iText数字签名例外：无法恢复密钥？

Question

我正在运行从此link获取的代码，但我收到以下异常，

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)

1. 我正在使用itext 5.3.4.jar，bcpkix-jdk15on-1.47.jar，bcprov-jdk15on-1.47.jar
2. 使用以下命令创建密钥库 $ keytool -genkey -alias foobar -keyalg RSA -keystore .keystore

任何人都可以帮我确定异常的原因.....