web_diagram_src.txt

title Model Validation and Authentication

Phone->Phone: install app

Server->Server: set up server

IT_Dept->Phone: Generate and Register PR / SU
IT_Dept->Server: Register PU


loop while behavioral model not trained
Phone->Phone: train model
end


#Initial Registration of Phone to Server
#Server->+Key Server: prove PU valid*/
#Key Server -->-Server: validated 

note right of Server
The server's public key will need to be exchanged
on initialization (in the real world, they could
set up a certificate authority to get the server's
public key to the client).
end note

Phone-->+Server: E( session request || username || PU || PSK || HMAC(MSG, PSK), SU )
#I added the HMAC using HK as the key. As long as the PSK is unique to the phone and the server, HMAC will also provide source authentication - prevent random people registering and creating log info (which could be used in a DoS attack).

Server-->-Phone: E( E( SK0 || TL, PU), SR) || HMAC(MSG, PSK)

#Phone msg:
#    - Recent Confidence Scores (for logging)
#    - Nonce (to prevent replay)
#    - HMAC (for data integrity)

#Server msg:
#    - Nonce (same as phone)
#    - Times Left value
#        OR
#    - Nonce (new)
#    - SKn (new session key)
#    - Nonce
#        OR
#    - STOP message (Times Left = -1)
# I think we should use timestamps instead of nonce's. A timestamp would prevent replay from phone to server which could be done to pollute the logging files.

# Standard Communication

loop lifetime of device
#Phone to Server
Phone->+Server: E(IV, SU) || E( RCS || TS1, SK ) || HMAC(MSG, HK)


alt if SK is old (but still the most recent)
# We could encrypt with the Phone Public Key to make the reset key messages uniform (though not vital)
Server->-Phone: E( SK_new || TS1 || TL, PU) || HMAC(MSG, HK)

# Phone confirms receipt of the new key
Phone-->-Server: E(IV, SU) || E( Confirm_Message || TS1, SK_new) || HMAC(MSG, HK)

#Server updates the phone session key in database
end


alt if phone is reported as compromised
Server->-Phone: E(IV, PU) || E( TS1 || TL = -1, SK) || HMAC(MSG, HK)

note right of Server
After the user's phone is unlocked, 
the TL should be set to zero and 
it should resume regular behavior 
(it's session key will be set to 'expired' on the server).
end note

#After user inputs password and unlocks phone
Phone-->+Server: E(IV, SU) || E( RCS || TS1, SK ) || HMAC(MSG, HK)

Server-->-Phone: E( SK_new || TS1 || TL, PU) || HMAC(MSG, HK)

# Phone confirms receipt of the new key
Phone-->-Server: E(IV, SU) || E( Confirm_Message || TS1, SK_new) || HMAC(MSG, HK)

end


alt else
Server->-Phone: E(IV,PU) || E( TS1 || TL, SK) || HMAC(MSG, HK)
end
end



note right of Phone
 I used added numbers in designating TS 
to show that the TS was used as a timestamp 
then reflected (when number is repeated) like a nonce
end note


note left of IT_Dept
 PSK = Pre-shared Key
 SK = Current Session Key
 SK_new = New Session Key
 PU = Phone public key
 PR = Phone private key
 SU = Server public key
 SR = Server private key
 TL = Times Left (Behav model checks)
 RCS = Recent Confidence Scores
 TS = Timestamp
 HK = HMAC Key (from PSK & SK)
 MSG = given message sent 
    end note