Two-Part Code

March 5, 2024

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Two-Part Code

Created OnMarch 5, 2024

Last Updated OnMay 1, 2025

What is Two Part-Code?

A “Two-Part Code” in cryptography refers to a code system that uses two separate codebooks, one for encoding and one for decoding. This system enhances security compared to single-part codes by requiring access to both books to decipher messages.

Definition:

Two-Part Code is code consisting of an encoding section, in which the vocabulary items (with their associated code groups) are arranged in alphabetical or other systematic order, and a decoding section, in which the code groups (with their associated meanings) are arranged in a separate alphabetical or numeric order.
SOURCE: CNSSI-4009

Two Codebooks:

This consists of two distinct codebooks. The encoding book lists plain-language words in alphabetical order, and the decoding book lists the corresponding code groups in either alphabetical or numerical order.

Encoding:

When encoding a message, the sender uses the encoding codebook to find the corresponding code group for each plain-language word or phrase.

Decoding:

The receiver uses the decoding codebook to find the plain-language word associated with a given code group.

Security:

This system is more secure than single-part codes. It requires both the encoding and decoding books to decipher a message.

Example:

The Zimmermann Telegram in World War I used this code, making it more difficult to break.

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