1CSC 4103 - Operating SystemsSpring 2007Tevfik KoşarLouisiana State UniversityApril 17th, 2007Lecture - XXIProtection and Security - IISymmetric Encryption• Same key used to encrypt and decrypt– E(k) can be derived from D(k), and vice versa• DES is most commonly used symmetric block-encryption algorithm(created by US Govt)– Encrypts a block of data at a time (64 bit messages, with 56 bit key)• Triple-DES considered more secure (repeat DES three times withthree different keys)• Advanced Encryption Standard (AES) replaces DES– Key length upto 256 bits, working on 128 bit blocks• Twofish, RC4, RC5 .. other symmetric algorithms• RC4 is most common symmetric stream cipher (works on bits, notblocks), but known to have vulnerabilities– Encrypts/decrypts a stream of bytes (i.e wireless transmission, webbrowsers)– Key is a input to psuedo-random-bit generator• Generates an infinite keystreamSymmetric EncryptionAsymmetric Encryption• Encryption and decryption keys are different• Public-key encryption based on each user having twokeys:– public key – published key used to encrypt data– private key – key known only to individual user used to decryptdata• Must be an encryption scheme that can be made publicwithout making it easy to figure out the decryptionscheme– Most common is RSA (Rivest, Shamir, Adleman) block cipherAsymmetric Encryption (Cont.)• Formally, it is computationally infeasible to derive D(kd, N) from E(ke , N), and so E(ke , N) need not be keptsecret and can be widely disseminated– E(ke , N) (or just ke) is the public key– D(kd , N) (or just kd) is the private key– N is the product of two large, randomly chosen prime numbersp and q (for example, p and q are 512 bits each)– Select ke and kd, where ke satisfies kekd mod (p− 1 )(q − 1) = 1– Encryption algorithm is E(ke , N)(m) = mke mod N,– Decryption algorithm is then D(kd , N)(c) = ckd mod NAsymmetric Encryption Example• For example. choose p = 7 and q = 13• We then calculate N = 7∗13 = 91 and (p− 1 )(q− 1 ) = 72• We next select ke relatively prime to 72 and< 72, yielding 5• Finally,we calculate kd such that kekd mod 72 = 1, yielding 29• We how have our keys– Public key, ke, N = 5, 91– Private key, kd , N = 29, 91• Encrypting the message 69 with the public key results in thecyphertext 62 (E=695 mod 91)• Cyphertext can be decoded with the private key– Public key can be distributed in cleartext to anyone who wants tocommunicate with holder of public keyEncryption and Decryption using RSA AsymmetricCryptographyCryptography (Cont.)• Note symmetric cryptography based ontransformations, asymmetric based on mathematicalfunctions– Asymmetric much more compute intensive– Typically not used for bulk data encryption– Used for authentication, confidentiality, key distributionAuthentication• Constraining set of potential senders of a message– Complementary and sometimes redundant to encryption– Also can prove message unmodified• Algorithm components– A set K of keys– A set M of messages– A set A of authenticators– A function S : K → (M→ A)• That is, for each k ∈ K, S(k) is a function for generatingauthenticators from messages• Both S and S(k) for any k should be efficiently computablefunctions– A function V : K → (M× A→ {true, false}). That is, for each k ∈ K, V(k) is afunction for verifying authenticators on messages• Both V and V(k) for any k should be efficiently computablefunctionsAuthentication (Cont.)• For a message m, a computer can generate an authenticator a ∈ Asuch that V(k)(m, a) = true only if it possesses S(k)• Thus, computer holding S(k) can generate authenticators onmessages so that any other computer possessing V(k) can verifythem• Computer not holding S(k) cannot generate authenticators onmessages that can be verified using V(k)• Since authenticators are generally exposed (for example, they aresent on the network with the messages themselves), it must not befeasible to derive S(k) from the authenticatorsKey Distribution• Delivery of symmetric key is huge challenge– Sometimes done out-of-band, via paper documents orconversation• Asymmetric keys can proliferate – stored on key ring– Even asymmetric key distribution needs care – man-in-the-middle attackMan-in-the-middle Attack on AsymmetricCryptographyDigital Certificates• Proof of who or what owns a public key• Public key digitally signed a trusted party• Trusted party receives proof of identification fromentity and certifies that public key belongs to entity• Certificate authority are trusted party – their publickeys included with web browser distributions– They vouch for other authorities via digitally signing their keys,and so onEncryption Example - SSL• Insertion of cryptography at one layer of the ISO network model(the transport layer)• SSL – Secure Socket Layer (also called TLS)• Cryptographic protocol that limits two computers to only exchangemessages with each other– Very complicated, with many variations• Used between web servers and browsers for secure communication(credit card numbers)• The server is verified with a certificate assuring client is talking tocorrect server• Asymmetric cryptography used to establish a secure session key(symmetric encryption) for bulk of communication during session• Communication between each computer then uses symmetric keycryptographyUser Authentication• Crucial to identify user correctly, as protection systems depend onuser ID• User identity most often established through passwords, can beconsidered a special case of either keys or capabilities– Also can include something user has and /or a user attribute• A password can be associated with each resource (eg. File)• Different passwords may be associated with different access rights– Eg. Reading, updating, and deleting files• Passwords must be kept secret– Frequent change of passwords– Use of “non-guessable” passwords– Log all invalid access attempts• Passwords may also either be encrypted or allowed to be used onlyoncePassword Vulnerabilities• Password length– A four digit password would take less than 5 seconds to crack• Password combination– Should use combination of digits, upper and lower case letters,and other characters• Never write your password somewhere, memorize it• Periodically change your password• Do not use the following in your password:– Name,