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.

0 1 2 3 4 5 6 7 8 9
A   S I N   T O E R
1 B C D F G H J K L M
5 P Q U V W X Y Z . /

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
Plaintext H E L L O W O R L D
Encoded 15 8 18 18 7 54 7 9 18 12
• Example of converting keystream to digits using straddling checkerboard
Keystream O C R Y N N D E T T M A
Encoded 7 11 9 56 4 4 12 8 6 6 19 0
• Encrypting
Plaintext 1 5 8 1 8 1 8 7 5 4 7 9 1 8 1 2
Keystream 7 1 1 9 5 6 4 4 1 2 8 6 6 1 9 0
Ciphertext 8 6 9 0 3 7 2 1 6 6 5 5 7 9 0 2