1. Ciphers:
    1. Easy [text]
    2. Intermediate [text]
    3. Hard [PDF]
  2. Hints for decrypting the ciphers:
    1. a general discussion on how to break ciphers [PDF]
    2. a brief guide on how to break affine ciphers [PDF]
    3. a table of letter frequencies in English (and other languages) [PDF]
    4. some statistics related to the "Hard" ciphers above [xls]
  3. A useful frequency analysis tool
  4. Miscellaneous – some extra fun material intended for those who may wish to read a bit further:
    1. WWII pigeon message stumps GCHQ decoders
    2. The silent war against the Japanese Navy
    3. Simon Singh explaining the Enigma Machine
    4. JN25 Japanese Codes [PDF]
    5. How did the NSA hack our emails?‚Äč