BACKUP PROCESS

 SENDER --> [ UNTRUSTED STORAGE ] --> Receiver 
 * Backup         (STACK)              * Verify
 * Encrypt                       	   * Decrypt

Pre-requisites:

1. Sender has a private key unique to that machine (SEN_PRIV)
2. Sender has the public key from the receiver (RECV_PUB).
3. Sender has openssl installed and can generate its own ECDH-keys. (TEMP_PRIV / TEMP_PUB)
4. Receiver has public key from Sender (SEN_PUB)
5. Receiver has the private key of the public key referenced in 2. (RECV_PRIV)

BACKUP PROCESS (encrypt.sh) :

1. Generate the files to be backedup (e.g. a sqldump. Outside scope of this document)
2. Generate a new temporary private key TEMP_PRIV 2a) Derive a "SharedSecret" using TEMP_PRIV and RECV_PUB - using Elliptic Curve Diffi Helman (ECDH) 2b) Create the SHA 256-hash of SharedSecret.. To be used as AES_ENCRYPTION_KEY
3. Encrypt the file using AES_ENCRYPTION_KEY
4. Sign the encrypted contents with SEN_PRIV. This provides Integrity, Authentication and Non-Repudiation of the encrypted contents.
5. Save TEMP_PUB as filename.encrypted.pubkey. Remove TEMP_PRIV.
6. Send : filename.encrypted the encrypted contents filename.encrypted.pubkey the ephemereal public key / TEMP_PUB filename.encrypted.signed the encrypted contents signed with SEN_PRIV

RESTORE PROCSS (decrypt.sh) :

0. Make sure you have at least filename.encrypted, filename.encrypted.pubkey (TEMP_PUB) and SEN_PRIV
1. Verify the encrypted file comes from Sender by using your copy of SEN_PUB
2. Regenerate AES_ENCRYPTION_KEY using your RECV_PRIV and TEMP_PUB (aka filename.encrypted.pubkey)
3. Decrypt the contents using AES_ENCRYPTION_KEY
4. Make sure the contents match what you expect

     It may be theoretically possible an attacker changes filename.encrypted.pubkey
     The ECDH will then create a different AES_ENCRYPTION_KEY, and decryption using
     that key may result in a possible malicious plaintext !