Two questions about applet security

I have two questions about applet security.
1. Can an unsigned JAR be trusted by Java Plugin?
2. A JAR is signed. If I add/delete/modify some files to/from/in it, can it stilly be verified?
Thanks.

for applet to be trusted.
it HAS TO BE Digitally signed.
unsigned applet is NOT TRUSTED
if a signed jar is modified, it can be verified..but verification process will fail ..data integreity check is done by public/private key algo

What do you mean "verification process will fail"?
Do you mean if I use "jarsigner -verify a.jar", some failure infomation will appear?

For example, if someone has exploded a signed JAR file, has modified
one of its files, and has rebuilt the JAR file, when we try to verify the
signature using jarsigner, we get the following error message:

jarsigner: java.lang.SecurityException: SHA1 digest error for
itso.class

Originally posted by sowmya thiru:
For example, if someone has exploded a signed JAR file, has modified
one of its files, and has rebuilt the JAR file, when we try to verify the
signature using jarsigner, we get the following error message:

jarsigner: java.lang.SecurityException: SHA1 digest error for
itso.class

That's true.
But if I ADD or DELETE files in the JAR, the verification still succeeds.

Originally posted by Leo Liao:

But if I ADD or DELETE files in the JAR, the verification still succeeds.

I believe the signature is on a per-file bases (files in the Jar).
Any files that are unchanged after signing the JAR should still verify
successfully even if other files were added or deleted.
The added files would not verify unless the Jar is signed again after the Add. (I'm too lazy to try this out, however.