From 0a829ed1ee08d167e55864a9696a59914d589810 Mon Sep 17 00:00:00 2001
From: Alex Svetkin <alex.svetkin@gmail.com>
Date: Sun, 26 Oct 2025 20:42:51 +0100
Subject: [PATCH] fixed rsa2 to work with binary strings; added fuzz test

---
 cipher/rsa/rsa2.go           | 20 ++++++++++----------
 cipher/rsa/rsa2_fuzz_test.go | 31 +++++++++++++++++++++++++++++++
 cipher/rsa/rsa2_test.go      |  4 ++++
 3 files changed, 45 insertions(+), 10 deletions(-)
 create mode 100644 cipher/rsa/rsa2_fuzz_test.go

diff --git a/cipher/rsa/rsa2.go b/cipher/rsa/rsa2.go
index 18b6e1391..9890291fe 100644
--- a/cipher/rsa/rsa2.go
+++ b/cipher/rsa/rsa2.go
@@ -10,9 +10,9 @@ package rsa
 
 import (
 	"encoding/binary"
-	"fmt"
 	"math/big"
 	"math/rand"
+	"strings"
 
 	"github.com/TheAlgorithms/Go/math/gcd"
 	"github.com/TheAlgorithms/Go/math/lcm"
@@ -61,24 +61,24 @@ func New() *rsa {
 // EncryptString encrypts the data using RSA algorithm
 // returns the encrypted string
 func (rsa *rsa) EncryptString(data string) string {
-	var nums []byte
+	var result strings.Builder
 
-	for _, char := range data {
+	for i := 0; i < len(data); i++ {
 		slice := make([]byte, 8)
 		binary.BigEndian.PutUint64( // convert uint64 to byte slice
 			slice,
-			encryptDecryptInt(rsa.publicKey, rsa.modulus, uint64(char)), // encrypt each character
+			encryptDecryptInt(rsa.publicKey, rsa.modulus, uint64(data[i])), // encrypt each byte
 		)
-		nums = append(nums, slice...)
+		result.Write(slice)
 	}
 
-	return string(nums)
+	return result.String()
 }
 
 // DecryptString decrypts the data using RSA algorithm
 // returns the decrypted string
 func (rsa *rsa) DecryptString(data string) string {
-	result := ""
+	var result strings.Builder
 	middle := []byte(data)
 
 	for i := 0; i < len(middle); i += 8 {
@@ -86,12 +86,12 @@ func (rsa *rsa) DecryptString(data string) string {
 			break
 		}
 
-		slice := middle[i : i+8]
+		slice := middle[i : i+8] // temp slice
 		num := binary.BigEndian.Uint64(slice) // convert byte slice to uint64
-		result += fmt.Sprintf("%c", encryptDecryptInt(rsa.privateKey, rsa.modulus, num))
+		result.WriteByte(byte(encryptDecryptInt(rsa.privateKey, rsa.modulus, num)))
 	}
 
-	return result
+	return result.String()
 }
 
 // GetPublicKey returns the public key and modulus
diff --git a/cipher/rsa/rsa2_fuzz_test.go b/cipher/rsa/rsa2_fuzz_test.go
new file mode 100644
index 000000000..ad759cc9c
--- /dev/null
+++ b/cipher/rsa/rsa2_fuzz_test.go
@@ -0,0 +1,31 @@
+package rsa
+
+import (
+	"testing"
+)
+
+func FuzzRSA2RoundTrip(f *testing.F) {
+	// Add seed corpus
+	f.Add("Hello")
+	f.Add("World")
+	f.Add("A")
+	f.Add("Test123")
+	f.Add("")
+	f.Add("Special chars: !@#")
+	f.Add("Unicode: αβγ")
+
+	f.Fuzz(func(t *testing.T, message string) {
+		rsaInstance := New()
+
+		if rsaInstance == nil {
+			t.Fatal("Failed to create RSA instance")
+		}
+
+		encrypted := rsaInstance.EncryptString(message)
+		decrypted := rsaInstance.DecryptString(encrypted)
+
+		if decrypted != message {
+			t.Errorf("RSA2 roundtrip failed:\n  Original: %q\n  Decrypted: %q", message, decrypted)
+		}
+	})
+}
diff --git a/cipher/rsa/rsa2_test.go b/cipher/rsa/rsa2_test.go
index 29e15c557..0bdc2fd9d 100644
--- a/cipher/rsa/rsa2_test.go
+++ b/cipher/rsa/rsa2_test.go
@@ -19,6 +19,10 @@ func TestRSA(t *testing.T) {
 			name:    "Encrypt 'Hello, World!' and decrypt it back",
 			message: "Hello, World!",
 		},
+		{
+			name: "Encrypt an unicode string and decrypt it back",
+			message: "こんにちは世界",
+		},
 	}
 
 	for _, tt := range tests {