= BB84 intercept-resend error rate
{c}
{title2=$Q=q/4$}
In ideal <BB84> with uniform independent basis choices, intercepting a fraction $q$ of signals, measuring in a random protocol basis and resending that result introduces sifted-key <bit error rate> $q/4$. Conditional on a retained position, the attacker chose the wrong basis with probability $1/2$; then the receiver disagrees with the original bit with probability $1/2$. The calculation describes one particular attack and does not establish general security.
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