On Hiding a Plaintext Length by Preencryption

It is a well known fact that encryption schemes cannot hide a plaintext length when it is unbounded. We thus admit that an approximation of it may leak and we focus on hiding its precise value. Some standards such as TLS or SSH offer to do it by applying some pad-then-encrypt techniques. In this study, we investigate the information leakage when these techniques are used. We define the notion of padding scheme and its associated security. We show that when a padding length is uniformly distributed, the scheme is nearly optimal. We also show that the insecurity degrades linearly with the padding length.


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For an encryption scheme, instead of Shannon's perfect security definition, Goldwasser and Micali defined a realistic provable security called semantic security. Using indistinguishability notion, one can define security levels according to the polynomial time adversaries' capabilities such as chosen plaintext attacks (CPA) and chosen ciphertext attacks (CCA) for both symmetric and asymmetric encryption schemes in addition to the hard mathematical problems the algorithms based on. Precautions to prevent the...
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Citation Formats
C. Tezcan, “On Hiding a Plaintext Length by Preencryption,” 2011, vol. 6715, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/29853.