Encryption

What it is

• The transformation of data from one form to another to prevent an unauthorised third part from being able to understand it.

Why we need it

• More and more data is being transmitted online (websites, web forms, emails, messaging)
• There is always a danger of data being hijacked
• People conduct more of their lives online with more sensitive data than ever being transmitted

Terminology

• Plaintext
  • no encryption; the original data
• Ciphertext
  • encrypted data
• Cipher
  • the encryption method or algorithm
• Key
  • secret information to encrypt or decrypt the message

People have been using encryption for as long as there has been written communication - not just since the introduction of computers

One of the oldest known encryption methods is the Caesar Cipher

Caesar Cipher

• Also known as the shift cipher
• It is a form of substitution cipher

It works by shifting the number of letters of the alphabet along by a certain number of characters.

Breaking the Caesar Cipher

• Even without the key, the Caesar Cipher is very easy to decrypt.

• To decrypt this message:

  DGYDQFH WR ERUGHU DQG DWWDFN DW GDZQ

  You could attempt a Brutus Brute Force Attack (trying to crack a code/password by trying hundreds/thousands of possibilities by trial and error)

  • there are only 25 different possibilities:

    EHZERGI XS FSVHIV ERH EXXEGO EX HEAR
    FIAFSHJ YT GTWIJW FSI FYYFHP FY IFBS
    GJBGTIK ZU HUXJKX GTJ GZZGIQ GZ JGCT
    HKCHUJL AV IVYKLY HUK HAAHJR HA KHDU
    ILDIVKM BW JWZLMZ IVL IBBIKS IB LIEV
    JMEJWLN CX KXAMNA JWM JCCJLT JC MJFW
    KNFKXMO DY LYBNOB KXN KDDKMU KD NKGX
    LOGLYNP EZ MZCOPC LYO LEELNV LE OLHY
    MPHMZOQ FA NADPQD MZP MFFMOW MF PMIZ
    NQINAPR GB OBEQRE NAQ NGGNPX NG QNJA
    ORJOBQS HC PCFRSF OBR OHHOQY OH ROKB
    PSKPCRT ID QDGSTG PCS PIIPRZ PI SPLC
    QTLQDSU JE REHTUH QDT QJJQSA QJ TQMD
    RUMRETV KF SFIUVI REU RKKRTB RK URNE
    SVNSFUW LG TGJVWJ SFV SLLSUC SL VSOF
    TWOTGVX MH UHKWXK TGW TMMTVD TM WTPG
    UXPUHWY NI VILXYL UHX UNNUWE UN XUQH
    VYQVIXZ OJ WJMYZM VIY VOOVXF VO YVRI
    WZRWJYA PK XKNZAN WJZ WPPWYG WP ZWSJ
    XASXKZB QL YLOABO XKA XQQXZH XQ AXTK
    YBTYLAC RM ZMPBCP YLB YRRYAI YR BYUL
    ZCUZMBD SN ANQCDQ ZMC ZSSZBJ ZS CZVM
    ADVANCE TO BORDER AND ATTACK AT DAWN
    BEWBODF UP CPSEFS BOE BUUBDL BU EBXO
    CFXCPEG VQ DQTFGT CPF CVVCEM CV FCYP

Vernam Cipher

• Invented in 1917 - the only cipher that is still unbreakable
• Uses a one-time pad (also known as the encryption key)
  • is equal length to (or longer than) the plaintext
  • is completely random
  • only used once (one-time)
• One-time pads are used in pairs - sender and recipient have the key.
  • key is shared securely (in person)

How it works

• The plaintext is given in ASCII (binary)
• The one-time pad is given in binary
• The XOR operation is carried out between the two binary strings

Example:

• Plaintext character M in ASCII: 1001101
• One-time pad (+ in ASCII): 0101011
• XOR them

1001101 M
0101011 +
- XOR - ⊻
1100110 f

Encrypt E with key w

01000101 E
01110111 w
-- XOR - ⊻
00110010 2