Cryptographic Tools• Cryptographic algorithms – important element in security services• review various types of elements – symmetric encryption – public-key (asymmetric) encryption – digital signatures and key management – secure hash functions
Symmetric Encryption• universal technique for providing confidentiality• also referred to as single-key encryption• two requirements for secure use: – need a strong encryption algorithm – sender and receiver must have obtained copies of the secret key in a secure fashion – and must keep the key secure
Attacking Symmetric Encryption Cryptanalytic Attacks Brute-Force Attack• rely on: try all possible keys on some – nature of the algorithm ciphertext until an intelligible – plus some knowledge of the general translation into plaintext is characteristics of the plaintext obtained – even some sample plaintext- ciphertext pairs on average half of all possible keys must be tried to achieve success• exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or the key being used• if successful all future and past messages encrypted with that key are compromised
Average Time for Exhaustive Search Number of Time Required at 1 Time Required atKey Size (bits) Alternative Keys Decryption/µs 106 Decryptions/µs 32 232 = 4.3 × 109 231 µs = 35.8 minutes 2.15 milliseconds 56 256 = 7.2 × 1016 255 µs = 1142 years 10.01 hours 128 2128 = 3.4 × 1038 2127 µs = 5.4 × 1024 years 5.4 × 1018 years 168 2168 = 3.7 × 1050 2167 µs = 5.9 × 1036 years 5.9 × 1030 years26 characters 26! = 4 × 1026 2 × 1026 µs = 6.4 × 1012 years 6.4 × 106 years(permutation)
Data Encryption Standard (DES)• most widely used encryption scheme – referred to as the Data Encryption Algorithm – uses 64 bit plaintext block and 56 bit key to produce a 64 bit ciphertext block• strength concerns: – concerns about algorithm – DES is the most studied encryption algorithm in existence – use of 56-bit key – Electronic Frontier Foundation (EFF) announced in July 1998 that it had broken a DES encryption
Data Encryption Standard (DES)• Linux machines have deleted DES from VPN negotiations because it takes minutes or less to break.• This breaks the standard for VPN negotiations.• Windows 2000 server shows 3DES but negotiates DES.
Time to Break a Codeassuming 106 decryptions/ms
Triple DES (3DES)• repeats basic DES algorithm three times using either two or three unique keys• attractions: – 168-bit key length overcomes the vulnerability to brute-force attack of DES – underlying encryption algorithm is the same as in DES• drawbacks: – algorithm is sluggish in software – uses a 64-bit block size
Advanced Encryption Standard (AES) needed a NIST called for selectedreplacement for proposals for a Rijndael in 3DES new AES in 1997 November 2001 should have a security strength equal to or better than 3DES significantly improved 3DES was not efficiency reasonable for long published as FIPS 197 term use symmetric block cipher 128 bit data and 128/192/256 bit keys
Practical Security Issues• typically data unit is larger than a single 64-bit or 128-bit block• electronic codebook (ECB) mode – the simplest approach to multiple-block encryption – each block is encrypted using the same key – exploit regularities in the plaintext• modes of operation – alternative techniques to increase the security for large sequences – overcomes the weaknesses of ECB
Block & Stream Ciphers Block Cipher• processes the input one block of elements at a time• produces an output block for each input block• can reuse keys• more commonStream Cipher• processes the input elements continuously• produces output one element at a time• primary advantage is that they are almost always faster and use far less code• encrypts plaintext one byte at a time• pseudorandom stream is one that is unpredictable without knowledge of the input key
Random and XORPseudorandom is nort random. Linux has areal random number generator./dev/randomAnd the pseudorandom is /dev/urandom.The pseudorandom is fast.You can program it to collect real randomdata.Random XOR with real random is notbreakable until you get access to the key.
Quantum computingA formula to factor large numbers can breakmany formulas.Quantum computers can break almost anynormal encryption.Random XOR is theoretically not breakableby the availability of both.Many people who want to control informationshall not like the use of RXOR and shallresist it.
GPGGPG is available for windows also. Need asimple way to run it.Assignment.Gnu Privacy Guard.