In this digital age, security of online documents and web connections is paramount. Digital signatures are used to authenticate website connections, emails and legal documents in some countries. They work because they are unique to the file or software that is signed, as they are created from the contents of the signed file. Therefore, if someone tries to cut a digital signature from one document and stick it to another, the signature fails because it no longer matches the document. However, recently exposed cracks in the digital signature algorithm make it possible for someone to extract a signature from one file, and use it with another! This means that attackers could potentially forge legal documents, load certified software with bogus code, or turn a digitally-signed letter of recommendation into one that authorises access to private information.

How Digital Signature Works (Courtesy: Microsoft)The signature is generated using a public algorithm, called the Hash function. These algorithms convert a digital file into a fixed-length string of bits (made up of “0”s and “1”s) called a hash, which is considered unique. The hash is then bound up with the digital signatory’s key to generate their signature. The signature is verified by a trusted third party that removes the key and compares the remaining number with a hash of the document.
Cracks first appeared last year, when Xiaoyun Wang and colleagues at the Shandong University of Technology in China generated two documents that had the same MD5 signature. In February 2005 Wang demonstrated the same thing - called a collision - but with the US Government’s gold-standard algorithm SHA-1, which was considered more secure than MD-5!!
Stefan Lucks of the University of Mannheim and Magnus Daum of the Ruhr-University, Bochum, both in Germany, combined Wang’s work with a clever trick in order to produce two meaningful documents with the same hash function. They used a capability in a file-type known as postscript, which is similar to the PDF format. Postscript allowed them to bind up two documents in the same file, but to reveal only one document and hide the other, and vice versa, without changing the hash of the whole file:)).
According to Dan Kaminsky, an independent security consultant based in Seattle, Washington, It’s not the end of the world yet, but we need to stop using MD-5 and SHA-1 before it is!