Homomorphic, sortable or searchable encryption
Homomorphic, sortable and searchable encryption are three alternative concepts to the method used by eperi, each of which has a number of weaknesses.
Homomorphic encryption is a class of encryption methods in which arithmetic operations with encrypted values are possible, such as the addition of two encrypted values. Homomorphic encryption is a field of cryptography research. So far the methods are too CPU-intensive to be really practicable and the functionality is still severely limited. In addition, the methodology is so innovative that no statement can yet be made about the safety level of such methods.
Sortable encryption significantly reduces the security level. The problem is the following: when data is encrypted, the order of the encrypted data is different from the order of the original data. This is an important and necessary feature of encryption. If, on the other hand, the encrypted data is in the same order to be sortable, this considerably reduces the security of the process. For example, if Alice, Bob and Chris are encrypted to X, Y and Z, any attacker who knows the plain text can deduce the corresponding encrypted values. By cleverly encrypting plain text, an attacker can determine the position – and thus the value – of the encrypted data set at any time.
Searchable encryption uses non-verifiable and therefore untrustworthy encryption methods. In order to find an encrypted original value, the encrypted value must be decrypted. This is a basic principle of encryption technology. The string “Bo” can only be found in the encrypted “Bob” if the encrypted text is decrypted first. The methods of searchable encryption encrypt – roughly speaking – each part of the word separately, so that it is possible to search for word fragments. Bob” becomes “Xyz”. If you search for “Bo” (encrypted “Xy”), then the search is also successful with alleged encryption.
However, this is not strong encryption, but only a kind of Caesar cipher. This was already used in antiquity (ca. 50 B.C.) and is completely unsafe.