# types of substitution cipher

The Polybius Square is essentially identical to the simple substitution cipher, except that each plaintext character is enciphered as 2 ciphertext characters. On modern computer systems, about 280 keys can be tried in a “short” amount of time, so any algorithm employed by the defender to resist attack should have a key space at least this large. Complexity is no guarantee of security, however, so a detailed understanding of the risk and reward is a must. In a transposition cipher, the plaintext is repositioned, but the letters are left unchanged. A well-known example of a substitution cipher is the Caesar cipher. If the key space is small enough that an adversary can try all possible keys in a “short” amount of time, then it doesn’t matter what the algorithm is, it is essentially useless. In the early 1970s, Whitfield Diffie teamed up with Martin Hellman reasoned that for some applications this two-way symmetrical key capability was unnecessary. The best methods now to secure encryption is to combine both substitution and transposition ciphers to create a powerful encryption schema (AES and DES algorithms combine both ciphers to create very powerful algorithms). This can be found by factoring N into p and q then solving the equation to find d.xxix However, this factorization would take millions of years using current knowledge and technology.19, A simpler method is Pretty Good Privacy (PGP) developed by Phil Zimmerman in 1991xxx and subsequently multiply revised. Both the Caesar cipher and the mono-alphabetic substitution have a block size of one—only one character is encrypted at a time. The recipient then decrypts the session key using their private key, with which they decrypt the message.20. The whole key space is quite large. There’s simply only so many ways you can reposition the letters of a message, therefore most of these techniques can be cracked by hand without the necessity for a computer. Types of Ciphers. The English language is very repetitive; for example, the most common letter is usually “E,” the second most common is “T,” and a three-letter word that appears repeatedly is probably “The.” By applying this type of “frequency analysis,” an eavesdropper can easily guess which letters in the ciphertext represent “E,” “T,” and so on.3 Voice is also very repetitive, in that sounds and vowels are readily replicated due to sampling. The spaces in the plaintext are ignored, for now. Second, the larger number of symbols requires correspondingly more ciphertext to productively analyze letter frequencies. The messages were encoded with a “substitution cipher” used by Caesar. A typical distribution of letters in English language text [10]. However, if the defender does not want to have to change the cipher relatively soon, we suggest a rather larger key space, and so does NIST (National Institute of Standards and Technology) [5]. One method of frustrating frequency attacks on the underlying plaintext is to increase the block size of the cipher. The simple substitution cipher is quite easy to break. Polyalphabetic Cipher is also known as Vigenere Cipher which is invented by Leon Battista Alberti. Examples of similar weak ciphers are Caesar Shift, Atbash, and Keyword. And, if two different one-way keys are used, then security can be enhanced. The answers and/or solutions by chapter can be found in the Online Instructor’s Solutions Manual. The reader will observe that we did not cover the Public Key Infrastructure (PKI) due to lack of space in the chapter. Though its historical origins are unclear, the Pigpen Cipher was used by several groups throughout the years, most notably including Union soldiers imprisoned by the Confederacy during the American Civil War, and by the semi-secret society of Freemasons from the 18th century. Over the past decade, we have seen the application of quantum theory to encryption in what is termed quantum cryptology, which is used to transmit the secret key securely over a public channel. And in the mid-19th century a robust method for discovery of the key period of the cipher was developed. Littlejohn Shinder, Michael Cross, in Scene of the Cybercrime (Second Edition), 2008. So we write 32 % 27=5, read “32 modulo 27” or “32 mod 27” for short. The set of letters used can be more complex. Technically speaking, the Caesar cipher may be differentiated from other, more complex substitution ciphers by terming it either a shift cipher or a mono-alphabetic cipher; both are correct. Such an approach is referred to as a monoalphabetic substitution cipher, because a single cipher alphabet (mapping from plain alphabet to cipher alphabet) is used per message. A device may be fully encrypted (i.e. A person in possession of the key could either send or receive messages. The cipher is conceptually like using multiple different mono-alphabetic cipher keys in sequence. Polybius Square Cipher. Transposition cipher is also susceptible to many different attacks. Then he uses the “one-way function” to work out the corresponding encoding key. Encryption is, of course, a very old science. It is a cipher key, and it is also called a substitution alphabet. There is a subject known as cryptoanalysis that attempts to crack the encryption to expose the key, partially or fully. This is a relatively common puzzle in newspapers, so it should not be surprising it’s easy to break. The operation is technically to divide by 27 and then take the whole number remainder that is left. There is, however, another line of attack. Devices may use multiple keys for different partitions, thereby not being fully compromised if one key is discovered. We simply reverse the text. Encryption can also be used on data “at rest”, i.e. In this chapter we have attempted to cover cryptography from its very simple structure such as substitution ciphers to the complex AES and elliptic curve cryptosystems. Because Various logics and functions works in cryptography. #Types of Substitution Techniques#Ceaser Cipher#Monoalphabetic Cipher#Polyalphabetic Cipher#Playfair Cipher#Types of Asymmetric Cryptography Modern algorithms try to be robust against this in a variety of ways, which will be discussed later. This example also uses the comma character as the final character of the alphabet, 26. Steganography is a process of hiding files within other files, often at bit level – image files are therefore very suitable for this, as reducing 24-bit colour depth to (say) 16-bit is rarely noticeable to the human eye (which is the basic rationale behind lossy compression) and so the other 8 bits can be used for the hidden information. Each letter of the alphabet is assigned a number—that is, A is 0, B is 1, and so on, through Z at 25. Checksums were originally developed due to the unreliability of electronic transmission. The transposition cipher does not replace the original text with different text, but rather moves the original values around. A type of simple substitution cipher, very easy to crack. The Gronsfeld cipher. Finally, let us move on to the real interactive part of this chapter: review questions/exercises, hands-on projects, case projects, and the optional team case project. Stahl, Fred A., On Computational Security, University of Illinois, 1974. The reader will observe that we did not cover the Public Key Infrastructure owing to a lack of space in the chapter. it employs only 2 characters. This means that the keys for encryption and decryption were the same. The key used to encrypt and decrypt and it also needs to be a number. There is no good algorithmic way around the problem of short key periods—once it starts to repeat, the cipher is breakable. The number of changes in the key per element of plaintext before the key repeats is called the period of the key; both preceding cipher examples have a key period of 1 as well as a block size of 1. Whenever we gets in touch of Cryptography then we gets touch in terms like Cipher, Cypher, Caesar Cipher, Ciphertext, Plaintext, Cipher System and Cipher Code. In a polygraphic substitution cipher, plaintext letters are substituted in larger groups, instead of substituting letters individually. Baconian Cipher. Substitution of single letters separately â simple substitution â can be demonstrated by writing out the alphabet in some order to represent the substitution. Gronsfeld This is also very similar to vigenere cipher. However, sometimes the best course of action for the defender to resist such frequency attacks is for the defender to modify the contents of the actual message, before encryption, to remove these regularities. Dr.Bhushan Kapoor, Dr.Pramod Pandya, in Cyber Security and IT Infrastructure Protection, 2014. You can check out my latest â¦ To decrypt the message, one could quickly try all 26 keys. This problem persists to this day. As an example, consider 9. Only Bob has the private key, so he and he alone can decrypt the message. It was invented by Charles Wheatstone in 1854, but is named after the person who promoted its use. Afternoon: substitution cipher â¢ Discussion of frequency analysis (for general substitution ciphers). Substitution ciphers have literally thousands of different implementations, some of which include serious complexity. If the result was odd, a bit with the value 1 would be added to the end of the message. Because of the private key limitation, Alice knows that no eavesdropper can capture and decode the information; thus, she feels safe in conducting the funds transfer. Difference between Substitution Cipher Technique and Transposition Cipher Technique Last Updated: 15-06-2020 Both Substitution cipher technique and Transposition cipher technique are the types of Traditional cipher which are used to convert the plain text into cipher text. The reader will observe that we did not cover the Public Key Infrastructure (PKI) due to lack of space in the chapter. More sophisticated substitution ciphers, in which the alphabet is thoroughly scrambled, are nevertheless easy enough to break, even for a novice. In PGP a random key is first generated and is encrypted using the recipient’s public key. Let’s take a look at an example. Today, the complexity of substitution ciphers has increased tremendously since the creation of the computer. Block ciphers are ciphers with a block size greater than 1, and they will be discussed in more detail in the context of modern encryption in the section “Block Ciphers”. If it contains word boundaries (spaces and punctuation), it is called an Aristocrat. But that does not mean the algorithm resists all attempts to subvert it. Each message recipient (Bob) chooses a “private key” that he will use to decode messages. Simple substitution is a method often used by children in their first experiments with secret code. 1. The number of possible keys is called the key space. Over the past 10 years, we have seen the application of quantum theory to encryption in what is termed quantum cryptology, which is used to transmit the secret key securely over a public channel. Monoalphabetic substitution Each letter is represented by another letter or character in a one-to-one relationship. Early ciphers were relatively simple systems, easy for both sender and receiver to use. The most common encryption in use today is RSA developed by Rivest, Shamir and Adelman in 1978 and relies upon the difficulty of factoring into prime numbers. So encryption must be sufficient to keep this from happening. To modern readers, the Caesar cipher is perhaps better known through the Captain Midnight Code-O-Graph and secret decoder rings that even came inside Kix cereal boxes [4]. Homophonic substitutionmakes it possible to have each letter of the plaintext message correspond tâ¦ Note a feature of the math here that did not arise in the previous example. A VoIP call may have 50 samples/s using 160 bytes of voice. The first one who studied frequencies of letters was the Muslim Arab mathematician Al-Kindi (c.801–873 CE), who formally developed this method. One such program is Audacity that allows the manipulation of words or sentences. Alice, a customer using Bob’s banking service, wants to tell Bob to move funds from her account to her mortgage company account, paying her monthly mortgage. This understanding, that cryptosystems did not have to be symmetrical, opened the door to more intense use of cryptography. In this chapter we have attempted to cover cryptography from its very simple structure such as substitution ciphers to the complex AES and elliptic curve crypto-systems. This means that malicious tampering can lead to two different files having the same computed hash. Caesar’s cipher can be cracked simply by moving each letter in the encoded message back three spaces in the alphabet. Other substitution methods can be much more difficult to crack. Computers can also do it reliably when they have at least 150 characters [6, p. 131]. The mono-alphabetic cipher is subject to frequency attacks or guessing. The encryption per character is the same as the Caesar cipher—letters are converted to numbers and added. This is shown in Figure 4.2. Crypto Wiki is a FANDOM Lifestyle Community. Polyalphabetic cipher was adopted to reduce the effectiveness of frequency analysis attacks on the ciphertext, because each letter in the ciphertext is shifted by a different amount, determined according to the key used. To decrypt the message, d is also required. Today, the computer is a cryptanalyst’s strongest weapon. We use cookies to help provide and enhance our service and tailor content and ads. This computing power also allows the ease of combining substitution and transposition into one ciphering technique. So too with encryption ciphers. Use f(x) for encryption, g(x) for decryption. This is known as the sufficient key space principle[1, p. 11]. The issue is that simple substitution ciphers do not really encrypt effectively in terms of computer evaluation â with the rise of the personal computer, substitution ciphers became relatively easy for computers to crack. âIt is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. See Cryptanalysis of the Substitution Cipher for a guide on how to automatically break this cipher. For every measure created there is a countermeasure developed. A single columnar transposition could be attacked by guessing possible column lengths, writing the message out in its columns, and then looking for possible anagrams. SUBSTITUTION TECHNIQUES. Quotes and word puzzles in the local newspapers are examples of this type of scrambling. is about equal to 288, which is large enough to resist brute-force attacks that try all the possible keys; that is, it satisfies the sufficient key space principle. 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