: Provide application processes with strong agreement guarantees despite failures is a fundamental problem of fault-tolerant distributed computing. Correct processes have not to be "polluted" by the erroneous behavior of faulty processes. This paper considers the consensus agreement problem in a setting where some processes can behave arbitrarily (Byzantine behavior). In such a context it is possible that Byzantine processes collude to direct the correct processes to decide on a "bad" value (a value proposed only by faulty processes). The paper has several contributions. It presents a family of consensus algorithms in which no bad value is ever decided by correct processes. These processes always decide a value they have proposed (and this is always the case when they all propose the same value) or a default value . These algorithms are called intrusion-free consensus algorithms. To that end, each consensus algorithm is an appropriate underlying broadcast algorithm....