Explicit support and optimizations for cleartext input

[matthiasgeihs]

Cleartext input

• No optimizations for unpadded strings: The original bounty description stated that all strings should be 0-padded. Later, this requirement was relaxed (see note in bounty description) to allow for unpadded strings that are indentifiable as such without decryption. Due to time constraints, unpadded strings, or any optimizations in that regard, are currently not implemented. However, we do list potential optimizations further below.

• No optimizations for partial cleartext input: We did not implement a dedicated cleartext API or any optimizations for it. We support these operations by first encrypting the cleartext inputs and then calling the corresponding ciphertext API.