Chapter 6 : Exam Essentials

Transposition Ciphers

Transposition ciphers use an encryption algorithm to rearrange the letters of a plaintext message, forming the ciphertext message. The decryption algorithm simply reverses the encryption transformation to retrieve the original message. In the following example, we're attempting to encrypt the message "The fighters will strike the enemy bases at noon" using the secret key attacker. Our first step is to take the letters of the keyword and number them in alphabetical order. The first appearance of the letter A receives the value 1; the second appearance is numbered 2. The next letter in sequence, C, is numbered 3, and so on. This results in the following sequence: A T T A C K E R 1 7 8 2 3 5 4 6 Next, the letters of the message are written in order underneath the letters of the keyword: A T T A C K E R 1 7 8 2 3 5 4 6 T H E F I G H T E R S W I L L S T R I K E T H E E N E M Y B A S E S A T N O O N Finally, the sender enciphers the message by reading down each column; the order in which the columns are read corresponds to the numbers assigned in the first step. This produces the following ciphertext: T E T E E F W K M T I I E Y N H L H A O G L T B O T S E S N H R R N S E S I E A

Symmetric key cryptography Weaknesses

- Key distribution is a major problem. Parties must have a secure method of exchanging the secret key before establishing communications with a symmetric key protocol. - Symmetric key cryptography does not implement nonrepudiation. Because any communicating party can encrypt and decrypt messages with the shared secret key, there is no way to prove where a given message originated. - The algorithm is not scalable. It is extremely difficult for large groups to communicate using symmetric key cryptography. Secure private communication between individuals in the group could be achieved only if each possible combination of users shared a private key. Keys must be regenerated often. Each time a participant leaves the group, all keys known by that participant must be discarded.

Asymmetric Key Strengths

- The addition of new users requires the generation of only one public-private key pair. This same key pair is used to communicate with all users of the asymmetric cryptosystem. This makes the algorithm extremely scalable. - Users can be removed far more easily from asymmetric systems. Asymmetric cryptosystems provide a key revocation mechanism that allows a key to be canceled, effectively removing a user from the system. - Key regeneration is required only when a user's private key is compromised. If a user leaves the community, the system administrator simply needs to invalidate that user's keys. No other keys are compromised and therefore key regeneration is not required for any other user. - Asymmetric key encryption can provide integrity, authentication, and nonrepudiation. If a user does not share their private key with other individuals, a message signed by that user can be shown to be accurate and from a specific source and cannot be later repudiated. - Key distribution is a simple process. Users who want to participate in the system simply make their public key available to anyone with whom they want to communicate. There is no method by which the private key can be derived from the public key. - No preexisting communication link needs to exist. Two individuals can begin communicating securely from the moment they start communicating. Asymmetric cryptography does not require a preexisting relationship to provide a secure mechanism for data exchange.

One-Time Pads

A one-time pad is an extremely powerful type of substitution cipher. One-time pads use a different substitution alphabet for each letter of the plaintext message. They can be represented by the following encryption function, where K is the encryption key used to encrypt the plaintext letter P into the ciphertext letter C: C = (P + K) mod 26 Usually, one-time pads are written as a very long series of numbers to be plugged into the function. Several requirements must be met to ensure the integrity of the algorithm. - The one-time pad must be randomly generated. Using a phrase or a passage from a book would introduce the possibility that cryptanalysts could break the code. - The one-time pad must be physically protected against disclosure. If the enemy has a copy of the pad, they can easily decrypt the enciphered messages. - Each one-time pad must be used only once. If pads are reused, cryptanalysts can compare similarities in multiple messages encrypted with the same pad and possibly determine the key values used. - The key must be at least as long as the message to be encrypted. This is because each character of the key is used to encode only one character of the message.

The Kerckhoffs's Principle

All cryptography relies on algorithms. An algorithm is a set of rules, usually mathematical, that dictates how enciphering and deciphering processes are to take place. Most cryptographers follow the Kerckhoffs's principle, a concept that makes algorithms known and public, allowing anyone to examine and test them. Specifically, the Kerckhoffs's principle (also known as Kerckhoffs's assumption) is that a cryptographic system should be secure even if everything about the system, except the key, is public knowledge. The principle can be summed up as "The enemy knows the system."

Ciphers

Are always meant to hide the true meaning of a message. They use a variety of techniques to alter and/or rearrange the characters or bits of a message to achieve confidentiality. Ciphers convert messages from plaintext to ciphertext on a bit basis (that is, a single digit of a binary code), character basis (that is, a single character of an American Standard Code for Information Interchange (ASCII) message), or block basis (that is, a fixed-length segment of a message, usually expressed in number of bits).

Codes

Are cryptographic systems of symbols that represent words or phrases, are sometimes secret, but they are not necessarily meant to provide confidentiality. Some codes are secret. They may convey confidential information using a secret codebook where the meaning of the code is known only to the sender and recipient. For example, a spy might transmit the sentence "The eagle has landed" to report the arrival of an enemy aircraft.

Triple DES (3DES)

As mentioned in previous sections, the Data Encryption Standard's (DES) 56-bit key is no longer considered adequate in the face of modern cryptanalytic techniques and supercomputing power. However, an adapted version of DES, Triple DES (3DES), uses the same algorithm to produce a more secure encryption. There are four versions of 3DES. The first simply encrypts the plaintext three times, using three different keys: K1, K2, and K3. It is known as DES-EEE3 mode (the Es indicate that there are three encryption operations, whereas the numeral 3 indicates that three different keys are used). DES-EEE3 can be expressed using the following notation, where E(K,P) represents the encryption of plaintext P with key K: E(K1,E(K2,E(K3,P))) DES-EEE3 has an effective key length of 168 bits. The second variant (DES-EDE3) also uses three keys but replaces the second encryption operation with a decryption operation. E(K1,D(K2,E(K3,P))) The third version of 3DES (DES-EEE2) uses only two keys, K1 and K2, as follows: E(K1,E(K2,E(K1,P))) The fourth variant of 3DES (DES-EDE2) also uses two keys but uses a decryption operation in the middle. E(K1,D(K2,E(K1,P))) Both the third and fourth variants have an effective key length of 112 bits.

Asymmetric cryptosystems

Asymmetric cryptosystems use individual combinations of public and private keys for each user of the system Asymmetric cryptosystems provide a key revocation mechanism that allows a key to be canceled, effectively removing a user from the system.

Asymmetric Key Algorithms

Asymmetric key algorithms, also known as public key algorithms, provide a solution to the weaknesses of symmetric key encryption. In these systems, each user has two keys: a public key, which is shared with all users, and a private key, which is kept secret and known only to the user. Asymmetric key algorithms also provide support for digital signature technology.

Know how cryptosystems can be used to achieve authentication goals

Authentication provides assurances as to the identity of a user. One possible scheme that uses authentication is the challenge-response protocol, in which the remote user is asked to encrypt a message using a key known only to the communicating parties. Authentication can be achieved with both symmetric and asymmetric cryptosystems.

Goals of Cryptography: Authentication

Authentication verifies the claimed identity of system users and is a major function of cryptosystems. For example, suppose that Bob wants to establish a communications session with Alice and they are both participants in a shared secret communications system. Alice might use a challenge-response authentication technique to ensure that Bob is who he claims to be.

Block Ciphers

Block ciphers operate on "chunks," or blocks, of a message and apply the encryption algorithm to an entire message block at the same time. The transposition ciphers are examples of block ciphers. The simple algorithm used in the challenge-response algorithm takes an entire word and reverses its letters. The more complicated columnar transposition cipher works on an entire message (or a piece of a message) and encrypts it using the transposition algorithm and a secret keyword. Most modern encryption algorithms implement some type of block cipher.

Cipher Feedback (CFB)

Cipher Feedback (CFB) mode is the streaming cipher version of CBC. In other words, CFB operates against data produced in real time. However, instead of breaking a message into blocks, it uses memory buffers of the same block size. As the buffer becomes full, it is encrypted and then sent to the recipients. Then the system waits for the next buffer to be filled as the new data is generated before it is in turn encrypted and then transmitted. Other than the change from preexisting data to real-time data, CFB operates in the same fashion as CBC. It uses an IV, and it uses chaining.

Goals of Cryptography: Confidentiality

Confidentiality ensures that data remains private in three different situations: when it is at rest, when it is in transit, and when it is in use. Confidentiality is perhaps the most widely cited goal of cryptosystems—the preservation of secrecy for stored information or for communications between individuals and groups. Two main types of cryptosystems enforce confidentiality. Symmetric cryptosystems use a shared secret key available to all users of the cryptosystem. Asymmetric cryptosystems use individual combinations of public and private keys for each user of the system.

Understand the importance of key security

Cryptographic keys provide the necessary element of secrecy to a cryptosystem. Modern cryptosystems utilize keys that are at least 128 bits long to provide adequate security. It's generally agreed that the 56-bit key of the Data Encryption Standard (DES) is no longer sufficiently long to provide security.

Data Encryption Standard (DES)

DES is a 64-bit block cipher that has five modes of operation: Electronic Code Book (ECB) mode, Cipher Block Chaining (CBC) mode, Cipher Feedback (CFB) mode, output feedback (OFB) mode, and Counter (CTR) mode. All of the DES modes operate on 64 bits of plaintext at a time to generate 64-bit blocks of ciphertext. The key used by DES is 56 bits long. DES uses a long series of exclusive OR (XOR) operations to generate the ciphertext. This process is repeated 16 times for each encryption/decryption operation. Each repetition is commonly referred to as a round of encryption, explaining the statement that DES performs 16 rounds of encryption.

Counter (CTR)

DES that is run in Counter (CTR) mode uses a stream cipher similar to that used in CFB and OFB modes. However, instead of creating the seed value for each encryption/decryption operation from the results of the previous seed values, it uses a simple counter that increments for each operation. As with OFB mode, errors do not propagate in CTR mode.

Electronic Code Book (ECB)

Each time the algorithm processes a 64-bit block, it simply encrypts the block using the chosen secret key. This means that if the algorithm encounters the same block multiple times, it will produce the same encrypted block. If an enemy were eavesdropping on the communications, they could simply build a "code book" of all the possible encrypted values. After a sufficient number of blocks were gathered, cryptanalytic techniques could be used to decipher some of the blocks and break the encryption scheme. This vulnerability makes it impractical to use ECB mode on all but the shortest transmissions. In everyday use, ECB is used only for exchanging small amounts of data, such as keys and parameters used to initiate other DES modes as well as the cells in a database.

Cipher Block Chaining (CBC)

In Cipher Block Chaining (CBC) mode, each block of unencrypted text is XORed with the block of ciphertext immediately preceding it before it is encrypted using the DES algorithm. The decryption process simply decrypts the ciphertext and reverses the XOR operation. CBC implements an IV and XORs it with the first block of the message, producing a unique output every time the operation is performed. The IV must be sent to the recipient, perhaps by tacking the IV onto the front of the completed ciphertext in plain form or by protecting it with ECB mode encryption using the same key used for the message. One important consideration when using CBC mode is that errors propagate—if one block is corrupted during transmission, it becomes impossible to decrypt that block and the next block as well.

Advanced Encryption Standard (AES)

In October 2000, the National Institute of Standards and Technology announced that the Rijndael (pronounced "rhine-doll") block cipher had been chosen as the replacement for DES. In November 2001, NIST released FIPS 197, which mandated the use of AES/Rijndael for the encryption of all sensitive but unclassified data by the U.S. government. The AES cipher allows the use of three key strengths: 128 bits, 192 bits, and 256 bits. AES only allows the processing of 128-bit blocks, but Rijndael exceeded this specification, allowing cryptographers to use a block size equal to the key length. The number of encryption rounds depends on the key length chosen: - 128-bit keys require 10 rounds of encryption. - 192-bit keys require 12 rounds of encryption. - 256-bit keys require 14 rounds of encryption.

Output Feedback (OFB)

In output feedback (OFB) mode, DES operates in almost the same fashion as it does in CFB mode. However, instead of XORing an encrypted version of the previous block of ciphertext, DES XORs the plaintext with a seed value. For the first encrypted block, an initialization vector is used to create the seed value. Future seed values are derived by running the DES algorithm on the previous seed value. The major advantages of OFB mode are that there is no chaining function and transmission errors do not propagate to affect the decryption of future blocks.

Running Key Ciphers (Book Cipher)

In this cipher, the encryption key is as long as the message itself and is often chosen from a common book. For example, the sender and recipient might agree in advance to use the text of a chapter from Moby-Dick, beginning with the third paragraph, as the key. They would both simply use as many consecutive characters as necessary to perform the encryption and decryption operations.

Goals of Cryptography: Integrity

Integrity ensures that data is not altered without authorization. If integrity mechanisms are in place, the recipient of a message can be certain that the message received is identical to the message that was sent. Similarly, integrity checks can ensure that stored data was not altered between the time it was created and the time it was accessed. Integrity controls protect against all forms of alteration, including intentional alteration by a third party attempting to insert false information, intentional deletion of portions of the data, and unintentional alteration by faults in the transmission process. Message integrity is enforced through the use of encrypted message digests, known as digital signatures, created upon transmission of a message. The recipient of the message simply verifies that the message's digital signature is valid, ensuring that the message was not altered in transit. Integrity can be enforced by both public and secret key cryptosystems.

Running Key Ciphers (Book Cipher) Example

Let's look at an example. Suppose you wanted to encrypt the message "Richard will deliver the secret package to Matthew at the bus station tomorrow" using the key just described. This message is 66 characters in length, so you'd use the first 66 characters of the running key: "With much interest I sat watching him. Savage though he was, and hideously marred." Any algorithm could then be used to encrypt the plaintext message using this key. Let's look at the example of modulo 26 addition, which converts each letter to a decimal equivalent, adds the plaintext to the key, and then performs a modulo 26 operation to yield the ciphertext. If you assign the letter A the value 0 and the letter Z the value 25, you have the following encryption operation for the first two words of the ciphertext: Plaintext R I C H A R D W I L L Key W I T H M U C H I N T Numeric plaintext 17 8 2 7 0 17 3 22 8 11 11 Numeric key 22 8 19 7 12 20 2 7 8 13 19 Numeric ciphertext 13 16 21 14 12 11 5 3 16 24 4 Ciphertext N Q V O M L F D Q Y E When the recipient receives the ciphertext, they use the same key and then subtract the key from the ciphertext, perform a modulo 26 operation, and then convert the resulting plaintext back to alphabetic characters.

Symmetric memorization chart

Name Block size Key size Advanced Encryption Standard (AES) 128 128, 192, 256 Rijndael Variable 128, 192, 256 Blowfish (often used in SSH) 64 32-448 Data Encryption Standard (DES) 64 56 IDEA (used in PGP) 64 128 Rivest Cipher 2 (RC2) 64 128 Rivest Cipher 5 (RC5) 32, 64, 128 0-2,040 Skipjack 64 80 Triple DES (3DES) 64 112 or 168 Twofish 128 1-256

Goals of Cryptography: Nonrepudiation

Nonrepudiation provides assurance to the recipient that the message was originated by the sender and not someone masquerading as the sender. It also prevents the sender from claiming that they never sent the message in the first place (also known as repudiating the message). Secret key, or symmetric key, cryptosystems (such as simple substitution ciphers) do not provide this guarantee of nonrepudiation. If Jim and Bob participate in a secret key communication system, they can both produce the same encrypted message using their shared secret key. Nonrepudiation is offered only by public key, or asymmetric, cryptosystems

Stream Ciphers

Stream ciphers operate on one character or bit of a message (or data stream) at a time. The Caesar cipher is an example of a stream cipher. The one-time pad is also a stream cipher because the algorithm operates on each letter of the plaintext message independently. Stream ciphers can also function as a type of block cipher. In such operations there is a buffer that fills up to real-time data that is then encrypted as a block and transmitted to the recipient.

Substitution Ciphers

Substitution ciphers use the encryption algorithm to replace each character or bit of the plaintext message with a different character. The Caesar cipher is a good example of a substitution cipher.

Symmetric cryptosystems

Symmetric cryptosystems use a shared secret key available to all users of the cryptosystem The fact that symmetric cryptosystems require each pair of potential communicators to have a shared private key makes the algorithm nonscalable. The total number of keys required to completely connect n parties using symmetric cryptography is given by the following formula: Formula shows number of keys equal to n times n minus 1 divided by 2.

Symmetric Key Algorithms

Symmetric key cryptography can also be called secret key cryptography and private key cryptography. Symmetric key algorithms rely on a "shared secret" encryption key that is distributed to all members who participate in the communications. This key is used by all parties to both encrypt and decrypt messages, so the sender and the receiver both possess a copy of the shared key. The sender encrypts with the shared secret key and the receiver decrypts with it. When large-sized keys are used, symmetric encryption is very difficult to break. It is primarily employed to perform bulk encryption and provides only for the security service of confidentiality.

Know the differences between symmetric and asymmetric cryptosystems.

Symmetric key cryptosystems (or secret key cryptosystems) rely on the use of a shared secret key. They are much faster than asymmetric algorithms, but they lack support for scalability, easy key distribution, and nonrepudiation. Asymmetric cryptosystems use public-private key pairs for communication between parties but operate much more slowly than symmetric algorithms.

Know the Advanced Encryption Standard (AES)

The Advanced Encryption Standard (AES) uses the Rijndael algorithm and is the U.S. government standard for the secure exchange of sensitive but unclassified data. AES uses key lengths of 128, 192, and 256 bits and a fixed block size of 128 bits to achieve a much higher level of security than that provided by the older DES algorithm.

Be able to explain the basic operational modes of the Data Encryption Standard (DES) and Triple DES (3DES).

The Data Encryption Standard operates in five modes: Electronic Code Book (ECB) mode, Cipher Block Chaining (CBC) mode, Cipher Feedback (CFB) mode, Output Feedback (OFB) mode, and Counter (CTR) mode. ECB mode is considered the least secure and is used only for short messages. 3DES uses three iterations of DES with two or three different keys to increase the effective key strength to 112 or 168 bits, respectively.

Zero-Knowledge Proof

The classic example of a zero-knowledge proof involves two individuals: Peggy and Victor. Peggy knows the password to a secret door located inside a circular cave Victor would like to buy the password from Peggy, but he wants Peggy to prove that she knows the password before paying her for it. Peggy doesn't want to tell Victor the password for fear that he won't pay later. Victor can stand at the entrance to the cave and watch Peggy depart down the path. Peggy then reaches the door and opens it using the password. She then passes through the door and returns via path 2. Victor saw her leave down path 1 and return via path 2, proving that she must know the correct password to open the door.

Asymmetric Key Weakness

The major weakness of public key cryptography is its slow speed of operation. For this reason, many applications that require the secure transmission of large amounts of data use public key cryptography to establish a connection and then exchange a symmetric secret key. The remainder of the session then uses symmetric cryptography.

Be familiar with the basic terminology of cryptography.

When a sender wants to transmit a private message to a recipient, the sender takes the plaintext (unencrypted) message and encrypts it using an algorithm and a key. This produces a ciphertext message that is transmitted to the recipient. The recipient then uses a similar algorithm and key to decrypt the ciphertext and re-create the original plaintext message for viewing.

Goals of Cryptography: Confidentiality - Continued

When developing a cryptographic system for the purpose of providing confidentiality, you must think about three different types of data. Data at rest, or stored data, is that which resides in a permanent location awaiting access. Examples of data at rest include data stored on hard drives, backup tapes, cloud storage services, USB devices, and other storage media. Data in motion, or data on the wire, is data being transmitted across a network between two systems. Data in motion might be traveling on a corporate network, a wireless network, or the public internet. Data in use is data that is stored in the active memory of a computer system where it may be accessed by a process running on that system.

Split Knowledge

When the information or privilege required to perform an operation is divided among multiple users, no single person has sufficient privileges to compromise the security of an environment. This separation of duties and two-person control contained in a single solution is called split knowledge. The best example of split knowledge is seen in the concept of key escrow. Using key escrow, cryptographic keys, digital signatures, and even digital certificates can be stored or backed up in a special database called the key escrow database. In the event a user loses or damages their key, that key can be extracted from the backup. However, if only a single key escrow recovery agent exists, there is opportunity for fraud and abuse of this privilege. M of N Control requires that a minimum number of agents (M) out of the total number of agents (N) work together to perform high-security tasks. So, implementing three of eight controls would require three people out of the eight with the assigned work task of key escrow recovery agent to work together to pull a single key out of the key escrow database (thereby also illustrating that M is always less than or equal to N).

Work Function

You can measure the strength of a cryptography system by measuring the effort in terms of cost and/or time using a work function or work factor. Usually the time and effort required to perform a complete brute-force attack against an encryption system is what the work function represents. The size of the work function should be matched against the relative value of the protected asset. The work function need be only slightly greater than the time value of that asset. In other words, all security, including cryptography, should be cost effective and cost efficient. Spend no more effort to protect an asset than it warrants, but be sure to provide sufficient protection.

Symmetric Cryptography

several common symmetric cryptosystems: - Data Encryption Standard (DES) - Triple DES (3DES) - International Data Encryption Algorithm (IDEA) - Blowfish - Skipjack - Advanced Encryption Standard (AES)