README.md in trackler-2.2.1.27

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 # Simple Cipher
 
-Implement a simple shift cipher like Caesar and a more secure substitution cipher.
+Implement a simple shift cipher like Caesar and a more secure
+substitution cipher.
 
 ## Step 1
 
-"If he had anything confidential to say, he wrote it in cipher, that is,
-by so changing the order of the letters of the alphabet, that not a word
-could be made out. If anyone wishes to decipher these, and get at their
-meaning, he must substitute the fourth letter of the alphabet, namely D,
-for A, and so with the others."
-—Suetonius, Life of Julius Caesar
+"If he had anything confidential to say, he wrote it in cipher, that
+is, by so changing the order of the letters of the alphabet, that not
+a word could be made out. If anyone wishes to decipher these, and get
+at their meaning, he must substitute the fourth letter of the
+alphabet, namely D, for A, and so with the others."  —Suetonius, Life
+of Julius Caesar
 
 Ciphers are very straight-forward algorithms that allow us to render
 text less readable while still allowing easy deciphering. They are
 vulnerable to many forms of cryptoanalysis, but we are lucky that
 generally our little sisters are not cryptoanalysts.
 
 The Caesar Cipher was used for some messages from Julius Caesar that
-were sent afield. Now Caesar knew that the cipher wasn't very good, but
-he had one ally in that respect: almost nobody could read well. So even
-being a couple letters off was sufficient so that people couldn't
+were sent afield. Now Caesar knew that the cipher wasn't very good,
+but he had one ally in that respect: almost nobody could read well. So
+even being a couple letters off was sufficient so that people couldn't
 recognize the few words that they did know.
 
 Your task is to create a simple shift cipher like the Caesar Cipher.
 This image is a great example of the Caesar Cipher:
 
 ![Caesar Cipher][1]
 
 For example:
 
-Giving "iamapandabear" as input to the encode function returns the cipher "ldpdsdqgdehdu". Obscure enough to keep our message secret in transit.
+Giving "iamapandabear" as input to the encode function returns the
+cipher "ldpdsdqgdehdu". Obscure enough to keep our message secret in
+transit.
 
 When "ldpdsdqgdehdu" is put into the decode function it would return
 the original "iamapandabear" letting your friend read your original
 message.
 
+Initially you will implement a [Caesar Cipher][cc] with a fixed shift
+distance of 3 (*namely D, for A*).
+
 ## Step 2
 
-Shift ciphers are no fun though when your kid sister figures it out. Try
-amending the code to allow us to specify a key and use that for the
-shift distance. This is called a substitution cipher.
+Fixed distance Shift Ciphers are no fun though when your kid sister
+figures it out. Try amending the code to allow us to specify a shift
+distance.
 
+You will implement a more generic Shift Cipher with a flexible shift
+distance.
+
+# Step 3
+
+With only 26 true possible shift values, your kid sister will figure
+this out too. Next lets define a more complex cipher using a string as
+key value: a [Vigenère cipher][vc].
+
 Here's an example:
 
-Given the key "aaaaaaaaaaaaaaaaaa", encoding the string "iamapandabear"
-would return the original "iamapandabear".
+Given the key "aaaaaaaaaaaaaaaaaa", encoding the string
+"iamapandabear" would return the original "iamapandabear".
 
 Given the key "ddddddddddddddddd", encoding our string "iamapandabear"
-would return the obscured "lpdsdqgdehdu"
+would return the obscured "ldpdsdqgdehdu"
 
-In the example above, we've set a = 0 for the key value. So when the
-plaintext is added to the key, we end up with the same message coming
-out. So "aaaa" is not an ideal key. But if we set the key to "dddd", we
-would get the same thing as the Caesar Cipher.
+In the example above, we've set a = 0 for all the key values, with a
+shift distance of 0. So when the plaintext is added to the key, we end
+up with the same message coming out. So "aaaa" is not an ideal
+key. But if we set the key to "dddd", we would get the same thing as
+the Shift Cipher with all shift distances set to 4.
 
-## Step 3
+These keys are not much of an improvement over the fixed distance Shift
+Cipher. However, we can put many different lengths into the key if we
+use strings with different characters:
 
-The weakest link in any cipher is the human being. Let's make your
-substitution cipher a little more fault tolerant by providing a source
-of randomness and ensuring that the key is not composed of numbers or
-capital letters.
+Given the key "adadadadad", encoding the string "iamapandabear" would
+return the obscured "idmdpdngaeedr".
 
-If someone doesn't submit a key at all, generate a truly random key of
-at least 100 characters in length, accessible via Cipher#key (the #
-syntax means instance variable)
+Each character in the key is used to shift the corresponding character
+by index. If the key is shorter than the text, repeat the key as
+needed.
 
-If the key submitted has capital letters or numbers, throw an
-ArgumentError with a message to that effect.
-
 ## Extensions
 
 Shift ciphers work by making the text slightly odd, but are vulnerable
 to frequency analysis. Substitution ciphers help that, but are still
-very vulnerable when the key is short or if spaces are preserved. Later
-on you'll see one solution to this problem in the exercise
-"crypto-square".
+very vulnerable when the key is short or if spaces are
+preserved. You'll see one solution to this problem in the exercise
+"[crypto-square](https://github.com/exercism/go/tree/master/exercises/crypto-square)".
 
-If you want to go farther in this field, the questions begin to be about
-how we can exchange keys in a secure way. Take a look at [Diffie-Hellman
-on Wikipedia][dh] for one of the first implementations of this scheme.
+If you want to go farther in this field, the questions begin to be
+about how we can exchange keys in a secure way. Take a look at
+[Diffie-Hellman on Wikipedia][dh] for one of the first implementations
+of this scheme.
 
 [1]: https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Caesar_cipher_left_shift_of_3.svg/320px-Caesar_cipher_left_shift_of_3.svg.png
-[dh]: http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
+[cc]: https://en.wikipedia.org/wiki/Caesar_cipher
+[vc]: https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
+[dh]: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
+
+## Implementation
+
+The definition of the Cipher interface is located in
+[cipher.go](./cipher.go).
+
+Your implementations should conform to the Cipher interface.
+
+```go
+type Cipher interface {
+    Encode(string) string
+    Decode(string) string
+}
+```
+
+It is expected that `Encode` will ignore all values in the string that
+are not A-Za-z, they will not be represented in the output. The output
+will be also normalized to lowercase.
+
+The functions used to obtain the ciphers are:
+
+```go
+func NewCaesar() Cipher { }
+
+func NewShift(distance int) Cipher { }
+
+func NewVigenere(key string) Cipher { }
+```
+
+Argument for `NewShift` must be in the range 1 to 25 or -1 to -25.
+Zero is disallowed.  For invalid arguments `NewShift` returns nil.
+
+Argument for `NewVigenere` must consist of lower case letters a-z
+only.  Values consisting entirely of the letter 'a' are disallowed.
+For invalid arguments `NewVigenere` returns nil.
+
+
 
 ## Running the tests
 
 To run the tests run the command `go test` from within the exercise directory.
 