Maths(一)---Sets and Numbers(3)

hashing function

h(k ) = k modm

例一.

Assign a memory location to each of the following studentnumbers when m = 1024. Solve the problem if collisions occur.

h(1234567) = 1234567 mod 1024 = 647

h(1352467) = 1352467 mod 1024 = 787

h(1347347) = 1347347 mod 1024 = 787

Name Student number Memory Location

Peter 1234567 647

Michael 1 352467 787

John 1347347 788

Pseudorandom Number Generator

例一.

Cryptology

例一.

Find the secret message to represent the message “MEET YOUIN THE PARK” using Caesar cipher with k = 3.

Original message: MEET YOU IN THE PARK

p: 　　　　　　　12-4-4-19 24-14-20 8-13 19-7-4 15-0-17-10

f (p): 　　　　　　15-7-7-22 1-17-23 11-16 22-10-7 18-3-20-13

Encrypted message: PHHW BRX LQ WKH SDUN

例二.

Find the original message of “FXAT QJAM” using Caesarcipher with k = 9.

Original message: FXAT QJAM

p: 　　　　　　　5-23-0-19 16-9-0-12

f −1(p): 　　　　　22-14-17-10 7-0-17-3

Decrypted message: WORK HARD