Add affine cipher exercise

config.json

@@ -1255,6 +1255,15 @@
           "transforming"
         ]
       },
+      {
+         "slug": "affine-cipher",
+         "name": "Affine Cipher",
+         "uuid": "cf62a3c5-af1a-4d05-a004-11b35eb79442",
+         "practices": [],
+         "prerequisites": [],
+         "difficulty": 4,
+         "topics": ["algorithms", "cryptography", "strings"]
+      },
       {
         "slug": "flatten-array",
         "name": "Flatten Array",

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 Roman alphabet `m` is `26`.
+- `a` and `b` are integers which make 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": [
+    "KasimKaizer"
+  ],
+  "files": {
+    "solution": [
+      "affine_cipher.go"
+    ],
+    "test": [
+      "affine_cipher_test.go"
+    ],
+    "example": [
+      ".meta/example.go"
+    ]
+  },
+  "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.go

@@ -0,0 +1,113 @@
+// Package affinecipher contains tools that implement affine-cipher.
+package affinecipher
+
+import (
+	"errors"
+	"strings"
+)
+
+type operation int
+
+const (
+	encode operation = iota + 1
+	decode
+)
+
+const _totalAlphabets = 26 // total number of letters in alphabet.
+
+// Encode encodes the provided message with the provided keys, using affine-cipher.
+func Encode(text string, a, b int) (string, error) {
+	if gcd(a, _totalAlphabets) != 1 {
+		return "", errors.New("affinecipher.Encode: a and b must be co-prime")
+	}
+	return cipher(encode, text, a, b)
+}
+
+// Decode decodes the provided encoded message with the provided keys, using affine-cipher.
+func Decode(text string, a, b int) (string, error) {
+	if gcd(a, _totalAlphabets) != 1 {
+		return "", errors.New("affinecipher.Decode: a and b must be co-prime")
+	}
+	mmi := multiInv(a, _totalAlphabets)
+	return cipher(decode, text, mmi, b)
+}
+
+// cipher functions takes a operation, the text and as well as key values, and performs the
+// operation on the text using the provided keys.
+func cipher(op operation, text string, a, b int) (string, error) {
+	text = strings.ToLower(text)
+	var output strings.Builder
+	accum := 0
+	opFunc := operationFunc(op)
+	for _, char := range []byte(text) {
+		if !(char >= 'a' && char <= 'z') && // if not a letter and
+			!(char >= '1' && char <= '9') { // if not a number
+			continue
+		}
+
+		// if its encoding then write a space every 5 letters.
+		if op == encode && accum != 0 && (accum%5) == 0 {
+			output.WriteByte(' ')
+		}
+		accum++
+
+		if char >= '1' && char <= '9' { // write numbers as it is.
+			output.WriteByte(char)
+			continue
+		}
+		output.WriteByte(opFunc(char, a, b))
+	}
+	return output.String(), nil
+}
+
+// gcd function finds the greatest common divisor for the numbers passed into it.
+func gcd(a, b int) int {
+	if a < b {
+		a, b = b, a
+	}
+	for b > 0 {
+		a, b = b, (a % b)
+	}
+	return a
+}
+
+// multiInv finds the modular multiple inverse of the passed num and mod.
+func multiInv(num, mod int) int {
+	t1, t2 := 0, 1
+	a, b := num, mod
+
+	if a < b {
+		a, b = b, a
+	}
+
+	for b > 0 {
+		t := t1 - (t2 * (a / b))
+		a, b = b, (a % b)
+		t1, t2 = t2, t
+	}
+
+	return t1
+}
+
+// operationFunc function returns the appropriate function for the passed operation.
+func operationFunc(op operation) func(byte, int, int) byte {
+	if op == encode {
+		return encryptionFunc
+	}
+	return decryptionFunc
+}
+
+// encryptionFunc function is for encrypting text.
+func encryptionFunc(letter byte, num1, num2 int) byte {
+	temp := ((num1 * int(letter-'a')) + num2) % _totalAlphabets
+	return byte(temp) + 'a'
+}
+
+// decryptionFunc function is for decrypting text.
+func decryptionFunc(letter byte, mmi, num2 int) byte {
+	temp := (mmi * (int(letter-'a') - num2)) % _totalAlphabets
+	if temp < 0 {
+		temp += _totalAlphabets
+	}
+	return byte(temp) + 'a'
+}

exercises/practice/affine-cipher/.meta/gen.go

@@ -0,0 +1,90 @@
+package main
+
+import (
+	"log"
+	"text/template"
+
+	"../../../../gen"
+)
+
+func main() {
+	t, err := template.New("").Parse(tmpl)
+	if err != nil {
+		log.Fatal(err)
+	}
+	j := map[string]interface{}{
+		"encode": &[]testCase{},
+		"decode": &[]testCase{},
+	}
+	if err := gen.Gen("affine-cipher", j, t); err != nil {
+		log.Fatal(err)
+	}
+}
+
+type testCase struct {
+	Description string `json:"description"`
+	Input       struct {
+		Phrase string `json:"phrase"`
+		Key    struct {
+			Num1 int `json:"a"`
+			Num2 int `json:"b"`
+		} `json:"key"`
+	} `json:"input"`
+	Expected interface{} `json:"expected"`
+}
+
+func (t testCase) ExpectedString() string {
+	m, ok := t.Expected.(string)
+	if !ok {
+		return ""
+	}
+	return m
+}
+
+func (t testCase) Error() bool {
+	m, ok := t.Expected.(map[string]interface{})
+	if !ok {
+		return ok
+	}
+	_, ok = m["error"].(string)
+	return ok
+}
+
+// Template to generate encode and decode test cases.
+var tmpl = `package affinecipher
+
+{{.Header}}
+
+type testCase struct {
+	description    string
+	inputPhrase    string
+	inputA          int
+	inputB          int
+	expectError     bool
+	expected       string
+}
+
+var encodeTests = []testCase{
+{{range .J.encode}}{
+		description:         {{printf "%q" .Description}},
+		inputPhrase:         {{printf "%q" .Input.Phrase}},
+		inputA:              {{printf "%d" .Input.Key.Num1}},
+		inputB:			     {{printf "%d" .Input.Key.Num2}},
+		expectError:         {{printf "%t" .Error}},
+		expected:            {{printf "%q" .ExpectedString}},
+	},
+{{end}}
+}
+
+var decodeTests = []testCase{
+{{range .J.decode}}{
+		description:         {{printf "%q" .Description}},
+		inputPhrase:         {{printf "%q" .Input.Phrase}},
+		inputA:              {{printf "%d" .Input.Key.Num1}},
+		inputB:			     {{printf "%d" .Input.Key.Num2}},
+		expectError:         {{printf "%t" .Error}},
+		expected:            {{printf "%q" .ExpectedString}},
+	},
+{{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/affine_cipher.go

@@ -0,0 +1,9 @@
+package affinecipher
+
+func Encode(text string, a, b int) (string, error) {
+	panic("Please implement the Encode function")
+}
+
+func Decode(text string, a, b int) (string, error) {
+	panic("Please implement the Decode function")
+}