# Application of uni-directional proxy re-encryption?

What kinds of applications is uni-directional proxy re-encryption applicable to?

I learned some applications that proxy re-encryption can be used, which are email and file sharing scenarios.

All of the scenarios that I am aware do make sense when we use bidirectional proxy re encryption. When would uni-directional proxy re encryption can make sense?

In bidirectional proxy re-encryption, there's an efficient algorithm to transform a re-encryption key from Alice to Bob in a re-encryption key from Bob to Alice. For example, the BBS98 proxy re-encryption scheme has re-encryption keys of the form $rk_{A \to B} = \frac{b}{a}$, where $a$ and $b$ are the private keys of Alice and Bob, respectively; it is easy to see that this scheme is bidirectional, since the algorithm to transform $rk_{A \to B}$ into $rk_{B \to A}$ is simply the modular inversion: $$rk_{B \to A} = (rk_{A \to B})^{-1} = (\frac{b}{a})^{-1} = \frac{a}{b}$$