TLS.spthy

theory TLS 
begin

section{* TLS Handshake *}

text{*
  Modeled after Paulson`s TLS model in Isabelle/src/HOL/Auth/TLS.thy. Notable
  differences are:

    1. We use explicit global constants to differentiate between different
       encryptions instead of implicit typing.

    2. We model session keys directly as hashes of the relevant information.
       Due to our support for composed keys, we do not need any custom
       axiomatization as Paulson does.

    3. We do not model the ClientResume, ServerResume, and Oops steps. They
       are currently outside the scope of our modelling language.

*}

protocol TLS {
  1. C -> S: C, nc, sid, pc
  2. C <- S: ns, sid, ps

  // getting the server certificate from the certificate-authority
  ca1. C -> CA: S
  ca2.   <- CA: CA, sign{'cert', S, pk(S)}pk(CA)
       C <-   : CA, sign{'cert', S, pkS  }pk(CA)

  3. C ->  : { '31', pms                     }pkS ,
             sign{ '32', h('32', ns, S, pms) }pk(C) ,
             { '33', sid, h('PRF', pms, nc, ns),
               nc, pc, C, ns, ps, S
             } 
             h('clientKey', nc, ns, h('PRF', pms, nc, ns))

       -> S: { '31', pms                     }pk(S) ,
             sign{ '32', h('32', ns, S, pms) }pk(C) ,
             { '33', sid, h('PRF', pms, nc, ns),
               nc, pc, C, ns, ps, S
             } 
             h('clientKey', nc, ns, h('PRF', pms, nc, ns))

  4. C <- S: { '4', sid, h('PRF', pms, nc, ns),
               nc, pc, C, ns, ps, S
             } 
             h('serverKey', nc, ns, h('PRF', pms, nc, ns))
}

text{* 
We assume that all clients talk to uncompromised certificate authorities. 
*}

axiom (of TLS) uncompromised_CA:
  premises
    "role(1) = C"
  imply
    "uncompromised(CA#1)"


subsection{* Secrecy Properties *}

properties (of TLS)
  C_pms_sec:  secret(C, -, pms,                   {S}) 
  C_PRF_sec:  secret(C, -, h('PRF', pms, nc, ns), {S})
  C_clientKey_sec:  
    secret(C, -, h('clientKey', nc, ns, h('PRF', pms, nc, ns)), {S})
  C_serverKey_sec:  
    secret(C, -, h('serverKey', nc, ns, h('PRF', pms, nc, ns)), {S})

  S_pms_sec:  secret(S, 4, pms,                   {C,S})
  S_PRF_sec:  secret(S, 4, h('PRF', pms, nc, ns), {C,S})

  S_clientKey_sec:  
    secret(S, 4, h('clientKey', nc, ns, h('PRF', pms, nc, ns)), {C,S})
  S_serverKey_sec:  
    secret(S, 4, h('serverKey', nc, ns, h('PRF', pms, nc, ns)), {C,S})


subsection{* Authentication Properties *}

text{*
  First, we prove two first send properties in order to simplify proof search
  for the authentication properties.
*}

property (of TLS) nc_first_send:
  premises
    "role(1) = C"
    "knows(nc#1)"
  imply
    "St(1, C_1) < Ln(nc#1)"

property (of TLS) ns_first_send:
  premises
    "role(1) = S"
    "knows(ns#1)"
  imply
    "St(1, S_2) < Ln(ns#1)"

property (of TLS) C_ni_synch:
  premises
    "role(1) = C"
    "step(1, C_4)"
    "uncompromised(C#1,S#1)"
  imply a thread 2 such that
    "  role(2) = S
    &  C#1  = C#2
    &  S#1  = S#2
    &  nc#1 = nc#2
    &  ns#1 = ns#2
    &  pc#1 = pc#2
    &  ps#1 = ps#2
    &  sid#1 = sid#2
    &  pms#1 = pms#2
    &    St(1, C_1) < St(2, S_1)
       < St(2, S_2) < St(1, C_2)
       < St(1, C_3) < St(2, S_3)
       < St(2, S_4) < St(1, C_4)
    "

property (of TLS) S_ni_synch:
  premises
    "role(2) = S"
    "step(2, S_4)"
    "uncompromised(C#2,S#2)"
  imply a thread 1 such that
    "  role(1) = C
    &  C#1  = C#2
    &  S#1  = S#2
    &  nc#1 = nc#2
    &  ns#1 = ns#2
    &  pc#1 = pc#2
    &  ps#1 = ps#2
    &  sid#1 = sid#2
    &  pms#1 = pms#2
    &    St(1, C_1) < St(2, S_1)
       < St(2, S_2) < St(1, C_2)
       < St(1, C_3) < St(2, S_3)
       < St(2, S_4) 
    "

end