iText Java签名PDF文档异常：空间不足

Question

我正在使用iText 5.5.5 for Java,我想创建带有外部签名的签名PDF,如下所示：

PdfReader reader = new PdfReader(src);
FileOutputStream os = new FileOutputStream(dest);
PdfStamper stamper = PdfStamper.createSignature(reader, os, '\0');
PdfSignatureAppearance appearance = stamper.getSignatureAppearance();
appearance.setVisibleSignature(new Rectangle(36, 748, 144, 780), 1, "test");
appearance.setCertificate(chain[1]);
ExternalSignatureContainer external = new ExternalBlankSignatureContainer(PdfName.ADOBE_PPKLITE, PdfName.ADBE_PKCS7_DETACHED);
MakeSignature.signExternalContainer(appearance, external, 8192);

InputStream is = appearance.getRangeStream();
byte[] toSign = getBytes(is);
this.b64String = new String(Base64.encode(toSign));

使用外部签名机制对b64string进行签名，该机制提供作为Base64中的PKCS#7签名数据的签名。

创建ExternalSignatureContainer以仅使用来自外部签名机制的PKCS#7签名数据：

public class MyExternalSignatureContainer implements ExternalSignatureContainer {
    protected byte[] sig;

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

    @Override
    public void modifySigningDictionary(PdfDictionary arg0) {
    }

    @Override
    public byte[] sign(InputStream arg0) throws GeneralSecurityException {
        return sig;
    }
}

使用MyExternalSignatureContainer创建签名PDF文档：

PdfReader reader = new PdfReader(dest);
FileOutputStream os = new FileOutputStream(signedpdf);
ExternalSignatureContainer external = new MyExternalSignatureContainer(signedData);
MakeSignature.signDeferred(reader, "test", os, external);

com.itextpdf.text.DocumentException: Not enough space

问题出在哪里，如何化解？

Answer 1

您估计签名可以容纳8192个字节。但是，签名byte[]的字节数超过8192，因此出现空间不足的例外情况。例如：外部签名容器返回一个度量为10000字节的签名。iText告诉你10000比8192大，你问的是不可能的事情。

如何解决这个问题：在创建带有空签名的PDF文件时做一个更好的估计。