Encryption is a good way to learn the beauty of Mathematics.  I
  remember the first time I put a message through this encyption
  system I was amazed that it worked the way it did.

  I think encryption is pointless. But I still wrote RSA.

  Source Code.

  Orignal Github Repository

  This is an example key.  Notice the public and private keys are
  together.

#S(RSA-KEY
   :NAME "me"
   :NAME-BASE64 "bWU="
   :LENGTH 2048
   :N 21337566008785129485769556105641647502289044850192296801400455594751853404768992120257911240444939522444606221954442261615762112584491294113836356022632541409370111873314402962878892829576661318557136977107241919865330261349291843709440919335276687398754227925596047115243189961128028883177777041198792994272733494135451979463362129066883533468025865478176665077312237100709076832187281315192914259834491057434902403409989286897134492157855558984187367973158020231497571267004090375959686441678226447350713301032648144174339829601731498703174862839187415973984707473382237671895730453359811608847327969111886473412009
   :E 17
   :D 17572113183705400752986693263469592060708625170746597365859198725089761627456817040212397492131126665542616888668364215448274680951934006917276999077462092925363621542729508322370852918474897556458818687029493345771448450522946224231304286511404330798974070056373215271376744673870141433205228151575476583518480867951218709515018655916296511364290014796065266825309774570441700773804056511473141487910890023466983240875239978926948775055171826208836138597933241105376808424906695445746677789042451848139305083225046327068317121527354573488293252780350948512968116376882470592040285173381251093602609934599605994462193 )

  It is using modulus 17 as the exponent, both a prime and my
  favourite number.  You can change it in the source code to something
  bigger.  I don't know if that really matters.  I'm not a
  cryptography expert, but I learned a lot implementing RSA.

  Your buffer length is your key length.  You cannot encypt more data
  than the length of your key, meaning that (I think) the bigger your
  key length the larger your block size and the better encrypted it
  is.

  There is also a small implementation of a key database in there.