Cryptology — Encoding & Decoding

Plain Text and Cipher Text

Meaning: Plain text is the original message. Cipher text is the coded message.

Definition: Cipher text is a message after it has been encoded.

If CAT is encoded as FDW using a shift of 3, CAT is plain text and FDW is cipher text.

• Keep spaces and punctuation as the question instructs.

Encoding

Meaning: Encoding changes a normal message into a code.

Definition: Encoding applies a fixed rule to each part of a message.

With shift 2, A becomes C, B becomes D, and C becomes E.

• Use the same shift for each letter.
• Wrap around after Z.
• Write each coded letter clearly.

Decoding

Meaning: Decoding reverses the code.

Definition: Decoding changes cipher text back into plain text.

If the code used shift 2 forward, decoding uses shift 2 backward.

• Identify the shift.
• Move in the opposite direction.
• Check if the decoded word makes sense.

Caesar Cipher

Meaning: A Caesar cipher is a lette$r-s$hift code.

Definition: A Caesar cipher replaces each letter by another letter a fixed number of positions away.

With shift 3, A becomes D and X becomes A because the alphabet wraps around.

Easy

Question: Encode CAT using shift 2.

1. C shifted by 2 becomes E.
2. A shifted by 2 becomes C.
3. T shifted by 2 becomes V.
4. CAT becomes ECV.

Medium

Question: Decode KHOOR if it was encoded using shift 3.

1. Move each letter 3 places backward.
2. K becomes H.
3. H becomes E.
4. O becomes L.
5. O becomes L.
6. R becomes O.
7. KHOOR decodes to HELLO.

Exam-level

Question: Encode XYZ using shift 4.

1. X shifted by 4 goes Y, Z, A, B, so X becomes B.
2. Y shifted by 4 becomes C.
3. Z shifted by 4 becomes D.
4. XYZ becomes BCD.