---
title: "Inverse Cryptology"
id: 76469
type: "computer_media"
slug: "inverse-cryptology"
url: "http://localhost/computer_media/inverse-cryptology/"
markdown_url: "http://localhost/computer_media/inverse-cryptology.md"
published_at: "2026-11-11T08:02:40+00:00"
modified_at: "2026-11-11T08:02:40+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2026/11/inverse-cryptology-1.png"
excerpt: "A two-player code-breaking game: one player enters a phrase, the computer enciphers it, and the other player guesses the letters one at a time."
post_tag:
  - name: "Downloadable"
    slug: "downloadable"
    taxonomy: "post_tag"
    url: "http://localhost/tag/downloadable/"
  - name: "ZX80"
    slug: "zx80"
    taxonomy: "post_tag"
    url: "http://localhost/tag/zx80/"
model:
  - name: "Sinclair ZX80"
    slug: "zx80"
    taxonomy: "model"
    url: "http://localhost/model/zx80/"
indiv:
  - name: "Harold Lamkin"
    slug: "harold-lamkin"
    taxonomy: "indiv"
    url: "http://localhost/indiv/harold-lamkin/"
genre:
  - name: "Game"
    slug: "game"
    taxonomy: "genre"
    url: "http://localhost/type/game/"
media_type: "Program"
programmers:
  - name: "Harold Lamkin"
    slug: "harold-lamkin"
    taxonomy: "indiv"
    url: "http://localhost/indiv/harold-lamkin/"
download_url: "https://archive.org/download/timex-sinclair-software-archive/Inverse%20Cryptology%20%281981%29%28Lamkin%2C%20Harold%29%28ZX80%29%28US%29%28Program%29.zip"
mediadate: "1981"
images:
  - url: "http://localhost/wp-content/uploads/2026/11/inverse-cryptology-1.png"
  - url: "http://localhost/wp-content/uploads/2026/11/inverse-cryptology-2.png"
  - url: "http://localhost/wp-content/uploads/2026/11/inverse-cryptology-3.png"
article_media:
  - id: 19739
    title: "Inverse Cryptology"
    type: "article"
    url: "http://localhost/article/inverse-cryptology/"
related_content:
  - id: 19711
    title: "Dear Editor"
    type: "article"
    url: "http://localhost/article/dear-editor-22/"
media_type_tags: "Game"
license: false
---

# Inverse Cryptology

A code-breaking game for two players on a 1K ZX80. Player 1 types a phrase, and the program prints it in a random substitution cipher. Player 2 guesses with coded letter = real letter; a right guess replaces that letter everywhere with its inverse-video original. Typing the whole phrase solves it, and the program reports the number of wrong guesses.

The cipher is never stored: the program re-encodes the phrase from a 26-element array each time it prints it, to save memory. Harold Lamkin sent a better way to exit the game to the July 1981 Dear Editor column.

Published in Syntax June 1981 ([Inverse Cryptology](https://timexsinclair.com/article/inverse-cryptology/)).

## Source Code

```
  10 DIM A(25)
  20 RANDOMISE
  30 FOR Z=0 TO 25
  40 LET A(Z)=RND(26)+37
  60 FOR X=0 TO 25
  70 IF A(Z)=A(X) AND NOT Z=X THEN GO TO 40
  80 NEXT X
  90 NEXT Z
 100 PRINT "STATE A PHRASE"
 110 INPUT W$
 120 GO SUB 600
 400 PRINT
 410 PRINT "ENTER PHRASE OR LETTER=LETTER"
 420 INPUT U$
 425 IF U$=W$ THEN GO TO 900
 430 IF NOT CODE(TL$(U$))=227 THEN GO TO 700
 440 LET Y=CODE(TL$(TL$(U$)))
 450 IF Y<38 OR Y>63 THEN GO TO 700
 460 IF NOT A(Y-38)=CODE(U$) THEN GO TO 700
 470 LET A(Y-38)=Y+128
 480 CLS
 485 PRINT "CORRECT"
 490 GO SUB 600
 491 IF Z=0 THEN GO TO 900
 495 GO TO 400
 600 PRINT
 605 LET Z=0
 610 LET U$=W$
 620 LET Y=CODE(U$)
 630 IF Y<38 OR Y>63 THEN PRINT CHR$(Y);
 640 IF Y<38 OR Y>63 THEN GO TO 650
 645 PRINT CHR$(A(Y-38));
 646 IF NOT A(Y-38)=Y+128 THEN LET Z=Z+1
 650 IF Y=1 THEN GO TO 680
 660 LET U$=TL$(U$)
 670 GO TO 620
 680 PRINT
 690 RETURN
 700 LET X=X+1
 705 CLS
 710 PRINT "TRY AGAIN"
 720 GO SUB 600
 730 GO TO 400
 900 CLS
 910 PRINT "YOU BROKE THE CODE WITH ";X-26;" ERRORS ";W$
```