To work well, a code must have thousands of plaintext words or phrases with a codetext equivalent for each. Usually, for the sake of manageability, a codetext word is limited to three to five letters or numbers. All the codetexts and their plaintext meanings are listed in a codebook.
Source: wiktionary
The repetitions of the codetext matched those of the proposed plaintext perfectly.[…]The two spent the morning inserting his equivalents into the codetexts, but none stood close enough together in the texts to obtain confirmation through new identifications.
Source: wiktionary
DNA encryption of images, part 1: Alice uses her library of primers to make codetextᶜ-plaintextᶜ pairs.[…]The result of adding the whole library to the long DNA, letting the copying happen, and then dissociating the copied DNA from the original is that Alice will have a collection of plaintextᶜ-codetextᶜ pairs, the codetextᶜ being photosensitive (the superscript c indicates a strand complementary, so able to bind to the original DNA). Alice then adds her mixture of plaintextᶜ-codetextᶜ pairs to her chip, and the strands will hybridize to the complementary, immobile strands on the chip.
Source: wiktionary