Editing Ciphertext (section)

== Conceptual underpinnings ==
Let <math>m\!</math> be the plaintext message that Alice wants to secretly transmit to Bob and let <math>E_k\!</math> be the encryption cipher, where <math>_k\!</math> is a [[secret key|cryptographic key]]. Alice must first transform the plaintext into ciphertext, <math>c\!</math>, in order to securely send the message to Bob, as follows:

: <math>c = E_k(m). \!</math><ref name="Fundamentals">{{cite book|isbn=0-7923-8675-2 |title=Fundamentals of Cryptology |publisher=Kluwer Academic Publishers |year=2000 |first=Henk C.A. |last=van Tilborg |page=3}}</ref>

In a symmetric-key system, Bob knows Alice's encryption key. Once the message is encrypted, Alice can safely transmit it to Bob (assuming no one else knows the key). In order to read Alice's message, Bob must decrypt the ciphertext using <math>{E_k}^{-1}\!</math> which is known as the decryption cipher, <math>D_k: \!</math>

: <math>D_k(c) = D_k(E_k(m)) = m.\!</math><ref name="Fundamentals" />

Alternatively, in a non-symmetric key system, everyone, not just Alice and Bob, knows the encryption key; but the decryption key cannot be inferred from the encryption key. Only Bob knows the decryption key <math>D_k,</math> and decryption proceeds as

:<math>D_k(c)=m.</math>