Fast Search on Encrypted Feild

I have a encrypted column use encrypt function.

Querying against this column is almost not acceptable - returning 12
rows took 25,908 ms.

The query was simply Select decrypt(phn_phone_enc) FROM phn WHERE
decrypt(phn_phone_enc,'xxx','xxx') LIKE '123%'

So I built index like: CREATE INDEX idx_phn_phone_dec ON phn
(decrypt(phn_phone_enc, 'xxx', 'xxx'))

This returns 12 rows in 68 ms.

Would this be the solution for the fast encrypted field search or does
this raise the security issue?

Kindest regards,

Naoko Reeves