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Say I start with a 26-letter keyword and convert the letters into 26 integers (A=0, B=1...Z=25) nr[0] to nr[25]. Then I create a stream, nr[26] onwards, with a lagged fibonacci sequence where nr[n]= (sum of nr[n-26] to nr[n-1] )mod 26. And I use the stream to encipher a brief message (no longer than 200 letters) in such a way that the stream is hidden. How secure is that encipherment?
example of cipher:
ciphertext letter[n] = plaintext letter[n] + key letter[n mod 26] + nr[n+26]
Your key only seems to consist of n, which should be easily brute forced. So not very secure is the only answer that can be given. n could be very large (a 128 bit number or so), but in that case this scheme seems very inefficient.
Please note the remarks on this kind of generator here and here. So even if it is secure, it may be very tricky to show that it is secure (for a large n). There are plenty of better stream ciphers to choose from.
