c# – iTextSharp – 如何获取用于签名的PDF内容,然后在以后签名

原来服务器上的进程必须如下：

>获取未签名的PDF并添加一个空的签名字段
>根据文件的修改内容计算需要签名的字节
>将带有空签名的已修改PDF保存到临时文件中
>将计算的字节发送到客户端
>当客户端使用签名响应时,打开临时文件并将签名插入到先前创建的字段中

相关的服务器代码：

public static byte[] GetBytesToSign(string unsignedPdf,string tempPdf,string signatureFieldName)
{
    using (PdfReader reader = new PdfReader(unsignedPdf))
    {
        using (FileStream os = File.OpenWrite(tempPdf))
        {
            PdfStamper stamper = PdfStamper.CreateSignature(reader,os,'');
            PdfSignatureAppearance appearance = stamper.SignatureAppearance;
            appearance.SetVisibleSignature(new Rectangle(36,748,144,780),1,signatureFieldName);
            IExternalSignatureContainer external = new ExternalBlankSignatureContainer(PdfName.ADOBE_PPKMS,PdfName.ADBE_PKCS7_SHA1);
            MakeSignature.SignExternalContainer(appearance,external,8192);

            return SHA1Managed.Create().ComputeHash(appearance.GetRangeStream());
        }
    }
}
public static void EmbedSignature(string tempPdf,string signedPdf,string signatureFieldName,byte[] signedBytes)
{
    using (PdfReader reader = new PdfReader(tempPdf))
    {
        using (FileStream os = File.OpenWrite(signedPdf))
        {
            IExternalSignatureContainer external = new MyExternalSignatureContainer(signedBytes);
            MakeSignature.SignDeferred(reader,signatureFieldName,external);
        }
    }
}

private class MyExternalSignatureContainer : IExternalSignatureContainer
{
    private readonly byte[] signedBytes;

    public MyExternalSignatureContainer(byte[] signedBytes)
    {
        this.signedBytes = signedBytes;
    }

    public byte[] Sign(Stream data)
    {
        return signedBytes;
    }

    public void ModifySigningDictionary(PdfDictionary signDic)
    {
    }
}

旁注：在所有这些iText样本中困扰我的是魔术数字的存在(如8192这里),没有任何评论.这使得使用这个库变得更加困难和烦人.