Microsoft is banning certain cryptographic functions from new computer code, citing increasingly sophisticated attacks that make them less secure, according to a company executive.

The Redmond, Wash., software company instituted a new policy for all developers that bans functions using the DES, MD4, MD5 and, in some cases, the SHA1 encryption algorithm, which is becoming "creaky at the edges," said Michael Howard, senior security program manager at the company, Howard said.

MD4 and MD5 are instances of the Message Digest algorithm that was developed at MIT in the early 1990s and uses a cryptographic hash function to verify the integrity of data.

The algorithms are used to create digital signatures and check the integrity of information passed within Microsoft Corp.'s products.

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DES (Data Encryption Standard) is a cipher that is used to encrypt information that is used in many networking protocols.

All three algorithms show signs of "extreme weakness" and have been banned, Howard said.

Microsoft is recommending using the Secure Hash Algorithm (SHA)256 encryption algorithm and AES (Advanced Encryption Standard) cipher instead, he said.

The change is part of a semi-yearly update to Microsoft's Secure Development Lifecycle policies by engineers within Microsoft's Security Business & Technology Unit.

Developers who use one of the banned cryptographic functions in new code will have it flagged by automated code scanning tools and will be asked to update the function to something more secure, Howard said.

Eventually, the company will also remove vulnerable cryptographic functions from older code, though that will take longer, he said.

"Threats are constantly evolving, so it's important to stay one step ahead," he said.

"It's about time," added Bruce Schneier of Counterpane Security Inc.

Microsoft should have ended use of DES, MD4 and MD5 "years ago," and is only being prudent in doing so now, Schneier said.

However, the company's "case by case" approach to banning SHA1 is more aggressive, considering that theoretical attacks on that algorithm only appeared in February, Schneier said.

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The theoretical attacks on SHA0 and SHA1 were developed by Chinese researchers and have some experts predicting that those algorithms will soon be considered too vulnerable to rely on.

The NIST (National Institute of Standards and Technology) has scheduled a workshop in October to discuss alternatives to SHA1.

Using vulnerable encryption algorithms could expose sensitive data in Microsoft systems. But attacks on those algorithms are still unlikely, given other, easier to exploit holes in the software, Schneier said.

"There's just so much that's worse," he said of the other security holes.