Self-synchronizing stream ciphers is another technique that uses part of the previous "N" ciphertext digits to compute the keystream. Such schemes are known also as asynchronous stream ciphers or ciphertext autokey (CTAK). The idea of self-synchronization was patented in 1946, and has the advantage that the receiver will automatically synchronise with the keystream generator after receiving "N" ciphertext digits, making it easier to recover if digits are dropped or added to the message stream. Single-digit errors are limited in their effect, affecting only up to "N" plaintext digits. It is somewhat more difficult to perform active attacks on self-synchronising stream ciphers rather than synchronous counterparts.