Semantic-security of individual plaintext bits given the corresponding ciphertext is a fundamental notion in modern cryptography. We initiate the study of this basic problem for Order-Preserving Encryption (OPE), asking “what plaintext information can be semantically hidden by OPE encryptions?” OPE has gained much attention in recent years due to its usefulness for secure databases, and has received a thorough formal treamtment with innovative and useful security notions. However, all previous notions are one-way based, and tell us nothing about partial-plaintext indistinguishability (semantic security).
In this paper, we propose the first indistinguishability-based security notion for OPE, which can ensure
secrecy of lower bits of a plaintext
(under essentially a random ciphertext probing setting). We then justify the definition, from the theoretical plausibility and practicality aspects. Finally, we propose a new scheme satisfying this security notion (the first one to do so). In order to be clear, we note that the earlier security notions, while innovative and surprising, nevertheless tell us nothing about the above partial- plaintext indistinguishability because they are limited to being one-way-based.