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Ransomware Trojan code break 'impractical'

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Updated A cryptographic expert has questioned the practicality of a code breaking initiative geared to cracking the key used in the dangerous Gpcode-AK ransomware virus.

Gpcode-AK encrypts content on compromised PCs using a 1024-bit RSA key. In response, Russian net security firm Kaspersky Labs launched an international code-breaking initiative dubbed Stop Gpcode. It wants cryptography experts, other antivirus vendors and independent researchers to get involved in an effort to combat the mendacious malware.

Kaspersky reckons it would take approximately 15 million PCs, running for about a year, to crack the code.

But cryptographic expert Bruce Schneier argues that the effort is impractical, misguided and little better than a publicity stunt.

"We've never factored a 1024-bit number - at least, not outside any secret government agency - and it's likely to require a lot more than 15 million computer years of work. The current factoring record is a 1023-bit number, but it was a special number that's easier to factor than a product-of-two-primes number used in RSA," he writes.

"Breaking that Gpcode key will take a lot more mathematical prowess than you can reasonably expect to find by asking nicely on the internet," he adds.

Kaspersky said that such criticisms might be valid of a brute force attack but of uncovering weaknesses in the cryptographic techniques used by GPcode's anonymous authors.

"The initiative launched with Aleks Gostev's blog post isn't intended to crack 1024-bit encryption per se. Rather, our aim is to find a way of cracking Gpcode's implementation of it," David Emm, senior technology consultant at Kaspersky Lab explained.

"This is a much more realistic proposition; it may be that, as in previous variants, the implementation is flawed. That's what we're looking for; and that's what we've invited the wider community to help us with," he added.

Cyber-extortion

Ransomware strains of malware have been seen before, notably the earlier example of the Gpcode virus. Previous versions featured shorter keys and therefore weaker encryption.

The latest version of the virus uses a 1024-bit key, making the task of cracking the key particularly onerous. Cryptographic analysis by Kaspersky researchers thus far has broken key lengths of up to 660 bits. Crooks made errors in implementing the encryption algorithm in those cases, reducing the difficulty of cracking the code.

Strains of the Gpcode virus scramble user files of particular types (.doc, .txt, .pdf, .xls, .jpg, .png, .cpp, .h etc.) using an RSA encryption algorithm with varying bit lengths. After rendering ciphertext files unreadable the malware generates a message telling victims that their files have been encrypted in an attempt to extort payment for a decryption utility.

As Kaspersky Labs notes, backing up data and keeping up to date with security safeguards is the best defence against the malware. ®

Internet Security Threat Report 2014

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