Cryptography, Padding Oracles

Notes

Secret messages

• First, simply hide the message:
  • Herodotus wrote of messages written on wood and covered in wax so that they appeared blank and, more outlandishly, writing a message on someone's head and waiting for their hair to regrow, swallowing balls of silk sealed in wax, invisible ink, etc.
  • Known as Steganography (steganos = covered, graphein = to write)
  • Works quite well but has a fundamental weakness: if the message is discovered, all information is revealed
• So the development of Cryptography (kryptos = hidden), the message is not hidden but its meaning is
• Transposition and substitution:
  • Transposition: where the letters retain their identity but change position, e.g. using a scytale
  • Substitution: where the letters retain their position but change their identity, e.g. the Caesar cipher
• To simplify sharing algorithms (e.g. a substitution alphabet), can introduce the idea of a key which helps generate the substitution, e.g. a key word/phrase for the Caesar cipher
• Attack on substitution cipher if we know the language of the plain text: frequency analysis both of letters and their relative positions to one another (e.g. how vowels and consonants often appear)

References

• Simon Singh, "The Code Book"
• Dan Boneh, Stanford University, "Cryptography I"