RDF: fix the issue of using encryption/decryption hashed key as the real key for AES

[tian3rd]

Just found out that in the encryption/decryption process, the methods use the hashed key as the real key, and this key is actually stored in the encrypted .enc.ttl file in plaintext. This is not right. The right way to do it is to use the original key, but needs to find a way to convert different length to a fixed 32-long in order to use AES.

• Convert any key to a unique 32 length key
• Use this secret key to encrypt and decrypt

[tian3rd]

• Rather than converting to 32 length key, use sha512 first 32 chars instead for guaranteed security

Hashed key is generated with sha256 for quick verification and is stored in the encrypted file, but the real key is hidden and not stored anywhere, so only when the user's passphrase matches the sh256 key in the encrypted file, then we proceed to decrypt the file:

1. first generate the real key for AES (256 bits / 32 bytes) with the first 32 chars of sha512
2. then use this real key to decrypt the content