Chapter 2 Cryptographic Tools
Symmetric Encryption • The universal technique for providing confidentiality for transmitted or stored data • Also referred to as conventional encryption or single-key encryption • Two requirements for secure use: • Need a strong encryption algorithm • Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure
Symmetric Encryption Terminology • Plaintext: 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.
Plaintext input Y = E[ K , X ] X = D[ K , Y ] X K K Transmitted ciphertext Plaintext output Secret key shared by sender and recipient Secret key shared by sender and recipient Encryption algorithm (e.g., DES) Decryption algorithm (reverse of encryption algorithm) Figure 2.1 Simplified Model of Symmetric Encryption
Attacking Symmetric Encryption Cryptanalytic Attacks Brute-Force Attack Rely on: Nature of the algorithm Some knowledge of the general characteristics of the plaintext Some sample plaintext- ciphertext pairs Exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or the key being used If successful all future and past messages encrypted with that key are compromised Try all possible keys on some ciphertext until an intelligible translation into plaintext is obtained On average half of all possible keys must be tried to achieve success
Attacking Symmetric Encryption • Example ciphertext ovd dpss dl iylhr aopz zljbyl jvkl • Frequency analysis – the study of letters or groups of letters in a cipher o ‘E’ is the most common letter, occurring ~13% of the time o ‘LL EE SS OO TT FF RR NN PP CC’ are the most common doubles • The above cipher is a Caesar Cipher, one of the first encryption methods invented
Attacking Symmetric Encryption • Brute-force example +1 NUCCORRCKHXKGQZNOYYKIAXKIUJK +2 MTBBNQQBJGWJFPYMNXXJHZWJHTIJ +3 LSAAMPPAIFVIEOXLMWWIGYVIGSHI +4 KRZZLOOZHEUHDNWKLVVHFXUHFRGH +5 JQYYKNNYGDTGCMVJKUUGEWTGEQFG +6 IPXXJMMXFCSFBLUIJTTFDVSFDPEF +7 HOWWILLWEBREAKTHISSECURECODE +8 GNVVHKKVDAQDZJSGHRRDBTQDBNCD +9 FMUUGJJUCZPCYIRFGQQCASPCAMBC +10 ELTTFIITBYOBXHQEFPPBZROBZLAB +11 DKSSEHHSAXNAWGPDEOOAYQNAYKZA +12 CJRRDGGRZWMZVFOCDNNZXPMZXJYZ +13 BIQQCFFQYVLYUENBCMMYWOLYWIXY +14 AHPPBEEPXUKXTDMABLLXVNKXVHWX +15 ZGOOADDOWTJWSCLZAKKWUMJWUGVW
Symmetric Block Encryption •
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