Searchable symmetric encryption (SSE) is a form of encryption that allows one to efficiently search over a collection of encrypted documents or files without the ability to decrypt them. SSE can be used to outsource files to an untrusted cloud storage server without ever revealing the files in the clear but while preserving the server’s ability to search over them. Keyword search using an SSE scheme can be illustrated in Figure 1. ID is s4001812. Email address is [email protected]

Figure 1. Create Search Token, Search, Decrypt Retrieved Documents

Suppose you wish to store the following five documents with given keywords on a cloud server using SSE:

Document 1: {School, University, Lecture}

Document 2: {Tutorial, Assignment}

Document 3: {University, School, Tutorial}

Document 4: {Team, Scores}

Document 5: {School, Codes}

Assume that the secret key for token generation: k1 =  ID and the secret key for encryption key generation:  k2 = Email address, and the Pseudorandom Function (PRF) is HMAC-SHA1 and the encryption algorithm is Advanced Encryption Standard (AES) with 128 bits key size.

(1) Based on the information provided, create a complete encrypted index table for SSE to be uploaded to the cloud server.

(2) Assuming you wish to find documents containing the keyword “University”, what do you need to send to the cloud server? (you need to give some explanations and show the computation steps)

(3) How does the cloud server process your query? What does it return to you? (you need to give some explanations and show the computation steps)

(4) Discuss what the cloud server can learn from the encrypted index table initially. What can it learn after it receives your query?

(5) Discuss why different keys are needed to encrypt document identifiers in different rows.

Please use the following links for online HMAC and AES calculators

https://www.devglan.com/online-tools/aes-encryption-decryption

https://www.freeformatter.com/hmac-generator.html