Foundations of Computer SecurityLecture 41: Using InformationDr. Bill YoungDepartment of Computer SciencesUniversity of Texas at AustinLecture 41: 1 Using InformationCryptanalysisAttacks on an encryption algorithm are classified according to whatinformation is available to the attacker.Ciphertext-only: attacker has only encrypted textKnown plaintext: attacker has some ciphertext/plaintext pairs.Chosen plaintext: attacker can cause messages of his choosing tobe encrypted.Adaptive chosen plaintext: chosen plaintext attack adjustedaccording to earlier results.Chosen ciphertext: attacker can decrypt selected ciphertext.Lecture 41: 2 Using InformationBreaking a Cipher... ...... ...{M}K?SenderAnalystA cryptanalyst’stask is extracting the correctdecryption from the space of possibledecryptions, given limited information.How much can she glean fromthe ciphertext and the circumstancesto reduce the search space?Lecture 41: 3 Using InformationThought Experiment: Using InformationQuestion 1: Suppose you know that “xyy” encodes a string inthe English alphabet (26 letters) using a substitution cipher. Howmany decryptions are possible?Answer 1: 263= 17576Question 2: Add the information that it’s a simple substitutioncipher.Answer 2: 26 × 25 = 650. (Reduce search space by a factor of27.)Question 3: Add that you know the plaintext is an English word:Answer 3: around 40. (Reduce original search space by a factor of439.)Lecture 41: 4 Using InformationPerfect CiphersA perfect cipher would be one for which no reduction of the searchspace is gained from knowing:1the encryption algorithm, and2the ciphertext.The attacker’s uncertainty (the likelihood of guessing theplaintext) of the message is exactly the same whether or not shehas access to the ciphertext.Do you think a perfect cipher is possible?Lecture 41: 5 Using InformationLessonsThe cryptanalytic task is to reduce the uncertainty in themessage (plaintext) using all available information.A perfect cipher would be one in which no reduction of thesearch space is possible, even given access to the ciphertextand algorithm.Next lecture: A Perfect CipherLecture 41: 6 Using
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