Network Security Md. Kamrul Hasan Assistant Professor and Chairman Computer and Communication Engineering Dept.

Classifying security attacks: Passive Attacks & Active Attacks A passive attack attempts to learn or make use of information from the system but does not affect system resources. An active attack attempts to alter system resources or affect their operation. Passive attacks are in the nature of eavesdropping on, or monitoring of, transmissions eavesdropping on transmissions to obtain information release of possibly sensitive/confidential message contents traffic analysis which monitors frequency and length of messages to get info on senders difficult to detect can be prevented using encryption: emphasis in dealing with passive attacks is on prevention rather than detection A useful means of classifying security attacks (RFC 2828) is in terms of passive attacks and active attacks. A passive attack attempts to learn or make use of information from the system but does not affect system resources. An active attack attempts to alter system resources or affect their operation. Passive attacks are in the nature of eavesdropping on, or monitoring of, transmissions. The goal of the opponent is to obtain information that is being transmitted. Two types of passive attacks are release of message contents (which may contain sensitive or confidential information), and traffic analysis which is subtler. Even with encryption protecting message contents, an opponent might still be able to observe the pattern of these messages. The opponent could determine the location and identity of communicating hosts and could observe the frequency and length of messages being exchanged. This information might be useful in guessing the nature of the communication that was taking place. Passive attacks are very difficult to detect because they do not involve any alteration of the data. Typically, the message traffic is sent and received in an apparently normal fashion and neither the sender nor receiver is aware that a third party has read the messages or observed the traffic pattern. However, it is feasible to prevent the success of these attacks, usually by means of encryption. Thus, the emphasis in dealing with passive attacks is on prevention rather than detection.

Passive Attacks

Active Attacks

Active Attacks Active attacks involve some modification of the data stream or the creation of a false stream and can be subdivided into four categories: masquerade, replay, modification of messages, and denial of service. masquerade pretending to be a different entity replay modification of messages denial of service easy to detect detection may lead to deterrent hard to prevent focus on detection and recovery Active attacks involve some modification of the data stream or the creation of a false stream and can be subdivided into four categories: masquerade, replay, modification of messages, and denial of service. Masquerade attacks take place when one entity pretends to be a different entity. A masquerade attack usually includes some other forms of active attack. Replay involves the passive capture of a data unit and its subsequent retransmission to produce an unauthorized effect. Modification of messages simply means that some portion of a legitimate message is altered, or that messages are delayed or reordered, to produce an unauthorized effect. The denial of service prevents or inhibits the normal use or management of communications facilities. This attack may have a specific target. Another form of service denial is the disruption of an entire network or a server. Active attacks present the opposite characteristics of passive attacks. Whereas passive attacks are difficult to detect, measures are available to prevent their success. On the other hand, it is quite difficult to prevent active attacks absolutely, because to do so would require physical protection of all communications facilities and paths at all times. Instead, the goal is to detect them and to recover from any disruption or delays caused by them. Because the detection has a deterrent effect, it may also contribute to prevention.

Symmetric Encryption Symmetric encryption, also referred to as conventional encryption or single-key encryption, was the only type of encryption in use prior to the introduction of public-key encryption Symmetric encryption, also referred to as conventional encryption or single-key encryption, was the only type of encryption in use prior to the introduction of public-key encryption in the late 1970s. It remains by far the more widely used of the two types of encryption. A symmetric encryption scheme has five ingredients , as shown in Stallings DCC8e Figure 21.1 above: • Plaintext: This is the original message or data that is fed into the algorithm as input. • Encryption algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext. • Secret key: The secret key is also input to the encryption algorithm. The exact substitutions and transformations performed by the algorithm depend on the key. • Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the secret key. For a given message, two different keys will produce two different ciphertexts. • Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the original plaintext.

Requirements for Security strong encryption algorithm even known, unable to decrypt without key even if many plaintexts & ciphertexts available sender and receiver must obtain secret key securely once key is known, all communication using this key is readable There are two requirements for secure use of symmetric encryption: 1. We need a strong encryption algorithm. At a minimum, we would like the algorithm to be such that an opponent who knows the algorithm and has access to one or more ciphertexts would be unable to decipher the ciphertext or figure out the key. This requirement is usually stated in a stronger form: The opponent should be unable to decrypt ciphertext or discover the key even if he or she is in possession of a number of ciphertexts together with the plaintext that produced each ciphertext. 2. Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. If someone can discover the key and knows the algorithm, all communication using this key is readable.

Attacking Encryption There are two general approaches to attacking a symmetric encryption scheme: cryptanalysis relay on nature of algorithm plus some knowledge of general characteristics of plaintext attempt to deduce plaintext or key brute force try every possible key until plaintext is recovered rapidly becomes infeasible as key size increases 56-bit key is not secure There are two general approaches to attacking a symmetric encryption scheme. The first attack is known as cryptanalysis. Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of the general characteristics of the plaintext or even some sample plaintext-ciphertext pairs. This type of attack exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or to deduce the key being used. If the attack succeeds in deducing the key, the effect is catastrophic: All future and past messages encrypted with that key are compromised. The second method, known as the brute-force attack, is to try every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained. On average, half of all possible keys must be tried to achieve success. Table 21.1 shows how much time is involved for various key sizes. The table shows results for each key size, assuming that it takes 1 µs to perform a single decryption, a reasonable order of magnitude for today's computers. With the use of massively parallel organizations of microprocessors, it may be possible to achieve processing rates many orders of magnitude greater. The final column of the table considers the results for a system that can process 1 million keys per microsecond. As one can see, at this performance level, a 56-bit key can no longer be considered computationally secure.

Block Ciphers most common symmetric algorithms process plain text in fixed block sizes producing block of cipher text of equal size most important current block ciphers: Data Encryption Standard (DES) Advanced Encryption Standard The most commonly used symmetric encryption algorithms are block ciphers. A block cipher processes the plaintext input in fixed-size blocks and produces a block of ciphertext of equal size for each plaintext block. The two most important symmetric algorithms, both of which are block ciphers, are the Data Encryption Standard (DES) and the Advanced Encryption Standard (AES).

Data Encryption Standard US standard 64 bit plain text blocks 56 bit key broken in 1998 by Electronic Frontier Foundation special purpose US$250,000 machine with detailed published description less than three days DES now worthless DES has been the dominant encryption algorithm since its introduction in 1977. However, because DES uses only a 56-bit key, it was only a matter of time before computer processing speed made DES obsolete. In 1998, the Electronic Frontier Foundation (EFF) announced that it had broken a DES challenge using a special-purpose "DES cracker" machine that was built for less than $250,000. The attack took less than three days. The EFF has published a detailed description of the machine, enabling others to build their own cracker. And, of course, hardware prices will continue to drop as speeds increase, making DES worthless.

Triple DEA The life of DES was extended by the use of triple DES (3DES), which involves repeating the basic DES algorithm three times, using either two or three unique keys, for a key size of 112 or 168 bits. Drawback of 3DES is that the algorithm is relatively sluggish in software. A secondary drawback is that both DES and 3DES use a 64-bit block size. The life of DES was extended by the use of triple DES (3DES), which involves repeating the basic DES algorithm three times, using either two or three unique keys, for a key size of 112 or 168 bits. The principal drawback of 3DES is that the algorithm is relatively sluggish in software. A secondary drawback is that both DES and 3DES use a 64-bit block size. For reasons of both efficiency and security, a larger block size is desirable.

Advanced Encryption Standard NIST issued call for proposals for an Advanced Encryption Standard (AES) in 1997 security strength equal to or better than 3DES significantly improved efficiency symmetric block cipher with block length 128 bits key lengths 128, 192, and 256 bits evaluation include security, computational efficiency, memory requirements, hardware and software suitability, and flexibility AES issued as FIPS (federal information processing standard )197 in 2001 Because of these drawbacks, 3DES is not a reasonable candidate for long-term use. As a replacement, the National Institute of Standards and Technology (NIST) in 1997 issued a call for proposals for a new Advanced Encryption Standard (AES), which should have a security strength equal to or better than 3DES and significantly improved efficiency. In addition to these general requirements, NIST specified that AES must be a symmetric block cipher with a block length of 128 bits and support for key lengths of 128, 192, and 256 bits. Evaluation criteria include security, computational efficiency, memory requirements, hardware and software suitability, and flexibility. In 2001, AES was issued as a federal information processing standard (FIPS 197).

AES Description assume key length 128 bits input a 128-bit block (square matrix of bytes) copied into state array, modified at each stage after final stage, state copied to output matrix 128-bit key (square matrix of bytes) expanded into array of 44 32-bit key schedule words byte ordering by column 1st 4 bytes of 128-bit input occupy 1st column 1st 4 bytes of expanded key occupy 1st column In the description of this section, we assume a key length of 128 bits, which is likely to be the one most commonly implemented. The input to the encryption and decryption algorithms is a single 128-bit block. In FIPS 197, this block is depicted as a square matrix of bytes. This block is copied into the State array, which is modified at each stage of encryption or decryption. After the final stage, State is copied to an output matrix. Similarly, the 128-bit key is depicted as a square matrix of bytes. This key is then expanded into an array of key schedule words; each word is four bytes and the total key schedule is 44 words for the 128-bit key. The ordering of bytes within a matrix is by column. So, for example, the first four bytes of a 128-bit plaintext input to the encryption cipher occupy the first column of the in matrix, the second four bytes occupy the second column, and so on. Similarly, the first four bytes of the expanded key, which form a word, occupy the first column of the w matrix.

AES Encryption and Decryption 1. The key that is provided as input is expanded into an array of forty-four 32-bit words. 2. Four different stages are used, one of permutation and three of substitution Substitute bytes: Uses a table, referred to as an S-box, to perform a byte-by-byte substitution of the block Shift rows: A simple permutation that is performed row by row Mix columns: A substitution that alters each byte in a column Add round key: A simple bitwise XOR of the current block with a portion of the expanded key Stallings DCC8e Figure 21.2 shows the overall structure of AES. The following comments give some insight into AES: 1. The key that is provided as input is expanded into an array of forty-four 32-bit words. Four distinct words (128 bits) serve as a round key for each round. 2. Four different stages are used, one of permutation and three of substitution: Substitute bytes: Uses a table, referred to as an S-box, to perform a byte-by-byte substitution of the block Shift rows: A simple permutation that is performed row by row Mix columns: A substitution that alters each byte in a column as a function of all of the bytes in the column Add round key: A simple bitwise XOR of the current block with a portion of the expanded key 3. The structure is quite simple. For both encryption and decryption, the cipher begins with an Add Round Key stage, followed by nine rounds that each includes all four stages, followed by a tenth round of three stages. Figure 21.3 depicts the structure of a full encryption round. 4. Only the Add Round Key stage makes use of the key. For this reason, the cipher begins and ends with an Add Round Key stage. Any other stage, applied at the beginning or end, is reversible without knowledge of the key and so would add no security.

AES Encryption and Decryption 3. The structure is quite simple. For both encryption and decryption, the cipher begins with an Add Round Key stage, followed by nine rounds that each includes all four stages, followed by a tenth round of three stages. 4. Only the Add Round Key stage makes use of the key. For this reason, the cipher begins and ends with an Add Round Key stage. Stallings DCC8e Figure 21.2 shows the overall structure of AES. The following comments give some insight into AES: 1. The key that is provided as input is expanded into an array of forty-four 32-bit words. Four distinct words (128 bits) serve as a round key for each round. 2. Four different stages are used, one of permutation and three of substitution: Substitute bytes: Uses a table, referred to as an S-box, to perform a byte-by-byte substitution of the block Shift rows: A simple permutation that is performed row by row Mix columns: A substitution that alters each byte in a column as a function of all of the bytes in the column Add round key: A simple bitwise XOR of the current block with a portion of the expanded key 3. The structure is quite simple. For both encryption and decryption, the cipher begins with an Add Round Key stage, followed by nine rounds that each includes all four stages, followed by a tenth round of three stages. Figure 21.3 depicts the structure of a full encryption round. 4. Only the Add Round Key stage makes use of the key. For this reason, the cipher begins and ends with an Add Round Key stage. Any other stage, applied at the beginning or end, is reversible without knowledge of the key and so would add no security.

AES Encryption Round 5. The Add Round Key stage by itself would not be formidable. The other three stages together scramble the bits, but by themselves would provide no security because they do not use the key. We can view the cipher as alternating operations of XOR encryption (Add Round Key) of a block, followed by scrambling of the block (the other three stages), followed by XOR encryption, and so on. This scheme is both efficient and highly secure. 6. Each stage is easily reversible. For the Substitute Byte, Shift Row, and Mix Columns stages, an inverse function is used in the decryption algorithm. For the Add Round Key stage, XOR is its own inverse. More comments on AES: 5. The Add Round Key stage by itself would not be formidable. The other three stages together scramble the bits, but by themselves would provide no security because they do not use the key. We can view the cipher as alternating operations of XOR encryption (Add Round Key) of a block, followed by scrambling of the block (the other three stages), followed by XOR encryption, and so on. This scheme is both efficient and highly secure. 6. Each stage is easily reversible. For the Substitute Byte, Shift Row, and Mix Columns stages, an inverse function is used in the decryption algorithm. For the Add Round Key stage, XOR is its own inverse. 7. As with most block ciphers, the decryption algorithm makes use of the expanded key in reverse order. However, the decryption algorithm is not identical to the encryption algorithm. This is a consequence of the particular structure of AES. 8. Once it is established that all four stages are reversible, it is easy to verify that decryption does recover the plaintext. Figure 21.2 lays out encryption and decryption going in opposite vertical directions. At each horizontal point State is the same for both encryption and decryption. 9. The final round of both encryption and decryption consists of only three stages. Again, this is a consequence of the particular structure of AES and is required to make the cipher reversible.

AES Encryption Round 7. As with most block ciphers, the decryption algorithm makes use of the expanded key in reverse order. However, the decryption algorithm is not identical to the encryption algorithm. This is a consequence of the particular structure of AES. 8. Once it is established that all four stages are reversible, it is easy to verify that decryption does recover the plaintext. Figure 21.2 lays out encryption and decryption going in opposite vertical directions. At each horizontal point State is the same for both encryption and decryption. More comments on AES: 5. The Add Round Key stage by itself would not be formidable. The other three stages together scramble the bits, but by themselves would provide no security because they do not use the key. We can view the cipher as alternating operations of XOR encryption (Add Round Key) of a block, followed by scrambling of the block (the other three stages), followed by XOR encryption, and so on. This scheme is both efficient and highly secure. 6. Each stage is easily reversible. For the Substitute Byte, Shift Row, and Mix Columns stages, an inverse function is used in the decryption algorithm. For the Add Round Key stage, XOR is its own inverse. 7. As with most block ciphers, the decryption algorithm makes use of the expanded key in reverse order. However, the decryption algorithm is not identical to the encryption algorithm. This is a consequence of the particular structure of AES. 8. Once it is established that all four stages are reversible, it is easy to verify that decryption does recover the plaintext. Figure 21.2 lays out encryption and decryption going in opposite vertical directions. At each horizontal point State is the same for both encryption and decryption. 9. The final round of both encryption and decryption consists of only three stages. Again, this is a consequence of the particular structure of AES and is required to make the cipher reversible.

AES Encryption Round 9. The final round of both encryption and decryption consists of only three stages. Again, this is a consequence of the particular structure of AES and is required to make the cipher reversible. More comments on AES: 5. The Add Round Key stage by itself would not be formidable. The other three stages together scramble the bits, but by themselves would provide no security because they do not use the key. We can view the cipher as alternating operations of XOR encryption (Add Round Key) of a block, followed by scrambling of the block (the other three stages), followed by XOR encryption, and so on. This scheme is both efficient and highly secure. 6. Each stage is easily reversible. For the Substitute Byte, Shift Row, and Mix Columns stages, an inverse function is used in the decryption algorithm. For the Add Round Key stage, XOR is its own inverse. 7. As with most block ciphers, the decryption algorithm makes use of the expanded key in reverse order. However, the decryption algorithm is not identical to the encryption algorithm. This is a consequence of the particular structure of AES. 8. Once it is established that all four stages are reversible, it is easy to verify that decryption does recover the plaintext. Figure 21.2 lays out encryption and decryption going in opposite vertical directions. At each horizontal point State is the same for both encryption and decryption. 9. The final round of both encryption and decryption consists of only three stages. Again, this is a consequence of the particular structure of AES and is required to make the cipher reversible.

Location of Encryption Devices The most powerful, and most common, approach to countering the threats to network security is encryption. In using encryption, we need to decide what to encrypt and where the encryption gear should be located. As Stallings DCC8e Figure 21.4 indicates, there are two fundamental alternatives: link encryption and end-to-end encryption. Encryption can be done in two fundamental alternatives: Link encryption and End-to-end encryption.

Link Encryption With link encryption each communication link equipped at both ends all traffic secure high level of security although it requires lots of encryption devices Disadvantage: message must be decrypted at each switch to read address (virtual circuit number) to route the packet. security vulnerable at switches particularly on public switched network With link encryption, each vulnerable communications link is equipped on both ends with an encryption device. Thus, all traffic over all communications links is secured. Although this requires a lot of encryption devices in a large network, it provides a high level of security. One disadvantage of this approach is that the message must be decrypted each time it enters a packet switch; this is necessary because the switch must read the address (virtual circuit number) in the packet header to route the packet. Thus, the message is vulnerable at each switch. If this is a public packet-switching network, the user has no control over the security of the nodes.

End to End Encryption encryption done at ends of system data in encrypted form crosses network unaltered destination shares key with source to decrypt Weak Spot: host can only encrypt user data otherwise switching nodes could not read header or route packet hence traffic pattern not secure solution is to use both link and end to end With end-to-end encryption, the encryption process is carried out at the two end systems. The source host or terminal encrypts the data. The data, in encrypted form, are then transmitted unaltered across the network to the destination terminal or host. The destination shares a key with the source and so is able to decrypt the data. This approach would seem to secure the transmission against attacks on the network links or switches. There is, however, still a weak spot. The host may only encrypt the user data portion of the packet and must leave the header in the clear, so that the switching nodes in the network can read it in order to route the packet to its destination. Thus, with end-to-end encryption, the user data are secure. However, the traffic pattern is not, because packet headers are transmitted in the clear. To achieve greater security, both link and end-to-end encryption are needed, as is shown in Stallings DCC8e Figure 21.4.

Key Distribution symmetric encryption needs key distribution protected for access by others changed frequently possibilities for key distribution key selected by A and delivered to B third party selects key and delivers to A and B use old key to encrypt & transmit new key from A to B use old key to transmit new key from third party to A and B For symmetric encryption to work, the two parties to an exchange must share the same key, and that key must be protected from access by others. Furthermore, frequent key changes are usually desirable to limit the amount of data compromised if an attacker learns the key. Therefore, the strength of any cryptographic system rests with the key distribution technique, a term that refers to the means of delivering a key to two parties that wish to exchange data, without allowing others to see the key. Key distribution can be achieved in a number of ways. For two parties A and B, 1. A key could be selected by A and physically delivered to B. 2. A third party could select the key and physically deliver it to A and B. 3. If A and B have previously and recently used a key, one party could transmit the new key to the other, encrypted using the old key. 4. If A and B each have an encrypted connection to a third party C, C could deliver a key on the encrypted links to A and B. Options 1 and 2 call for manual delivery of a key. For link encryption, this is a reasonable requirement. However, for end-to-end encryption, manual delivery is awkward. Option 3 is a possibility for either link encryption or end-to-end encryption, but if an attacker ever succeeds in gaining access to one key, then all subsequent keys are revealed. Even if frequent changes are made to the link encryption keys, these should be done manually. To provide keys for end-to-end encryption, option 4 is preferable.

Automatic Key Distribution Stallings DCC8e Figure 21.5 illustrates an implementation of option 4 for end-to-end encryption. For this scheme, two kinds of keys are identified: • Session key: used for the duration of that logical connection, to encrypt all user data are with a one-time session key, which is then destroyed. • Permanent key: A permanent key is a key used between entities for the purpose of distributing session keys. The configuration consists of the following elements: • Key distribution center: The key distribution center determines which systems are allowed to communicate with each other and provides a one-time session key for that connection. • Security service module (SSM): This module, which may consist of functionality at one protocol layer, performs end-to-end encryption and obtains session keys on behalf of users. The steps involved in establishing a connection are shown in the figure. The automated key distribution approach provides the flexibility and dynamic characteristics needed to allow a number of terminal users to access a number of hosts and for the hosts to exchange data with each other.

Message Authentication protection against active attacks with falsification of data falsification of source authentication allows receiver to verify that message is authentic has not been altered is from claimed/authentic source timeliness Encryption protects against passive attack (eavesdropping). A different requirement is to protect against active attack (falsification of data and transactions). Protection against such attacks is known as message authentication which allows communicating parties to verify that received messages are authentic. The two important aspects are to verify that the contents of the message have not been altered and that the source is authentic. We may also wish to verify a message's timeliness (it has not been artificially delayed and replayed) and sequence relative to other messages flowing between two parties.

Authentication Using Symmetric Encryption assume sender & receiver only know key only sender could have encrypted message for other party message must include one of: error detection code sequence number time stamp It is possible to perform authentication simply by the use of symmetric encryption. If we assume that only the sender and receiver share a key (which is as it should be), then only the genuine sender would be able successfully to encrypt a message for the other participant. Furthermore, if the message includes an error-detection code and a sequence number, the receiver is assured that no alterations have been made and that sequencing is proper. If the message also includes a timestamp, the receiver is assured that the message has not been delayed beyond that normally expected for network transit.

Authentication Without Encryption authentication tag generated and appended to each message message not encrypted useful when don’t want encryption because: messages broadcast to multiple destinations have one destination responsible for authentication one side heavily loaded encryption adds to workload can authenticate random messages programs authenticated without encryption can be executed without decoding Now examine several approaches to message authentication that do not rely on message encryption, but where an authentication tag is generated and appended to each message for transmission. The message itself is not encrypted and can be read at the destination independent of the authentication function at the destination. Because the approaches discussed in this section do not encrypt the message, message confidentiality is not provided. There is a place for both authentication and encryption in meeting security requirements. Here are three situations in which message authentication without confidentiality is preferable: 1. There are a number of applications in which the same message is broadcast to a number of destinations. eg, notification to users that the network is now unavailable or an alarm signal in a control center. It is cheaper and more reliable to have only one destination responsible for monitoring authenticity. The responsible system performs authentication. If a violation occurs, the other destination systems are alerted by a general alarm. 2. Another possible scenario is an exchange in which one side has a heavy load and cannot afford the time to decrypt all incoming messages. Authentication is carried out on a selective basis, with messages chosen at random for checking. 3. Authentication of a computer program in plaintext is an attractive service. The computer program can be executed without having to decrypt it every time, which would be wasteful of processor resources. However, if a message authentication tag were attached to the program, it could be checked whenever assurance is required of the integrity of the program.

Message Authentication Code generate authentication code based on shared key and message common key shared between A and B if only sender and receiver know key and code matches: receiver assured message has not altered receiver assured message is from alleged sender if message has sequence number, receiver assured of proper sequence can use various algorithms, eg. DES One authentication technique involves the use of a secret key to generate a small block of data, known as a message authentication code, that is appended to the message. This technique assumes that two communicating parties, say A and B, share a common secret key KAB. If we assume that only the receiver and the sender know the identity of the secret key, and if the received code matches the calculated code, then 1. The receiver is assured that the message has not been altered. If an attacker alters the message but does not alter the code, then the receiver's calculation of the code will differ from the received code. Because the attacker is assumed not to know the secret key, the attacker cannot alter the code to correspond to the alterations in the message. 2. The receiver is assured that the message is from the alleged sender. Because no one else knows the secret key, no one else could prepare a message with a proper code. 3. If the message includes a sequence number (such as is used with X.25, HDLC, and TCP), then the receiver can be assured of the proper sequence, because an attacker cannot successfully alter the sequence number. A number of algorithms could be used to generate the code. The National Bureau of Standards, in its publication DES Modes of Operation, recommends the use of DES. DES is used to generate an encrypted version of the message, and the last number of bits of ciphertext are used as the code. A 16- or 32-bit code is typical. The process just described is similar to encryption, but is less vulnerable.

Message Authentication Code Assume A and B, share a common secret key KAB. When A has a message M to send to B, it calculates the message authentication code as a function of the message and the key: MACM = F(KAB, M). The message plus code are transmitted to the intended recipient. The recipient performs the same calculation on the received message, using the same secret key, to generate a new message authentication code. The received code is compared to the calculated code, as shown in Stallings DCC8e Figure 21.6.

One Way Hash Function accepts variable size message and produces fixed size tag (message digest) but without use of a secret key send digest with message in manner that validates authenticity advantages of authentication without encryption encryption is slow encryption hardware expensive encryption hardware optimized for large data sets algorithms covered by patents algorithms subject to export controls (from USA) A variation on the message authentication code that has received much attention recently is the one-way hash function. As with the message authentication code, a hash function accepts a variable-size message M as input and produces a fixed-size message digest H(M) as output. Unlike the MAC, a hash function does not also take a secret key as input. To authenticate a message, the message digest is sent with the message in such a way that the message digest is authentic. These two approaches have an advantage over approaches that encrypt the entire message in that less computation is required. Reasons for this are: • Encryption software is quite slow. Even though the amount of data to be encrypted per message is small, there may be a steady stream of messages into and out of a system. • Encryption hardware costs are nonnegligible. Low-cost chip implementations of DES are available, but the cost adds up if all nodes in a network must have this capability. • Encryption hardware is optimized toward large data sizes. For small blocks of data, a high proportion of the time is spent in initialization/invocation overhead. • Encryption algorithms may be covered by patents. Some encryption algorithms, such as the RSA public-key algorithm, are patented and must be licensed, adding a cost. • Encryption algorithms may be subject to export control.

Using One Way Hash Functions The message digest can also be encrypted using public-key encryption (part b); The public-key approach has two advantages: it provides a digital signature as well as message authentication, and it does not require the distribution of keys to communicating parties. A hash function but no encryption for message authentication. This technique assumes that two communicating parties, say A and B, share a common secret value SAB. When A has a message to send to B, it calculates the hash function over the concatenation of the secret value and the message: MDM = H(SAB||M). It then sends [M||MDM] to B. Because B possesses SAB, it can recompute H(SAB||M) and verify MDM. Because the secret value itself is not sent, it is not possible for an attacker to modify an intercepted message. As long as the secret value remains secret, it is also not possible for an attacker to generate a false message. Stallings DCC8e Figure 21.7 illustrates three ways in which the message can be authenticated. The message digest can be encrypted using symmetric encryption (part a); if it is assumed that only the sender and receiver share the encryption key, then authenticity is assured. The message digest can also be encrypted using public-key encryption (part b); this is explained in Stallings DCC8e Section 21.4. The public-key approach has two advantages: it provides a digital signature as well as message authentication, and it does not require the distribution of keys to communicating parties. Stallings DCC8e Figure 21.7c shows a technique that uses a hash function but no encryption for message authentication. This technique assumes that two communicating parties, say A and B, share a common secret value SAB. When A has a message to send to B, it calculates the hash function over the concatenation of the secret value and the message: MDM = H(SAB||M). It then sends [M||MDM] to B. Because B possesses SAB, it can recompute H(SAB||M) and verify MDM. Because the secret value itself is not sent, it is not possible for an attacker to modify an intercepted message. As long as the secret value remains secret, it is also not possible for an attacker to generate a false message. This third technique, using a shared secret value, is the one adopted for IP security; it has also been specified for SNMPv3, discussed in Ch 22.

Secure Hash Functions produce a “fingerprint” of message/file must have the following properties: can be applied to any size data block produce fixed length output easy to compute not feasible to reverse not feasible to find two messages with the same hash giving “weak” & “strong” hash functions also used for data integrity The one-way hash function, or secure hash function, is important not only in message authentication but in digital signatures. The purpose of a hash function is to produce a "fingerprint" of a file, message, or other block of data. To be useful for message authentication, a hash function H must have the following properties: 1. H can be applied to a block of data of any size. 2. H produces a fixed-length output. 3. H(x) is relatively easy to compute for any given x, making both hardware and software implementations practical. 4. For any given code h, it is computationally infeasible to find x such that H(x) = h. 5. For any given block x, it is computationally infeasible to find y ≠ x with H(y) = H(x). 6. It is computationally infeasible to find any pair (x, y) such that H(x) = H(y). A hash function that satisfies the first five properties in the preceding list is referred to as a weak hash function. If the sixth property is also satisfied, then it is referred to as a strong hash function. In addition to providing authentication, a message digest also provides data integrity. It performs the same function as a frame check sequence: if any bits in the message are accidentally altered in transit, the message digest will be in error.

Secure Hash Algorithm Secure Hash Algorithm (SHA) SHA defined in FIPS 180 (1993), 160-bit hash SHA-1 defined in FIPS 180-1 (1995) SHA-256, SHA-384, SHA-512 defined in FIPS 180-2 (2002), 256/384/512-bit hashes SHA-1 being phased out, attack known SHA-512 processes input message with total size less than 2128 bits in 1024 bit blocks to produce a 512-bit digest The Secure Hash Algorithm (SHA) was developed by NIST and published as a federal information processing standard (FIPS 180) in 1993; a revised version was issued as FIPS 180-1 in 1995 and is generally referred to as SHA-1. SHA-1 produces a hash value of 160 bits. In 2002, NIST produced a new version of the standard, FIPS 180-2, that defined three new versions of SHA, with hash value lengths of 256, 384, and 512 bits, known as SHA-256, SHA-384, and SHA-512. These new versions have the same underlying structure and use the same types of modular arithmetic and logical binary operations as SHA-1. In 2005, NIST announced the intention to phase out approval of SHA-1 and move to a reliance on the other SHA versions by 2010. Shortly thereafter, a research team described an attack in which two separate messages could be found that deliver the same SHA-1 hash using 269 operations, far fewer than the 280 operations previously thought needed to find a collision with an SHA-1 hash. This result should hasten the transition to the other versions of SHA. The SHA-512 algorithm takes as input a message with a maximum length of less than 2128 bits and produces as output a 512-bit message digest. The input is processed in 1024-bit blocks.

SHA-512 Hash Function Stallings DCC8e Figure 21.8 depicts the overall processing of a message to produce a digest. The processing consists of the following steps: Step 1: Append padding bits. The message is padded so that its length is congruent to 896 modulo 1024 [length mod 1024 = 896). Step 2: Append length. A block of 128 bits is appended to the message. This block is treated as an unsigned 128-bit integer (most significant byte first) and contains the length of the original message (before the padding) Step 3: Initialize MD buffer. A 512-bit buffer is used to hold intermediate and final results of the hash function. Step 4: Process message in 512-bit (16-word) blocks. The heart of the algorithm is a module that consists of 80 rounds of processing. The 80 rounds have a the same structure, but vary some constants and logical functions. Step 5: Output. After all N 1024-bit blocks have been processed, the output from the Nth stage is the 512-bit message digest. The SHA-512 algorithm has the property that every bit of the hash code is a function of every bit of the input. The complex repetition of the basic function F produces results that are well mixed; that is, it is unlikely that two messages chosen at random, even if they exhibit similar regularities, will have the same hash code.

SHA-512 The processing consists of the following steps: Step 1: Append padding bits. The message is padded so that its length is congruent to 896 modulo 1024 [length mod 1024 = 896). Step 2: Append length. A block of 128 bits is appended to the message. This block is treated as an unsigned 128-bit integer (most significant byte first) and contains the length of the original message (before the padding) Step 3: Initialize MD buffer. A 512-bit buffer is used to hold intermediate and final results of the hash function. Step 4: Process message in 512-bit (16-word) blocks. The heart of the algorithm is a module that consists of 80 rounds of processing. The 80 rounds have a the same structure, but vary some constants and logical functions. Step 5: Output. After all N 1024-bit blocks have been processed, the output from the Nth stage is the 512-bit message digest.

Public Key Encryption A public-key encryption scheme has six ingredients: • Plaintext: the readable message or data fed into the algorithm as input. • Encryption algorithm: performs various transformations on the plaintext. • Public and private key: a pair of keys where one is used for encryption and the other for decryption. The public key of the pair is made public for others to use, while the private key is known only to its owner. • Ciphertext: the scrambled message produced as output, which depends on the plaintext and key. • Decryption algorithm: accepts the ciphertext and the matching key and produces the original plaintext. Of equal importance to symmetric encryption is public-key encryption, which finds use in message authentication and key distribution. Public-key encryption, first publicly proposed by Diffie and Hellman in 1976, is the first truly revolutionary advance in encryption in literally thousands of years. They are based on mathematical functions rather than on simple operations on bit patterns, and are asymmetric, involving the use of two separate keys. The use of two keys has profound consequences in the areas of confidentiality, key distribution, and authentication. A public-key encryption scheme has six ingredients, as shown in Stallings DCC8e Figure 21.9: • Plaintext: the readable message or data fed into the algorithm as input. • Encryption algorithm: performs various transformations on the plaintext. • Public and private key: a pair of keys where one is used for encryption and the other for decryption. The public key of the pair is made public for others to use, while the private key is known only to its owner. • Ciphertext: the scrambled message produced as output, which depends on the plaintext and key. • Decryption algorithm: accepts the ciphertext and the matching key and produces the original plaintext.

Public Key Encryption - Operation public key is used for encryption private key is used for decryption infeasible to determine decryption key given encryption key and algorithm steps: user generates pair of keys user places one key in public domain to send a message to user, encrypt using public key user decrypts using private key A general-purpose public-key cryptographic algorithm relies on one key for encryption and a different but related key for decryption, and have the following important characteristics: It is computationally infeasible to determine the decryption key given only knowledge of the cryptographic algorithm and the encryption key, and for most public-key schemes, either of the two related keys can be used for encryption, with the other for decryption. The essential steps to perform public key encryption are: 1. Each user generates a pair of keys to be used for the encryption and decryption of messages. 2. Each user places one of the two keys in a public register or other accessible file. This is the public key. The companion key is kept private. As Figure 21.9 suggests, each user maintains a collection of public keys obtained from others. 3. If Bob wishes to send a private message to Alice, Bob encrypts the message using Alice's public key. 4. When Alice receives the message, she decrypts it using her private key. No other recipient can decrypt the message because only Alice knows Alice's private key. With this approach, all participants have access to public keys, and private keys are generated locally by each participant and therefore need never be distributed. As long as a user protects his or her private key, incoming communication is secure. At any time, a user can change the private key and publish the companion public key to replace the old public key.

Digital Signatures In this case Bob uses his own private key to encrypt the message. When Alice receives the ciphertext, she finds that she can decrypt it with Bob's public key, thus proving that the message must have been encrypted by Bob. No one else has Bob's private key and therefore no one else could have created a ciphertext that could be decrypted with Bob's public key. Therefore, the entire encrypted message serves as a digital signature. In addition, it is impossible to alter the message without access to Bob's private key, so the message is authenticated both in terms of source and in terms of data integrity. Public-key encryption can be used in another way, as illustrated in Stallings DCC8e Figure 21.9b. Suppose that Bob wants to send a message to Alice and, although it is not important that the message be kept secret, he wants Alice to be certain that the message is indeed from him. In this case Bob uses his own private key to encrypt the message. When Alice receives the ciphertext, she finds that she can decrypt it with Bob's public key, thus proving that the message must have been encrypted by Bob. No one else has Bob's private key and therefore no one else could have created a ciphertext that could be decrypted with Bob's public key. Therefore, the entire encrypted message serves as a digital signature. In addition, it is impossible to alter the message without access to Bob's private key, so the message is authenticated both in terms of source and in terms of data integrity.

Digital Signatures sender encrypts message with private key receiver decrypts with senders public key authenticates sender does not give privacy of data must send both original and encrypted copies more efficient to sign authenticator a secure hash of message send signed hash with message In the preceding scheme, the entire message is encrypted, which, although validating both author and contents, requires a great deal of storage. Each document must be kept in plaintext to be used for practical purposes. A copy also must be stored in ciphertext so that the origin and contents can be verified in case of a dispute. A more efficient way of achieving the same results is to encrypt a small block of bits that is a function of the document. Such a block, called an authenticator, must have the property that it is infeasible to change the document without changing the authenticator. If the authenticator is encrypted with the sender's private key, it serves as a signature that verifies origin, content, and sequencing. A secure hash code such as SHA-1 can serve this function. It is important to emphasize that the digital signature does not provide confidentiality. That is, the message being sent is safe from alteration but not safe from eavesdropping. This is obvious in the case of a signature based on a portion of the message, because the rest of the message is transmitted in the clear. Even in the case of complete encryption, there is no protection of confidentiality because any observer can decrypt the message by using the sender's public key.

RSA Algorithm One of the first public-key schemes was developed in 1977 by Ron Rivest, Adi Shamir, and Len Adleman at MIT and first published in 1978. The RSA scheme has since that time reigned supreme as the only widely accepted and implemented approach to public-key encryption. RSA is a block cipher in which the plaintext and ciphertext are integers between 0 and n – 1 for some n. Encryption and decryption are of the following form, for some plaintext block M and ciphertext block C: C = Me mod n M = Cd mod n = (Me)d mod n = Med mod n Both sender and receiver must know the values of n and e, and only the receiver knows the value of d. This is a public-key encryption algorithm with a public key of PU = {e, n} and a private key of PR = {d, n}. For this algorithm to be satisfactory for public-key encryption, some requirements must be met. The first two are easy. The third can be met for large values of e and n. Stallings DCC8e Figure 21.10 summarizes the RSA algorithm. Begin by selecting two prime numbers, p and q and calculating their product n, which is the modulus for encryption and decryption. Next, we need the quantity (n), referred to as the Euler totient of n, which is the number of positive integers less than n and relatively prime to n. Then select an integer e that is relatively prime to (n) [i.e., the greatest common divisor of e and (n) is 1]. Finally, calculate d such that de mod (n) = 1. It can be shown that d and e have the desired properties.

RSA Example An example is shown in Stallings DCC8e Figure 21.11. For this example, the keys were generated as follows: 1. Select two prime numbers, p = 17 and q = 11. 2. Calculate n = pq = 17  11 = 187. 3. Calculate (n) = (p – 1)(q – 1) = 16  10 = 160. 4. Select e such that e is relatively prime to (n) = 160 and less than (n); we choose e = 7. 5. Determine d such that de mod 160 = 1 and d < 160. The correct value is d = 23, because 23  7 = 161 = 10  160 + 1. The resulting keys are public key PU = {7, 187} and private key PR = {23, 187}. The example shows the use of these keys for a plaintext input of M = 88. For encryption, we need to calculate C = 887 mod 187. For decryption, we calculate M = 1123 mod 187.

Secure Sockets Layer / Transport Layer Security Secure Sockets Layer (SSL) is a widely used set of general purpose security protocols use TCP to provide reliable end-to-end service Transport Layer Security (TLS) in RFC 2246 two implementation options incorporated in underlying protocol suite embedded in specific packages minor differences between SSLv3 and TLS One of the most widely used security services is the Secure Sockets Layer (SSL) and the follow-on Internet standard known as Transport Layer Security (TLS), the latter defined in RFC 2246. SSL is a general-purpose service, implemented as a set of protocols that rely on TCP, to provide a reliable end-to-end secure service. At this level, there are two implementation choices. For full generality, SSL (or TLS) could be provided as part of the underlying protocol suite and therefore be transparent to applications. Alternatively, SSL can be embedded in specific packages. For example, Netscape and Microsoft Explorer browsers come equipped with SSL, and most Web servers have implemented the protocol. This section discusses SSLv3. Only minor changes are found in TLS.

SSL Connection and Session a transport connection providing suitable service are peer-to-peer, transient associated with one session multiple secure connections between parties possible SSL session an association between client and server created by Handshake Protocol define set of cryptographic security parameters to avoid negotiation of new security parameters for each connection  multiple simultaneous sessions between parties possible but not used in practice Two important SSL concepts are the SSL session and the SSL connection, which are defined in the specification as follows: • Connection: A connection is a transport (in the OSI layering model definition) that provides a suitable type of service. For SSL, such connections are peer-to-peer relationships. The connections are transient. Every connection is associated with one session. • Session: An SSL session is an association between a client and a server. Sessions are created by the Handshake Protocol. Sessions define a set of cryptographic security parameters, which can be shared among multiple connections. Sessions are used to avoid the expensive negotiation of new security parameters for each connection. Between any pair of parties (applications such as HTTP on client and server), there may be multiple secure connections. In theory, there may also be multiple simultaneous sessions between parties, but this feature is not used in practice.

Handshake Protocol most complex protocol allows parties to authenticate each other and negotiate encryption and MAC algorithm and cryptographic keys series of messages with four phases: phase 1 Initiate Connection phase 2 Certificate/Key Exchange phase 3 Client Verifies Certificate, Parameters phase 4 Complete Secure Connection Setup The most complex part of SSL is the Handshake Protocol. This protocol allows the server and client to authenticate each other and to negotiate an encryption and MAC algorithm and cryptographic keys to be used to protect data sent in an SSL record. The Handshake Protocol is used before any application data is transmitted. The Handshake Protocol consists of a series of messages exchanged by client and server, which can be viewed as having four phases. Phase 1 is used to initiate a logical connection and to establish the security capabilities that will be associated with it. The details of phase 2 depend on the underlying public-key encryption scheme that is used. In some cases, the server passes a certificate to the client, possibly additional key information, and a request for a certificate from the client. The final message in Phase 2, and one that is always required, is the server_done message, which is sent by the server to indicate the end of the server hello and associated messages. After sending this message, the server will wait for a client response. In phase 3, upon receipt of the server_done message, the client should verify that the server provided a valid certificate if required and check that the server_hello parameters are acceptable. If all is satisfactory, the client sends one or more messages back to the server, depending on the underlying public-key scheme. Phase 4 completes the setting up of a secure connection.

SSL Handshake Protocol Stallings DCC8e Figure 21.15 shows the initial exchange needed to establish a logical connection between client and server. Phase 1 is used to initiate a logical connection and to establish the security capabilities that will be associated with it. The details of phase 2 depend on the underlying public-key encryption scheme that is used. The final message in Phase 2, and one that is always required, is the server_done message, which is sent by the server to indicate the end of the server hello and associated messages. After sending this message, the server will wait for a client response. In phase 3, the client should verify that the server provided a valid certificate if required and check that the server_hello parameters are acceptable. If all is satisfactory, the client sends one or more messages back to the server, depending on the underlying public-key scheme. Phase 4 completes the setting up of a secure connection. The client sends a change_cipher_spec message and copies the pending CipherSpec into the current CipherSpec. Note that this message is not considered part of the Handshake Protocol but is sent using the Change Cipher Spec Protocol. The client then immediately sends the finished message under the new algorithms, keys, and secrets. The finished message verifies that the key exchange and authentication processes were successful. In response to these two messages, the server sends its own change_cipher_spec message, transfers the pending to the current CipherSpec, and sends its finished message. At this point the handshake is complete and the client and server may begin to exchange application layer data.