Tutorial02 of security(using sageMath)

In this exercise students exercise mono-alphabetic and Playfair cipher encryption and decryption using SageMath tool.

First:Mono-alphabetic cipher

Monoalphabetic cipher Security(some artributes)

1. with so many keys, might think is secure
2. But would be wrong

Encrypt "Ifwewishtoreplaceletters" 
using mono-alphabetic cipher with the key k="DKVQFIBJWPESCXHTMYAUOLRGZN"
Decrypt back the ciphertext
Encrypt "Welcometomysecuritycourse" 
using mono-alphabetic cipher with the key k="DKVQFIBJWPESCXHTMYAUOLRGZN"
Decrypt back the ciphertext

alpha="abcdefghijklmnopqrstuvwxyz"

def is_alpha_char(c):
    return(c.lower()in alpha)

def get_alpha_index(c):
    return alpha.index(c.lower())

def get_key_index(c,key):
    return key.index(c.upper())

def encrypt_ma(k,plaintext):
    ciphertext=""
    if len(k)==26:
        key=k
        for j in range(len(plaintext)):
            p=plaintext[j]
            if is_alpha_char(p):
                x=get_alpha_index(p)
                c=key[x]
            else:
                c=p
            ciphertext +=c
        return ciphertext
    else:
        print("wrong key size")
def decrypt_ma(k,ciphertext):
    plaintext=""
    if len(k)==26:
        key=k
        for j in range(len(ciphertext)):
            c=ciphertext[j]
            if is_alpha_char(c):
                x=get_key_index(c,key)
                p=alpha[x]
            else:
                p=c
            plaintext +=p
        return plaintext
    else:
        print("wrong key size")

ptext="Ifwewishtoreplaceletters"
k="DKVQFIBJWPESCXHTMYAUOLRGZN"
print(encrypt_ma(k,ptext))
ctext=encrypt_ma(k,ptext)
print(decrypt_ma(k,ctext))

the result：
WIRFRWAJUHYFTSDVFSFUUFYA
ifwewishtoreplaceletters

PlayFair cipher algorithm

if pair in same row, replace with letter to right(OA->NR)
pair in same column, replace with one beneath(DT->KZ)
replace row letter with letter in row of other letter’s column(组成矩形的另外俩）

Security of playfair Cipher

• security much improved over monoalphabetic
• 26x26=676 digrams
• correspondingly more ciphertext
• it can be broken.still has much of plaintext structure

原文地址：https://blog.csdn.net/yanlingyun0210/article/details/142300591

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