It's trivial to rewrite plaintext on the fly and the number of people in a position to do so is staggering. Like passing a note in class, it's basically about trust. You want to ensure the message hasn't been tampered with by the time it reaches you. HTTPS is one way, even if the recent past has backed it into a tiny corner (RIP SSLv2, SSLv3, TLSv1.0 and a long list of cipher suites).
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It's trivial to rewrite plaintext on the fly and the number of people in a position to do so is staggering. Like passing a note in class, it's basically about trust. You want to ensure the message hasn't been tampered with by the time it reaches you. HTTPS is one way, even if the recent past has backed it into a tiny corner (RIP SSLv2, SSLv3, TLSv1.0 and a long list of cipher suites).