1. AVISPA Automated Validation of Internet Security Protocols and Applications
Slides adapted from Duminda Wijesekera as well as from Alessandro Armando

2. What is AVISPA? Push-button security protocol analyzer.
Supports the specification of security protocols and properties by means of a modular and expressive specification language.
Integrates different back-ends implementing a variety of state-of-the-art automatic analysis techniques for
protocol falsification (by finding an attack on the input protocol)
abstraction-based verification methods
both for finite and infinite numbers of sessions.
User interaction facilitated by an emacs mode and a Web interface.

3. Architecture

4. AVISPA Back-end Analyses
Protocol falsification, and bounded and un-bounded verification.
On-the-fly Model-Checker (OFMC)
employs several symbolic techniques to explore the state space in a demand-driven way
CL-AtSe (Constraint-Logic-based Attack Searcher)
applies constraint solving with simplification heuristics and redundancy elimination techniques
SAT-based Model-Checker (SATMC)
builds a propositional formula encoding all the possible attacks (of bounded length) on the protocol and feeds the result to a state-of-the-art SAT solver.
TA4SP (Tree Automata based on Automatic Approximations for the Analysis of Security Protocols)
approximates the intruder knowledge by using regular tree languages and rewriting to produce under and over approximations.

5. High Level Protocol Specification Language. (HLPSL)
Supports symmetric and asymmetric keys, non-atomic keys, key-tables, Diffie-Hellman key-agreement, hash functions, algebraic functions, typed and untyped data, etc.
Security properties: different forms of authentication and secrecy.
The intruder is modeled by the channel(s) over which the communication takes places:
Dolev-Yao intruder and (preliminarily) other intruder models.
Role-based language:
a role for each (honest) agent,
parallel and sequential composition glue roles together.

6. High Level Protocol Specification Language (HLPSL)
Syntax used to specify protocols in AVISPA:
Strongly typed
Supports
modularity: composition, hiding
control flow
explicit intruder knowledge
cryptographic primitives:
nonces,
hashes,
signatures
algebraic properties:
Xor
exp

7. Entities in HLPSL Basic types and terms State-based formalism Roles
Simple (agents such as Alice, Bob etc)
Composite (communities of agents playing their roles as Alice Bob and the Dolev-Yao Intruder, Key Server synchronizing with each other)
The environment
(where all evil lives = Intruder = environment)
Security Goals
String authentication, authorization, anonymity, secrecy, etc.

8. States and Variables Kinds of variables:
State variables: Those that are within the scope of a role.
Declared at the top of a role
Unprimed versions indicate current state
Primed versions indicate next state

9. An example role Alice (A, B: agent, Ka, Kb: public_key,
SND, RCV: channel (dy))
played_by A def=
local State:nat, Na:text (fresh), Nb:text
init State = 0
transition
1. State =0 /\ RCV(start)
=|> State'=2 /\ SND({Na'.A}_Kb)
/\ witness(A,B,na,Na')
2. State =2 /\ RCV({Na.Nb'}_Ka)
=|> State'=4 /\ SND({Nb'}_Kb)
/\ request(A,B,nb,Nb')
/\ secret(Na,B)
end role

10. Basic types in HLPSL Agent: names of principles
public_key: asymmetric keys
symmetric key: symmetric keys
nat: natural numbers
function: to model hash functions etc
bool: Boolean values for modeling flags

11. Aggregate types Lists Example: KeyMap: (agent, public_key) list
init KeyMap = []
in((B,Kb’), KeyMap)

12. State and transition predicates
State predicate: First order formulas written using unprimed state variables.
Example:
State = Init
State= done
Transition predicates First order formulas written using primed and unprimed state variables.
(State‘=2) /\ SND({Na'.A}_Kb)
/\ witness(A,B,na,Na‘)

13. Roles Description of entity behavior Two kinds:
Basic Roles:
Schematic descriptions of atomic behavior
Composed Roles:
Instantiations of other roles composed using operators
Roles are translated to TLA for operational semantics

14. Role Definition Role declaration:
its name and the list of formal arguments, along with (in the case of basic roles)
a player declaration;
Declaration of local variables and ownership rules, if any;
Initialization of variables, if required;
Declaration of accepting states, if any;
Knowledge declarations, if applicable;
Either (optionally) :
a transition section (for basic roles) or
a composition section (for composed roles).

15. Basic Roles role Basic_Role (…) played_by … def= owns {θ: Θ} local {ε}
General Pattern
Initiator Role in NSPK
role Basic_Role (…)
played_by … def=
owns {θ: Θ}
local {ε}
init Init
accepts Accept
transition
event1  action1
event2  action2
end role
role Alice (A, B: agent,
Ka, Kb: public_key,
SND, RCV: channel (dy))
played_by A def=
local State:nat, Na:text (fresh), Nb:text
init State = 0
transition
1. State =0 /\ RCV(start)
=|> State'=2 /\ SND({Na'.A}_Kb)
/\ witness(A,B,na,Na')
2. State =2 /\ RCV({Na.Nb'}_Ka)
=|> State'=4 /\ SND({Nb'}_Kb)
/\ request(A,B,nb,Nb')
/\ secret(Na,B)
end role

16. Composed Roles: Parallel Composition
Pattern
role Par_Role (…)
def= owns {θ:Θ}
local {ε}
init Init
accepts Accept
composition
A  B
end role
Example
role Kerberos (..)
composition
Client /\ Authn_Server /\ TGS /\ Server
end role

17. Composed Roles: Sequential Composition
General Pattern
role Seq_Role (…)
def= owns {θ:Θ}
local {ε}
init Init
accepts Accept
composition
A ; B
end role
Example
role Alice (..)
establish_TLS_Tunnel(server_ authn_only);
present_credentials;
main_protocol(request, response)
end role

18. State predicates, events and actions
A state predicate: Predicates that do not have primed variables.
Stuttering step: A transition predicate that does not change any value
Example: X’=X /\ Y=Y’
Action: transition predicates p(v,v’) satisfying v  v’ p(v,v’)
Events: transition predicates containing at least one X ≠X’

19. Communication in HLPSL
Synchronous, via immediate transitions
Runtime ensures that SND and RCV are executed simultaneously, over channels
How communication is modeled:
SND(msg) in RHS of rule shorthand for SND’=msg
RCV(masg) in LHS is shorthand for
(RCV-flag’≠RCV-flag)/\(RCV’=Msg) where
RCV-flag is a binary flag toggled whenever the channel has a new message.

20. Role composition No transition section
Have a composition section that instantiate other roles
Operators:
Parallel /\
Sequential ;
Top level role is named Environment

21. The NSPK example: Alice
role Alice (A,B:agent,Ka,Kb:public_key,SND,RCV: channel(dy))
played_by A def=
exists State : nat, Na : text (fresh), Nb: text
init State=0
knowledge(A) = { inv(Ka) }
transition
step1. State=0 /\ RCV(start)=|>
State’=1/\ SND({Na’.A}Kb)
step2. State=1 /\ RCV({Na.Nb’}Ka) =|>
State’=2 /\ SND({Nb’}Kb)
end role

22. The NSPK: Composition role NSPK(S, R: agent -> channel (dy),
Instances: (agent,agent, public_key,public_key) set)
def=
exists A, B: agent, Ka, Kb: public_key
composition
/\_{in((A,B,Ka,Kb),Instances)}
Alice(A,B,Ka,Kb,S(A),R(A))
/\ Bob(A,B,Ka,Kb,S(B),R(B))
end role

23. References http://www.avispa-project.org