End-to-end encrypted datagram service

Features:

low network overhead for the security offered ~10 bytes for dns ~10 bytes for every group member ~250bytes for an encrypted public key for broadcasting (if necessary)
supports messages, files and streams
to multiple recipients
anonymous data content (end-to-end encrypted)
anonymous senders
only the end users know the group compositions

Problems:

sender bandwidth increases with group size (fixed by adding 1 or 2 additional broadcast proxies depending on the load situation)
keeping participants anonymous whilst verifying their authenticity (solved: use a passphrase for adding people)
sending data to anonymous receivers (no need for anonymous receivers due unknown message contents and anonymous transmitters)
The proxies (especially L2) should not know CS_private
Latency due to a triple proxy (can be shortened to a double proxy during high traffic, getting rid of L3, masking patterns)
If you have control over all L23 and CS you can trace the path of the packet.

Setup:

All devices keep a connection with the central server for receiving messages. All non-mobile (not network limited) devices not behind a NAT (ie. company servers) act as proxies for sending traffic. Get data from the central server: its public key and a list of proxies with their public keys.

Creating a group:

generate a key pair
add yourself to the recipient list

Adding a person:

The adder

prompt for a target id
prompt for a passphrase
encode the group key pair and recipient list with the passphrase
send the message to the target id

The addee

if their private keys don't work
prompt for a passphrase
if it works, add the recipient list and private key to its cache
for every recipient except itself generate a temporary key -- hide similar looking messages for the central server encode the recipient list with the temporary key prepend the temporary key encrypt the entire string with the groups public key and send

Sending a message

encrypt with group_pub (or passphrase) and append the encrypted (with CS_pub) recipients
pick a proxy at random (L2)
S sends L2 the group_pub encrypted message with (1) encrypt(L2_pub,group_pub) (2) a list of targets encrypted with CS_pub

L2 decodes the group public key and generates a temp key.
L2 scrambles the message using the temp key (encode and prepend the key)
L2 encrypts the scrambled message with group_pub
L2 sends individual messages for the different targets with the (now different) 'encrypted_scrambled_encrypted_message's to L3
L3 proxy forwards the message to CS
CS decrypts the target of the message and sends it there 10.T applies group_private, retrieves and decrypts the scrambling and finally decrypts the message with group_private to get cleartext.

Layers:

S : encrypting and sending the message with encrypt(L2_pub, group_pub) for scrambling, a choice for L2 and a list of encrypt(CS_pub, target) knows everything about the message

L2 : splices the message into separate, scrambled parts (is allowed to know group_pub because it doesn't know the source or destinations) knows the L1 it came from, knows the L3 it moves toward, knows group_pub, knows the encrypted_scrambled_encrypted_message, knows all encrypted targets does not know the target

L3 : hiding that messages are part of the same group and forwarding them to CS (can be omitted if the outgoing link of L2 is congested with random interleaving) knows the L2 the message came from, knows the encrypted target, knows the encrypted_scrambled_encrypted_message does not know the source or target

CS : sending the messages to their final destinations knows the choice of L3, knows the target address, knows the encrypted_scrambled_encrypted_message does not know the source

T : receives the message (target) knows the content of the message doesn't know the source or the other recipients

Gladdy/CryptNet