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Given a valid, signed X.509 certificate using the MD5 digest algorithm and an RSA public key, how feasible would it be to generate a public key which would, all other parts of the certificate being identical, not invalidate the existing digital signature when substituted into the certificate?

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Given an existing certificate, it isn’t practical.

MD5 is preimage-resistant, so it is not practical (given $x, y: x = \text{MD5}(y)$) to find $z: z \neq y \land \text{MD5}(z) = x$

But MD5 is still a TERRIBLE choice

MD5 allows for trivial collisions. This has been used by malware to forge code-signing certificates.

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