(*  Decrypt Vignere cipher  *)

  (*  cipher key to numbers  *)
kText = ToCharacterCode("Christmas");

  (*  ciphertext to numbers  *)
cTextRaw = ToCharacterCode("of eiex us");

    (*  get system value for " " symbol  *)
spaceCharCode = Part(ToCharacterCode(" "),-1);

  (*  find the spaces  *)
wordBreaks =  Flatten(Position(cTextRaw,spaceCharCode));

  (*  drop all spaces  *)
cTextCook = Cases(cTextRaw, x_ :> x/; x!= spaceCharCode);

  (*  repeat cipher key so it is longer than ciphertext  *)
kTextPad = Flatten(Table(kText, IntegerPart(Length(cTextCook)/Length(kText))+1));

  (*  make same length as ciphertext for list addition  *)
kTextPad = Part(kTextPad, 1;; Length(cTextCook));

  (*  convert cipher key into offsets to ASCII alphabet  *)
kTextPadOffset = Times(-1,Map(Function(x, Which(x>=65 && x <= 90, x - 65,
   x>=97 && x <= 122, x - 97, true,0)), kTextPad));

  (*  convert ciphertext into offsets to ASCII alphabet  *)
cTextCookOffset = Map(Function(x, Which(x>=65 && x <= 90, x - 65,
   x>=97 && x <= 122, x - 97, true,0)), cTextCook);

  (*  add offsets and perform modulo arithmetic  *)
alphaStart = Part(ToCharacterCode("a"), -1);
pText = Replace(cTextCookOffset + kTextPadOffset, x_ :> alphaStart + Mod(x + 26, 26));

  (*  insert word-breaks  *)
pString = FromCharacterCode(pText);
Do( pString = StringInsert(pString," ", k), {k, wordBreaks});

  (*  plain text  *)
pString