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Stream Ciphers

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   Stream ciphers are symmetric-key ciphers (i.e., encryption and decryption use the same key) that perform bit-by-bit encryption. In a stream cipher, any bit of the ciphertext depends on a single bit of the plaintext (and the encryption key). Stream ciphers differ from the other big class of symmetric ciphers - the  block ciphers,  which encrypt block-by-block. In a block cipher, the bits in a ciphertext block depend on (ideally) all bits in the corresponding plaintext block. To the extreme, a stream cipher is a particular type of block cipher with the block's length equal to 1. Compared to block ciphers, stream ciphers are usually faster, but the test of time shows them less secure in general. A stream cipher can be seen as the analog of the One Time Pad (OTP) (see Perfect Secrecy and the One Time Pad (OTP) ) in computational security. Recall OTP, for which encryption is simply a bitwise XOR between the plaintext and the encryption key. The same holds for the str...
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Unconditional vs. Conditional Security

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The cryptographic constructions (also referred to later on as cryptographic schemes) are built to stand against  adversaries  that mount intentional  attacks  (see  Crypt(?) ).  In general, we aim for powerful adversaries: a cryptographic construction that stands against one adversary also stands against  weaker  adversaries (i.e., adversaries with less capabilities). So, the stronger the adversary is, the better (i.e., more secure) the scheme is. The most powerful adversary we can think of is  unbounded  in the sense that he/she can use infinite resources (e.g., unlimited computational power and time). A cryptographic construction that fully stands against an unbounded adversary is called  unconditionally secure  or  information-theoretically secure.  Examples of  information-theoretically secure  schemes include  One Time Pad (OTP) [1]   and  Shamir's secret sharing scheme  [2 ] . ...
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Symmetric vs. Asymmetric Encryption

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Encryption  aims to provide  confidentiality , the first listed objective in the  CIA Triad  (see  Crypt(?) ). It transforms meaningful data (the  cleartext ) into a form that conceals its meaning (the  ciphertext ). Of course, concealing should hold for all parties except the one(s) that are intended to read the data, i.e., to  decrypt. Decryption is thus the reverse operation of encryption; it means to obtain - by proper means (owning and using a  decryption  key) - the  plaintext  from the  ciphertext .  Decryption allows restoring the data in its original form by  legitimate  parties. Others (e.g.,  adversaries ) should not be able to reverse the encryption, and if they succeed (in an efficient time), this is the result of successful  cryptanalysis . If the adversary manages to revert (even partially) the ciphertext into the plaintext, we say that the ...
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