General Report on Tunny

22A Page 45


22A Introductory
22B The Chi Stream
22C The Motor Stream
22D The Psi Stream
22E The Sum of Two Streams
22F The Key Stream
22G The Plain Language Stream
22H The De-chi Stream
22J The Cipher Stream
22K Sampling Errors in Alphabetical Counts
22W Some further Streams
22X The Algebra of Proportional Bulges
22Y The Amount of Evidence derived from a Letter Count


Statistical methods of Tunny breaking are possible because (and only because) cipher, plain, key, chi, extended psi, de-chi and motor streams can - with suitable treatment - be made to exhibit marked characteristics which will distinguish them from a random sequence of letters. In this chapter we analyse these characteristics, and in subsequent chapters we show how they are exhibited.

(a) Notation.

The letters Z, P, K, Χ, Ψ', D are used to denote the operative letters of the Cipher, Plain, Key, Chi, Extended psi and de-chi Streams at any given ciphering position. They are connected by the equations:

Z = P + K        
K = Χ + Ψ'        
D = Z + Χ = P + Ψ'

The suffixes 1, 2, 3, 4, 5 are used when a particular impulse is specified so that (using a generalised form) Ui denotes the operative

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