Read the excerpt from “Tools of the Spymaster."
The numbers that Major Tallmadge assigned to members of the Culper Ring were from a secret writing system he invented. He substituted digits for words that would be used in messages. "Long Island," for example, was 728, "arms" was 7, and "city" was 88. There was a number for each month, such as 341 for "January." He made four copies of his codes. He kept one and gave the others to Woodhull, Townsend, and General Washington. For words that did not have a number code, Tallmadge gave his agents a cipher. In a cipher, each letter in a message is replaced by another letter or a number.
Which statement best expresses the central idea of the excerpt?