# Substitution Cipher

Hiding some data is known as encryption. When plain text is encrypted it becomes unreadable and is known as ciphertext. In a Substitution cipher, any character of plain text from the given fixed set of characters is substituted by some other character from the same set depending on a key. For example with a shift of 1, A would be replaced by B, B would become C, and so on.

Note: Special case of Substitution cipher is known as Caesar cipher where the key is taken as 3.

#### Mathematical representation

The encryption can be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1,…, Z = 25. Encryption of a letter by a shift n can be described mathematically as. (Encryption Phase with shift n) (Decryption Phase with shift n) Examples:

```Plain Text: I am studying Data Encryption
Key: 4
Output: M eq wxyhCmrk Hexe IrgvCtxmsr

Plain Text: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Key: 4
Output: EFGHIJKLMNOPQRSTUVWXYZabcd
```

Algorithm for Substitution Cipher:
Input:

• A String of both lower and upper case letters, called PlainText.
• An Integer denoting the required key.

Procedure:

• Create a list of all the characters.
• Create a dictionary to store the subtitution for all characters.
• For each character, transform the given character as per the rule, depending on whether we’re encrypting or decrypting the text.
• Print the new string generated.

Below is the implementation.

 `# Python program to demonstrate ` `# Substitution Cipher ` ` `  ` `  `import` `string ` ` `  ` `  `# A list containing all characters ` `all_letters``=` `string.ascii_letters ` `  `  `     `  `"""  ` `create a dictionary to store the substitution  ` `for the given alphabet in the plain text  ` `based on the key ` `"""` `  `  `     `  `dict1 ``=` `{} ` `key ``=` `4` `  `  `for` `i ``in` `range``(``len``(all_letters)): ` `    ``dict1[all_letters[i]] ``=` `all_letters[(i``+``key)``%``len``(all_letters)] ` `  `  `  `  `plain_txt``=` `"I am studying Data Encryption"` `cipher_txt``=``[] ` `  `  `# loop to generate ciphertext ` `  `  `for` `char ``in` `plain_txt: ` `    ``if` `char ``in` `all_letters: ` `        ``temp ``=` `dict1[char] ` `        ``cipher_txt.append(temp) ` `    ``else``: ` `        ``temp ``=``char ` `        ``cipher_txt.append(temp) ` `         `  `cipher_txt``=` `"".join(cipher_txt) ` `print``(``"Cipher Text is: "``,cipher_txt) ` `  `  `     `  `"""  ` `create a dictionary to store the substitution ` `for the given alphabet in the cipher  ` `text based on the key ` `"""` `  `  `     `  `dict2 ``=` `{}      ` `for` `i ``in` `range``(``len``(all_letters)): ` `    ``dict2[all_letters[i]] ``=` `all_letters[(i``-``key)``%``(``len``(all_letters))] ` `      `  `# loop to recover plain text ` `decrypt_txt ``=` `[] ` ` `  `for` `char ``in` `cipher_txt: ` `    ``if` `char ``in` `all_letters: ` `        ``temp ``=` `dict2[char] ` `        ``decrypt_txt.append(temp) ` `    ``else``: ` `        ``temp ``=` `char ` `        ``decrypt_txt.append(temp) ` `         `  `decrypt_txt ``=` `"".join(decrypt_txt) ` `print``(``"Recovered plain text :"``, decrypt_txt)  `

Output:

```Cipher Text is:  M eq wxyhCmrk Hexe IrgvCtxmsr
Recovered plain text : I am studying Data Encryption
```

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My Personal Notes arrow_drop_up Ami Munshi is an Asst Professor with MPSTME, Mumbai (NMIMS University) with the Electronics and Telecommunications specialization Focus areas on GfG Application of Python3 libs for Data/Image compression, Encryption, Data Science and Analytics applications

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