 Intercept-resend attack

An eavesdropper measures quantum carriers and forwards states consistent with the outcomes. The resulting disturbance is tested statistically by the legitimate users. For a singlet measured by Eve on both sides along ${\mathbf{n}}_a,{\mathbf{n}}_b$, the forwarded correlation is $-(\mathbf{a}\cdot {\mathbf{n}}_a)({\mathbf{n}}_a\cdot {\mathbf{n}}_b)(\mathbf{b}\cdot {\mathbf{n}}_b)$. It follows by averaging the product of the conditional local outcome means over Eve's correlated outcomes. This model does not cover every general quantum attack.