Privacy Amplification, introduced by Bennett et. al in [1] and generalized in [2], is the technique of shrinking a partially secret string to a highly secret one by public discussion. More precisely, in the privacy amplification phase, two legitimate parties, Alice and Bob, who share a partially secret string S about an opponent Eve has partial information to extract, and a shorter but highly-secret string S' by communication through an public channel such that Eve’s information about S' is negligible, i.e., ε= ≥ −S' U uH ( / ) log S2 | '| holds with very high probability for some small ε > 0, where the random variable U summarizes Eve’s complete knowledge about S', and u is the particular value known to Eve [2]. Privacy amplification is an important step in both quantum cryptography and key theoretically secret information agreement [3].
Privacy Amplification, introduced by Bennett et. al in [1] and generalized in [2], is the technique of shrinking a partially secret string to a highly secret one by public discussion. More precisely, in the privacy amplification phase, two legitimate parties, Alice and Bob, who share a partially secret string S about an opponent Eve has partial information to extract, and a shorter but highly-secret string S' by communication through an public channel such that Eve’s information about S' is negligible, i.e., ε= ≥ −S' U uH ( / ) log S2 | '| holds with very high probability for some small ε > 0, where the random variable U summarizes Eve’s complete knowledge about S', and u is the particular value known to Eve [2]. Privacy amplification is an important step in both quantum cryptography and key theoretically secret information agreement [3].
This is the third part of a three-part paper on se- cret-key agreement secure against active adversaries. Here, we con- sider the special case where the legitimate partners already share a mutual string which might, however, be partially known to the adversary. The problem of generating a key in this case has been well studied in the passive-adversary model—for instance, in the context of quantum key agreement—under the name of privacy amplification. We consider the same problem with respect to an ac- tive adversary and propose two protocols, one based on universal hashing and one based on extractors, allowing for privacy ampli- fication secure against an adversary whose knowledge about the initial partially string is limited to one third of the length of this string. Our results are based on novel techniques for au- thentication secure even against adversaries knowing a substantial amount of the secret key. information about . From Eve's point of view, the length of can in this case be roughly equal to the Renyi entropy of . In this paper, we investigate the same problem under the as- sumption that the communication between Alice and Bob is not authenticated. Note that, in contrast to the model where many independent repetitions of the involved random variables are given (15), the same piece of information must be used here both for authentication and as the input for privacy amplifica- tion. Two problems that arise in this context are authentication with an only partially key, and the fact that this authenti- cation leaks information about , hence potentially also about , to the adversary. We show that for our purpose, a new, inter- active, authentication method is better than one-way authentica- tion by strongly universal hashing, and that so-called extractors, requiring fewer random bits (i.e., shorter messages to be com- municated), are a better technique for privacy amplification than
We study the classical problem of privacy amplification, where two parties Alice and Bob share a weak secret X of min-entropy k, and wish to agree on secret key R of length m over a public communication channel completely controlled by a computationally unbounded attacker Eve. Despite being extensively studied in the literature, the problem of designing “optimal ” efficient privacy amplification protocols is still open, because there are several optimization goals. The first of them is (1) minimizing the entropy loss L = k−m (it is known that the optimal value for L = O(λ), where ε = 2−λ is the desired security of the protocol). Other important considerations include (2) minimizing the number of communication rounds, (3) maintaining security even after the secret key is used (this is called post-application robustness), and (4) ensuring that the protocol P does not leak some “useful information ” about the source X (this is called source privacy). Additionally, when dealing with a very long source X, as happens in the so-called Bounded Retrieval Model (BRM), extracting as long a key as possible is no longer the goal. Instead, the goals are (5) to touch as little of X as possible (for efficiency), and (6) to be able to run the protocol many times on the same X, extracting multiple secure keys.
This paper, provides a general treatment of privacy amplification by public discussion, a concept introduced by Bennett, Brassard, and Robert for a special scenario. Privacy amplification is a process that allows two parties to distil a secret key from a common random variable about which an eavesdropper has partial information. The two parties generally know nothing about the eavesdropper's information except that it satisfies a certain constraint. The results have applications to unconditionally secure secret-key agreement protocols and quantum cryptography, and they yield results on wiretap and broadcast channels for a considerably strengthened definition of secrecy capacity.
Communications over the public channel should be authenticated to make privacy amplification against active adversaries possible. A new method of constructing authentication codes from error correcting codes is proposed and analyzed, and an arbitrary q ary code( q is a prime power) can be transformed into an authentication code according to the method. When the two communicants share n bit partially secret string S , an authentication scheme can be constructed from a ( 2 m,k,2 m-k+1 ) extended Reed Solomon code, where m=[XXZS-ZSX*3Y0]n/k[XXYS-YSX*3Y0] and 2 mk . As long as the two parties share an authentication key of at least 3 m bit, there exists a strong protocol for privacy amplification which ensures that the probability that the adversary successfully implements active attacks is at most k/2 m , and a highly secret string can be distilled of length about the Renyi entropy of S when given Eve's complete knowledge U=u about S . As n is large enough, more than 3 m bits secret string can always be distilled, thus the protocol is practical.
For pt. II see ibid., vol.49, no.4, p.832-38 (2003). Here, we consider the special case where the legitimate partners already share a mutual string which might, however, be partially known to the adversary. The problem of generating a secret key in this case has been well studied in the passive-adversary model - for instance, in the context of quantum key agreement - under the name of privacy amplification. We consider the same problem with respect to an active adversary and propose two protocols, one based on universal hashing and one based on extractors, allowing for privacy amplification secure against an adversary whose knowledge about the initial partially secret string is limited to one third of the length of this string. Our results are based on novel techniques for authentication secure even against adversaries knowing a substantial amount of the "secret" key.
In this paper, we investigate how the use of a channel with perfect authenticity but no privacy can be used to repair the defects of a channel with imperfect privacy but no authenticity. More precisely, let us assume that Alice and Bob wish to agree on a secret random bit string, and have at their disposal an imperfect private channel and a perfect public channel. The private channel is imperfect in various ways: transmission errors can occur, and partial information can leak to an eavesdropper, Eve, who also has the power to suppress, inject, and modify transmissions arbitrarily. On the other hand, the public channel transmits information accurately, and these transmissions cannot be modified or suppressed by Eve, but their entire contents becomes known to her. We consider the situation in which a random bit string x has already been transmitted from Alice to Bob over the private channel, and we describe interactive public channel protocols that allow them, with high probability: (1) to assess the extent to which the private channel transmission has been corrupted by tampering and channel noise; and (2) if this corruption is not too severe, to repair Bob’s partial ignorance of the transmitted string and Eve’s partial knowledge of it by distilling from the transmitted and received versions of the string another string, in general shorter than x, upon which Alice and Bob have perfect information, while Eve has nearly no information (or in some cases exactly none), except for its length. These protocols remain secure against unlimited computing power.