# many caesers

## Description

I took an old cipher and modified it to make it secure again. By breaking the regular patterns I made sure the known attacks don't work. Now I can use it to encrypt texts with my favourite Chat partner.

## Source

- `chall.py`
```
import string
import re

text = open('text.txt','r').read().lower()
flag = open('flag.txt','r').read().strip()[4:-1].replace('_','+')
chars = string.ascii_letters + string.digits + '+/='
regex = re.compile('[' + chars + ']{5,70}')
assert re.fullmatch(regex, flag)

def caesar(msg, shift):
return ''.join(chars[(chars.index(c) + shift) % len(chars)] for c in msg)

i = 0
count = 0
while i < len(text):
if text[i] not in string.ascii_lowercase:
print(text[i], end = '')
i += 1
else:
j = i
while text[j] in string.ascii_lowercase: j += 1
print(caesar(text[i:j], chars.index(flag[count % len(flag)])), end = '')
count += 1
i = j

```

- `output.txt`
```
AtvDxK lAopjz /i + vhw c6 uwnshnuqjx ymfy kymhi Kyv 47+3l/eh Bs kpfkxkfwcnu Als 9phdgj9 +ka ymzuBGxmFq 6fdglk8i CICDowC, sjxir bjme+pfwfkd 6li=fj=kp, nCplEtGtEJ, lyo qeb INKLNBM vm ademb7697. ollqba lq DitCmA xzhm fx ef7dd7ii, wIvv eggiww GB kphqtocvkqp, 3d6 MAx ilsplm /d rpfkd vnloov hc nruwtAj xDxyjrx vexliv KyrE +3hc Gurz, jcemgt ixlmgw 9f7gmj5/9k obpmlkpf/ib mzp 8k/=64c ECo sj qb=eklildv. =k loGznlEpD qzC qo+kpm+obk=v, vHEEtuHKtMBHG, huk h7if75j/d9 mofs+=v, zkloh lqAkwCzioqvo rfqnhntzx fhynAnynjx b/a7 JKvrCzEx hexe BE ecwukpi 63c397. MAxLx wypujpwslz 3/c ql irvwhu 9bbcj1h9cb fsi f tswmxmzi zDGrtK ed FBpvrGL vjtqwij ixlmgep 5f8 =lkpqor=qfsb tmowuzs.
```

## Solution
- Starting with empty string , in each iteration I add one character to the flag from the charset and check what the decrypted message would be if this were the flag.
- If that corresponding plaintext for that particular character reveals an extra correct word, I accepted that as the next character of the flag
- To get the plaintext from the ciphertext, I simply changed the shift in the encrypt function to subtract

- `solve.py`
```
import string

text = "AtvDxK lAopjz /i + vhw c6 uwnshnuqjx ymfy kymhi Kyv 47+3l/eh Bs kpfkxkfwcnu Als 9phdgj9 +ka ymzuBGxmFq 6fdglk8i CICDowC, sjxir bjme+pfwfkd 6li=fj=kp, nCplEtGtEJ, lyo qeb INKLNBM vm ademb7697. ollqba lq DitCmA xzhm fx ef7dd7ii, wIvv eggiww GB kphqtocvkqp, 3d6 MAx ilsplm /d rpfkd vnloov hc nruwtAj xDxyjrx vexliv KyrE +3hc Gurz, jcemgt ixlmgw 9f7gmj5/9k obpmlkpf/ib mzp 8k/=64c ECo sj qb=eklildv. =k loGznlEpD qzC qo+kpm+obk=v, vHEEtuHKtMBHG, huk h7if75j/d9 mofs+=v, zkloh lqAkwCzioqvo rfqnhntzx fhynAnynjx b/a7 JKvrCzEx hexe BE ecwukpi 63c397. MAxLx wypujpwslz 3/c ql irvwhu 9bbcj1h9cb fsi f tswmxmzi zDGrtK ed FBpvrGL vjtqwij ixlmgep 5f8 =lkpqor=qfsb tmowuzs."

chars = string.ascii_letters + string.digits + '+/='
print(chars)

def caesar(msg, shift):
return ''.join(chars[(chars.index(c) - shift) % len(chars)] for c in msg)

flag_ = 'th3+d1ffer3nce5+m4ke+4ll+th3+diff3renc3th'
for a in chars:
flag = flag_+a
print(flag)
i = 0
count = 0
while i < len(text):
if text[i] not in chars:
print(text[i], end = '')
i += 1
else:
j = i
while text[j] in chars: j += 1
print(caesar(text[i:j], chars.index(flag[count % len(flag)])), end = '')
count += 1
i = j
print()
```

- command : `python3 solve.py > dump`

# Flag
`ENO{th3_d1ffer3nce5_m4ke_4ll_th3_diff3renc3}`