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❆✉t❤❡♥t✐❝❛t✐♦♥ ❙❤❛♥❦❛r ❆♣r✐❧ ✶✶✱ ✷✵✶✼

❖✉t❧✐♥❡ ♦✈❡r✈✐❡✇ ❖✈❡r✈✐❡✇ ❆✉t❤❡♥t✐❝❛t✐♦♥ ❜❛s✐❝s ❆✉t❤❡♥t✐❝❛t✐♥❣ ❤✉♠❛♥s ❙t♦r✐♥❣ ♣❛ss✇♦r❞s ❛t s❡r✈❡rs ❙❝❛❧✐♥❣ t♦ ♠❛♥② ✉s❡rs ❛♥❞ ❞♦♠❛✐♥s ❑❉❈✿ ❑❡② ❉✐str✐❜✉t✐♦♥ ❈❡♥t❡r ❈❆✿ ❈❡rt✐✜❝❛t✐♦♥ ❆✉t❤♦r✐t②

❆✉t❤❡♥t✐❝❛t✐♦♥ ❢r❛♠❡✇♦r❦ ♦✈❡r✈✐❡✇ Pr✐♥❝✐♣❛❧s ❛r❡ ❛♣♣❧✐❝❛t✐♦♥ ❝❧✐❡♥ts ❛♥❞ s❡r✈❡rs ✐♥t❡r❛❝t✐♥❣ ♦✈❡r ❚❈P✴❯❉P ✐♥ ❛♥ ✐♥s❡❝✉r❡ ♥❡t✇♦r❦ ✭❡❣✱ ■♥t❡r♥❡t✮ Pr✐♥❝✐♣❛❧s ❡st❛❜❧✐s❤ s❡ss✐♦♥s✱ ❡①❝❤❛♥❣❡ ❞❛t❛✱ ❝❧♦s❡ s❡ss✐♦♥s ❆tt❛❝❦s ◆❡t✇♦r❦ ❛tt❛❝❦s✿ ❧✐st❡♥✱ ✐♥t❡r❝❡♣t ♠s❣s✱ r❡s❡♥❞ ♠♦❞✐❢❡❞ ♠s❣s ❊♥❞♣♦✐♥t✿ ♠❛❧✐❝✐♦✉s✴❝♦♠♣r♦♠✐s❡❞ ✉s❡r ❆✉t❤❡♥t✐❝❛t✐♦♥ ❣♦❛❧s✿ ❊♥s✉r❡ t❤❛t s❡ss✐♦♥ ♣❡❡rs ❛r❡ ✇❤♦ t❤❡② s❛② t❤❡② ❛r❡ ❊st❛❜❧✐s❤ s❡ss✐♦♥ ❦❡②✭s✮ ❢♦r ❞❛t❛ ❝♦♥✜❞❡♥t✐❛❧✐t②✴✐♥t❡❣r✐t② ❜❡tt❡r t♦ ✉s❡ t❡♠♣♦r❛r② ❦❡②s t❤❛♥ ❧♦♥❣✲t❡r♠ ❦❡②s

❚❈P✲❜❛s❡❞ s❡ss✐♦♥✿ ✇✐t❤♦✉t ❛✉t❤❡♥t✐❝❛t✐♦♥ ♦✈❡r✈✐❡✇ client tcp tcp server <ip addr, port #> A x1 x2 B accept( ) connect(x2) [x1, x2, SYN, ...] connecting accepting [x2, x1, SYNACK, ...] open open to x1 [x1, x2, ACK, ...] open open send(data) send(data) [x1, x2, ..., [data] ] recv( ) recv( ) [x2, x1 ..., [data] ] data data close( ) close( ) closing closing tcp closing closed closed

❚❈P✲❜❛s❡❞ s❡ss✐♦♥✿ ❛tt❛❝❦s ♦✈❡r✈✐❡✇ client tcp tcp server <ip addr, port #> A x1 x2 B accept( ) connect(x2) [x1, x2, SYN, ...] connecting accepting [x2, x1, SYNACK, ...] open open to x1 [x1, x2, ACK, ...] open open send(data) send(data) [x1, x2, ..., [data] ] recv( ) recv( ) [x2, x1 ..., [data] ] data data close( ) close( ) closing closing tcp closing closed closed endpoint attacks network attacks endpoint attacks

❚❈P✲❜❛s❡❞ s❡ss✐♦♥✿ ✇✐t❤ ❛✉t❤❡♥t✐❝❛t✐♦♥ ♦✈❡r✈✐❡✇ client tcp tcp server <ip addr, port #> A x1 x2 B accept( ) connect(x2) connecting tcp conn establishment accepting open open to x1 open open tcp data exchange authenticated secure data exchange close( ) close( ) closing closing tcp closing closed closed

❉✐❝t✐♦♥❛r② ✭❛❦❛ ♣❛ss✇♦r❞✲❣✉❡ss✐♥❣✮ ❛tt❛❝❦s ♦✈❡r✈✐❡✇ ❲❡❛❦ s❡❝r❡t ✭❛❦❛ ❧♦✇✲q✉❛❧✐t② s❡❝r❡t✮ ❝♦♠❡s ❢r♦♠ ❛ s♣❛❝❡ s♠❛❧❧ ❡♥♦✉❣❤ ❢♦r ❛ ❜r✉t❡✲❢♦r❝❡ s❡❛r❝❤ ❡❣✿ ♣❛ss✇♦r❞s✱ ❛♥❞ ❦❡②s ♦❜t❛✐♥❡❞ ❢r♦♠ t❤❡♠ ❙tr♦♥❣ s❡❝r❡t ✭❛❦❛ ❤✐❣❤✲q✉❛❧✐t② s❡❝r❡t✮✿ ♥♦t ✇❡❛❦ ❡❣✿ ❦❡② ✇✐t❤ ✶✷✽ r❛♥❞♦♠ ❜✐ts ❉✐❝t✐♦♥❛r② ❛tt❛❝❦s ✭❛❦❛ ♣❛ss✇♦r❞✲❣✉❡ss✐♥❣ ❛tt❛❝❦s✮ ●✐✈❡♥ ❝✐♣❤❡rt❡①t ❢r♦♠ str✉❝t✉r❡❞ ♣❧❛✐♥t❡①t ❛♥❞ ✇❡❛❦ ❦❡②✱ ❞❡❝r②♣t ✇✐t❤ ❡✈❡r② ♣♦ss✐❜❧❡ ❦❡② ✉♥t✐❧ str✉❝t✉r❡ ❛♣♣❡❛rs ❖♥❧✐♥❡ ❛tt❛❝❦✿ ✐♥t❡r❛❝t ✇✐t❤ ❛✉t❤❡♥t✐❝❛t♦r ❛t ❡✈❡r② ❣✉❡ss ❉❡❢❡♥s❡✿ ❧✐♠✐t ♥✉♠❜❡r✴❢r❡q✉❡♥❝② ♦❢ ❛tt❡♠♣ts ❖✤✐♥❡ ❛tt❛❝❦✿ ✐♥t❡r❛❝t ✇✐t❤ ❛✉t❤❡♥t✐❝❛t♦r ❥✉st ♦♥❝❡ ❉❡❢❡♥s❡✿ ❞♦♥✬t ❡①♣♦s❡ r❡❧❡✈❛♥t ❝✐♣❤❡rt❡①t

❈♦♥✈❡♥t✐♦♥s✿ ❈r②♣t♦ ♦✈❡r✈✐❡✇ ❙②♠♠❡tr✐❝ ❝r②♣t♦ E ( key , msg ) ✿ ❡♥❝r②♣t msg ✇✐t❤ key / ✐♥❝❧✉❞❡s ❛♥② ■❱ / D ( key , ctx ) ✿ ❞❡❝r②♣t ctx ✇✐t❤ key / ✐♥❝❧✉❞❡s ❛♥② ■❱ / ❍❛s❤ H ( msg ) ✿ ❤❛s❤ ♦❢ msg / ❡❣✱ ❙❍❆✲✶ / H ( key , msg ) ✿ ❦❡②❡❞✲❤❛s❤ / ❡❣✱ ❍▼❆❈✲❙❍❆✲✶ / ❆s②♠♠❡tr✐❝ ❝r②♣t♦ / ♣✉❜❧✐❝✲❦❡② ♣❛✐r [ sk , pk ] / E P ( pk , msg ) ✿ ❡♥❝r②♣t msg ✭✇✐t❤ ♣✉❜❧✐❝ ❦❡②✮ D P ( sk , msg ) ✿ ❞❡❝r②♣t msg ✭✇✐t❤ s❡❝r❡t ❦❡②✮ Sgn ( sk , msg ) ✿ s✐❣♥❛t✉r❡ ♦❢ msg ✭✉s✐♥❣ s❡❝r❡t ❦❡②✮ Vfy ( pk , msg , s ) ✿ ✈❡r✐❢② s✐❣♥❛t✉r❡ s ♦❢ msg ✭✇✐t❤ ♣✉❜❧✐❝ ❦❡②✮

❈♦♥✈❡♥t✐♦♥s✿ ◆♦♥❝❡s ♦✈❡r✈✐❡✇ ◆♦♥❝❡✿ ♥❡✇ ✈❛❧✉❡ / ♥❡✇ ❂ ♥❡✈❡r ❜❡❢♦r❡ s❡❡♥ / ❈❛♥ ❜❡ ♣r❡❞✐❝t❛❜❧❡ ♦r r❛♥❞♦♠ Pr❡❞✐❝t❛❜❧❡✿ ❣✐✈❡♥ ♦♥❡ ✈❛❧✉❡✱ ❛tt❛❝❦❡r ❝❛♥ ❣✉❡ss t❤❡ ♥❡①t ♦♥❡ ❘❛♥❞♦♠✿ ♥♦t ♣r❡❞✐❝t❛❜❧❡ / / ♣❤②s✐❝❛❧ r❛♥❞♦♠♥❡ss✱ ❝r②♣t♦ ♦✉t♣✉t

❖✉t❧✐♥❡ ❛✉t❤ ❜❛s✐❝ ❖✈❡r✈✐❡✇ ❆✉t❤❡♥t✐❝❛t✐♦♥ ❜❛s✐❝s ❆✉t❤❡♥t✐❝❛t✐♥❣ ❤✉♠❛♥s ❙t♦r✐♥❣ ♣❛ss✇♦r❞s ❛t s❡r✈❡rs ❙❝❛❧✐♥❣ t♦ ♠❛♥② ✉s❡rs ❛♥❞ ❞♦♠❛✐♥s ❑❉❈✿ ❑❡② ❉✐str✐❜✉t✐♦♥ ❈❡♥t❡r ❈❆✿ ❈❡rt✐✜❝❛t✐♦♥ ❆✉t❤♦r✐t②

A , B s❤❛r❡ s②♠♠❡tr✐❝ ❦❡② k ❛✉t❤ ❜❛s✐❝ ❝❧✐❡♥t A ✭❦❡② k ❢♦r s❡r✈❡r B ✮ s❡r✈❡r B ✭❤❛s ❦❡② k ❢♦r ✉s❡r A ✮ s❡♥❞ [ A , B , ❝♦♥♥ ] r❝✈ [ A , B , ❝♦♥♥ ] c B ← r❛♥❞♦♠ / / s❡r✈❡r ❝❤❛❧❧❡♥❣❡ s❡♥❞ [ B , A , c B ] r❝✈ [ B , A , c B ] c A ← r❛♥❞♦♠ / / ❝❧✐❡♥t ❝❤❛❧❧❡♥❣❡ r B ← E ( k , c B ) / / ❝❧✐❡♥t r❡s♣♦♥s❡ s❡♥❞ [ c A , r B ] r❝✈ [ c A , r B ] ✐❢ ✭ r B � = E ( k , c B ) ✮ ❋❆■▲ r A ← E ( k , c A ) / / s❡r✈❡r r❡s♣♦♥s❡ s❡♥❞ [ r A ] r❝✈ [ r A ] s❡ss✐♦♥ ❦❡② ← Func ( c A , c B , k ) ✐❢ ✭ r A � = E ( k , c A ) ✮ ❋❆■▲ s❡ss✐♦♥ ❦❡② ← Func ( c A , c B , k )

A , B s❤❛r❡ s②♠♠❡tr✐❝ ❦❡② k ✭❝♦♥t✮ ❛✉t❤ ❜❛s✐❝ ▼❛♥② ✈❛r✐❛t✐♦♥s ♦❢ ❝❤❛❧❧❡♥❣❡✴r❡s♣♦♥s❡ ♦♣❡♥ ❝❤❛❧❧❡♥❣❡✱ ❡♥❝r②♣t❡❞ r❡s♣♦♥s❡ / c A → E ( k , c A ) / ❡♥❝r②♣t❡❞ ❝❤❛❧❧❡♥❣❡ ❛♥❞ r❡s♣♦♥s❡ / E ( k , c A ) → E ( k , c A + ✶ ) / ❖✤✐♥❡ ❞✐❝t✐♦♥❛r② ❛tt❛❝❦ ✐❢ k ✐s ✇❡❛❦ ❛♥❞ ❛tt❛❝❦❡r ❝❛♥ ❡❛✈❡s❞r♦♣✱ ♦r ❛tt❛❝❦❡r ❝❛♥ ❛tt❛❝❤ t♦ B ✬s ♥❡t ❛❞❞r❡ss ■❢ ❝❧✐❡♥t ✐ss✉❡s ❝❤❛❧❧❡♥❣❡ ✜rst ❛♥❞ k ✐s ✇❡❛❦✱ ❝❛♥ ❞♦ ♦✤✐♥❡ ❞✐❝t✐♦♥❛r② ❛tt❛❝❦ ✇✐t❤♦✉t ❛tt❛❝❦✐♥❣ ♥❡t✇♦r❦ ❛tt❛❝❦❡r s❡♥❞s ❝❤❛❧❧❡♥❣❡✱ ❣❡ts r❡s♣♦♥s❡

A , B ❤❛✈❡ ❡❛❝❤ ♦t❤❡r✬s ♣✉❜❧✐❝ ❦❡② ❛✉t❤ ❜❛s✐❝ ❝❧✐❡♥t A ✭❤❛s [ sk A , pk A ] ✱ pk B ✮ s❡r✈❡r B ✭❤❛s [ sk B , pk B ] ✱ pk A ✮ c A ← r❛♥❞♦♠ / ❝❤❛❧❧❡♥❣❡ / r❝✈ [ A , B , ❝♦♥♥ , y A ] s❡♥❞ [ A , B , ❝♦♥♥ , E P ( pk B , c A )] c A ← D P ( sk B , y A ) c B ← r❛♥❞♦♠ / ❝❤❛❧❧❡♥❣❡ / r❝✈ [ B , A , y B ] s❡♥❞ [ B , A , E P ( pk A , [ c B , c A ])] / / r❡s♣ [ c B , r A ] ← D P ( sk A , y B ) ✐❢ ✭ r A � = c A ✮ ❋❆■▲ s❡♥❞ [ E P ( pk B , c B )] / r❡s♣♦♥s❡ / s❡ss✐♦♥ ❦❡② ← Func ( c A , c B ) r❝✈ [ y B ] r B ← D P ( sk B , y B ) ✐❢ ✭ r B � = c B ✮ ❋❆■▲ s❡ss✐♦♥ ❦❡② ← Func ( c A , c B ) ❙❛❢❡ ❢r♦♠ ❞✐❝t✐♦♥❛r② ❛tt❛❝❦ / ❛s②♠♠❡tr✐❝ ❦❡②s ❛❧✇❛②s str♦♥❣ /