Nicodemus Cipher

The Nicodemus Cipher is a classical polygraphic transposition cipher designed to encrypt messages using a keyword-driven arrangement of letter pairs. Named after cryptographer Nicodemus, this cipher extends the principles of traditional substitution and transposition by working on multiple letters at a time, often in digraphs or trigraphs, and incorporating padding to maintain regular block sizes. It provides a higher level of complexity than simple monoalphabetic or basic polyalphabetic ciphers.

In the Nicodemus Cipher, a keyword determines the sequence of columns or positions used to rearrange the plaintext, while a padding character (commonly "X") fills any incomplete blocks. This ensures that every block aligns with the cipher’s structure, maintaining consistent encryption and avoiding ambiguity during decryption.

Nicodemus Cipher: Encoding

To encrypt using the Nicodemus Cipher, the plaintext is first normalized (uppercase, no spaces), and divided into blocks according to the cipher’s internal structure. Padding is added if necessary. The keyword is then applied to permute letters within each block:

Plaintext: HELLONICODEMUS
Keyword:   KEY
Padding:   X (added to complete blocks)

Step 1: Normalize and pad plaintext
Step 2: Apply keyword-driven transposition on each block

Ciphertext:
DBMMIOPILHYLXP

Each letter’s final position depends on the keyword’s columnar mapping, providing a polygraphic rearrangement that increases security over simple substitution methods.

Nicodemus Cipher: Decoding

Decoding reverses the columnar rearrangement using the same keyword and removes any padding characters added during encryption:

Ciphertext: DBMMIOPILHYLXP
Keyword:    KEY
Padding:    X

Step 1: Apply inverse keyword transposition to each block
Step 2: Remove padding
Step 3: Reassemble plaintext

Plaintext:
HELLONICODEMUS

Correct decryption depends on using the identical keyword and block size, and accurately handling padding.

Nicodemus Cipher: Notes

Key points and practical considerations for the Nicodemus Cipher:

• Type: Polygraphic transposition cipher
• Key: keyword determining columnar rearrangement
• Input: normalized uppercase plaintext
• Padding: uses a character (e.g., "X") to fill incomplete blocks
• Output: ciphertext letters
• Strengths: obscures letter patterns with multi-letter blocks; increases complexity over monoalphabetic and simple polyalphabetic ciphers
• Weaknesses: still vulnerable to analysis if the keyword is short or predictable
• Use: educational tool for understanding polygraphic transposition; classical cipher exercises

The Nicodemus Cipher illustrates the power of keyword-based polygraphic encryption combined with structured padding. While obsolete for modern cryptography, it remains a valuable method for learning advanced transposition techniques.