Does privacy require true randomness?

Carl Bosley, Yevgeniy Dodis

Research output: Chapter in Book/Report/Conference proceedingConference contribution


Most cryptographic primitives require randomness (for example, to generate their secret keys). Usually, one assumes that perfect randomness is available, but, conceivably, such primitives might be built under weaker, more realistic assumptions. This is known to be true for many authentication applications, when entropy alone is typically sufficient. In contrast, all known techniques for achieving privacy seem to fundamentally require (nearly) perfect randomness. We ask the question whether this is just a coincidence, or, perhaps, privacy inherently requires true randomness? We completely resolve this question for the case of (information-theoretic) private-key encryption, where parties wish to encrypt a b-bit value using a shared secret key sampled from some imperfect source of randomness script J sign. Our main result shows that if such n-bit source script J sign allows for a secure encryption of b bits, where b > log n, then one can deterministically extract nearly b almost perfect random bits from script J sign. Further, the restriction that b > log n is nearly tight: there exist sources script J sign allowing one to perfectly encrypt (log n -loglog n) bits, but not to deterministically extract even a single slightly unbiased bit. Hence, to a large extent, true randomness is inherent for encryption: either the key length must be exponential in the message length b, or one can deterministically extract nearly b almost unbiased random bits from the key. In particular, the one-time pad scheme is essentially "universal". Our technique also extends to related computational primitives which are perfectly-binding, such as perfectly-binding commitment and computationally secure private- or public-key encryption, showing the necessity to efficiently extract almost b pseudorandom bits.

Original languageEnglish (US)
Title of host publicationTheory of Cryptography - 4th Theory of Cryptography Conference, TCC 2007, Proceedings
PublisherSpringer Verlag
Number of pages20
ISBN (Print)9783540709350
StatePublished - 2007
Event4th Theory of Cryptography Conference, TCC 2OO7 - Amsterdam, Netherlands
Duration: Feb 21 2007Feb 24 2007

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume4392 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Other4th Theory of Cryptography Conference, TCC 2OO7

ASJC Scopus subject areas

  • Theoretical Computer Science
  • General Computer Science


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