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  1. A private information retrieval (PIR) protocol is a protocol that allows a user to retrieve an item from a server in possession of a database without revealing which item is retrieved.

  2. An oblivious transfer (OT) protocol is a type of protocol in which a sender transfers one of the potentially many pieces of information to a receiver, but remains oblivious as to what piece (if any) has been transferred.

As the Wikipedia said, PIR is a weaker version of 1-out-of-n oblivious transfer, but I do not really know why is a weaker version.

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In general, PIR mainly guarantees that the server should not know which item is retrieved, as does OT. However, in contrast to PIR, OT also guarantees that the user should not learn information about other items.

----update based on Lindell's comment----

Another difference between PIR and OT is the requirement of sublinear communication cost for PIR. Note that PIR with linear communication is trivial and hence not interesting as the server can simply send the whole database to the user. However, this does not work for OT because sending the whole database reveals all other database items to the user.

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  • $\begingroup$ In PIR, maybe the databse is public? So the user can see all the data, but only privately retriveal the data corresponding to the index he want? $\endgroup$
    – w.qi
    Dec 5 '18 at 3:52
  • $\begingroup$ @w.qi The database is private to the server but some data items can be uploaded by the user (and then the user may delete his copy). $\endgroup$
    – Shan Chen
    Dec 5 '18 at 7:01
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    $\begingroup$ PIR also has a requirement of sub-linear communication, whereas OT does not require this. $\endgroup$
    – Yehuda Lindell
    Dec 5 '18 at 13:13
  • $\begingroup$ @ShanChen No problem; maybe update your answer to reflect this. $\endgroup$
    – Yehuda Lindell
    Dec 5 '18 at 17:11
  • $\begingroup$ Hi, what is the difference between symmetric PIR and OT? $\endgroup$
    – mallea
    Sep 9 '19 at 5:57

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