![Message Padding
at end of message must handle a possible last
short block
which is not as large as blocksize of cipher
pad either with known non-data value (eg nulls)
or pad last block along with count of pad size
• eg. [ b1 b2 b3 0 0 0 0 5]
• means have 3 data bytes, then 5 bytes pad+count
this may require an extra entire block over those in
message
there are other, more esoteric modes, which
avoid the need for an extra block](https://image.slidesharecdn.com/4-231008141947-4d8ae202/85/4-ppt-12-320.jpg)
4.ppt
- 1. Multiple Encryption & DES • clear a replacement for DES was needed – theoretical attacks that can break it – demonstrated exhaustive key search attacks • AES is a new cipher alternative • prior to this alternative was to use multiple encryption with DES implementations • Triple-DES is the chosen form
- 2. Double-DES? • could use 2 DES encrypts on each block – C = EK2(EK1(P)) • issue of reduction to single stage • and have “meet-in-the-middle” attack – works whenever use a cipher twice – since X = EK1(P) = DK2(C) – attack by encrypting P with all keys and store – then decrypt C with keys and match X value – can show takes O(256) steps
- 3. Meet In the Middle Attack • For the double-DES cipher & a given (P,C) pair, this attack works as follow: • Encrypt the plaintext P with all possibilities of Ka, store the results in a table, & sort that table by the value of 2length(ka). • Decrypt C with all possible values of Kb, check each resulted value with the entries in the table, in case of match, check these two keys against another known pair (P1,C1), if match, accept them as the correct keys.
- 4. Triple-DES with Two-Keys • hence must use 3 encryptions – would seem to need 3 distinct keys • but can use 2 keys with E-D-E sequence – C = EK1(DK2(EK1(P))) – nb encrypt & decrypt equivalent in security – if K1=K2 then can work with single DES • standardized in ANSI X9.17 & ISO8732 • no current known practical attacks – several proposed impractical attacks might become basis of future attacks
- 5. Triple-DES with Three-Keys • although are no practical attacks on two-key Triple-DES have some indications • can use Triple-DES with Three-Keys to avoid even these – C = EK3(DK2(EK1(P))) • has been adopted by some Internet applications, eg PGP, S/MIME
- 6. Modes of Operation • block ciphers encrypt fixed size blocks – eg. DES encrypts 64-bit blocks with 56-bit key • need some way to en/decrypt arbitrary amounts of data in practise • NIST SP 800-38A defines 5 modes • have block and stream modes • to cover a wide variety of applications • can be used with any block cipher
- 7. Electronic Codebook Book (ECB) • message is broken into independent blocks which are encrypted • each block is a value which is substituted, like a codebook, hence name • each block is encoded independently of the other blocks Ci = EK(Pi) • uses: secure transmission of single values
- 9. Advantages and Limitations of ECB message repetitions may show in ciphertext if aligned with message block particularly with data such graphics or with messages that change very little, which become a code-book analysis problem weakness is due to the encrypted message blocks being independent main use is sending a few blocks of data
- 10. Cipher Block Chaining (CBC) • message is broken into blocks • linked together in encryption operation • each previous cipher blocks is chained with current plaintext block, hence name • use Initial Vector (IV) to start process Ci = EK(Pi XOR Ci-1) C-1 = IV • uses: bulk data encryption, authentication
- 12. Message Padding at end of message must handle a possible last short block which is not as large as blocksize of cipher pad either with known non-data value (eg nulls) or pad last block along with count of pad size • eg. [ b1 b2 b3 0 0 0 0 5] • means have 3 data bytes, then 5 bytes pad+count this may require an extra entire block over those in message there are other, more esoteric modes, which avoid the need for an extra block
- 13. Advantages and Limitations of CBC a ciphertext block depends on all blocks before it any change to a block affects all following ciphertext blocks need Initialization Vector (IV) which must be known to sender & receiver if sent in clear, attacker can change bits of first block, and change IV to compensate hence IV must either be a fixed value (as in EFTPOS) or must be sent encrypted in ECB mode before rest of message
- 14. Stream Modes of Operation • block modes encrypt entire block • may need to operate on smaller units – real time data • convert block cipher into stream cipher – cipher feedback (CFB) mode – output feedback (OFB) mode – counter (CTR) mode • use block cipher as some form of pseudo- random number generator
- 15. Cipher FeedBack (CFB) • message is treated as a stream of bits • added to the output of the block cipher • result is feed back for next stage (hence name) • standard allows any number of bit (1,8, 64 or 128 etc) to be feed back – denoted CFB-1, CFB-8, CFB-64, CFB-128 etc • most efficient to use all bits in block (64 or 128) Ci = Pi XOR EK(Ci-1) C-1 = IV • uses: stream data encryption, authentication
- 17. Advantages and Limitations of CFB appropriate when data arrives in bits/bytes most common stream mode limitation is need to stall while do block encryption after every n-bits note that the block cipher is used in encryption mode at both ends errors propogate for several blocks after the error
- 18. Output FeedBack (OFB) • message is treated as a stream of bits • output of cipher is added to message • output is then feed back (hence name) • feedback is independent of message • can be computed in advance Oi = EK(Oi-1) Ci = Pi XOR Oi O-1 = IV • uses: stream encryption on noisy channels
- 20. Advantages and Limitations of OFB needs an IV which is unique for each use if ever reuse attacker can recover outputs bit errors do not propagate more vulnerable to message stream modification sender & receiver must remain in sync only use with full block feedback subsequent research has shown that only full block feedback (ie CFB-64 or CFB-128) should ever be used
- 21. Counter (CTR) • a “new” mode, though proposed early on • similar to OFB but encrypts counter value rather than any feedback value • must have a different key & counter value for every plaintext block (never reused) Oi = EK(i) Ci = Pi XOR Oi • uses: high-speed network encryptions
- 22. Counter (CTR)
- 23. Advantages and Limitations of CTR • efficiency – can do parallel encryptions in h/w or s/w – can preprocess in advance of need – good for bursty high speed links • random access to encrypted data blocks • provable security (good as other modes) • but must ensure never reuse key/counter values, otherwise could break (cf OFB)