Geffe generator
Three LFSRs combined through a multiplexer-like switch, which still leaks information about two of them.
The Geffe generator was described by Philip Geffe in 1973 and uses three LFSRs. Register x2 chooses whether the output follows x1 or x3, following the formula (x1 AND x2) XOR (NOT x2 AND x3). The linear complexity looks good on paper, but the output agrees with x1 and with x3 in three cases out of four. A correlation attack can therefore target one register at a time instead of searching the entire key space in one go.