the data safety is maintained even from the user side himself. So all the embedded systems must get into or apply some function or any apt protocol to make sure that the data is transferred securely. Thus the main idea of such implementation and beefing up the security is to not allow any unwanted and people without proper authority to get the data accessed or corrupt or steal away the data and make the system highly secured. Thus the Security of data transfer and Security at device level are the two basic protections that are concerned in an embedded device. 4.1 Data transfer Security In a public network data need to traverse along various most untrusting and unreliable points that are intermediate. So the information need to be protected so that the date hacked in between the source and correct destination will be in a confusing, useless or beyond the reach of other s intelligence. This is easily felt and realized taking the aid of number of crypto graphical ways such as encrypting and decrypting the data, Algorithms related to key agreement, issuing of certificates and signatures. Data Encryption Encryption is the method of changing the form of part or whole of the text so as to hide the contents from third parties and thereby securing the data by applying a secret key so as to make only the desired recipient with the corresponding key to receive and decrypt or descramble the data on the other side.

3 Among the available so many number of available cryptographic algorithm for the general public like DES, triple DES or AES or any algorithm invented by the particular system manufacturer to suite a particular demand any one can be applied for this purpose.[40] Only the end users i.e. the communicating ends will know the keys and can be even of length 100s of bits. The keys used for encryption and decryption should be kept highly secured and secret if we are using any publicly available algorithms. Distributing and maintaining the keys in a secret way between the end users involved without any unauthorized hackers getting the information regarding the keys is very important and vital for a successful and foolproof data communication. It is also possible to embed the keys within the device prior to the communication, i.e. they are exchanged offline in a secured way or using any key agreement algorithm to get established online. 4.2 Public-key Agreement Algorithm The key maintenance is very easy in case of a couple of few number of systems, but one can feel a really tough task which will be felt even as unrealistic in a network where there are more than hundreds or even thousands or devices interconnected. Then the algorithm called key agreement one will come to the rescue. By applying a an algorithm for Agreement of keys the secret shared will be established between the parties that communicate with least need secret keys or any kind of secret elements exchange, that take place during online or during offline. The algorithm functions as illustrated below. In the category of public-key cryptography algorithm the key agreement one will fall. A private and an public key that is associated with it are contained in device that makes use of key agreement algorithm. The secret key generally contains a few number of 100s or even 1000s of bits and by applying the single way function that the algorithm that is key agreement mentions helps in getting the public keys. The functions applying upon

4 which one can get easily the result in the forward way but its impossible to apply the same in the reverse order and arrive at the initial data and so this function is called forward function. By applying the private key on the forward operation of the one way function the public key can be retrieved whereas the reverse operation of getting the secret key with the help of the public one is really impossible. The devices that are involved in the communication does exchange the public keys and other public constants to enable them share the data or information in a secured way. To obtain the secret information both the sender device and receiver device uses the other s public keys to generate the key using its private key. The success of secret data exchange is acknowledged through the comparison of the data. If both the data s are same then the communication is a great success. 4.3 Digital Signature In a public network the communication can happen between two devices which my be hundreds or thousands of km apart from one another. This way of communication also demands the active involvement of many intermediate routers to aid the communication of data. While in the effort to establish a private key in the agreement procedure any third person will be able to remove and change the key which is public and thus it culminates in secret key sharing with the concerned system. So in order to maintain shared secret with the help of the algorithm related to key agreement and is very vital to get the authorized public key for the system from the other end. For the public key distribution with trust and authenticated method the digital signature and certificates are being employed. The main purpose of a digital signature is to check the authenticity that is to cross verify that the data received is really from the peer and is basically a public-key method. Very much alike that followed in the algorithm for key agreement a device makes use even in the digital signature a pair of keys that is both signed private and public key

5 respectively. Here the signed private key is kept secured and safe by the device but all the systems that take part in the communication are being given the public key. With the aid of signature algorithm the system signs the plain text that need to be communicated and it keeps the signed private key which in fact helps the signature generation and it allows all the systems having signed public key accessibility can cross check the text with the aid of signature available and also by making use of the signature verification algorithm. The verification miserably fails if the message or the signature is modified by any third party. The private key is kept known only by the particular signed device so that it will be practically without reach of any device other than the concerned ones to know the signature and forge or will be able to do any modification. 4.4 Digital Certificate Though the digital signature ensures the transfer of authenticated transfer of signed public key from one peer device to another to make sure that the received message is an authenticated one that s its from the destined source itself then it also takes care of the public key distribution in key agreement process or in the establishment of digital signature in an authenticated way through a large network is difficult and is impossible without a central trusted authority. All the devices in the network have to abide and follow the s centralized authority in the network. This central authorized body is popularly called as reliable Certificate Authority which is denoted by simply CA.[24] With the aid of secret key of CA and the public key and the ID that is associated with each device the certificate authority signs and generates the signature. Thus the certificate is the signature thus formed clubbed together with the data that are signed by CA like the public key and IDs which are kept in a format that is standard. So if the public key issued by CA is available with any system then it can cross check the origin and ownership of the certificate that is to check the originality and also confirm the same for public key.

6 4.5 Certificate Hierarchy With the huge number of systems that involve itself in the data transfer in the network has gone up and geographically diversified locations of these systems or instruments over diversified areas over the globe a single authority to issue the certificate will not be capable enough to handle, give, distribute and support certificates for all the instruments or systems involved. In this juncture the Digital Certificate is the only solution seems to be practically possible and feasible. In Hierarchy of certificates there will be a main base acting from the center certificate authority is entitled to give directions and its consent to other certificate authorities to distribute the certificates for all the devices or bodies that take part in communication. The main certificate authority that acts as the root gives up the certificate and that is taken by the midway corresponding certificate authority. The central certificate Authority will be releasing to the in between certificate authority the certificates. This is followed by the intermediate CA which in turn will issue certificates to the device. Not only the intermediate CA issues certificates to the devices It also takes care of the certificates that are being issued by the root certificate authority. to the devices. The intermediate certificate authorities then gives its consent to other authorities that permits and gives certificate to release certificate to different devices that communicate based on the different levels of certificate hierarchy. As for an instance let a device A gets not only a certificate from an intermediate certificate authority but also the certificate of the intermediate certificate authority that was issued by the root certificate authority. When we have several level of intermediate Certificate authorities besides the CA which issues to the corresponding devices. Then the particular system will be receiving certificates that was send by all the intermediate certificate authorities till the fundamental main root Certificate authority. In the case of communication that is secure type a system may request every intermediate

7 certificates through the whole of its peer to do a successful verification completely of the device certificate that is being recieved. 4.6 Device level security The main important point in the security aspect is that the security of the data and it lies in the fact that it solely depends in the secrecy of the keys. The fact that is taken into account based on it the key that need to be secured is a private-key of any public key algorithm and it can be also that any secret that has got from last transaction has been taken into consideration. To reinforce and incorporate new security there are some cryptographic algorithms do mention a set of values that are constant in nature and that need to be protected and secured from the system concerned. Here the secret key mentioned often in the document includes both secret keys and the secured values that are stored within the particular system and that requires to be protected from unauthorized access. Some of these protected secret keys may last even through out the life time of the system and is kept inside it. Now the main task of the designer is to design a highly secured implementation that are embedded in both hardware and software side of the system and should be capable enough to defeat any attempt to corrupt or access the data without permission. Many information like the certificate that was directed from the root certificate authority in the particular system which can be shared with the outside world where as it need to be protected from unwanted modification or deletion and access. When some modification is done to the root CA certificate then it will be very easy for the attackers or hackers to take away the data and can fool the device by making it accept the duplicate certificate that is fake root CA one and can mislead and defeat the whole idea for a certificate and can cause heavy damage to the system and secured communication. So it is very vital and of prime importance that the security outside and inside the system need to be cent percentage secured and the root certificate authority issued

8 certificates are not being accessed and tampered. Based on the content and its nature the hierarchy of protection or the security that need to be implemented is decided. The intention or providing system security lies heavily on the case of system handling restricted data of the user such as copy protected one unlike in the case of a particular secret data like files or sensitive bank details such as the transactions that happen. The prime motive behind this is in the private data of the particular user since the person involved will have to undergo the loss directly by loosing such data and the responsibility for such restrictions that is to access physically the secret keys or to the information that are kept protected inside the system lies on the user directly. Individual key is the suggested best one for every system as recommended by the secure data transfer protocol implementation. Therefore if the security with respect to hardware of any of the device is compromised and has nothing to do with respect to security of all the other systems present within the network. Where as in the case of highly protected and limited data of users the protection of the whole copy protected information manipulated by the system one has to compromise by giving up the secret key of a single system.[101] The existence of such culprit instrument will only aid the unwanted and those not authorized person in helping in accessing the contents that are copy protected or realize its contents, or else he can decrypt it and there by share any number of copy of the copy protected content. During execution time the secure key that are protected are loaded to the RAM in clear text from the Secure ROM. The accessing of private keys that are protected can be seen by the bus. By keeping the keys protected and safe in special memory areas called buffers the inside the RAM of the secure SoC the hacking and unauthorized theft can be prevented. So to take away the keys from Secure Soc which are protected is nearly impossible in this method. The protected Boot loader also ensures the safety and originality of the OS operating system or the software having the whole and

9 privileges for the right process by keeping itself inside the secure SoC. The buffers within Internal Random access memory are being allowed to get accessed by the operating system that configure the unit that manages the memory called as MMU which allows secret key functions only to those processes which are safe with operating systems special privileges. Where as in some cases where the Secure Read only memory is a restricted and limited resource or preprogrammed directly at the manufacturing end with the help of a master key the Secure ROM can be programmed. For the purpose of encryption and to store the secret keys within the internal ROM the master key is being used.[59] Real model case of a Secure SoC 1. Directly accessing and retrieving the secret keys is beyond the scope of Secure ROM. 2. The buses cannot be viewed or seen so as to retrieve the protected data or keys within the Secure SoC. 3. Any attempt to remove or add a component or replace the existing with another should not be entertained and if at all they are successful in doing manipulations then the secure SoC should stop its functions for ever. Based on the hierarchy of information sensitivity that is intended to be secured the level of protection provided also varies. The protection would be either detection of tamper or nullifying and neutralizing of all the information stored within the SoC if any possibility of theft is seen or felt. In this method its not guaranteeing full security of the data from the device whereas it will be able to make it to understand if any tamper has happened to the chip or not. To make it exceptional and standard one one require dedicated power supply and a committed support to the hardware which indeed increased the price of the chip.[115]

10 4.8 Secure ROM Before storing the system s private keys which are secret in the memory area allocated for it the encryption process is done to make it very safe and secure. Thus even if somebody is able to hack the system and secure the private keys that are kept as secret they wont be able to understand it as it has already been secured with encryption methods. The algorithm meant for encryption and the keys associated are the two elements that one need to encrypt any message. Like Advance encryption standard a well established and highly recommended is being employed for encryption of the private keys, then we can see that how much the keys are secured only that much will be the security of the whole system lies. The only solution that is visible to get rid of the above mentioned one is to design and develop a new algorithm dedicated for encryption and known only to the system manufacturer so that its impossible for the hackers to crack the system at any cost. When an algorithm totally new is developed as mentioned above then the security of the keys lies with the security of the algorithm rather than the secret keys alone. The probability of getting away the secret keys cannot be neglected as there are a very large number of tools that are available for remodeling the codes and this makes use of the binary code that is kept in a clear readable format in the system memory. Keeping the secret keys within the secure ROM is another method of storing the keys in a secured manner. The Secure ROM is kept within the secure SoC inside the particular system. The information that is kept inside the secure ROM is descrambled by the hardware controller before taking it back. So no externals can take away the information that is kept inside the secure ROM and it protects the information from external hacking. Within the RAM that resides internally of secure SoC that the buffer with the private keys and the sensitive values of cryptographic functions are stored. So by doing so any bus outside the

11 secure SoC are stopped from getting any information. By using a master key of device the secure ROM can be programmed if it has a limited or if its programmed before by the manufacturer. The main master keys of the system generally a key dedicated singly to every system hardware or dedicated to each and every Secure SoC that will be applied in future for encryption and there by storing the system secret keys within a less Secure ROM. In the implementation of Secure ROM the possible attacks and vulnerabilities are: 1. The physical connection between Secure ROM and the Secure SoC will be removed and keep it over another system so as to make it function and there by to get backthe protected keys. 2. To retrieve the protected keys one way is to access bus between Secure ROM and RAM. 3.The running over Application program interface by some illegal and unwanted applications and there by can make an effort to retrieve the keys. 4.9 The interior Random Access Memory and Protected Processes The private keys that are secured are stored in special memory areas called buffers and when some functions in the cryptographical algorithm demands then they are given to the internal ram of the secure SoC and by doing so it can be prevented from getting into wrong hands. Secure memory area is the place where the secret keys and other intermediate values that result in an operation are being stored and that lies inside the RAM. The access to memory areas are not given to all the systems to access it. Here the processes that have the special operating system privilege such as secure process will be allowed to get and take the data from secure memory area. Here this is somewhat similar to the special privileges given to the administrator or special privileges provided at the root of an operating system.

12 With the help of secure processes the secure memory area ia accessed by the memory management unit which in deed is configured by the operating system during the boot up process. So for other outsiders or unauthorized people don t have the accessing power to modify the memory management unit with the configured code in the operating system. This security level can be achieved with the apt use of secure boot loader and signing the code. Even when the booting process take place the configuration of the secure processes are completed. Any other secure process whether new one or some other will not be allowed and entertained by the operating system. So by doing so any application that has been down loaded can be prevented by the system to break into the memory area that is secure and can read the secured private keys. Usually public key or that is open as in encrypted data are the results that occur in the operations carried out by a process which is secure over the secret keys. The main of these results of the manipulations done above is to pass it to other devices to perform some functions having less privilege functions. There are several methods by which a lower privileged process calls a secure process 4.10 Secure Code Signing and Boot-loader Though protected by hardware security measure, the secret keys should be revealed outside because certain application peripheral interface will take their use. So its very vital to keep into consideration that the software of the system may not get altered or modified and any one like hackers or unauthorized people can get into the system and take the information like secret keys from the application peripheral interface. In a system there are also some very vital and important codes that the firmware contain which contains information to handle the most important configurations the hardware as the internal RAM configuration that stated the

13 permissions regarding the accessibility. So any attempt to change or overwrite or to reveal the device s firmware component cannot be considered at all. If the system has secure Bootloader then it can help a lot in handling such situations. Here the secure bootloader first verifies and makes sure that the software has been altered in any way or replaced even before the loading happens when initially started the genuineness is checked thoroughly. Secure Bootloader is kept inside the SoC in a ROM that too as write protected manner. KeeThus by placing the Secure Bootloader within ROM that is write protected makes sure that it is not at all changed or accessed unwantedly. Besides the usual initialization section by the Bootloader during booting it also has a crosscheck module related to signature of the software code and it also cross check public key. The code in the firmware gets signed with the system private key that the manufacturer code possess. During the boot up process the Bootloader verifies and validates the code by cross checking the signature with the help of the public key of the code. Even if the private key that is used to sign the code in the firmware is not kept open during the shipping of the device it needs to be protected as secret by the producer of the device. Any if any lapse in the private key secrecy which is taken to sign the code in the firmware permits anyone who do have the permission or authorized for the private-key or to do any changes and signing the code which is in an permissible form to the Secure Bootloader. Even if the firmware cannot be upgraded in a device its security an be increased and strengthened by a very easy method even without availing the secure boot loader. For meeting such situations and to eliminate the risk of unauthorized firmware entering and disrupting the entire program can be achieved by writing or developing from the scratch the entire firmware and storing the same in a memory of read only type and the boot loader should be programmed in such a way that it gets loaded only from this memory area, thereby preventing unauthorized

14 entries. If the concerned firmware is not properly upgraded then it invites too many problems and limitations to the product. The whole of the device can be under threat if certain files like the root certificate authority certificate are altered. So one way to get this secured is to make sure that the code in the firmware of the system is signed. Here each and every individual device files such as encrypted secret keys or certificates which if at all undergo some change or alteration caused inconvenience and stopping of secure data communication but they never bother or hinter the security of the device as a whole. While the manufacturing of the device is happening or when the up gradation of the firmware concerned is taking place the particular files concerned are not signed as it can cause overhead and it is practically impossible to sign each and every files of the each device at that time. With the alteration of changing of the files do not endu up in the lose of the security of the system they can be signed later as per our convenience Encryption and decryption engine Generally in the case of implementations involving secure protocol the out put of key agreement algorithm that is the shared secret key is the master key and for the purpose of encryption and decryption the sub keys are being used. Depending upon the data transfer s protocol the device keeps the shared secret keys till the mentioned expiry time in the protocol and the lifetime can take days or even several months even though the master key is used. The lifetime is just a very few second or in that order for the sub keys associated with it. The main purpose of storing inside the device is the high requirement of the security with respect to the shared secret keys. The keys that are kept as secret like the one shared and the one used in certificates enjoys much importance over he the sub keys. Using the sub keys the master keys cannot be taken out this is made possible through the design of sub key generation protocol. The cryptographic engine has the provision reside outside the secure SoC in conditions where the need of protection of the

15 keys that are concerned in encrypting and decrypting is not that much important. The keys that are generated within the SoC are distributed to the cryptographic engine if it is residing outside the secure SoC. While in some cases or protocols for the encryption and decryption the shared secret or secret keys are themselves taken. If that is the condition then the decryption and encryption has to be stored within the SoC in order to safeguard the keys from being openly available to the bus external to the secure SoC Which is based upon the level of protection that is needed and importance of the keys to be employed for cryptographic process and it has the choice to reside within or external to the secure SoC System Time There is a validity period associated with the digital certificate of a device. As per the different protocol implementations the validity periods will differ. Certain protocols as SSL used to have a limited validity period which can be from few years to decades where as some protocols will have an unlimited validity for the certificate example the DTCP. [94] To set or update or reset the time associated with a system which is an embedded system have an interface with the user to do so. In order to avoid the system from accepting the expired certificates the cross verification process is a must and for theis purpose the system keeps the system time even though the user changes the system time for his convenience and with this it can prevent the malfunctions and hacking from happening. The timer keeps the counting at all the states of the device whether its on or off it goes on internally. In cases where the certificate is valid for more than 20 years any attempt to change the system time without any permission is not a big issue and can be allowed. This is because this time span is itself very big when

16 compared to the life time of the device as such. Within the time frame like this allocated the updating of the certificate by the root CA is very high. Any modification in the root CA certificate by the CA then it should be intimated to the root and all other in between and must get from the CA a fresh device certificate.