Assuming we have fixed pairing friendly elliptic curve groups $G_1$, $G_2$ and $G_T$ where for $a \in G_1$ and $b \in G_T$ it holds $e(a,b) \in G_T$.
Let's put some more context and we are working in bls12-381. what is a standard way to compute $e(a,b)*r$, where r is a random bit string. Putting in another way how do we map bitstrings to the target group $G_T$ ?
Using bls12-381 the group $G_T$ us a multiplicative subgroup of $GF(p^{12})$ of order $\ell$ for a certain 381-bit prime $p$ and 256-bit prime $\ell$. We can convert a random bitstring to an element of $GF(p^{12})$ by hashing a to a uniform integer in $[0,p^{12}-1)$, expressing this integer in base $p$ and extracting the digits to use as coefficients in whatever basis we choose to represent $GF(p^{12})$. We can then map this element of $GF(p^{12})$ to our subgroup $G_T$ by raising to the power $(p^{12}-1)/\ell$.
