1The Elements of Cryptography(March 31, 2014)© Abdou Illia – Spring 20142Learning Objectives Discuss Cryptography Terminology Discuss Symmetric Key Encryption Discuss Asymmetric Key Encryption Distinguish between Hashing and Encryption3Cryptography? Traditionally, cryptography refers to The practice and the study of encryption Transforming information in order to prevent unauthorized people to read it. Today, cryptography goes beyond encryption/decryption to include Techniques for making sure that encrypted messages are not modified en route Techniques for secure identification/authentication of communication partners.24Your knowledge about Cryptography1) Which of the following do cryptographic systems protect?a) Data stored on local storage media (like hard drives) from access by unauthorized users.b) Data being transmitted from point A to point B in a networkc) Both a and b#$%^@#$5hh&*9(?>/@$#)>5Your knowledge about Cryptography2) Which of the following security issues is addressed by cryptographic systems?a) Confidentiality; i.e. protection against eavesdroppingb) Authentication; i.e. assurance parties involved in a communication are who they claim to bec) Message integrity; i.e. assurance that messages are not altered en routed) Availability; i.e. making sure that communication systems are not shut down by intruders.e) All of the above6Basic Terminology 1NetworkPlaintext“Hello”Ciphertext “11011101”Ciphertext “11011101”Plaintext“Hello”DecryptionAlgorithmInterceptorParty AParty B Plaintext: original message to be sent. Could be text, audio, image, etc. Encryption/Decryption Algorithm: mathematical tool (software) used to encrypt or decrypt Key: A string of bits used by to encrypt the plaintext or decrypt the ciphertext Ciphertext: encrypted message. Looks like a random stream of bits+ Decryption keyEncryptionAlgorithm+ Encryption keyHello37Basic Terminology 2 Encryption: Converting plaintext into ciphertext using algorithms and keys The size of the ciphertext is proportional to the size of the plaintext Ciphertext is reversible to plaintext Symmetric Key Encryption: Same key is used both for encryption and decryption Keys are usually identical or trivially identical* Asymmetric Key Encryption: Also called Public/Private Key Encryption Two different keys are used: one for encryption, one for decryptionParty AParty BParty AParty B* Trivially identical means simple transformation could lead from one key to the another.Flexcrypt: http://www.flexcrypt.com/flexcryptfree.html8Your knowledge about Cryptography3) Based on how symmetric encryption systems work, which of the following is the worst thing to happen?a) An attacker gets a copy of the encryption and decryption algorithmsb) An attacker gets the decryption keyc) a and b are equally damaging4) Which of the following presents more challenge for exchanging keys between partners?a) Asymmetric encryptionb) Symmetric encryptionc) A and b are equally challenging9Exhaustive search and Key lengthKey Length in bits Number of possible keys (2key length in bits)12244168 25616 6553656 72057594037927900112 5192296858534830000000000000000000 or 5.1923E+33168 3.74144E+50256 1.15792E+77512 1.3408E+154 Attacker could use the right algorithm and do an exhaustive search (i.e. try all possible keys) in order to decrypt the ciphertext Most attacks require the capture of large amount of ciphertext Every additional bit in the length of the key doubles the search time Every additional bit in the length of the key doubles the requirements in terms of minimum processor’s speed to crack the key.410Your knowledge about Cryptography4) If you increase the key length from 56 bits to 66 bits. How much more key combinations an attacker who captures enough ciphertext will have to try in order to decipher the captured ciphertext using the appropriate algorithm?_______________________________________5) Assuming that it takes 7 days to try all possible combinations of a 56 bit key, how much time it would take to try all possible combinations when the key length is increased to 58 bits?________________11Weak vs. Strong Keys Symmetric Key Encryption Usually used for customer e-business Keys with lengths of less than 100 bits are considered weak today.  Keys with lengths of more than 100 bits are considered strong today. Asymmetric Key Encryption Usually used for B2B financial e-business Key pairs must be much longer (512 bit and more) because of the disastrous consequences of breaking the decryption keyKey Length in bitsNumber of possible keys (2key length in bits)122416 6553656 72057594037927900112 5192296858534830000000000000000000 or 5.1923E+33168 3.74144E+50256 1.15792E+77512 1.3408E+15412Your knowledge about Cryptography6) Most attacks require the capture of large amount of ciphertext, which can take a certain amount of time. Beside using strong keys what else can be done to make it harder to crack the key?5Symmetric Key Encryption14Symmetric Key Encryption methods Two categories of methods Stream cipher: algorithm operates on individual bits (or bytes); one at a time Block cipher: operates on fixed-length groups of bits called blocks Only a few symmetric methods are used todayMethods Year approved CommentsData Encryption Standard - DES 1977 1998: Electronic Frontier Foundation’s Deep Crack breaks a DES key in 56 hoursDES-Cipher Block ChainingTriple DES – TDES or 3DES 1999Advanced Encryption Standard – AES 2001 Its versions among the most used todayOther symmetric encryption methodsIDEA (International Data Encryption Algorithm), RC5 (Rivest Cipher 5), CAST (Carlisle Adams Stafford Tavares), Blowfish 15Data Encryption Standard (DES)DES EncryptionProcess64-Bit CiphertextBlock64-Bit DES Symmetric Key(56 bits + 8 redundant bits)64-Bit PlaintextBlock DES is a block encryption method, i.e. uses block cipher DES uses a 64 bit key; actually 56 bits + 8 bits computable from the other 56 bits Problem: same input plaintext gives same output ciphertext616DES-Cipher Block ChainingFirst64-Bit Plaintext BlockDES EncryptionProcessSecond64-Bit Plaintext BlockFirst64-Bit Ciphertext BlockInitializationVector (IV)DES EncryptionProcessSecond64-Bit Ciphertext BlockDES KeyDES Key DES-CBC uses ciphertext from previous block as input making decryption by attackers even harder An 64-bit initialization vector is used for first