Biba model
The integrity counterpart to Bell-LaPadula: No Read Down and No Write Up stop trusted data from being contaminated by less trustworthy sources. Confidentiality is out of scope.
Full guide: Biba Model: No Read Down, No Write Up (CISSP)
Biba is the integrity model, and the fastest way to hold it is as Bell-LaPadula inverted. Where Bell-LaPadula stops secrets flowing downward, Biba stops contamination flowing upward. Subjects and objects carry integrity levels, and two rules govern the flow: No Read Down (the Simple Integrity Property) stops a high-integrity subject consuming low-integrity data, and No Write Up (the Star Integrity Property) stops a low-integrity subject corrupting high-integrity data.
The intuition: a surgeon should not base a decision on gossip (no read down), and a random blog should not be able to edit the pharmacy formulary (no write up). Biba says nothing about confidentiality, and it addresses only the first of the three integrity goals; preventing unauthorised modification is its territory, while Clark-Wilson goes further into consistency and well-formed change. The full walkthrough is in the Biba guide.
Exam relevance: the most common confusion is swapping Biba’s rules with Bell-LaPadula’s. Anchor on the goal first: disclosure questions point to Bell-LaPadula, modification questions point to Biba, and the rule directions follow from the goal.