記一次jsch客戶端與sshd服務端建鏈異常

以下異常，提示加密算法不知足密鑰尺寸。 

從外網找到緣由以下，可能會是下面的。jdk兩端版本不一致可能會報此類錯誤。但實際上我是用本地同樣的jdk都是1.8-u191版本，仔細看一下，報錯的線程棧打印的是DSA的加密算法，隨後改了下服務端使用RSA並指定密鑰長度爲2048，以後就能夠了。

The error message means, that the key likely has 1024 bits strength, because it had been generated with any JDK version before JDK 8u151 - while at least 2048 bits key-strength are being expected:

The security strength of SHA-1 digest algorithm is not sufficient for this key size.

The reason behind this is, that the default key-size had changed from 1024 to 2048 bits with JDK 8u151. This change can also be found in the JRE/JDK crypto roadmap at datecode 2017-10-17:

Upgraded the jarsigner and keytool ‑sigalg default to SHA256withDSA and the ‑keysize default to 2048 for DSA keys.

The "Reverting Instructions" would be (which do not really apply for signage with Android Studio):

To use a different algorithm and/or smaller key size use the ‑sigalg and ‑keysize options of keytool and jarsigner to override the current defaults. Before this change the default values were SHA1withDSA and 1024 bits.

[org.apache.sshd.server.session.ServerSession] : Exception caught
java.security.InvalidKeyException: The security strength of SHA-1 digest algorithm is not sufficient for this key size
	at sun.security.provider.DSA.checkKey(DSA.java:111)
	at sun.security.provider.DSA.engineInitSign(DSA.java:143)
	at java.security.Signature$Delegate.engineInitSign(Signature.java:1177)
	at java.security.Signature.initSign(Signature.java:530)
	at org.apache.sshd.common.signature.AbstractSignature.init(AbstractSignature.java:47)
	at org.apache.sshd.server.kex.AbstractDHGServer.next(AbstractDHGServer.java:93)
	at org.apache.sshd.server.session.ServerSession.handleMessage(ServerSession.java:177)
	at org.apache.sshd.common.session.AbstractSession.decode(AbstractSession.java:587)
	at org.apache.sshd.common.session.AbstractSession.messageReceived(AbstractSession.java:253)
	at org.apache.sshd.common.AbstractSessionIoHandler.messageReceived(AbstractSessionIoHandler.java:54)
	at org.apache.sshd.common.io.nio2.Nio2Session$2.completed(Nio2Session.java:231)
	at org.apache.sshd.common.io.nio2.Nio2Session$2.completed(Nio2Session.java:217)
	at sun.nio.ch.Invoker.invokeUnchecked(Invoker.java:126)
	at sun.nio.ch.Invoker$2.run(Invoker.java:218)
	at sun.nio.ch.AsynchronousChannelGroupImpl$1.run(AsynchronousChannelGroupImpl.java:112)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)