Ment
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I came up with something else based on the ideas mentioned above, but I checked it with a program, and it seems to fail somewhere.
bit1=xor(i1,i2)
bit2=xor(i1,i3)
bit3=xor(i1,i4)
bit4=xor(i2,i3)
bit5=xor(i2,i4)
bit6=xor(i3,i4)
bit7=i1
It failed for 1001 and 0110.
1001:
0011001
0110:
0011000
If the Germans mess up the last bit in one version, they will match. Too bad... Well, I tried too.