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Why should one not use the same asymmetric key for encryption as they do for signing?

I'm new to IT Security. I still can't figure out why using the same RSA key pair for both (signing and encryption) & (signing and encryption) is a bad idea?

Most people talk about why we should not use same key pair for signing and encryption, but what about signing and decryption?

  • Can you reread your question? Your sentences look a bit mangled. – CodesInChaos May 13 '12 at 19:55
  • @CodeInChaos Yes, I wasn't sure what to make of those, so I left them as is for the OP – jonsca May 14 '12 at 2:05
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There's no technical or cryptography-based reason why you wouldn't want to use the same key for both. The only reasons why are:

  • it limits the exposure of your keys -- i.e. you don't have to have the same key available to the people who perform what may be separate tasks
  • ancillary legal reasons -- there might be legal requirements around electronic signatures in your country
  • damage control in the case of compromise -- i.e. if your private key gets out, then the less that key is useful for the fewer problems it will cause
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