Encrypt and Decrypt Excel Files in C# | https://blog.aspose.com/2021/01/14/encrypt-and-decrypt-excel-files-using-csharp/

decrypt-excel-file.cs

// Open an excel file
Workbook workbook = new Workbook("encryptedBook.xlsx", new LoadOptions { Password="1234" });

// Set the password to null
workbook.Settings.Password = null;

// Save the excel file.
workbook.Save("decryptedBook.xlsx");

encrypt-excel-file.cs

// Open an excel file
Workbook workbook = new Workbook("Book1.xls");

// Specify XOR encryption type
workbook.SetEncryptionOptions(EncryptionType.XOR, 40);

// Specify Strong Encryption type (RC4,Microsoft Strong Cryptographic Provider)
workbook.SetEncryptionOptions(EncryptionType.StrongCryptographicProvider, 128);

// Password protect the file
workbook.Settings.Password = "1234";

// Save the excel file
workbook.Save("encryptedBook.xls");

verify-password.cs

// Create a Stream object
FileStream fstream = new FileStream("EncryptedBook1.xlsx", FileMode.Open);

// Verify password
bool isPasswordValid = FileFormatUtil.VerifyPassword(fstream, "1234");

// Print results
Console.WriteLine("Password is Valid: " + isPasswordValid);

Read the complete article: https://blog.aspose.com/2021/01/14/encrypt-and-decrypt-excel-files-using-csharp/