# Secure Remote Storage of Logs with Search Capabilities
## 4 Proposed Solution
### Indexing
The use of deterministic techniques for trapdoor generation directly leaks the search pattern since the remote storage server may use statistical analysis and infer information about the query keywords. This requirement is known as predicate privacy [28]. Access patterns can be described as the set of search results (i.e., the collection of documents) that were obtained for a given keyword. This type of information might aid an attacker to learn information about the keywords since the remote storage server will always return the same set of encrypted documents for the same encrypted keywords. In practice, the leakage of search and access patterns can be reduced but not totally eliminated [13]. ### Threat Model

In scientific literature, there is no standard security model regarding solutions based on searchable encryption [93]. The adopted threat model assumes an honest-but-curious server that faithfully follows the protocol, but it is eager to learn confidential information by analyzing the received encrypted log records, search queries and matching results in order to obtain information about the content of those log records. Moreover, we assume the existence of external threats. Respectively, we consider that an attacking agent may read, alter or delete data, not only at rest but also in transit. Based on that, we focus on the protection against violation of privacy and integrity, data leakage, replay attacks, unauthorized access and spoofing. Additionally, the proposed solution must offer protection against two of the most common referred attacks in the searchable encryption field, these being the Chosen-Keyword Attack (CKA) and the Known Keyword Attack (KGA). A CKA attack occurs when an attacker gathers knowledge about the stored information by obtaining the decryption of chosen keyword ciphertexts. In other words, the attacker chooses an encrypted keyword and is handed the corresponding keyword in clear text. To assure the confidentiality of the keywords, the searchable encryption scheme must be Semantically Secure under CKA (SS-CKA) [57]. In an SS-CKA scheme, an attacker cannot learn information about the keywords that are present on the stored ciphertexts, if that keyword is not known to the server. A KGA attack can be performed on a searchable encryption scheme if the ciphertexts of all keywords were produced by the attacker. By knowing one trapdoor, an attacker can search the remaining ciphertexts for corresponding results. If the ciphertext that contains the keyword is found once, an attacker can guess the keyword that is correlated to the trapdoor. This type of attack is more frequent when the keyword space is small since the attacker can rapidly generate ciphertexts for of all keywords [94]. It also important to assert that this security analysis was inspired by the security definitions brought by [PERSON] [17]. He pointed out that the security of both the indexes and the trapdoors are inherently linked, introducing two security notions, Non-adaptive security against CKA (IND-CKA1) and Adaptive security against CKA (IND-CKA2). This two definitions state that nothing should be leaked from the remotely stored documents and searchable index except the outcome of previously searched queries, providing security for trapdoors and assuring that the trapdoors do not leak information about the keywords, except for what can be inferred from the search and access patterns [2]. ### Analysis

Prior to the analysis itself, it is important to denote that we assume that all cryptographic keys are saved on controlled locations, always remain is the possession of their owners and are not shared with any unauthorized entity. The indexing operation of the proposed solution must assure privacy and integrity of the data at rest, but also index confidentiality and data leakage prevention due to index information must be accomplished. The data privacy at rest is given by the symmetric encryption of each log record prior to its transmission to the SLaS Application.