Editing (Symmetric) Private Information Retrieval (section)

==Properties==
<!-- All properties that should be satisfied by any protocol achieving the concerned functionality and other common terminologies used in all the protocols.-->


===Security definitions===
(Quantum) private information retrieval protocols are said to be secure if they satisfy the following conditions:
*'''Correctness''': assuming that all the parties in the protocol are honest, then the output of the protocol on the user’s side must be the queried database element.
*'''User privacy''': assuming that the user is honest, then, throughout the protocol, any query of the user to a server leaks no information about the desired database item.
In addition to the above requirements, symmetric (quantum) private information retrieval protocols must also satisfy the following condition:
*'''Data(base) privacy''': assuming that the server(s) is (are) honest(s), then, throughout the protocol, the user is unable to obtain any information beyond a single database element.

===Cost parameters===
The most common cost parameter used to characterise a given (Q)(S)PIR protocol is:
*'''Communication complexity''': total number of (qu)bits exchanged between the user and the server(s) throughout the protocol.
For (Q)(S)PIR protocols in general:
*'''(Q)(S)PIR capacity''': maximal achievable ratio of the retrieved database element size to the total download size.
Some less common cost parameters include:
*'''Storage overhead''' (for multi-database (Q)(S)PIR protocols): ratio between the total number of (qu)bits stored on all servers and the number of (qu)bits in the (resp. quantum) classical database. 
*'''Access complexity''': total amount of data to be accessed by the server(s) for answering queries throughout a (Q)(S)PIR protocol.