Is there a way to recover the string that was signed having access to both the signature and the private key?

Question

As the title says, I have both the signature and the private key and would like to know the string that generated the signature.

To sign my string I would do this:

$ echo 'data to sign' > data.txt
$ openssl dgst -sha1 -sign privateKey.pem -out data.sha1 data.txt
$ openssl enc -base64 -in data.sha1 -out data.b64 -A

And I end up with a 172 bytes signature.

What I would like to know is if there's a way to reverse the process.

Thank you.

Answer 1

You wont be able to do this. A Hash such as SHA1 is a one way type of encryption. Once the encryption is done its done and there is no going back.

Answer 2

Basically no, but if you know the string was relatively short and had some defined structure, you could brute force it until you get a matching hash value. For long and random strings it is impossible.