ROT13 and upside-down text (flip text)

1. ROT13

rotN[str_String, n_Integer: 13] := Module[
  {
   rule = Flatten@Normal[
      AssociationThread[#, RotateRight[#, n]] & /@ {
        CharacterRange["a", "z"],
        CharacterRange["A", "Z"]
        }
      ]
   }
  , StringReplace[str, rule]
  ]

Example

rotN["Mathematica Stackexchange, Zngurzngvpn Fgnpxrkpunatr"]

"Zngurzngvpn Fgnpxrkpunatr, Mathematica Stackexchange"

2. Flip text

flipText[str_String] := Module[
  {
   rule = Normal@AssociationThread[
      Characters[
       "abcdefghijklmnopqrstuvwxyz.,ɐqɔpǝɟƃɥıɾʞlɯuodbɹsʇnʌʍxʎz˙'"],
      Characters[
       "ɐqɔpǝɟƃɥıɾʞlɯuodbɹsʇnʌʍxʎz˙'abcdefghijklmnopqrstuvwxyz.,"]
      ]
   },
  StringReplace[StringReverse[ToLowerCase[str]], rule]
  ]

Example

flipText["Mathematica Stackexchange"]

"ǝƃuɐɥɔxǝʞɔɐʇs ɐɔıʇɐɯǝɥʇɐɯ"

keeping it simple the rotation can be done just like this:

Rotate[#, Pi] &@"Hello World"
Rotate[Text[Style[#, FontSize -> 30]], Pi] &@"Hello World"

As a follow up on a comment I made, here's a way I tried to convert from an arbitrary character to a flipped equivalent (which can maybe be extended by someone more knowledgeable about the subject). Unfortunately it isn't so simple to do.

First pull in the base unicode table and write a function that tries to find the version with / without "TURNED", to use as the flipped variant:

If[! StringQ@$unicodeText,
  $unicodeText :=
   $unicodeText = 
    Import["ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt", 
     "Text"]
  ];
If[! AssociationQ@$unicodeTable,
  $unicodeTable :=
   $unicodeTable =
    First@# -> Rest@# & /@ 
      StringSplit[StringSplit[$unicodeText, "\n"], ";"] // 
     Association
  ];

unicodeNameSelect[name_] :=
  Select[$unicodeTable,
   StringMatchQ[First@#, name] &
   ];
unicodeLongFlip[charName_] :=

  With[{chunks = StringSplit[charName]},
   If[MemberQ[chunks, "TURNED"],
    unicodeNameSelect[StringRiffle@DeleteCases[chunks, "TURNED"]],
    Select[
     unicodeNameSelect[
      StringExpression @@ Riffle[chunks, " TURNED " | " "]],
     First@# =!= charName &
     ]
    ]
   ];
unicodeFlip[char : _String | _Integer] :=
 Replace[
  FromCharacterCode@FromDigits[#, 16] & /@ 
   Keys@unicodeLongFlip@CharacterName[char, "UnicodeName"], {
   {f_} :> f,
   {} -> None
   }]

Then we'll write something that tries to flip based on this (but with caching):

If[! AssociationQ@$flipEncoding, $flipEncoding = <||>];
flipEncode[char_String?(StringLength[#] == 1 &)] :=

  Lookup[$flipEncoding, char,
   $flipEncoding[char] =
    $flipEncoding[First@ToCharacterCode@char] =
     unicodeFlip[char]
   ];
flipEncode[char_Integer] :=
  Lookup[$flipEncoding, char,
   $flipEncoding[char] =
    $flipEncoding[FromCharacterCode@char] =
     unicodeFlip[char]
   ];
flipEncode[string : {(_String | _Integer) ..}] :=

  Replace[flipEncode[#], None :> "_"] & /@ string;
flipEncode[string_String] :=

 flipEncode[ToCharacterCode@string] // StringJoin

Then try to convert the alphabet:

In[181]:= flipEncode@StringJoin@{ToUpperCase@Alphabet[], Alphabet[]}

Out[181]= "Ɐ______Ɥ__ꞰꞀƜ______Ʇ_Ʌ____ɐ___ǝ_ᵷɥᴉ_ʞꞁɯ____ɹ_ʇ_ʌʍ_ʎ_"

And we'll see we have an atrocious success rate. That's largely because it seems most flips just use a different Latin character.

Plus there are some weird missing characters like turned capital k:

In[191]:= $unicodeTable["A7B0"]

Out[191]= {"LATIN CAPITAL LETTER TURNED K", "Lu", "0", "L", "", "", \
"", "", "N", "", "", "", "029E"}