G50 - Network Security 1. Kerb Kerbero eross Authen Authentica tication tion System System (2.5 points points))
a. (1 point)
1e investigate the (ossi#ility of sr(ing a 2er#eros 3 client4s tic/et5 Consi!er a (irate 6ho sniffs the net6or/ an! sees the tic/et that the tic/et granting server 7T8S9 sen!s to the client 7C95 The (irate also /no6s the i!entity of the client to 6hom the tic/et is !estine!5 1hat (revents the (irate from sing the tic/et to o#tain the service in (lace of the legitimate client: b. (1.5 (1.5 poin points ts))
;n the 2er#eros 3 athentication system, the symmetric /ey se! #y the client 7C9 an! the athenticatio athenticationn server 7
Secrity for (#lic-/ey !istri#tion can #e achieve! #y maintaining a !irectory of (#lic /eys5 'aintenance an! !istri#tion of the !irectory have to #e the res(onsi#ility of a central athority5 ;n sch a scheme, the athority maintains a !irectory 6ith a =i!entifier, (#lic /ey> entry for each (artici(ant5 0ach (artici(ant registers a (#lic /ey 6ith the athority5 ?egistration has to #e in (erson or #y some form of secre athenticate! commnication5 0ach (artici(ant relia#ly /no6s a (#lic /ey for the athority, 6ith only the athority /no6ing the corres(on!ing (rivate /ey5 a. (1 point)
Consi!er the follo6ing (#lic-/ey !istri#tion scheme5 < → P#lic-/ey athority ) ID B P#lic-/ey athority → < ) ath= KU B> < sen!s a message to the (#lic-/ey athority containing an i!entifier of @ to reAest for @4s (#lic /ey5 The athority res(on!s 6ith @4s (#lic /ey KU B signe! 6ith the athority4s (rivate /ey5 ;f an a!versary sccee!s in o#taining or com(ting the (rivate /ey of the (#lic/ey athority, then ho6 can he com(romise the secrity of (artici(ants4 (#lic /eys: b. (2 point ints)
Stronger Stronger secrity for (#lic-/ey (#lic-/ey !istri#tion can #e achieve! achieve! #y (rovi!ing (rovi!ing tighter control over the !istri#tion of (#lic /eys from the athority 6ith the follo6ing scheme5 'i!term 0.am
Net6or/ Secrity
"B
Vietnam National University, Hanoi College of Technology
Pôles Universitaires Français Université Paris-S! ""
< → P#lic-/ey athority ) ID B P#lic-/ey athority → < ) ath= KU B> < → @ ) 0 KU BD ID ID A E N A @ → P#lic-/ey athority ) ID A P#lic-/ey athority → @ ) ath= KU A> @ → < ) 0 KU AD N N A 6here, ID A an! ID B are i!entifiers i !entifiers of < an! @ res(ectively, KU A an! KU B are (#lic /eys /eys of < an! an! @ res(e res(ect ctive ively ly,, N A is a nonce generate! #y < to i!entify the transaction niAely5 ;n this scheme, if an a!versary sccee!s in o#taining or com(ting the (rivate /ey of the (#lic-/ey athority, can he com(romise the secrity of @4s (#lic /ey: 1hy: $. "G" ep#o ep#oym yment ent ($ poi points nts))
S((ose that
8iven that asymmetric encry(tion systems are mch slo6er than symmetric encry(tion systems, P8P !oes not !irectly se the reci(ient4s (#lic /ey to encry(t !ata5 0.(lain the (roce!re ( roce!re se! #y P8P5 c. (1 po point)
Satisfie! 6ith P8P4s services,
1hy !oes SB';'0 incl!e a (#lic-/ey certificate in signedData messsages, 6hereas this isnt the case in envelopedData messages: 5. "Sec "Sec Sec Securit urity y Assoc Associati iation on ($ points) points)
Ira6 the format of the ;Pv$ ;PSec (ac/ets as transmitte! #y a remote host to a 6or/station #ehin! the fire6all of an organiGation5 ;PSec is im(lemente! on all theses 'i!term 0.am
Net6or/ Secrity
%B
Vietnam National University, Hanoi College of Technology
Pôles Universitaires Français Université Paris-S! ""
three !evices 7remote host, 6or/station, an! fire6all95 The remote host esta#lishes a tnn tnnel el mo!e mo!e sec secri rity ty asso associ ciat atio ionn to the the fire fire6a 6allll to (rov (rovi! i!ee limi limite te!! traf traffi ficc flo6 flo6 confi!entiality5 Iata origin athentication an! !ata confi!entiality are s((orte! #y means of a trans(ort mo!e secrity association #et6een the remote host an! the 6or/station5
?e6rite the J53& three-6ay athentication (roce!re so that it corres(on!s to the ;S<2'P athentication only e.change5 ;n!icate 6hich (arameters in each message go in 6hich ;S<2'P (ayloa! ty(es5 . SS SS/& /&/ /S S (% (% poi point nts) s)
Consi!er Consi!er the SSK Han!sha/e Han!sha/e Protocol5 Protocol5 S((ose that the 0(hemeral 0(hemeral Iiffie-Hellma Iiffie-Hellmann /ey e.chan e.change ge metho! is se!5 se!5 The client nee!nt nee!nt sen! a Certificateerif! message to the server to (rove its i!entity5 a. (1 point)
Ira6 the message e.change e.(ecte! for this scenario5 b. (" point ints)
Ira6 the format of the TCP segments e.change! #et6een the client an! the server !ring (hases t6o an! three5
