***(
The informal journal-level description of this protocol is as follows:

A --> B: A ; B ; exp(g,N_A)
B --> A: A ; B ; exp(g,N_A)
A --> B: enc(exp(exp(g,N_B),N_A),secret(A,B))

where N_A and N_B are nonces, exp(x,y) means x raised to y, enc(x,y) means
message y encripted using key x, and secret(A,B) is a secret shared between
A and B. Moreover, exponentiation and encription/decription have the following 
algebraic properties:

  exp(exp(X,Y),Z) = exp(X, Y * Z)
  e(K,d(K,M)) = M .
  d(K,e(K,M)) = M .

where * is the xor operator, though no algebraic property is given, since 
they are not necessary for this protocol. However, note that the property 
for exponentiation is restricted below by using appopriate sorts
in such a way that variable X can be only the generator g. This is necessary
to have a finitary unification procedure based on narrowing.
)***

fmod PROTOCOL-EXAMPLE-SYMBOLS is
  --- Importing sorts Msg, Fresh, Public
  protecting DEFINITION-PROTOCOL-RULES .
  
  ----------------------------------------------------------
  --- Overwrite this module with the syntax of your protocol
  --- Notes:
  --- * Sort Msg and Fresh are special and imported
  --- * Every sort must be a subsort of Msg
  --- * No sort can be a supersort of Msg
  ----------------------------------------------------------

  --- Sort Information 
  sorts Name Nonce NeNonceSet Gen Exp Key GenvExp Secret .
  subsort Gen Exp < GenvExp .
  subsort Name NeNonceSet GenvExp Secret Key < Msg .
  subsort Exp < Key .
  subsort Name < Public . --- This is quite relevant and necessary
  subsort Gen < Public . --- This is quite relevant and necessary

  --- Secret
  op sec : Name Fresh -> Secret [frozen] .

  --- Nonce operator
  op n : Name Fresh -> Nonce [frozen] .

  --- Intruder
  ops a b i : -> Name .

  --- Encryption
  op e : Key Msg -> Msg [frozen] .
  op d : Key Msg -> Msg [frozen] .

  --- Exp
  op exp : GenvExp NeNonceSet -> Exp [frozen] .

  --- Gen
  op g : -> Gen .

  --- NeNonceSet
  subsort Nonce < NeNonceSet .
  op _*_ : NeNonceSet NeNonceSet -> NeNonceSet [frozen assoc comm] .

  --- Concatenation
  op _;_ : Msg Msg -> Msg [frozen gather (e E)] .

endfm

fmod PROTOCOL-EXAMPLE-ALGEBRAIC is
  protecting PROTOCOL-EXAMPLE-SYMBOLS .
  
  ----------------------------------------------------------
  --- Overwrite this module with the algebraic properties 
  --- of your protocol
  ----------------------------------------------------------
  
  eq exp(exp(W:Gen,Y:NeNonceSet),Z:NeNonceSet) 
   = exp(W:Gen, Y:NeNonceSet * Z:NeNonceSet) [variant] .
  eq e(K:Key,d(K:Key,M:Msg)) = M:Msg [variant] .
  eq d(K:Key,e(K:Key,M:Msg)) = M:Msg [variant] .

endfm

fmod PROTOCOL-SPECIFICATION is
  protecting PROTOCOL-EXAMPLE-SYMBOLS .
  protecting DEFINITION-PROTOCOL-RULES .
  protecting DEFINITION-CONSTRAINTS-INPUT .

  ----------------------------------------------------------
  --- Overwrite this module with the strands 
  --- of your protocol
  ----------------------------------------------------------
  
  vars NS1 NS2 NS3 NS : NeNonceSet .
  var NA NB N : Nonce .
  var GE : GenvExp .
  var G : Gen .
  vars A B : Name .
  vars r r' r1 r2 r3 : Fresh .
  var Ke : Key .
  vars XE YE : Exp .
  vars M M1 M2 : Msg .
  var Sr : Secret .

  eq STRANDS-DOLEVYAO = 
     :: nil :: [ nil | -(M1 ; M2), +(M1), nil ] &
     :: nil :: [ nil | -(M1 ; M2), +(M2), nil ] &
     :: nil :: [ nil | -(M1), -(M2), +(M1 ; M2), nil ] &
     :: nil :: [ nil | -(Ke), -(M), +(e(Ke,M)), nil ] &
     :: nil :: [ nil | -(Ke), -(M), +(d(Ke,M)), nil ] &
     :: nil :: [ nil | -(NS1), -(NS2), +(NS1 * NS2), nil ] &
     :: nil :: [ nil | -(GE), -(NS), +(exp(GE,NS)), nil ] &
     :: r ::   [ nil | +(n(i,r)), nil ] &
     :: nil :: [ nil | +(g), nil ] &
     :: nil :: [ nil | +(A), nil ]
  [nonexec] .

  eq STRANDS-PROTOCOL = 
     :: r,r' :: 
     [nil | +(A ; B ; exp(g,n(A,r))), 
            -(A ; B ; XE), 
            +(e(exp(XE,n(A,r)),sec(A,r'))), nil] &
     :: r :: 
     [nil | -(A ; B ; XE), 
            +(A ; B ; exp(g,n(B,r))), 
            -(e(exp(XE,n(B,r)),Sr)), nil]
  [nonexec] .

  eq EXTRA-GRAMMARS
   = (grl empty => (NS * n(a,r)) inL . ;
      grl empty => n(a,r) inL . ;
      grl empty => (NS * n(b,r)) inL . ;
      grl empty => n(b,r) inL .  
      ! S2 )
  [nonexec] .

  eq ATTACK-STATE(0)
   = :: r :: 
     [nil, -(a ; b ; XE), 
           +(a ; b ; exp(g,n(b,r))), 
           -(e(exp(XE,n(b,r)),sec(a,r'))) | nil]
     || empty
     || nil
     || nil
     || never
     *** Pattern for authentication
     (:: R:FreshSet ::
     [nil | +(a ; b ; XE), 
            -(a ; b ; exp(g,n(b,r))), 
            +(e(YE,sec(a,r'))), nil]
      & S:StrandSet || K:IntruderKnowledge)
  [nonexec] .

  eq ATTACK-STATE(1)
   = :: r :: 
     [nil, -(a ; b ; XE), 
           +(a ; b ; exp(g,n(b,r))), 
           -(e(exp(XE,n(b,r)),sec(a,r'))) | nil]
     || sec(a,r') inI
     || nil
     || nil
     || nil
  [nonexec] .

  eq ATTACK-STATE(2)
   = :: r :: 
     [nil, -(a ; b ; XE), 
           +(a ; b ; exp(g,n(b,r))), 
           -(e(exp(XE,n(b,r)),sec(a,r'))) | nil]
     || sec(a,r') inI
     || nil
     || nil
     || never(
     *** Avoid infinite useless path
      (:: nil :: 
      [ nil | -(exp(GE,NS1 * NS2)), -(NS3), 
          +(exp(GE,NS1 * NS2 * NS3)), nil ] 
      & S:StrandSet || K:IntruderKnowledge)
     *** Pattern to avoid unreachable states
     (:: nil :: 
      [nil | -(exp(#1:Exp, N1:Nonce)), 
             -(sec(A:Name, #2:Fresh)), 
             +(e(exp(#1:Exp, N2:Nonce), sec(A:Name, #2:Fresh))), nil] 
      & S:StrandSet || K:IntruderKnowledge)
     *** Pattern to avoid unreachable states
     (:: nil :: 
      [nil | -(exp(#1:Exp, N1:Nonce)), 
          -(e(exp(#1:Exp, N1:Nonce), S:Secret)), 
          +(S:Secret), nil]
      & S:StrandSet || K:IntruderKnowledge)
     *** Pattern to avoid unreachable states
     (S:StrandSet 
      || (#4:Gen != #0:Gen), K:IntruderKnowledge)
     )
  [nonexec] .

endfm

--- THIS HAS TO BE THE LAST LOADED MODULE !!!!
select MAUDE-NPA .