There is apparently a provably secure cipher that was proposed by Diffie but enhanced by another cryptographer that works like this:

 1. Measure the length of the plain-text, n.
 2. Multiply it by 128.
 3. Generate this much real random data and split it out in to 128 byte arrays of equal length to the plain-text. This can be thought as a two dimensional array:
    1. One of the indices gives the sequence number (0-127)
    2. One of the indices gives the position in the sequence, 0 to n-1.
 4. Use a 128-bit key to choose which of these streams to XOR together. Each bit of the key corresponds to "yes/no" on whether to use particular sequence. All the selected sequences are XORed together to make a single keystream, K.
 5. Compute P XOR K to give the cipher-text C.
 6. Serialize the two dimensional array and append to the cipher text.
 7. Send the whole package to Bob who can then decrypt by de-serializing the matrix and selecting the same rows.

Apparently, this scheme is completely secure. The attacker has to examine every possible combination of sequences (2^127 on average) in order to break the encryption scheme.

What is the proof of this? I can't find the paper that discusses this anywhere?