This sounds like a kind of fractionated transposition cipher.
My understanding is that some kinds of transposition-alone ciphers can be more difficult to crack than double-Playfair (WGBH 2000),
and fractionated transposition ciphers, such as ADFGVX Cipher
are considered more difficult to crack than transposition-alone ciphers
As others have pointed out, it is trivially easy, if you encode the letters using ASCII, to rule out some possible messages -- messages that have a different Hamming weight than the (transposed) ciphertext.
However, it is in principle for transposition alone (with other kinds of encoding) to give (with sufficiently unguessable shuffling) "perfect security", although this doesn't seem useful in practice.
(Because other, apparently simpler techniques also give "perfect security";
and because we find in practice that we're willing to give up perfect security in order to make key distribution easier).
For example: algorithm A:
- Ahead of time, Alice and Bob share one-time pads of random numbers.
- Alice encodes a message into a series of 40 character lines using some constant-weight code (every possible message in that encoding has exactly the same Hamming weight).
- Alice rearranges the bits in each encoded line with a Fisher-Yates shuffle to fairly permute all the bits in the 40-character line (thank you, Luis Casillas), using the numbers from her one-time transmit pad to decide which of all possible shuffles to use.
- Alice sends a series of lines to Bob, and destroys the used pages of her one-time transmit pad.
- Bob unshuffles the bits in each line of text using his one-time receive pad, then decodes each line into 40 characters of plaintext, and destroyed the used pages of his one-time receive pad.
From any possible ciphertext generated by this algorithm A,
a cryptanalyst can "decrypt" any possible message with the same number of lines of text, simply by using a different one-time pad, so algorithm A has perfect security.
(A particular ciphertext might represent "YEAH", "NOPE", "HAHA", "GOGO", etc.)
(Alas, this uses up the one-time pads of numbers much faster than enciphering the same messages with traditional XOR encryption one-time pad, which also has "perfect security").
My understanding is that a fixed, publicly-known transposition could be used in the inner block of a Even-Mansour cipher.
Another kind of encoding that seems better than ASCII for transposition ciphers is: encoding text into a complete deck of cards,
as suggested by Tim Warriner
and Lee J Haywood.