Gromark cipher
A running-key cipher whose key stream grows out of a short five-digit primer by addition modulo 10.
The Gromark cipher turns the plaintext into digits and adds a key stream to them modulo 10. The key stream is built from a five-digit primer, with each new digit computed as a sum of the ones before it. The whole stream is therefore fixed by the primer, which is both the system's appeal and its weakness.
Five digits make a small search space, so cryptanalysts in practice go after the primer rather than the full key. Compared with a one-time pad the cipher is weak: a key that grows from a few digits is never random, however random it looks on the page.