1. STREAM CIPHERS by
Jennifer Seberry

2. Symmetric vs Asymetric Algorithms (Public Key)
Symmetric (or classical) encryption: when the same key is used for encryption and decryption.
Asymmetric (or public key): when different (but related in a complex mathematical way) keys are used for encryption and decryption.
8 December 2018
Centre for Computer Security Research, University of Wollongong

3. Centre for Computer Security Research, University of Wollongong
Symmetric Algorithms
Block ciphers (DES, AES, Serpent, Loki, …..): work on blocks of data – currently 64, 128, 192 or 256 bit blocks
Stream ciphers: work on bits or bytes at a time
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Example:
Message
a b c d e f ……
Key is
…..
Ciphertext is
a+1 b+2 c+3 d+4 …..
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Stream ciphers
A stream cipher is a type of symmetric encryption algorithm.
Stream ciphers can be designed to be exceptionally fast, much faster than any block cipher .
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Stream ciphers usage
Washington-Moscow Hotline
GSM mobile phones use A5 encryption (Prince Charles)
PKZIP
Many, many, more
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Stream ciphers
Block ciphers operate on large blocks of data - stream ciphers typically operate on smaller units of plaintext, usually bits or bytes.
The encryption of any particular plaintext with a block cipher will result in the same ciphertext when the same key is used.
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Stream ciphers (Cont)
The generation of the keystream can be independent of the plaintext and ciphertext, yielding what is termed a synchronous stream cipher, or it can depend on the data and its encryption, in which case the stream cipher is said to be self-synchronizing.
Most stream cipher designs are for synchronous stream ciphers.
8 December 2018
Centre for Computer Security Research, University of Wollongong

9. Centre for Computer Security Research, University of Wollongong
One time pads
Current interest in stream ciphers is most commonly attributed to the appealing theoretical properties of the one-time pad.
A one-time pad, sometimes called the Vernam cipher uses a string of bits that is generated completely at random.
The keystream is the same length as the plaintext message and the random string is combined using bitwise XOR with the plaintext to produce the ciphertext.
8 December 2018
Centre for Computer Security Research, University of Wollongong

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One time pads (Cont)
Since the entire keystream is random, even an opponent with infinite computational resources can only guess the plaintext if he or she sees the ciphertext.
Such a cipher is said to offer perfect secrecy, and the analysis of the one-time pad is seen as one of the cornerstones of modern cryptography [Sha49].
8 December 2018
Centre for Computer Security Research, University of Wollongong

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the one-time pad was during wartime over diplomatic channels requiring exceptionally high security, the fact that the secret key (which can be used only once) is as long as the message introduces severe key management problems
While perfectly secure, the one-time pad is in general impractical.
Soldiers spotting Japanese ships in the Coral Sea
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Stream ciphers were developed as an approximation to the action of the one-time pad.
The most widely used stream cipher is RC4.
However, stream ciphers with a dedicated design are typically much faster.
8 December 2018
Centre for Computer Security Research, University of Wollongong

13. Stream ciphers from blocks ciphers
Interestingly, certain modes of operation of a block cipher effectively transform it into a keystream generator and in this way, any block cipher can be used as a stream cipher; as in AES or DES in CFB or OFB modes .
Extreme care must be taken as these stream ciphers are vulnerable to the birthday attack.
8 December 2018
Centre for Computer Security Research, University of Wollongong

14. Linear Feedback Shift Registers
An LFSR is a mechanism for generating a sequence of binary bits.
The register consists of a series of cells that are set by an initialization vector that is, most often, the secret key.
8 December 2018
Centre for Computer Security Research, University of Wollongong

15. Cyclic redundancy codes
Used in all computers are based on
Linear Feedback Shift Registers: they have wonderful statistical properties and look at first sight to be wonderful candidates for stream ciphers BUT
they are linear and can easily be broken
mathematically
8 December 2018
Centre for Computer Security Research, University of Wollongong

16. A5 mobile telephone algorithm
W. G. Chambers “On Random Mappings and Random Permutations,” Walter Fumy (Ed.) Advances in Cryptology --EUROCRYPT ' 97, Springer Verlag.
Eli Biham and his colleagues have now broken A5 during the set up phase and so it absolutely compromised but still used
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Other Stream Ciphers
There are a vast number of alternative stream ciphers that have been proposed in cryptographic literature as well as an equally vast number that appear in implementations and products world-wide.
Many are based on the use of LFSRs and so more amenable to analysis and security assessment.
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Breaking RC3/RC4
RC3 was broken soon after its release
RC4 has also been broken but it still in wide use
Developments of the RC4 algorithm has led to the SHA family of algorithms
At Crypto’04 Biham et al announced SHA0 was broken and in Dec04 Wang et al that SHA1 was broken (2^{69})
8 December 2018
Centre for Computer Security Research, University of Wollongong

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Why No Standard?
NESSIE the European encryption call some years ago yielded no stream cipher better that those currently known
NIST has never called for a stream cipher submission
Europe has now made the Ecrypt call for stream ciphers
8 December 2018
Centre for Computer Security Research, University of Wollongong

20. Centre for Computer Security Research, University of Wollongong
The Future
Current algorithms will be broken
Good non-linear boolean functions are still needed for their design
More complex structures using, clocks and cascades will be devised – however we must remember the adage “the more complicated the design the more likely weaknesses will be overlooked”
8 December 2018
Centre for Computer Security Research, University of Wollongong

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THANK YOU
8 December 2018
Centre for Computer Security Research, University of Wollongong