如何获得PDF内容的签名，然后在以后的时间签署

我自己想出来的。这段代码为我指明了正确的方向。

结果显示服务器上的进程必须如下所示:

• 取未签名PDF并添加一个空签名字段
• 根据修改后的文件
计算需要签名的字节数
• 将修改后的PDF以空签名保存到临时文件
• 发送计算字节到客户端
• 当客户端响应签名时，打开临时文件并将签名插入先前创建的字段

相关服务器代码:

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, ''0');
            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, os, 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ittext示例中，困扰我的是没有任何注释的魔术数字(如8192)的存在。

下面的答案摘自我们关于数字签名的白皮书，第4章，第4.3.3节，使用在客户端创建的签名在服务器端签署文档。这里的代码示例

期望的工作流可以看作是3个主要步骤:

1. Presign:
   要求:pdf，证书链
   服务器端，设置签名基础设施，提取消息摘要并将摘要作为字节数组发送给客户端

签署

2. :
   要求:消息摘要为字节数组，私钥
   客户端，将加密算法应用于消息摘要，以从散列生成签名摘要，并将此签名发送给服务器

3. Postsign: 必需:签名摘要作为字节数组，pdf服务器端将签名摘要插入准备好的签名中，插入签名到pdf-document

代码示例，iText5和c#:

Presign (服务器)

//hello : 
//location of the pdf on the server
//or
//bytestream variable with teh pdf loaded in
//chain: certificate chain
// we create a reader and a stamper
PdfReader reader = new PdfReader(hello);
Stream baos = new MemoryStream();
PdfStamper stamper = PdfStamper.CreateSignature(reader,  baos., ''0');
// we create the signature appearance
PdfSignatureAppearance sap = stamper.SignatureAppearance;
sap.Reason = "Test";
sap.Location = "On a server!";
sap.SetVisibleSignature ( new Rectangle(36, 748, 144, 780), 1, "sig");
sap.Certificate = chain[0];
// we create the signature infrastructure
PdfSignature dic = new PdfSignature(
PdfName.ADOBE_PPKLITE, PdfName.ADBE_PKCS7_DETACHED);
dic.Reason = sap.Reason;
dic.Location = sap.Location;
dic.Contact = sap.Contact;
dic.Date = new PdfDate(sap.SignDate);
sap.CryptoDictionary = dic;
Dictionary<PdfName, int> exc = new Dictionary<PdfName, int>();
exc.Add(PdfName.CONTENTS, (int)(8192 * 2 + 2));
sap.PreClose(exc);
PdfPKCS7 sgn = new PdfPKCS7(null, chain, "SHA256", false);
//Extract the bytes that need to be signed
Stream data = sap.GetRangeStream();
byte[] hash = DigestAlgorithms.Digest(data,"SHA256");
byte[] sh = sgn.getAuthenticatedAttributeBytes(hash,null, null, CryptoStandard.CMS);
//Store sgn, hash,sap and baos on the server
//...
//Send sh to client

(客户端)

// we receive a hash that needs to be signed
Stream istream = response.GetResponseStream();
MemoryStream baos = new MemoryStream();
data = new byte[0x100];
while ((read = istream.Read(data, 0, data.Length)) != 0)  
    baos.Write(data, 0, read);  
istream.Close();
byte[] hash = baos.ToArray();
// we load our private key from the key store
Pkcs12Store store = new Pkcs12Store(new FileStream(KEYSTORE, FileMode.Open), PASSWORD);
String alias = "";
// searching for private key
foreach (string al in store.Aliases)
    if (store.IsKeyEntry(al) && store.GetKey(al).Key.IsPrivate) {
        alias = al;
        break;
    }
AsymmetricKeyEntry pk = store.GetKey(alias);
// we sign the hash received from the server
ISigner sig = SignerUtilities.GetSigner("SHA256withRSA");
sig.Init(true, pk.Key);
sig.BlockUpdate(hash, 0, hash.Length);
data = sig.GenerateSignature();
// we make a connection to the PostSign Servlet
request = (HttpWebRequest)WebRequest.Create(POST);
request.Headers.Add(HttpRequestHeader.Cookie,cookies.Split(";".ToCharArray(), 2)[0]);
request.Method = "POST";
// we upload the signed bytes
os = request.GetRequestStream();
os.Write(data, 0, data.Length);
os.Flush();
os.Close();

Postsign (服务器)

// we read the signed bytes
MemoryStream baos = new MemoryStream();
Stream InputStream iStream = req.GetInputStream();
int read;
byte[] data = new byte[256];
while ((read = iStream.read(data, 0, data.Length)) != -1) {
    baos.Write(data, 0, read);
}
// we complete the PDF signing process
sgn.SetExternalDigest(baos.ToArray(), null, "RSA");
byte[] encodedSig = sgn.getEncodedPKCS7(hash, cal, null,
null, null, CryptoStandard.CMS);
byte[] paddedSig = new byte[8192];
paddedSig.
encodedSig.CopyTo(paddedSig, 0);
PdfDictionary dic2 = new PdfDictionary();
dic2.Put(PdfName.CONTENTS, new PdfString(paddedSig).SetHexWriting(true));
try
{
    sap.close(dic2);
}
catch (DocumentException e)
{
    throw new IOException(e);
}

我省略了大部分客户机-服务器通信代码，只关注签名逻辑。我也没有对这些代码段进行彻底的测试，因为我必须将它们从java代码转换过来，而且我目前没有客户机-服务器设置来测试它们，所以复制并运行这些代码段要自担风险。