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Using Cryptography to Secure Information

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Overview Introduction to Cryptography Using Symmetric Encryption Using Hash Functions Using Public Key Encryption / Asymmetric Encryption

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Lesson: Introduction to Cryptography What Is Cryptography? What Are Algorithms? What Are Keys? Common Uses of Cryptography

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What Is Cryptography? Cryptography is a set of techniques used to encode and decode information Ciphertext hides information in a message while it is transferred from one location to another or while stored Ciphertext hello Encryption Decryption Plaintext *A%$q

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Services Confidentiality: prevents others from reading the information Integrity checking: reassuring that the information has not been altered since it was generated Authentication: verifying someone's (or something's) identity

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Terminology Encryption/Decryption: o encode/decode: translating entire words or phrases o encipher/decipher: translating letters of symbols individually Plaintext/Ciphertext Key Cryptosystem Cryptography / Cryptanalysis Cryptology

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Terminology Encryption is the process of encoding a message so that its meaning is not obvious; decryption is the reverse process, transforming an encrypted message back into its normal, original form. we say that we encode, encrypt, or encipher the original message to hide its meaning. Then, we decode, decrypt, or decipher it to reveal the original message. A system for encryption and decryption is called a cryptosystem.

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Algorithm (Plaintext) = Ciphertext What Are Algorithms? You can publish algorithms or keep them secret Multiple users can use the same algorithm to encrypt or decrypt multiple data sources AlgorithmPlaintextCiphertext Advance 2 spacesAC KM Examples e=f(m) Algorithms are mathematical formulas to encrypt or decrypt data

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Encryption Algorithms The cryptosystem involves a set of rules for how to encrypt the plaintext and how to decrypt the ciphertext. The encryption and decryption rules, called algorithms, often use a device called a key, denoted by K, so that the resulting ciphertext depends on the original plaintext message, the algorithm, and the key value. We write this dependence as c = e k ( m ) Essentially, e is a set of encryption algorithms, and the key k selects one specific algorithm from the set. We see later in this chapter that a cryptosystem such as the Caesar cipher is keyless but that keyed encryptions are more difficult to break. An encryption algorithm, or cipher, is a means of transforming plaintext into ciphertext, under the control of a secret key. This process is called encryption or encipherment. given c depends totally on the secrecy of k.

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c = e k ( m ) m is the plaintext, e is the cipher function, k is the secret key, c is the ciphertext. The reverse process is called decryption or decipherment, and we write m = d k ( c ) d is the decipher function, Note, that the encryption and decryption algorithms e, d are public, the secrecy of m given c depends totally on the secrecy of k.

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Symmetric Cryptosytem the encryption and decryption keys are the same, This form is called symmetric encryption because d and e are mirror-image processes

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What Are Keys? Keys are normally secret Key length and complexity influence the strength of encryption A key is a string of bits that is used to vary the results of an algorithm Algorithm and Key (Plaintext) = Ciphertext Algorithm and Key (Plaintext) = Ciphertext Examples AlgorithmKeyPlaintextCiphertext Advance x spaces3AD 5AF

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Three Techniques in Encryption Shift Cipher each letter is translated to a letter a fixed number of places after it in the alphabet. ( Caesar Cipher ) Substitution One symbol is exchanged for another, This technique is called a monoalphabetic cipher or simple substitution. only one alphabet substitution was used to encrypt the whole alphabet. Polyalphabetic substitution cipher, encrypt each letter with a different alphabet ( Vigenere Cipher ) Transposition (Permutation) The order of the symbols is rearranged

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Shift Cipher The Caesar Cipher: A shiff of 3: c i =e(m i )=( m i +3 ) mod 26 Plaintext ABCDEFGHIJKLMNOPQRSTUVWXYZ Ciphertext defghijklmnopqrstuvwxyzabc Using this encryption, the message PLEASE CALL ME would be encoded as PLEASE CALL ME S …….. There are only 26 possible keys, so the time for this exhaustive key search is very small.

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Shift Cipher i=7 c i =e(m i )=m i +7 Using above encryption algorithm, encrypt this message, HELLO

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