# How many possible keys are in a columnar transposition cipher?

In a book I am reading it says that the message "Common sense is not so common." (which has 30 characters, space and punctuation included) has a range of possible keys of 2 to 15, half the message size.

I don't understand why? I know key 1 will encrypt the message as the plain text because of the encryption algorithm as will key 30, but if you see bellow there are more than 15 keys where the message is still 'encrypted'

Key #0: |

Key #1: Common sense is not so common.|

Key #2: Cmo es snts omnomnsnei o ocmo.|

Key #3: Cm n tooooossin mnmneesoscm.|

Key #4: Coe nsononnioom.m sst mmse co|

Key #5: Cns smo enomms o omeitcnons o.|

Key #6: C toosi mmessmmn ooosn nneoc.|

Key #7: Cssonoe .mnncmsoooetmn m iso|

Key #8: Cenoonommstmme oo snnio. s s c|

Key #9: Cntoos nmes.m ooi nsc osnmeom|

Key #10: Cssoeom micoson m nmsooetnn .|

Key #11: Ce o cmiomsmo mnno onst.e nsso|

Key #12: C ooimmsmm oonnno. ts esnos ec|

Key #13: Cimosom nmn.oont sseon sceo m|

Key #14: Csno .mnmootn ssoe ncsoem mio|

Key #15: C onmomto ns os ecnosmem oins.|

Key #16: Cnoomtm osno sceonmsmeo ni.s |

Key #17: Cootm msoon csoemnmsoen .is n|

Key #18: Cto msmoo nc osmemnosne. is no|

Key #19: C osmom ocno msmeonns.e is not|

Key #20: Csoom mcoonm msoenn.se is not |

Key #21: Coo mcmoomnm osne.nse is not s|

Key #22: C ocmommomno ns.ense is not so|

Key #23: Ccoommmmoonn .sense is not so |

Key #24: Coommmmoonn. sense is not so c|

Key #25: Cmommomno.n sense is not so co|

Key #26: Cmoomnm.on sense is not so com|

Key #27: Coonm.mon sense is not so comm|

Key #28: Cno.mmon sense is not so commo|

Key #29: C.ommon sense is not so common|