Caesar Cipher

aesher9o1 · aesher9o1 · commit b56915ff7fee · 2019-10-10T22:51:14.000+05:30
diff --git a/src/algorithms/cryptography/caesar-cipher/CaesarCipher.js b/src/algorithms/cryptography/caesar-cipher/CaesarCipher.js
@@ -0,0 +1,46 @@
+export default class CaesarCipher {
+  /**
+       * Function that encrypts the word
+       * @assumption : The version of the function assumes that the inputs are wording.
+       * No provision or testing was done for other types of input
+       * @param {wording} word : The text to encrypt
+       * @param {number} shifts : By how much amount the text should be shifted
+       * @return {wording}
+       */
+
+  encrypt(word, shifts) {
+    if (shifts < 0) {
+      return this.encrypt(word, shifts + 26);
+    }
+
+    let output = '';
+    for (let i = 0; i < word.length; i + 1) {
+      let c = word[i];
+      if (c.match(/[a-z]/i)) {
+        const code = word.charCodeAt(i);
+        if ((code >= 65) && (code <= 90)) {
+          c = String.fromCharCode(((code - 65 + shifts) % 26) + 65);
+        } else if ((code >= 97) && (code <= 122)) {
+          c = String.fromCharCode(((code - 97 + shifts) % 26) + 97);
+        }
+      }
+
+      output += c;
+    }
+    return output;
+  }
+
+  /**
+       * Function that encrypts the word
+       * @assumption : The version of the function assumes that the inputs are wording.
+       * No provision or testing was done for other types of input
+       * @param {wording} word : The text to decrypt
+       * @param {number} shifts : By how much amount the text should be shifted back to
+       * retrive the original text
+       * @return {wording}
+       */
+
+  decrypt(word, shifts) {
+    return (shifts < 0) ? this.encrypt(word, 26 - shifts) : this.encrypt(word, -shifts);
+  }
+}
diff --git a/src/algorithms/cryptography/caesar-cipher/README.md b/src/algorithms/cryptography/caesar-cipher/README.md
@@ -0,0 +1,17 @@
+# Ceasar Cipher
+**Caesar cipher**, also known as the shift cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on.
+
+
+## Encryption Function
+e(x) = (x+k) mod(26)
+## Decryption Function
+e(x) = (x-k) mod(26)
+
+Where x is the text to be encrypted/decrypted and k is the private key or the amount by which the characters need to be shifted
+
+## References
+
+- [Where to Use Polynomial String Hashing](https://www.mii.lt/olympiads_in_informatics/pdf/INFOL119.pdf)
+- [Hashing on uTexas](https://www.cs.utexas.edu/~mitra/csSpring2017/cs313/lectures/hash.html)
+- [Hash Function on Wikipedia](https://en.wikipedia.org/wiki/Hash_function)
+- [Rolling Hash on Wikipedia](https://en.wikipedia.org/wiki/Rolling_hash)
diff --git a/src/algorithms/cryptography/caesar-cipher/__test__/CaesarCipher.test.js b/src/algorithms/cryptography/caesar-cipher/__test__/CaesarCipher.test.js
@@ -0,0 +1,12 @@
+import CaesarCipher from '../CaesarCipher';
+
+describe('ceasarCipher', () => {
+  it('shifts the character by a particular amount as specified by the user', () => {
+    const cipher = new CaesarCipher();
+    const encText = cipher.encrypt('ABC', 3);
+    const decText = cipher.decrypt('DEF', 3);
+
+    expect(encText).toEqual('XYZ');
+    expect(decText).toEqual('ABC');
+  });
+});
