Confirming understanding of security protocol modelled in Scyther

Question

In university, I'm currently learning how to use Scyther to model security protocols. Currently I am trying to understand what is happening in an example protocol given to me which is:

# Key Establishment Protocol
#

usertype SessionKey;

protocol ke11(I,R,S)
{
    role I
    {
        var Kir: SessionKey;

        send_1(I,S, R); 
        recv_2(S,I,{Kir}k(I,S));

        claim_I1(I,Running,R,Kir);

        send_4(I,R,{I}Kir);
        recv_5(R,I,{R}Kir);

        claim_I3(I,Commit,R,Kir);
        claim_I1(I,Secret,Kir);
    }    
    
    role R
    {
        var Kir: SessionKey;

        recv_3(S,R, (I,{Kir}k(R,S)));
        recv_4(I,R,{I}Kir);

        claim_R3(R,Running,I,Kir);

        send_5(R,I,{R}Kir);

        claim_R2(R,Commit,I,Kir);
        
        claim_R1(R,Secret,Kir);
    }

    role S
    {
        fresh Kir : SessionKey;

        recv_1(I,S, R);
        send_2(S,I, {Kir}k(I,S));
        send_3(S,R, I,{Kir}k(R,S));
    }
}

This has also been given to me:

1. I → S : R
2. S → I : {Kir}k(I,S)
3. S → R : I,{Kir}k(R,S)
4. R → I : {I}Kir
5. I → R : {R}Kir

From my understanding:

I is sending S the value of R
S is sending I the encryption of Kir, which is encrypted using a shared symmetric key between I and S
S is sending R the encryption of Kir, which is encrypted using a shared symmetric key between R and S
I is sending R the value of I encrypted using Kir which it got from decrypting the value in 2 using the shared symmetric key
R is sending I the value of R encrypted using Kir which it got from decrypting the value in 3 using the shared symmetric key.

I'm not really sure if this is correct so could someone please comment on if my understanding is correct or not. Specifically, I don't really understand what is being sent when I have written that the value of I or R is being sent (I -> S:R).

Any comments would greatly be appreciated. Many thanks.

Confirming understanding of security protocol modelled in Scyther

0 Answers0