Add affine-cipher exercise

Author: keiravillekode

config.json

@@ -50,7 +50,7 @@
         "prerequisites": [],
         "difficulty": 2,
         "status": "deprecated"
-      },        
+      },
       {
         "slug": "trinary",
         "name": "Trinary",
@@ -412,6 +412,14 @@
         "prerequisites": [],
         "difficulty": 4
       },
+      {
+        "slug": "affine-cipher",
+        "name": "Affine Cipher",
+        "uuid": "e0127be0-4664-44d0-9fbf-6fd9d9d944e0",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "all-your-base",
         "name": "All Your Base",

exercises/practice/affine-cipher/.docs/instructions.md

@@ -0,0 +1,74 @@
+# Instructions
+
+Create an implementation of the affine cipher, an ancient encryption system created in the Middle East.
+
+The affine cipher is a type of monoalphabetic substitution cipher.
+Each character is mapped to its numeric equivalent, encrypted with a mathematical function and then converted to the letter relating to its new numeric value.
+Although all monoalphabetic ciphers are weak, the affine cipher is much stronger than the Atbash cipher, because it has many more keys.
+
+[//]: # " monoalphabetic as spelled by Merriam-Webster, compare to polyalphabetic "
+
+## Encryption
+
+The encryption function is:
+
+```text
+E(x) = (ai + b) mod m
+```
+
+Where:
+
+- `i` is the letter's index from `0` to the length of the alphabet - 1.
+- `m` is the length of the alphabet.
+  For the Latin alphabet `m` is `26`.
+- `a` and `b` are integers which make up the encryption key.
+
+Values `a` and `m` must be _coprime_ (or, _relatively prime_) for automatic decryption to succeed, i.e., they have number `1` as their only common factor (more information can be found in the [Wikipedia article about coprime integers][coprime-integers]).
+In case `a` is not coprime to `m`, your program should indicate that this is an error.
+Otherwise it should encrypt or decrypt with the provided key.
+
+For the purpose of this exercise, digits are valid input but they are not encrypted.
+Spaces and punctuation characters are excluded.
+Ciphertext is written out in groups of fixed length separated by space, the traditional group size being `5` letters.
+This is to make it harder to guess encrypted text based on word boundaries.
+
+## Decryption
+
+The decryption function is:
+
+```text
+D(y) = (a^-1)(y - b) mod m
+```
+
+Where:
+
+- `y` is the numeric value of an encrypted letter, i.e., `y = E(x)`
+- it is important to note that `a^-1` is the modular multiplicative inverse (MMI) of `a mod m`
+- the modular multiplicative inverse only exists if `a` and `m` are coprime.
+
+The MMI of `a` is `x` such that the remainder after dividing `ax` by `m` is `1`:
+
+```text
+ax mod m = 1
+```
+
+More information regarding how to find a Modular Multiplicative Inverse and what it means can be found in the [related Wikipedia article][mmi].
+
+## General Examples
+
+- Encrypting `"test"` gives `"ybty"` with the key `a = 5`, `b = 7`
+- Decrypting `"ybty"` gives `"test"` with the key `a = 5`, `b = 7`
+- Decrypting `"ybty"` gives `"lqul"` with the wrong key `a = 11`, `b = 7`
+- Decrypting `"kqlfd jzvgy tpaet icdhm rtwly kqlon ubstx"` gives `"thequickbrownfoxjumpsoverthelazydog"` with the key `a = 19`, `b = 13`
+- Encrypting `"test"` with the key `a = 18`, `b = 13` is an error because `18` and `26` are not coprime
+
+## Example of finding a Modular Multiplicative Inverse (MMI)
+
+Finding MMI for `a = 15`:
+
+- `(15 * x) mod 26 = 1`
+- `(15 * 7) mod 26 = 1`, ie. `105 mod 26 = 1`
+- `7` is the MMI of `15 mod 26`
+
+[mmi]: https://en.wikipedia.org/wiki/Modular_multiplicative_inverse
+[coprime-integers]: https://en.wikipedia.org/wiki/Coprime_integers

exercises/practice/affine-cipher/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "AffineCipher.pas"
+    ],
+    "test": [
+      "TestCases.pas"
+    ],
+    "example": [
+      ".meta/example.pas"
+    ]
+  },
+  "blurb": "Create an implementation of the Affine cipher, an ancient encryption algorithm from the Middle East.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Affine_cipher"
+}

exercises/practice/affine-cipher/.meta/example.pas

@@ -0,0 +1,77 @@
+unit AffineCipher;
+
+{$mode ObjFPC}{$H+}
+
+interface
+
+function encode(const phrase : string; const a, b : longword) : string;
+function decode(const phrase : string; const a, b : longword) : string;
+
+implementation
+
+uses SysUtils;
+
+function inverse(const a : longword) : longword;
+begin
+  result := 0;
+  if a = 1 then result := 1;
+  if a = 3 then result := 9;
+  if a = 5 then result := 21;
+  if a = 7 then result := 15;
+  if a = 9 then result := 3;
+  if a = 11 then result := 19;
+  if a = 15 then result := 7;
+  if a = 17 then result := 23;
+  if a = 19 then result := 11;
+  if a = 21 then result := 5;
+  if a = 23 then result := 17;
+  if a = 25 then result := 25;
+  if result = 0 then
+    raise Exception.Create('a and m must be coprime.');
+end;
+
+function process(const phrase : string; const a, b : longword; group : integer) : string;
+var
+  c           : char;
+  encoded     : char;
+  remaining   : integer;
+begin
+  remaining := group;
+  result := '';
+  for c in phrase do
+    begin
+      if (c >= '0') and (c <= '9') then
+        encoded := c
+      else if (c >= 'A') and (c <= 'Z') then
+        encoded := chr(((ord(c) - ord('A')) * a + b) mod 26 + ord('a'))
+      else if (c >= 'a') and (c <= 'z') then
+        encoded := chr(((ord(c) - ord('a')) * a + b) mod 26 + ord('a'))
+      else
+        continue;
+
+      if remaining = 0 then
+        begin
+          result := result + ' ';
+          remaining := group;
+        end;
+
+      result := result + encoded;
+      remaining := remaining - 1;
+    end;
+end;
+
+function encode(const phrase : string; const a, b : longword) : string;
+begin
+  inverse(a);
+  result := process(phrase, a, b, 5);
+end;
+
+function decode(const phrase : string; const a, b : longword) : string;
+var
+  inv         : longword;
+begin
+  inv := inverse(a);
+  result := process(phrase, inv, (26 - inv) * b, -1);
+end;
+
+end.

exercises/practice/affine-cipher/.meta/tests.toml

@@ -0,0 +1,58 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[2ee1d9af-1c43-416c-b41b-cefd7d4d2b2a]
+description = "encode -> encode yes"
+
+[785bade9-e98b-4d4f-a5b0-087ba3d7de4b]
+description = "encode -> encode no"
+
+[2854851c-48fb-40d8-9bf6-8f192ed25054]
+description = "encode -> encode OMG"
+
+[bc0c1244-b544-49dd-9777-13a770be1bad]
+description = "encode -> encode O M G"
+
+[381a1a20-b74a-46ce-9277-3778625c9e27]
+description = "encode -> encode mindblowingly"
+
+[6686f4e2-753b-47d4-9715-876fdc59029d]
+description = "encode -> encode numbers"
+
+[ae23d5bd-30a8-44b6-afbe-23c8c0c7faa3]
+description = "encode -> encode deep thought"
+
+[c93a8a4d-426c-42ef-9610-76ded6f7ef57]
+description = "encode -> encode all the letters"
+
+[0673638a-4375-40bd-871c-fb6a2c28effb]
+description = "encode -> encode with a not coprime to m"
+
+[3f0ac7e2-ec0e-4a79-949e-95e414953438]
+description = "decode -> decode exercism"
+
+[241ee64d-5a47-4092-a5d7-7939d259e077]
+description = "decode -> decode a sentence"
+
+[33fb16a1-765a-496f-907f-12e644837f5e]
+description = "decode -> decode numbers"
+
+[20bc9dce-c5ec-4db6-a3f1-845c776bcbf7]
+description = "decode -> decode all the letters"
+
+[623e78c0-922d-49c5-8702-227a3e8eaf81]
+description = "decode -> decode with no spaces in input"
+
+[58fd5c2a-1fd9-4563-a80a-71cff200f26f]
+description = "decode -> decode with too many spaces"
+
+[b004626f-c186-4af9-a3f4-58f74cdb86d5]
+description = "decode -> decode with a not coprime to m"

exercises/practice/affine-cipher/AffineCipher.pas

@@ -0,0 +1,28 @@
+unit AffineCipher;
+
+{$mode ObjFPC}{$H+}
+
+interface
+
+function encode(const phrase : string; const a, b : longword) : string;
+function decode(const phrase : string; const a, b : longword) : string;
+
+implementation
+
+uses SysUtils;
+
+function encode(const phrase : string; const a, b : longword) : string;
+begin
+
+  raise ENotImplemented.Create('Please implement your solution.'); result := Copy(phrase, a, b);
+
+end;
+
+function decode(const phrase : string; const a, b : longword) : string;
+begin
+
+  raise ENotImplemented.Create('Please implement your solution.'); result := Copy(phrase, a, b);
+
+end;
+
+end.

exercises/practice/affine-cipher/Makefile

@@ -0,0 +1,15 @@
+SHELL       = /bin/bash
+MAKEFLAGS  += --no-print-directory
+DESTDIR     = build
+EXECUTABLE  = $(DESTDIR)/test
+COMMAND     = fpc -l- -v0 -g -gl -Sa -Cr -Sehnw -Fu./lib test.pas -FE"./$(DESTDIR)"
+
+.ONESHELL:
+
+test:
+	@mkdir -p "./$(DESTDIR)"
+	@cp -r ./lib "./$(DESTDIR)"
+	@$(COMMAND) && ./$(EXECUTABLE) $(test)
+
+clean:
+	@rm -fr "./$(DESTDIR)"