A toy pen and paper cipher

1. Pick a page from the book that has not yet been used;
2. First three digits of ciphertext is page number, with leading zeroes;
3. Keystream is third letter of each line, excluding spaces and punctuation;
4. Convert keystream to decimal digits;
5. Convert plaintext to decimal digits;
6. Add plaintext and keystream digit by digit modulo 10 (without carry);

Use straddling checkerboard to convert text to digits and back.

Letters in the unmarked row are represented by one digit (column number), e.g., N becomes 4.

Letters in the other two rows are represented by two digits (row, then column), e.g., W becomes 54.

Numbers are enclosed in / symbols (59), then each digit is repeated twice, e.g., 42 becomes 59 44 22 59.

• Example of encoding of text as stream of digits using straddling checkerboard

• Example of converting keystream to digits using straddling checkerboard

• Encrypting