Can be attacked through frequency analysis vernam

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Can be attacked through frequency analysis Vernam Cipher - One time pad, random set of non-repeating characters Book or Running Key Cipher n Using text from a book as the key and performing modulo26 addition on it. n Would use specific line and page number Codes - Deal with words and phrases and represent them with other numbers or letter Concealment cipher : Every X number of words within a text, is a part of the real message. Steganogrophy n Hiding the existence of the message. n A digital watermark would be used to detect copying of digital images Secret Key Cryptography – Symmetric Key n Sender and receiver both know the key n Encrypt and decrypt with the same key n Secret key should be changed frequently n Requires secure distribution of keys – by alternate channel n Ideally only used once n Secret Key Cryptosystem does have both public and private information n Public n Algorithm for enciphering plaintext n Possibly some plaintext and cipher text n Possibly encipherment of chosen plaintext n Private n The KEY n One cryptographic transformation out of many possible transformations n Large keys like >128 bit are very hard to break n Very fast n Sender requires different key for each receiver n Time stamps can be associated to the key so valid only during time window (counters replay) n No Authentication or repudiation n Best known is DES developed by IBM in 1970’s for commercial use DES – Data Encryption Standard n Derived in 1972 as derivation of Lucifer algorithm developed by Horst Fiestel at IBM n Patented in 1974 - Block Cipher Cryptographic System n Commercial and non-classified systems n DES describes the Data Encryption Algorithm DEA n Federal Information Processing Standard FIPS adopted DES in 1977 n Re-certified in 1993 by National Institute of Standards and Technology but will be replaced by AES Advanced Encryption Standard by Rijndael. n DES uses 64 bit block size and 56 bit key, begins with 64 bit key and strips 8 parity bits n DEA is 16 round cryptosystem designed for implementation in hardware n 56 bit key = 2 56 or 70 quadrillion possible keys n Distributed systems can break it. U.S. Government no longer uses it n Triple DES – three encryptions using DEA are now being used until AES is adopted n DES uses confusion and diffusion as suggested by Claude Shannon n Confusion conceals statistical connection n Accomplished through s-boxes n Diffusion spread the influence of plaintext character over many ciphertext characters
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n Accomplished through p-boxes DES Operates in four modes n Electronic Code Book (ECB) n Cipher Block Chaining (CBC) n Cipher Feedback (CFB) n Output Feedback (OFB) Electronic Code Book n Native encryption mode n Provides the recipe of substitutions and permutations that will be performed on the block of plaintext.
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