## What will we cover?

- Understand what Caesar Cipher is
- Implement Caesar Cipher in Python
- Understand the weakness of Caesar Cipher

## What is Caesar Cipher

Caesar Cipher is a simple substitution cipher, which is limited to only shift the characters by fix number.

Imagine you got the message: BRX DUH DZHVRPH. What to make out of it. Makes no sense. Just ignore it, right?

Well, for the untrained eye, yes, ignore it. But why would anyone bother sending you that message? Aren’t you curious?

I would be. Let’s say your friend told you it was a Caesar Cipher with a 3-key shift. What does that mean?

In the picture below, you can see how the letters are shifted from the green letters (plain text), to the encrypted red letters (cipher text). As an example, A is encrypted to D, and B is encrypted to E, and so forth.

By using that strategy, you can also figure out that a cipher B (red letters) maps to the plain Y (green letters). A cipher R maps to the plain O. And so forth. That results in that your secret message decrypts to YOU ARE AWESOME.

What a nice message to get.

## Implementation of Caesar Cipher in Python

Below is an example of how you could implement the Caesar Cipher in Python.

```
def generate_key(n):
chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
key = {}
cnt = 0
for c in chars:
key[c] = chars[(cnt + n) % len(chars)]
cnt += 1
return key
def encrypt(key, message):
cipher = ""
for c in message:
if c in key:
cipher += key[c]
else:
cipher += c
return cipher
def get_decrypt_key(key):
dkey = {}
for k in key:
dkey[key[k]] = k
return dkey
key = generate_key(3)
dkey = generate_key(23)
cipher = encrypt(key, "YOU ARE AWESOME")
print(cipher)
message = encrypt(dkey, cipher)
print(message)
dkey = get_decrypt_key(key)
print(encrypt(dkey, cipher))
```

There are some things to notice. First of all, the generate_key function comes in handy, when we extend our cipher function to the more general substitution cipher.

Also, notice, that encryption and decryption are done with the same function, just different keys. The decryption key of key-3 is key-23.

See that you can actually calculate the decryption key quite nice, as done in the get_decrypt_key function, which can be used if this is extended to a general substitution cipher.

## How to de-cipher a Caesar Cipher message

Image you had received the message BRX DUH DZHVRPH, but didn’t know the key. What to do?

No worries, there is a way to solve that problem efficiently.

The scenario is that Alice wants to send Bob a secret message, but someone (you) get’s a copy of the message (and you are called Eve).

The secrecy of the message was intended to be, that you (Eve) does not know the Algorithm (how the encryption is done). That might seems naive today, but back in the time of Caesar, this was the state of the art, and people were not that knowledgable about the art of cryptography.

But you are in luck. Yes, I am talking to you Eve.

You know the Algorithm and you are soon to figure out, that the key space is quite small. Actually it only contains 26 possible keys.

That is in the reach of you manually trying out all possible keys.

But you are in more luck, because you realised that you also have Python. So let’s try to fix it in Python.

```
def generate_key(n):
chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
key = {}
cnt = 0
for c in chars:
key[c] = chars[(cnt + n) % len(chars)]
cnt += 1
return key
def encrypt(key, message):
cipher = ""
for c in message:
if c in key:
cipher += key[c]
else:
cipher += c
return cipher
cipher = "BRX DUH DZHVRPH"
for i in range(26):
key = generate_key(i)
message = encrypt(key, cipher)
print(message)
```

That will generate the following output.

```
CSY EVI EAIWSQI
DTZ FWJ FBJXTRJ
EUA GXK GCKYUSK
FVB HYL HDLZVTL
GWC IZM IEMAWUM
HXD JAN JFNBXVN
IYE KBO KGOCYWO
JZF LCP LHPDZXP
KAG MDQ MIQEAYQ
LBH NER NJRFBZR
MCI OFS OKSGCAS
NDJ PGT PLTHDBT
OEK QHU QMUIECU
PFL RIV RNVJFDV
QGM SJW SOWKGEW
RHN TKX TPXLHFX
SIO ULY UQYMIGY
TJP VMZ VRZNJHZ
UKQ WNA WSAOKIA
VLR XOB XTBPLJB
WMS YPC YUCQMKC
XNT ZQD ZVDRNLD
YOU ARE AWESOME
ZPV BSF BXFTPNF
AQW CTG CYGUQOG
```

That was awesome right.

## Conclusion

The security of Caesar Cipher was by keeping the Algorithm secret. That approach to security is not used anymore. ** Kerckhoffs’ Principle **states that the adversary (Eve) should not be able to break the cipher even when she knows the Algorithm.