Please help me. I want to encrypt using AES-GCM algorithm in C# code.

Here is my code – however, it can not decrypt. I want to keep the decryption code intact.

How to solve this problem? Please help me.

// Encryption code.
private static byte[] salt = new byte[16]; // Fix 1: consider salt
private static byte[] iv = new byte[12];
private const int iterations = 100000; // Fix 2: apply the same iteration count 

public string Encrypt(string key, string data)
{
     byte[] tag = new byte[16];

     using (var rng = RandomNumberGenerator.Create())
     {
         rng.GetBytes(salt);
         rng.GetBytes(iv);
     }

     byte[] dataBytes = Encoding.UTF8.GetBytes(data);
     byte[] privateKey = DeriveKey(key);

     using (AesGcm aes = new AesGcm(privateKey, tag.Length))
     {
         byte[] encrypted = new byte[dataBytes.Length];
         aes.Encrypt(iv, dataBytes, encrypted, tag);

         byte[] combinedBuffer = new byte[salt.Length + iv.Length + encrypted.Length];
         Buffer.BlockCopy(salt, 0, combinedBuffer, 0, salt.Length); // Fix 1: consider salt
         Buffer.BlockCopy(iv, 0, combinedBuffer, salt.Length, iv.Length);
         Buffer.BlockCopy(encrypted, 0, combinedBuffer, salt.Length + iv.Length, encrypted.Length);

         string base64Buff = BuffToBase64(combinedBuffer);
         return base64Buff;
     }
}

public static byte[] DeriveKey(string password)
{
     byte[] passwordBytes = Encoding.UTF8.GetBytes(password);

     using (var pbkdf2 = new Rfc2898DeriveBytes(passwordBytes, salt, iterations, HashAlgorithmName.SHA256))
     {
         byte[] key = pbkdf2.GetBytes(32); // Fix 3: use the same key size (256 bits)
         return key;
     }
}

public static string BuffToBase64(byte[] buff)
{
    return Convert.ToBase64String(buff);
}

public static byte[] Base64ToBuff(string b64)
{
    return Convert.FromBase64String(b64);
}
 
// Decryption code
 
public string Decrypt(string password, string encryptedString)
{
    Span<byte> encryptedData = Convert.FromBase64String(encryptedString).AsSpan();
    Span<byte> salt = encryptedData[..16]; 
    Span<byte> nonce = encryptedData[16..(16 + 12)];
    Span<byte> data = encryptedData[(16 + 12)..^16];
    Span<byte> tag = encryptedData[^16..];

    using Rfc2898DeriveBytes pbkdf2 = new(Encoding.UTF8.GetBytes(password), salt.ToArray(), 100000, HashAlgorithmName.SHA256);
    using AesGcm aes = new(pbkdf2.GetBytes(32), tag.Length); 

    Span<byte> result = new byte[data.Length];
    aes.Decrypt(nonce, data, tag, result);
    return Encoding.UTF8.GetString(result);
}

// Run the program.
var r = new AesGcmEncryption().Encrypt("my passphrase", "abcd");
var r2 = new AesGcmEncryption().Decrypt("my passphrase", r);

Console.WriteLine(r);
Console.WriteLine(r2);

Error:

Specified argument was out of the range of valid values

I tried changing all the cases, by changing the code, and searched but no one wrote similar code.