To a situation where fraud requires the collusion of

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to a situation where fraud requires the collusion of 2 or more members of staff, otherwise known as "shared control". This principle is extended in banking to ensure that one mem- ber of staff doesn't have too much influence over the systems that keep track of what they do. Giving Nick Leeson man- agement control over the Barings Bank Singapore dealing room and back office operations at the same time violated this principle. The events leading up to this and the con- sequent collapse of Barings Bank was described in the film "Rogue Trader".
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The Clark-Wilson integrity model This is based on an analysis of the procedures adopted by the banking industry based upon the concepts de- scribed above, formulated into a set of rules. The Bell-La Padua model relevant to Multi-Level Se- curity is primarily concerned with information con- fidentiality. The Clark Wilson model (CWM) is con- cerned with information integrity.
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Clark Wilson model terms: UDI, CDI, TP UDI - Unconstrained Data Item, e.g. an input to the system prior to authentication and validation. CDI - Constrained Data Item e.g. a validated and au- thenticated input the processing of which maintains accounting balance. TP - Transformation Procedure. A means of trans- forming input data to output CDI which maintains the integrity of CDIs and which write enough informa- tion to an append-only CDI (audit trail) to enable the transaction to be reconstructed.
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Clark Wilson model terms: IVP, user, triple IVP - Integrity Verification Procedure - a procedure used to check the validity of a CDI e.g. that books balance. user - a subject or an agent such as a bank clerk, ATM engineer, forex dealer systems programmer, se- curity officer, typically having insider access. triple - Access control is by means of triples: (user,TP,CDI) so that shared control is enforced.
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Clark Wilson rules C1, C2 source: The model consists of two sets of rules: Certification Rules (C) and Enforcement Rules (E). The nine rules ensure the external and internal integrity of the data items. To paraphrase these: C1 - When an IVP is executed, it must ensure the CDIs are valid. C2 - For some associated set of CDIs, a TP must transform those CDIs from one valid state to anoth- er.
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Clark Wilson rules E1, E2 source: Since we must make sure that these TPs are certified to operate on a particular CDI, we must have E1 and E2. E1 - System must maintain a list of certified rela- tions and ensure only TPs certified to run on a CDI change that CDI. E2 - System must associate a user with each TP and set of CDIs. The TP may access the CDI on behalf of the user if it is "legal".
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Clark Wilson rules C3, E3 source: This requires keeping track of triples (user, TP, {CDIs}) called "allowed relations".
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Christopher Reinemann
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