Ошибка VB.NET AES "Заполнение недопустимо и не может быть удалено" тот же ключ /iv и последний блок очищен

Question

Я использую AesCryptoServiceProvider для выполнения шифрования и дешифрования с использованием AES. Кажется, что шифрование работает, но дешифрование создает исключение, говорящее "Заполнение недопустимо и не может быть удалено".

Вот мой код:

Public Function AESEnc(ByVal plain As String, ByVal key As String, ByVal iv As String) As AesResult
    Dim ciphertext As String
    Dim k(31) As Byte
    Dim v(15) As Byte
    Dim ret As AesResult
    Dim pBytes As Byte() = StrToByteArray(plain)
    Dim cipher As New AesCryptoServiceProvider()
    Dim enc As ICryptoTransform
    Dim ms As New MemoryStream
    Dim cs As CryptoStream

    k = StrToByteArray(key)
    v = HexStringByteArrayConverter.HexStringToBytes(iv)
    cipher.Mode = CipherMode.CFB
    cipher.Padding = PaddingMode.PKCS7

    enc = cipher.CreateEncryptor(k, v)
    cs = New CryptoStream(ms, enc, CryptoStreamMode.Write)
    cs.Write(pBytes, 0, pBytes.Length)
    cs.FlushFinalBlock()
    cs.Close()
    ciphertext = HexStringByteArrayConverter.BytesToHexString(ms.ToArray())
    ret.data = ciphertext
    ret.iv = v
    ret.key = k

    Return ret
End Function 'AESEnc

Public Function AESDec(ByVal ciphertext As String, ByVal key As String, ByVal iv As String) As AesResult
    Dim plaintext As String
    Dim k(31) As Byte
    Dim v(15) As Byte
    Dim ret As AesResult
    Dim pBytes As Byte() = StrToByteArray(ciphertext)
    Dim cipher As New AesCryptoServiceProvider()
    Dim dec As ICryptoTransform
    Dim ms As New MemoryStream
    Dim cs As CryptoStream

    k = StrToByteArray(key)
    v = HexStringByteArrayConverter.HexStringToBytes(iv)
    cipher.Mode = CipherMode.CFB
    cipher.Padding = PaddingMode.PKCS7

    dec = cipher.CreateDecryptor(k, v)
    cs = New CryptoStream(ms, dec, CryptoStreamMode.Write)
    cs.Write(pBytes, 0, pBytes.Length)
    cs.FlushFinalBlock()
    cs.Close()
    plaintext = ByteArrayToString(ms.ToArray())
    ret.data = plaintext
    ret.iv = v
    ret.key = k

    Return ret
End Function 'AESDec