Agreement Protocol. First we describe how PBFT achieves agreement on a unique order of requests within a single view. Figure 26.9 shows how the nodes come to an agreement on a sequence number for a client request. Informally, the protocol has these five steps:
Agreement Protocol. The fact that the message history can be modified does not mean that both participants agree on anything. It just allows them to revise earlier statements, which is not sufficient to coordinate actions between agents. In order to coordi- nate, it is required to come to a shared understanding about facts, which requires that the agents state the facts and that they signal each other that they agree on them. In the Semantic Web, the core of such an agreement naturally is a set of triples. Consequently, the agreement protocol allows the participants to select a set of triples as agreed-upon.
Agreement Protocol. Each receiver chooses a secret key, S. ;;; The receiver publishes a public key, which gives a, p, and ;;; P=a^S(mod p). The receiver also supplies a procedure that the ;;; sender can call with the ciphertext. Given a message, m, the ;;; sender chooses his own secret, T, and composes a ciphertext that ;;; has two components, x=a^T(mod p) and y=m*P^T(mod p). The ;;; Xxxxxx-Xxxxxxx shared secret is x^S(mod p)=P^T(mod p). So the ;;; receiver decrypts the message by computing m=(y/x^S)(mod p). (define (eg-receiver dh-system) ;ElGamal receiver (let ((k (dh-system-size dh-system)) (p (dh-system-prime dh-system))) (let ((my-secret (random-k-digit-number k)) (mod-expt (exptmod p)) (mod-* (modular p *)) (mod-inv (inversemod p))) (let ((advertised-number (mod-expt (dh-system-primitive-root dh-system) my-secret))) (let ((public-key (eg-make-public-key dh-system advertised-number)) (decryption-procedure (lambda (ciphertext) (let ((x (eg-ciphertext-x ciphertext)) (y (eg-ciphertext-y ciphertext))) (let ((m (mod-* y (mod-inv (mod-expt x my-secret))))) (integer->string m)))))) (eg-make-receiver public-key decryption-procedure)))))) (define (eg-send-message message receiver) YOUR CODE HERE) ;;; Data abstractions supporting the ElGamal system (define (public-dh-system k) (list k (random-k-digit-prime k) 2)) (define (dh-system-size system) (car system)) (define (dh-system-prime system) (cadr system)) (define (dh-system-primitive-root system) (caddr system)) (define (eg-make-ciphertext x y) (cons x y)) (define (eg-ciphertext-x c) (car c)) (define (eg-ciphertext-y c) (cdr c)) (define (eg-make-public-key dh-system advertised-number) (cons dh-system advertised-number)) (define (eg-public-key-system key) (car key)) (define (eg-public-key-number key) (cdr key)) (define (eg-make-receiver public-key decryption-procedure) (cons public-key decryption-procedure)) (define (eg-receiver-public-key receiver) (car receiver)) (define (eg-receiver-decryption-procedure receiver) (cdr receiver)) #|
Agreement Protocol. First we describe how PBFT achieves agreement on a unique order of requests within a view. request pre-prepare prepare commit reply (r, c)c (v, s, r, n0 )n0 (v, s, r, ni)ni (v, s, ni)ni (r, ni)ni client c primary p backup n1 backup n2 backup n3
