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Message - first 4 bits.
In the 5th bit, we write 0 if the first 2 are 00 or 01, and 1 if they are 10 or 11.
In the 6th bit, we write 0 if the 3rd and 4th are 00 or 01, and 1 if they are 10 or 11.
In the 7th bit, we write 0 if the 5th and 6th are 00 or 01, and 1 if they are 10 or 11.
Let's say the message is 1001. We append - 1001101.
If nothing is corrupted, everything matches.
If the first bit of the message is corrupted, it will be visible because it won't match the 5th bit. At the same time, the 5th matches the 7th.
And if the 5th bit is corrupted, it won't match the 7th. And also not the first and second.
If the 7th is corrupted, it won't match the 5th and 6th.
If the 6th is corrupted, it won't match the 3rd, 4th, and 7th.
....
In general, I haven't checked all the options, but there's a chance that this is correct; I couldn't come up with anything better.
Let's say he wants to transmit the combination 0101. Then, according to your algorithm, the fifth bit is 0, the sixth is 0, the seventh is 0, and the entire message is 0101000. How can you tell, for example, that this is 0101, where no bit has been corrupted, and not 0001 or 0100, where the second (fourth) bit has been corrupted? After all, for 0001, the combination will be 0001000, and for 0100 - 0100000.
Всё не так плохо как Вы думаете. Всё намного хуже!