我正在尝试用Python的reportlab为生成的PDF文件添加密码，这个PDF文件是在Django的web应用程序中生成的，Django动态生成PDF文件的代码在Django的帮助页面中有详细的文档记录：

import io
from django.http import FileResponse
from reportlab.pdfgen import canvas

def some_view(request):
    # Create a file-like buffer to receive PDF data.
    buffer = io.BytesIO()

    # Create the PDF object, using the buffer as its "file."
    p = canvas.Canvas(buffer)

    # Draw things on the PDF. Here's where the PDF generation happens.
    # See the ReportLab documentation for the full list of functionality.
    p.drawString(100, 100, "Hello world.")

    # Close the PDF object cleanly, and we're done.
    p.showPage()
    p.save()

    # FileResponse sets the Content-Disposition header so that browsers
    # present the option to save the file.
    buffer.seek(0)
    return FileResponse(buffer, as_attachment=True, filename='hello.pdf')

但当我尝试向PDF文件添加密码时，生成的PDF文件是空白的。

# This results in a blank PDF document
p = canvas.Canvas(buffer, encrypt='password')

有人知道如何在reportlab中使用BytesIO缓冲区并在文件上设置密码吗？