Review For Exam 3 (April 15, 2010) © Abdou Illia – Spring 2010

2 Elements of Cryptography

3 Cryptography? 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.

4 Your knowledge about Cryptography 1) 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 network c) Both a and b

5 Your knowledge about Cryptography 2) Which of the following security issues is addressed by cryptographic systems? a) Confidentiality; i.e. protection against eavesdropping b) Authentication; i.e. assurance parties involved in a communication are who they claim to be c) Message integrity; i.e. assurance that messages are not altered en route d) Availability; i.e. making sure that communication systems are not shut down by intruders. e) All of the above

6 Basic Terminology 1 Network Plaintext “Hello” Ciphertext “ ” Plaintext “Hello” Decryption Algorithm Interceptor Party A Party 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 key Encryption Algorithm + Encryption key

7 Basic 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 decryption Party A Party B Party A Party B * Trivially identical means simple transformation could lead from one key to the another. Flexcrypt:

8 Your knowledge about Cryptography 3) 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 algorithms b) An attacker gets the decryption key c) a and b are equally damaging 4) Which of the following presents more challenge for exchanging keys between partners? a) Asymmetric encryption b) Symmetric encryption c) A and b are equally challenging

9 Exhaustive search and Key length Key Length in bitsNumber of possible keys or E E E E+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

10 Your knowledge about Cryptography 4)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? ________________

11 Weak 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 key Key Length in bitsNumber of possible keys or E E E E+154

12 Your knowledge about Cryptography 6)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?

Symmetric Key Encryption

14 Symmetric 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 methods are used today MethodsYear approvedComments Data Encryption Standard - DES : Electronic Frontier Foundation’s Deep Crack breaks a DES key in 56 hours DES-Cipher Block Chaining Triple DES – TDES or 3DES1999 Advanced Encryption Standard – AES2001Most used today Other symmetric encryption methods IDEA (International Data Encryption Algorithm), RC5 (Rivest Cipher 5), CAST (Carlisle Adams Stafford Tavares), Blowfish

15 Data Encryption Standard (DES) DES Encryption Process 64-Bit Ciphertext Block 64-Bit DES Symmetric Key (56 bits + 8 redundant bits) 64-Bit Plaintext Block 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 ciphertext

16 DES-Cipher Block Chaining First 64-Bit Plaintext Block DES Encryption Process Second 64-Bit Plaintext Block First 64-Bit Ciphertext Block Initialization Vector (IV) DES Encryption Process Second 64-Bit Ciphertext Block DES 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 block

17 Triple DES (3DES) SenderReceiver Encrypts plaintext with the 1 st key Decrypts ciphertext with the 3d key Decrypts output of first step with the 2 nd key Encrypts output of the first step with the 2 nd key Encrypts output of second step with the 3d key; gives the ciphertext to be sent Decrypts output of second step with the 1 st key; gives the original plaintext 168-Bit Encryption with Three 56-Bit Keys 1st 2nd 3rd 2nd 1st

18 Triple DES (3DES) SenderReceiver Encrypts plaintext with the 1 st key Decrypts ciphertext with the 1 st key Decrypts output with the 2 nd key Encrypts output with the 2 nd key Encrypts output with the 1 st key Decrypts output with the 1 st key 112-Bit Encryption With Two 56-Bit Keys 1st 2nd 1st 2nd 1st

19 Your knowledge about Cryptography 7) Based on the way DES and 3DES work, which of the following is true? a) 3DES requires more processing time than DES b) Compared 3DES, DES requires more RAM c) Both a and b 8) Given the increasing use of hand-held devices, 3DES will be more practical than DES. a)True b)False

20 Advanced Encryption Standard - AES Developed by two Belgian cryptographers, Joan Daemen and Vincent Rijmen, and submitted to the AES selection process under the name "Rijndael", a portmanteau of the names of the inventors Offers key lengths of 128 bit, 192 bit, and 256 bit Efficient in terms of processing power and RAM requirements compared to 3DES Can be used on a wide variety of devices including Cellular phones PDAs Etc.

Asymmetric Key Encryption

22 Public Key Encryption Party A Party B Decrypt with Party A’s Private Key Encrypt with Party A’s Public Key Encrypt with Party B’s Public Key Decrypt with Party B’s Private Key Encrypted Message Encrypted Message Each Party uses other party’s public key for encryption Each Party uses own private key for decryption No need to exchange private key, but key need to be very strong (512+ bit)

23 Public Key Encryption methods Asymmetric encryption methods are used both for Encryption in order to provide confidentiality Digital signature in order to provide partners’ authentication MethodsYear proposedComments RSA by Ron Rivest, Adi Shamir, and Leonard Adleman : First attack in lab conditions was reported Elliptic Curve Cryptosystem - ECC1985Becoming widely used Other symmetric encryption methods: Dieffe-Hellman, El-Gamal

24 Basic Terminology 3 Hashing: Mathematical process for converting inputs into fixed-length outputs Hash function: Algorithm that does the hashing. Uses an input + a shared secret or password. Example: MD5, Secure Hash Algorithm. Hash: Fixed-length output of the hashing

25 Encryption Versus Hashing Encryption Uses a key as an input to an encryption method Output is similar in length to input Reversible; ciphertext can be decrypted back to plaintext Use of Key Length of Result Reversibility Hashing Password is usually added to text; the two are combined, and the combination is hashed Output is of a fixed short length, regardless of input One-way function; hash cannot be “de-hashed” back to the original string

26 MS-CHAP * : Hashing for Authentication * Microsoft’s version of Challenge Handshake Authentication Protocol CHAP is an authentication scheme used by Point to Point Protocol (PPP) servers to validate the identity of remote clients 1)After the completion of the link establishment phase, the server sends a "challenge" message to the client. 2)The client responds with a value calculated using a one-way hash function, such as an MD5 or SHA (Secure Hash Algorithm). 3)The server checks the response against its own calculation of the expected hash value. If the values match, the server acknowledges the authentication; otherwise it should terminate the connection. 4)At random intervals the server sends a new challenge to the peer and repeats steps 1 through 3. Shared secret

27 Digital Signature for Message-by- Message Authentication To Create the Digital Signature: 1. Hash the plaintext to create a brief Message Digest; this is NOT the Digital Signature. 2. Sign (encrypt) the message digest with the sender’s private key to create the Digital Signature. 3. Transmit the plaintext + digital signature, encrypted with symmetric key encryption. Plaintext MD DS Hash Sign (Encrypt) with Sender’s Private Key 4. Encrypted with Session Key DSPlaintext Sender Receiver

28 Digital Signature for Message -by-Message Authentication To Test the Digital Signature 5. Hash the received plaintext with the same hashing algorithm the sender used. This gives the message digest. 6. Decrypt the digital signature with the sender’s public key. This also should give the message digest. 7. If the two match, the message is authenticated. MD Received Plaintext MD DS 5.6. Hash Decrypt with True Party’s Public Key 7. Are they equal? Plaintext MD DS Hash Sign (Encrypt) with Sender’s Private Key

29 What you should know Answer to questions on Your knowledge About Cryptography in these class notes Answer to questions in ReadingQuestionCh7.doc file in Notes’ section of web site.

30 Cryptographic Systems

31 Figure 8-1: Cryptographic System Phase 1: Initial Negotiation of Security Parameters Phase 2: Mutual Authentication Client PC Server Phase 3: Key Exchange or Key Agreement Three Initial “Hand-Shaking” Phases

32 Figure 8-1: Cryptographic System Phase 4: Ongoing Communication with Message-by-Message Confidentiality, Authentication, and Message Integrity Client PC Server The Initial Hand-Shaking Stages are Very Brief Almost All Messages are Sent During the Ongoing Exchange Phase

33 Figure 8-2: Major Cryptographic Systems Application Layer Transport Internet Data Link Physical PPTP, L2TP (really only a tunneling system) Not applicable. No messages are sent at this layer—only individual bits IPsec SSL/TLS Kerberos Cryptographic System

34 Figure 8-4: SSL/TLS Operation Protects All Application Traffic That is SSL/TLS-Aware SSL/TLS Works at Transport Layer Applicant (Customer Client) Verifier (Merchant Server)

35 Figure 8-4: SSL/TLS Operation Applicant (Customer Client) Verifier (Merchant Server) 1. Negotiation of Security Options (Brief) 2. Merchant Authenticates Self to Customer Uses a Digital Certificate Customer Authentication is Optional and Uncommon

36 Figure 8-4: SSL/TLS Operation Applicant (Customer Client) Verifier (Merchant Server) 3. Client Generates Random Session Key Client Sends Key to Server Encrypted with Public Key Encryption 4. Ongoing Communication with Confidentiality and Merchant Digital Signatures