Pdf generation iText数字签名异常：无法恢复密钥？

我正在运行从中提取的代码，但我得到以下异常

Exception in thread "main" java.security.UnrecoverableKeyException: Cannot recover key
    at sun.security.provider.KeyProtector.recover(Unknown Source)
    at sun.security.provider.JavaKeyStore.engineGetKey(Unknown Source)
    at sun.security.provider.JavaKeyStore$JKS.engineGetKey(Unknown Source)
    at java.security.KeyStore.getKey(Unknown Source)
    at com.seind.pdf.digitalsignature.SimpleSignature.main(SimpleSignature.java:89)

我正在使用itext 5.3.4.jar，bcpkix-jdk15on-1.47.jar，bcprov-jdk15on-1.47.jar

我使用以下命令创建密钥存储 $keytool-genkey-alias foobar-keyalg RSA-keystore.keystore

有人能帮我找出异常的原因吗