The transit secrets engine handles cryptographic functions on data in-transit. Vault doesn't store the data sent to the secrets engine. It can also be viewed as "cryptography as a service" or "encryption as a service". The transit secrets engine can also sign and verify data; generate hashes and HMACs of data; and act as a source of random bytes.
https://www.vaultproject.io/docs/secrets/transit/index.html
https://www.vaultproject.io/docs/secrets/transit/index.html