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2d
comment Attacking unuauthenticated RSA
@HocusPocus, I agree with your answer completely, up to the point where you say "thinking it comes from Alice". Bob can't assume anything about the sender of the message, unless RSA is used in the context of a protocol (such as SSH or TLS, whereby you would validate the peer based on a mutual certificate, password, etc), PKI envelope, etc. I think it's useful to think about practical attacks against real systems that employ asymmetric crypto. Your attack is practical, but misses the part about why it would work. (that is, why the key is unauthentic and yet trusted anyway.)
2d
answered Attacking unuauthenticated RSA
2d
comment Attacking unuauthenticated RSA
This isn't how you'd go about attacking RSA. You don't encrypt a message using your own public key and send it to someone else. You encrypt it with their public key. The key question is "how do you know you can trust the other peer's public key?". Without authentication, you could be sending your message using the attacker's public key. (example: that's why the first time you SSH to a host, you are prompted to confirm that you trust the public key. If an attacker intercepted the communication using their unverified/unauthenticated key, they could intercept your session.)
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