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Hashing exploit threatens digital security

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Cryptographers have found a way to snip a digital signature from one document and attach it to a fraudulent document without invalidating the signature and giving the fraud away.

The development 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.

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.

But now Stefan Lucks of the University of Mannheim and Magnus Daum of the Ruhr-University, Bochum, both in Germany, have come up with a way to create two documents that both have the same digital signature.

The attack exploits recently discovered holes in a type of publicly available algorithm called a 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.

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