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RSA as implemented by OpenSSL et al. needs to protect against side channel attacks, at a big performance penalty.

However, TLS certificate validation involves no secrets. Therefore, I should be able to use a fast variable-time algorithm for this purpose, right?

Is this actually secure, and does anyone actually do this?

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  • $\begingroup$ OpenSSL blinds only RSA privatekey operations (decrypt and sign) precisely because the publickey doesn't need protection. And although I don't have figures I don't believe the penalty is very much: privatekey operations are much more expensive to start with, and since public exponents are usually conventional values like F4=65537 they are basically constanttime already. $\endgroup$ – dave_thompson_085 Mar 11 '16 at 9:02
  • $\begingroup$ TL;DR: Only signature verification involves only public information. Encryption involves a secret plaintext, signature involves a secret key and decryption involves both. $\endgroup$ – SEJPM♦ Mar 11 '16 at 11:25
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Two answers to the question:

  • It is about principles and reusability of the cryptographic primitives. Once there are implemented by insecure manner, nothing prevents reusing or misusing the insecure functionality later.

  • TLS validation (during the SSL handshake) involves a secret as well - during the SSL handshake a piece of data is encrypted and sent over to decrypt to proof ownership of the private key. This is the part when the side-channel attack needs to be mitigated.

Carpe diem

g.

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