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Perfect Secrecy and the One Time Pad (OTP)

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I have explained unconditional  (or  information-theoretical)  security in my previous post (see Unconditional vs. Conditional Security ). As I have mentioned there, we can refer to unconditional security in the context of various cryptographic primitives, among which the  encryption schemes (see Symmetric vs. Asymmetric Encryption ). An encryption scheme that is information-theoretically secure provides  perfect secrecy (see, e.g., [1]) ,  because the  ciphertext  perfectly  hides  the  plaintext.  In other words, the adversary has the same probability to correctly indicate the message  m  regardless if he/she knows the corresponding ciphertext  c.  Hence, the knowledge of the ciphertext gives no new information about the plaintext. We ignore the length of the message - which, of course, is exposed - and assume that all the possible messages are equally long. More rigorously, the  a-posteriori  probability  Pr[M=m/C=c]  to guess the plaintext  m  ( a - posteriori  in the sense that
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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 ] . Information-theoretical security is a strong concept that sometimes (in fact, most of the time!) can
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