Cryptanalysis- Types of Attacks Ciphertext-Only Attack Attacker knows ciphertext of several messages encrypted with the same key and/or several keys Recover the plaintext of as many messages as possible or even better deduce the key(or keys) Given: CI=EK(PD, C2=EK(P2),C=Ek(P Deduce: Either P, P,,P k; or an algorithm to intel from Ci+= EkPi+v Known-Plaintext Attack Known ciphertext plaintext pair of several messages Deduce the key or an algorithm to decrypt further messages Given Pl, C- Ek(P), P2, C2- Ek(P Ek(P) Deduce: Either k, or an algorithm to infer Pi+i from Ek(PCryptanalysis - Types of Attacks • Ciphertext-Only Attack – Attacker knows ciphertext of several messages encrypted with the same key and/or several keys – Recover the plaintext of as many messages as possible or even better deduce the key (or keys) – Given: C1 = Ek (P1 ), C2 = Ek (P2 ),...Ci = Ek (Pi ) Deduce: Either P1 , P2 ,...Pi ; k; or an algorithm to infer Pi+1 from Ci+1 = Ek (Pi+1) • Known-Plaintext Attack – Known ciphertext / plaintext pair of several messages – Deduce the key or an algorithm to decrypt further messages – Given: P1 , C1 = Ek (P1 ), P2 , C2 = Ek (P2 ),...Pi , Ci = Ek (Pi ) – Deduce: Either k, or an algorithm to infer Pi+1 from Ci+1 = Ek (Pi+1)