We consider data confidentiality in a distributed detection scenario with a type-based multiple-access (TBMA) protocol where a large set of sensors sends local measurements to an ally fusion center (FC) over an insecure wireless medium called the main channel. Then, the ally FC makes a final decision to the physical environment. Although many wireless sensor networks are mission-specific and need data confidentiality due to the broadcast nature of wireless transmission, it can be easily wiretapped by unauthorized enemy FCs through eavesdropping channels. We propose a novel TBMA protocol called secure TBMA which provides data confidentiality by taking advantage of inherent properties of wireless channels, namely randomness and independence of the main and eavesdropping channels. In particular, the secure TBMA activates sensors having strong and weak main channel gains and makes the sensors follow different reporting rules based on the magnitudes of their channel gains. The reporting rules are carefully designed to confuse the enemy FC. The proposed secure TBMA delivers unconditional/perfect secrecy and does not assume any superiority of the ally FC over the enemy FC in terms of computational capability, secret key, and so on. For Rayleigh fading channels, we analyze the performance of the secure TBMA at both enemy and ally FCs by investigating conditions for perfect secrecy and an error exponent of detection error probability, respectively. On the one hand, the analysis at the enemy FC provides a design criterion of the reporting rules to achieve perfect secrecy. On the other hand, the analysis of the error exponent carried out with a Gaussian approximation shows that perfect secrecy is achievable at a marginal cost in detection error performance. All our claims are also verified with simulation results which have good matches with the analysis.