Re: [JSch-users] SignatureDSA: incorrect conversion from mpint to ASN.1

[Jurrie O. <jsc...@ju...>]

On 03-09-17 20:20, Jurrie Overgoor wrote:
> We are using Apache VFS, which in turn uses JSCH to connect to SFTP. 
> Recently, my team and I stumbled upon a bug in JSCH which presents 
> itself when using Java 8 and SHA1withDSA. We are using JSCH 0.1.53, 
> but I tried 0.1.54 and the bug is still present. Other users appear to 
> be hitting this bug as well: https://sourceforge.net/p/jsch/bugs/111/
>
> I traced the problem down to SignatureDSA.java, which does conversion 
> from ASN.1 to mpint and from mpint to ASN.1. When verifying a hash, a 
> conversion from mpint to ASN.1 is done, just before the hash is passed 
> on to Java to actually do the verification. In the conversion from 
> mpint to ASN.1 I believe things are not done correctly. Leading 0x00 
> bytes are not stripped from the ASN.1 INTEGER as they should be. 
> Please note that Java 6 allows leading 0s, but Java 7 and 8 do not. 
> So, an IOException is thrown with message "Invalid encoding: redundant 
> leading 0s".
>
> So, when either r or s are integers that can be represented in less 
> than 20 bytes, we see redundant leading 0s. Sadly, r and s are 
> dependent on a random value k. So I whipped up a quick test that does 
> signatureDSA.sign() and signatureDSA.verify() over and over again. I 
> can reproduce the problem this way. I can not seem to trigger an error 
> when using SignatureRSA (representing SHA1withRSA) or SignatureECDSA 
> (representing SHA256withECDSA).
>
> Now, as I'm by no means an expert on this matter, I'd like to ask this 
> list if my observations are correct :)
>
> I made changes to SignatureDSA and am currently testing. Things seem 
> to work fine now. I can try to create a patch if it would be handy?

Hello everyone,

My alternative implementation of SignatureDSA.java is available as a 
Maven artifact. It's available at https://github.com/Jurrie/jsch-111-bugfix
Please let me know if it helped you, or if something is wrong.

With kind regards,

Jurrie