2.6 Decryption Key

To produce understandable decrypted text, you need to decrypt the encrypted text. Sometimes we might say decode or decipher instead of decrypt. To decrypt, we undo encryption, i.e. ``run encryption in reverse'' to transform ciphertext into plaintext, as shown Figure 4.

  

Figure 4: Intermediate Decryption Example Using the Caesar Cipher

Observe that Figure 4 has flipped the arrows upside down to show that decryption transforms the bottom line into the top line. Now, compare the character mappings used in Figures 2 and 4:

• Figure 2: 'w' $\rightarrow$'z', 'e' $\rightarrow$'h', the space maps to a space, 'a' $\rightarrow$'d', 'p' $\rightarrow$'s', and so forth.
• Figure 4: 'w' $\leftarrow$'z', 'e' $\leftarrow$'h', the space maps to a space, 'a' $\leftarrow$'d', 'p' $\leftarrow$'s', and so forth.

The mappings in Figure 4 reverse the mappings in Figure 2, which corresponds to the idea of ``running encryption in reverse''. So, to form the decryption key for decoding ciphertext, reverse all the mappings in the encryption key. Figure 5 shows this process for the encryption key from Figure 1.

  

Figure 5: Intermediate Decryption Key for the Caesar Cipher

For conciseness and consistency omit arrows but understand that they implicitly point down. So, you must flip Figures 4 and 5 upside down, yielding Figures 6 and 7. Figure 8 summarizes the different types and arrangements of keys.

  

Figure 6: Final Decryption Example Using the Caesar Cipher

  

Figure 7: Final Decryption Key for the Caesar Cipher

  

Figure 8: Summary of Keys