Chapter 6 Cryptography and Symmetric Key Algorithms Flashcards
CAESAR CIPHER
One of the earliest known cipher systems was used by Julius Caesar to communicate with Cicero in Rome while he was conquering Europe. Caesar knew that there were several risks when sending messages—one of the messengers might be an enemy spy or might be ambushed while en route to the deployed forces. For that reason, Caesar developed a cryptographic system now known as the Caesar cipher. The system is extremely simple. To encrypt a message, you simply shift each letter of the alphabet three places to the right. For example, A would become D, and B would become E. If you reach the end of the alphabet during this process, you simply wrap around to the beginning so that X becomes A, Y becomes B, and Z becomes C. For this reason, the Caesar cipher also became known as the ROT3 (or Rotate 3) cipher. The Caesar cipher is a substitution cipher that is mono-alphabetic.
commercial code machine nicknamed Enigma
This machine used a series of three to six rotors to implement an extremely complicated substitution cipher. The only possible way to decrypt the message with contemporary technology was to use a similar machine with the same rotor settings used by the transmitting device. The Germans recognized the importance of safeguarding these devices and made it extremely difficult for the Allies to acquire one.
Ultra to attack the Enigma codes
Eventually, their efforts paid off when the Polish military successfully reconstructed an Enigma prototype and shared their findings with British and American cryptology experts. The Allies, led by Alan Turing, successfully broke the Enigma code in 1940, and historians credit this triumph as playing a significant role in the eventual defeat of the Axis powers. The story of the Allies’ effort to crack the Enigma has been popularized in famous films including U-571 and The Imitation Game.
Japanese Purple Machine, during World War II
A significant American attack on this cryptosystem resulted in breaking the Japanese code prior to the end of the war. The Americans were aided by the fact that Japanese communicators used very formal message formats that resulted in a large amount of similar text in multiple messages, easing the cryptanalytic effort.
cryptographic systems to meet four fundamental goals
confidentiality, integrity, authentication, and nonrepudiation. Achieving each of these goals requires the satisfaction of a number of design requirements, and not all cryptosystems are intended to achieve all four goals. In the following sections, we’ll examine each goal in detail and give a brief description of the technical requirements necessary to achieve it.
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. Both of these concepts are explored in the section “Modern Cryptography” later in this chapter.
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.
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.
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.
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,
keys, cryptovariables.
is nothing more than a number. It’s usually a very large binary number, but it’s a number nonetheless. Every algorithm has a specific key space. The key space is the range of values that are valid for use as a key for a specific algorithm. A key space is defined by its bit size. Bit size is nothing more than the number of binary bits (0s and 1s) in the key. The key space is the range between the key that has all 0s and the key that has all 1s. Or to state it another way, the key space is the range of numbers from 0 to 2n, where n is the bit size of the key. So, a 128-bit key can have a value from 0 to 2128 (which is roughly 3.40282367 × 1038, a very big number!). It is absolutely critical to protect the security of secret keys. In fact, all of the security you gain from cryptography rests on your ability to keep the keys used private.
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.”
CRYPTOGRAPHY CONCEPTS
The art of creating and implementing secret codes and ciphers is known as cryptography. This practice is paralleled by the art of cryptanalysis—the study of methods to defeat codes and ciphers. Together, cryptography and cryptanalysis are commonly referred to as cryptology. Specific implementations of a code or cipher in hardware and software are known as cryptosystems.
Boolean Mathematics
defines the rules used for the bits and bytes that form the nervous system of any computer. You’re most likely familiar with the decimal system. It is a base 10 system in which an integer from 0 to 9 is used in each place and each place value is a multiple of 10. It’s likely that our reliance on the decimal system has biological origins— human beings have 10 fingers that can be used to count.
AND
The AND operation (represented by the ∧ symbol) checks to see whether two values are both true. The truth table that follows illustrates all four possible outputs for the AND function. Remember, the AND function takes only two variables as input. In Boolean math, there are only two possible values for each of these variables, leading to four possible inputs to the AND function. It’s this finite number of possibilities that makes it extremely easy for computers to implement logical functions in hardware.
OR
The OR operation (represented by the ∨ symbol) checks to see whether at least one of the input values is true. Refer to the following truth table for all possible values of the OR function. Notice that the only time the OR function returns a false value is when both of the input values are false
NOT
The NOT operation (represented by the ∼ or ! symbol) simply reverses the value of an input variable.
Exclusive OR
The final logical function you’ll examine in this chapter is perhaps the most important and most commonly used in cryptographic applications—the exclusive OR (XOR) function. It’s referred to in mathematical literature as the XOR function and is commonly represented by the ⊕ symbol. The XOR function returns a true value when only one of the input values is true. If both values are false or both values are true, the output of the XOR function is false.
Modulo Function
The modulo function is, quite simply, the remainder value left over after a division operation is performed. The modulo function is usually represented in equations by the abbreviation mod, although it’s also sometimes represented by the % operator
One-Way Functions
a mathematical operation that easily produces output values for each possible combination of inputs but makes it impossible to retrieve the input values. Public key cryptosystems are all based on some sort of one-way function. In practice, however, it’s never been proven that any specific known function is truly one way. Cryptographers rely on functions that they believe are one way, but it’s always possible that they might be broken by future cryptanalysts.
Nonce
Cryptography often gains strength by adding randomness to the encryption process. One method by which this is accomplished is through the use of a nonce. A nonce is a random number that acts as a placeholder variable in mathematical functions. When the function is executed, the nonce is replaced with a random number generated at the moment of processing for one-time use. The nonce must be a unique number each time it is used. One of the more recognizable examples of a nonce is an initialization vector (IV), a random bit string that is the same length as the block size and is XORed with the message. IVs are used to create unique ciphertext every time the same message is encrypted using the same key.
Zero-Knowledge Proof
One of the benefits of cryptography is found in the mechanism to prove your knowledge of a fact to a third party without revealing the fact itself to that third party. This is often done with passwords and other secret authenticators.
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 security and protection offered by a cryptosystem is directly proportional to the value of the work function/factor. 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. Thus, if information loses its value over time, the work function needs to be only large enough to ensure protection until the value of the data is gone.
Codes
are cryptographic systems of symbols that represent words or phrases, are sometimes secret, but they are not necessarily meant to provide confidentiality. A common example of a code is the “10 system” of communications used by law enforcement agencies. Under this system, the sentence “I received your communication and understand the contents” is represented by the code phrase “10-4.” This code is commonly known by the public, but it does provide for ease of communication. 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.
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).
Transposition ciphers
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.
Substitution ciphers
use the encryption algorithm to replace each character or bit of the plaintext message with a different character. The Caesar cipher discussed in the beginning of this chapter is a good example of a substitution cipher. Now that you’ve learned a little bit about cryptographic math, we’ll take another look at the Caesar cipher. Recall that we simply shifted each letter three places to the right in the message to generate the ciphertext. However, we ran into a problem when we got to the end of the alphabet and ran out of letters. We solved this by wrapping around to the beginning of the alphabet so that the plaintext character Z became the ciphertext character C.
One-Time Pads
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
Running Key Ciphers (also known as a 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.
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.
