Preserving the integrity of cyclic-redundancy checks when protected text is intentionally altered

As a digitally encoded message traverses a series of point-to-point communication links, it may be necessary to change the contents of that message at an intermediate station. If bit errors are introduced by the intermediary while the text is unprotected, these errors will be subsequently undetectable by cyclic redundancy checks. An algorithm is presented here for ensuring that such errors will not go undetected. Since the cyclic redundancy check is based on a linear mathematical operation, the frame-check sequence may be modified, rather than recalculated, by each intermediary changing the protected text. A frame-check sequence constructed in this way will reveal any errors introduced in the transmission path when the message is finally examined at the ultimate destination. Examples of the proposed technique applied to various local-area network bridges are developed. The technique is shown to be beneficial in these examples when the internal bit-error ratio of the text-changing device exceeds 10
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on unprotected paths.