●●●●● ●●●● ●●0 Requirements ●●● ●●●● o two requirements for secure use of symmetric encryption a strong encryption algorithm: the opponent should be unable to decrypt ciphertext or discover the key even if he has a number of ciphextexts together with the plaintext that produced each ciphertext o sender and recipient must have the secret key in a secure fashion and must keep the key secure o assume encryption algorithm is known assume a secure channel to distribute the key COMP4690. HKBU
COMP4690, HKBU 6 Requirements ⚫ two requirements for secure use of symmetric encryption: ⚫ a strong encryption algorithm: the opponent should be unable to decrypt ciphertext or discover the key even if he has a number of ciphextexts together with the plaintext that produced each ciphertext ⚫ sender and recipient must have the secret key in a secure fashion, and must keep the key secure ⚫ assume encryption algorithm is known ⚫ assume a secure channel to distribute the key
●●●●● ●●●● ●●0 Cryptanalysis ●●● ●●●● To exploit the characteristics of the cipher algorithm to attem pt to deduce a specific plaintext or to deduce the key ciphertext only o only know the ciphertext, the most difficult known plaintext o know some plaintext, ciphertext] pairs, to deduce the key chosen plaintext Plaintext chosen by cryptanalyst, together with its corresponding ciphertext generated with the key chosen ciphertext o ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the key e chosen text chosen plaintext& chosen ciphertext COMP4690. HKBU
COMP4690, HKBU 7 Cryptanalysis ⚫ To exploit the characteristics of the cipher algorithm to attempt to deduce a specific plaintext or to deduce the key ⚫ ciphertext only ⚫ only know the ciphertext, the most difficult! ⚫ known plaintext ⚫ know some {plaintext, ciphertext} pairs, to deduce the key ⚫ chosen plaintext ⚫ Plaintext chosen by cryptanalyst, together with its corresponding ciphertext generated with the key ⚫ chosen ciphertext ⚫ ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the key ⚫ chosen text ⚫ chosen plaintext & chosen ciphertext
●●●●● ●●●● ●●0 Brute-Force Attack ●●● ●●●● o Attacker tries every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained proportional to key size assume either know /recognise plaintext Number of Alternative Time required at 10 Key Size(bits) Key Time required at I encryption/ps encryptions 32 232=43x10 21s=358mus 2. 15 milliseconds 20=72x10 As=ll42 years 10.01 hours 212=34x103 21271s=54x102yeas 5. x 10 years 2168=37x1030 s=59×10yean 59 x 100yean 26 characters 26=4×102x1°15s=64x10yean 6.4 x I( years (permutation)
COMP4690, HKBU 8 Brute-Force Attack ⚫ Attacker tries every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained. ⚫ proportional to key size ⚫ assume either know / recognise plaintext
Classical Encryption ●●●●● ●●●● ●●0 Technique ●●● ●●●● ● Substitution letters of plaintext are replaced by other letters or by numbers or symbols ● Transposition e Combine substitution transposition COMP4690. HKBU
COMP4690, HKBU 9 Classical Encryption Technique ⚫ Substitution ⚫ letters of plaintext are replaced by other letters or by numbers or symbols ⚫ Transposition ⚫ Combine substitution & transposition
●●●●● ●●●● ●●0 Caesar Cipher ●●● ●●●● by Julius Caesar o first attested use in military affairs o replaces each letter with the letter standing three places further down the alphabet ● example Plaintext: meet me after the toga party Ciphertext: PHHW PH DIWHU WKH WRUD SDUWB COMP4690. HKBU
COMP4690, HKBU 10 Caesar Cipher ⚫ by Julius Caesar ⚫ first attested use in military affairs ⚫ replaces each letter with the letter standing three places further down the alphabet ⚫ example: Plaintext: meet me after the toga party Ciphertext:PHHW PH DIWHU WKH WRJD SDUWB