Hash function

Hiding Since non-functional CLBs and functional CLBs unused outputs can be both removed without affecting the digital design, they should be well hidden. The operations available to hide these elements are a secure hash function to randomly replace the unused CLBs, randomly connecting their inputs and outputs to nearby passing lines, and to dont-care inputs of other CLBs. These techniques make the non-functional CLBs appear like functional ones. 

This scheme is introduced in [4, 8], and also used in [7]. It differs from the basic scheme in the preprocessing and hiding operations. A secure hash function is applied to the signature before embedding, in order to reduce its size. The locations of the embedded CLBs are also fingerprinted with the identity of the user in order to trace back the origin of mis-appropriation. Additional hiding operations are also put in. 

SHA-1 (secure hashing algorithm) is a NIST-sponsored hashing function that has been... 

In case the attacker has access to agent s code and data as well as to all environmental information the agent is able to gather, the attacker may try to analyze what the agent searches for and forge this information. One of the methods to prevent such analysis is the use of hash functions [2] in a way that the agent does not reveal the required environmental information, i.e., searched information is not embedded in agent s code or carried data. 

For that purpose a secure timeserver is introduced. In this role an agent running within the platform is acting. The time agent supports two modes of operation as described in [1] forward-time hash function and backward-time hash function. The forward-time constructions permit key generation only after a given time. And the backward-time constructions permit key generation only before it. 

If s denotes the secret belonging to the time agent and h denotes a hash function, the forward-time hash function construction has the following steps [1] ... 

Similarly, in order to use backward-time hash function construction there are additional steps required in the protocol. However, this time application first queries the Time Agent for the target time s secret and only then queries the Platform Manager for target container s secret. In the backward-time hash function construction, as described in the previous paragraph, the target... 

Time Agent is an agent that implements forward- and backward-time hash function constructions. There is a one instance of the Time Agent running in the agent platform. 

The forward- and backward time hash functions allows the creation of mobile agents that are able to display the content simultaneously on many target devices after certain point in time or the agents carrying the content with expiration date set. 

There are several ways to facilitate an efihcient search in this list d. First consider a generic method that works whenever each receiver is a member of relatively few subsets 5 the values i, 2, are put in a hash table and in addition a perfect hash function h of the list is transmitted as well (see [15] for a recent survey of such functions). The length of the description of h can be relatively small compared to the length of the list i.e. it can be o m log w). The receiver u should check for all i such that u Si whether i is in the list by computing h i). In our case this would mean checking log TV values. 

Check data integrity (see also section on Hash Functions )... 

With similar hash functions, the messages can be reduced to constant length before they are signed bit by bit [DiHe76]. 

In more or less this form, and with one-way and hash functions of high efficiency, but unproven security, the scheme is nowadays still being considered for practical applications [Merk88, MeMa82 pp. 396-409, Meye91, Vaud92, BlMa94]. 

The choice of good redundancy predicates or hash functions is not easy, in particular, if one wants the hash function to be fast. Weaknesses have been found in several proposed versions For redundancy predicates, see [JoCh86], for hash functions, [DaPr85, Gira88, MiOI91]. 

More generally, the proposed schemes with redundancy or fast hash functions (often based on symmetric cryptologic schemes such as DBS) have a rather chaotic overall structure, so that one cannot even hope that breaking them is equivalent to a well-examined problem such as factoring. 

A recent technique is to show that at least the general principle of such a combined construction is sound by proving that the construction would be secure if the chaotic element were replaced by a random oracle [BeRo93]. Soundness in this sense was shown for the combination of trap-door one-way families of permutations and hash functions. 

The first major step was to recognize that redundancy or hash functions are not some protocol around the real signature scheme. Instead, the signature scheme must comprise everything that happens with the message, and security must be proved for this complete scheme. 

One might say cryptologically strong instead, if one adheres to the convention from Footnote 3. However, this term is established, and the convention is not always respected anyway. Actually, they are simply called claw-free permutation pairs in [G0MR88]. The two name changes make the notation consistent with related collision-intractable or collision-free families of hash functions (see Section 8.5). The reasons are that the objects called claws do exist, it is only infeasible to find them, and that similar families without trap-doors are needed later. 

Subsequently, one tried to find constructions on possibly weaker abstract assumptions. In [BeMiSS, BeMi92], the assumption is the existence of a trap-door one-way family of permutations. This assumption was used for the efficient construction in [DiHe76] (see Section 2.4) however, a much more complicated construction was needed to avoid the problems mentioned in Section 2.5. It has a lot in common with one-time signatures and tree authentication. The constructions could be extended to arbitrary one-way permutations, i.e., not necessarily with trapdoors, in [NaYu89]. In a sense, this is not too surprising because no trap-doors were needed in the informal constructions of one-time signatures md tree authentication (see Section 2.4) either. Finally, the result was extended to any oneway function [Romp90]. The main problem in the last two cases was to construct appropriate hash functions. 

Moreover, [DaPP94] contains a construction from an arbitrary so-called collision-intractable family of hash functions, which is fairly efficient at least if one trusts a fast, but not cryptographically strong hash function, and may therefore be a reasonable alternative if number-theoretic assumptions should be disproved. 

One basic idea for these constructions is tree authentication. If one starts with the type sketched in Section 2.4, the same addition is needed as with message hashing The hash functions used must be collision-intractable, and their collisions count as proofs of forgery. 

If one considers all schemes that look moderately secure, such as RSA with the additional measures described in Section 2.5, ordinary digital signature schemes exist where the complexity of one authentication is independent of the overall number of messages to be authenticated. In this case, one usually decides to trust fast hash functions, too, so that long messages can be signed fast. 

If one uses such a fast hash function in bottom-up tree authentication for a fail-stop signature scheme, the overhead for the tree part (for trees of reasonable size, such as depth 20) is small in comparison with the actual signature, at least in time complexity. (This is why one-time signature schemes with tree authentication are still considered in practice, see Section 2.4.)... 

This property, which will guarantee availability of service in the chosen model, is only defined here, but not always required, because that would exclude the general abstract construction with message hashing, unless the notion of hash functions were modified significantly. However, the constructions with concrete collision-intractable families of hash functions can be modified to have this property (see Section 10.1). 

Remark 8.7. One can generalize the constructions from this section and their computational aspects treated in Section 8.5.4 to larger tuples of permutations than pairs, similar to the construction of hash functions in [Damg88]. (Some performance analysis of the hash functions can be found in [Fox91].) However,... 

The property that makes all the functions with bundling properties, and also some other functions such as hash functions, interesting for cryptology is that it is infeasible to find collisions, i.e., two values with the same image see Figure 8.4 (and Figure 6.9). For precision, this notion and two related ones are given formal definitions. 

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