Date of Submission


Date of Award


Institute Name (Publisher)

Indian Statistical Institute

Document Type

Doctoral Thesis

Degree Name

Doctor of Philosophy

Subject Name



Applied Statistics Unit (ASU-Kolkata)


Nandi, Mridul (ASU-Kolkata; ISI)

Abstract (Summary of the Work)

In this thesis, we provide quantitative and/or qualitative improvements in the provable security of several symmetric-key schemes, encompassing major information security goals, viz. data authentication, encryption, and authenticated encryption.AUTHENTICATION AND INTEGRITY: Among authentication schemes, we analyze the CBC-MAC family and counter-based MACs (XMACC, XMACR, PCS, LightMAC etc.), referred as the XMAC family. First, we revisit the security proofs for CBC-MAC and EMAC, and identify a critical flaw in the state-of-the-art results. We revise the security proofs and obtain significantly better bounds in case of EMAC, ECBC and FCBC. Second, we study the security of CBC-MAC family, when the underlying primitive is pseudorandom function (PRF), and derive tight security bounds for EMAC, ECBC, FCBC, XCBC and TMAC. Third, we study the counter-based input encoding used in XMAC family. We present a generalized view on counter-based encoding and propose some efficient alternatives to the classical fixed length counter. Further, based on the generalized view, we identify some necessary and sufficient conditions, which result in simplified security arguments. As a side-result we also prove second preimage security for HAIFA-based hash function (using Davies-Meyer compression function) in the ideal cipher model.ENCRYPTION: Among encryption schemes, we study the problem of constructing beyond-the-birthday bound (BBB) secure online encryption schemes using tweakable block ciphers (TBCs). First, we construct a birthday bound distinguisher for POEx, which invalidates the BBB security claims of POEx. Second, we propose a BBB secure online cipher, called XTC, and prove that it is optimally secure. As a by-product we suggest a generic distinguisher for a class of TBC-based online ciphers that encompasses both POEx and XTC. On a related topic, we study the problem of constructing BBB secure TBCs from block ciphers. Specifically, we derive a tight security bound for cascaded LRW2 under the assumption that the underlying hash functions are 3-wise almost XOR universal.AUTHENTICATED ENCRYPTION: Finally, among authenticated encryption schemes, we study a generalization of the OCB family, called GOCB, with an aim to achieve efficient random read access. We introduce a relaxed notion of universal hash functions, called locally imperfect XOR universal (LIXU), and prove that GOCB is secure under this relaxed notion of universality. Further, we instantiate GOCB with AES round function based LIXU hash functions. These instantiations achieve significantly better random read access than OCB3.


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Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.


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