I have a constant SALT that is appended to every cookie before it is encrypted with sha512. If I know the clear text and the final encrypted values of more than one cookie, is it possible to use a tool like john the ripper to guess the salt value?
The answers I found on the internet talk about finding the password, but i'm interested in finding the salt.
Short Answer:
No you can't.
Reasons:
First of all sha512 is a hashfunction. You can't "decrypt" hashfunctions. If it would be able to do this, sha512 would not be safe.
Days ago google found the first collision in 6,610 CPU-Years.
Source: First sha512 Hashcollsion
(This is not an attack!) They used an amount of distributed systems. So a normal program like john-the-ripper wouldn't be able to do this.