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The Security of Elastic Block Ciphers Against KeyRecovery Attacks
 In Proceedings of the Information Security Conference (ISC
, 2007
"... Abstract. We analyze the security of elastic block ciphers against keyrecovery attacks. An elastic version of a fixedlength block cipher is a variablelength block cipher that supports any block size in the range of one to two times the length of the original block. Our method for creating an elas ..."
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Abstract. We analyze the security of elastic block ciphers against keyrecovery attacks. An elastic version of a fixedlength block cipher is a variablelength block cipher that supports any block size in the range of one to two times the length of the original block. Our method for creating an elastic block cipher involves inserting the round function of the original cipher into a substitutionpermutation network. In this paper, we form a polynomialtime reduction between the elastic and original versions of the cipher by exploiting the underlying network structure. We prove that the elastic version of a cipher is secure against a given keyrecovery attack if the original cipher is secure against such an attack. Our analysis is based on the general structure of elastic block ciphers (i.e., the network’s structure, the composition methods between rounds in the network and the keying methodology) and is independent of the specific cipher.
Elastic Block Ciphers in Practice: Constructions and Modes of Encryption
 In Proceedings of the European Conference on Computer Network Defense (EC2ND
, 2007
"... We demonstrate the general applicability of the elastic block cipher method by constructing examples from existing block ciphers: AES, Camellia, MISTY1 and RC6. An elastic block cipher is a variablelength block cipher created from an existing fixedlength block cipher. The elastic version supports ..."
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We demonstrate the general applicability of the elastic block cipher method by constructing examples from existing block ciphers: AES, Camellia, MISTY1 and RC6. An elastic block cipher is a variablelength block cipher created from an existing fixedlength block cipher. The elastic version supports any block size between one and two times that of the original block size. We compare the performance of the elastic versions to that of the original versions and evaluate the elastic versions using statistical tests measuring the randomness of the ciphertext. The benefit, in terms of an increased rate of encryption, of using an elastic block cipher varies based on the specific block cipher and implementation. In most cases, there is an advantage to using an elastic block cipher to encrypt blocks that are a few bytes longer than the original block length. The statistical test results indicate no obvious flaws in the method for constructing elastic block ciphers. We also use our examples to demonstrate the concept of a generic key schedule for block ciphers. In addition, we present ideas for new modes of encryption using the elastic block cipher construction.
Elastic Block Ciphers: The Feistel Cipher Case
, 2004
"... We discuss the elastic versions of block ciphers whose round function processes subsets of bits from the data block differently, such as occurs in a Feistel network and in MISTY1. We focus on how specific bits are selected to be swapped after each round when forming the elastic version, using an ela ..."
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We discuss the elastic versions of block ciphers whose round function processes subsets of bits from the data block differently, such as occurs in a Feistel network and in MISTY1. We focus on how specific bits are selected to be swapped after each round when forming the elastic version, using an elastic version of MISTY1 and differential cryptanalysis to illustrate why this swap step must be carefully designed. We also discuss the benefit of adding initial and final key dependent permutations in all elastic block ciphers. The implementation of the elastic version of MISTY1 is analyzed from a performance perspective.
Research Statement
"... The need for secure systems and improved authentication methods is growing as online transactions and identity theft increases, among other reasons. When addressing security needs, forcing applications to design around cryptographic primitives and protocols makes designing such items easier; however ..."
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The need for secure systems and improved authentication methods is growing as online transactions and identity theft increases, among other reasons. When addressing security needs, forcing applications to design around cryptographic primitives and protocols makes designing such items easier; however, disregarding common applications ’ needs and human factors can easily result in any security measure being ineffective. Implementations end up ”modifying ” the primitives or misusing them in order to fit an application, resulting in insecure designs. Also, security software and protocols often prove to be burdensome or confusing to users. My current research straddles systems and theory by considering the context of cryptographic algorithms in terms of applications ’ needs. A natural extension of my work, which I have begun to investigate, is to incorporate human factors into the design of security protocols and utilities. When designing cryptographic algorithms, the security and the efficiency of both software and hardware implementations are considered but not applications ’ needs. Examples of useful properties which are not available in existing symmetric key ciphers are the support for variable sized blocks in block ciphers and the ability to efficiently convert ciphertext resulting from encrypting data with one key to ciphertext corresponding to encrypting with another key without exposing the plaintext (a concept referred to as proxy cryptography). Where data is encrypted and decrypted in a system is also important in preventing plaintext exposure in components where attacks are
Methods for Linear and Differential Cryptanalysis of Elastic Block Ciphers
"... Abstract. The elastic block cipher design employs the round function of a given, bbit block cipher in a black box fashion, embedding it in a network structure to construct a family of ciphers in a uniform manner. The family is parameterized by block size, for any size between b and 2b. The design a ..."
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Abstract. The elastic block cipher design employs the round function of a given, bbit block cipher in a black box fashion, embedding it in a network structure to construct a family of ciphers in a uniform manner. The family is parameterized by block size, for any size between b and 2b. The design assures that the overall workload for encryption is proportional to the block size. When considering the approach taken in elastic block ciphers, the question arises as to whether cryptanalysis results, including methods of analysis and bounds on security, for the original fixedsized cipher are lost or, since original components of the cipher are used, whether previous analysis can be applied or reused in some manner. With this question in mind, we analyze elastic block ciphers and consider the security against two basic types of attacks, linear and differential cryptanalysis. We show how they can be related to the corresponding security of the fixedlength version of the cipher. Concretely, we develop techniques that take advantage of relationships between the structure of the elastic network and the original version of the cipher, independently of the cipher.
Elastic Block Ciphers: Method, Security and Instantiations
"... We introduce the concept of an elastic block cipher, which refers to stretching the supported block size of a block cipher to any length up to twice the original block size while incurring a computational workload that is proportional to the block size. Our method uses the round function of an exist ..."
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We introduce the concept of an elastic block cipher, which refers to stretching the supported block size of a block cipher to any length up to twice the original block size while incurring a computational workload that is proportional to the block size. Our method uses the round function of an existing block cipher as a black box and inserts it into a substitution permutation network. Our method is designed to enable us to form a reduction between the elastic and the original versions of the cipher. Using this reduction, we prove that the elastic version of a cipher is secure against keyrecovery attacks if the original cipher is secure against such attacks. We note that while reductionbased proofs of security are a cornerstone of cryptographic analysis, they are typical when complete components are used as subcomponents in a larger design. We are not aware of use of such techniques in the case of concrete block cipher designs. We demonstrate the general applicability of the elastic block cipher method by constructing examples from existing block ciphers: AES, Camellia, MISTY1 and RC6. We compare the performance of the elastic versions to that of the original versions and evaluate the elastic versions using statistical tests measuring the randomness of the ciphertext. We also use our examples to demonstrate the concept of a generic key schedule for block ciphers. key words: elastic block ciphers, variablelength block ciphers, security analysis, reduction proof, key recovery attacks. 1