The A5/1 privacy algorithm is used to protect the privacy of about 80 percent of all mobile calls worldwide. The algorithm in question has been used to encrypt GSM calls since 1988. The resulting two terabyte “code book” is available on various BitTorrent websites. Some security experts say the new development could “reduce the time to break a GSM call from weeks to hours,” others say it’s not time to panic just yet, but some say it could reduce the time it takes to intercept calls.”]
Source: https://gizmodo.com/the-algorithm-protecting-gsm-calls-has-been-cracked-5435677