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OK we'll run with this
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vEnhance committed Aug 15, 2024
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<ot>

Alice has $n$ messages $x_1, dots, x_n$.
For simplicity we'll assume the messages are pairwise distinct
(if not, one can pad them with random bits).
We'll assume the messages are essentially unrelated to each other
(since we could always pad them with random bits).
Bob wants to request the $i$-th message,
without letting Alice learn anything about the value of $i$.
Alice wants to send Bob $x_i$,
without letting him learn anything about the other $n-1$ messages. An _oblivious transfer (OT)_ allows Alice to transfer a single message to Bob, but she remains oblivious as to which message she has transferred. We'll see two simple protocols to achieve this.
without letting him learn anything about the other $n-1$ messages.
An _oblivious transfer (OT)_ allows Alice to transfer a single message to Bob,
but she remains oblivious as to which message she has transferred.
We'll see two simple protocols to achieve this.

(In fact, for two-party computation,
we only need "1-of-2 OT":
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Our first oblivious transfer protocol is built on the commutative encryption we just described.

Alice has $n$ messages $x_1, dots, x_n$, which we may as well assume are elements of the group $G$. Alice chooses a secret key $a$, encrypts each message, and sends all $n$ ciphertexts to Bob:
Alice has $n$ messages $x_1, dots, x_n$, which we may as well assume are elements of the group $G$.
Alice chooses a secret key $a$, encrypts each message, and sends all $n$ ciphertexts to Bob:
$
Enc_a (x_1), dots, Enc_a (x_n).
$
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