Safe-cracking is the process of opening a safe without either the combination or the key.

Safes have widely different designs, construction methods, and locking mechanisms. A safe cracker needs to know the specifics of whichever will come into play.

Lock manipulation is a damage-free, combination-based method. A well known surreptitious bypass technique, it requires knowledge of the device and well developed touch, along with the senses of sight and possibly sound.

While manipulation of combination locks is usually performed on Group 2 locks, many Group 1 locks are also susceptible. The goal is to successfully obtain the combination one number at a time. Manipulation procedures vary, but all rely on exploiting mechanical imperfections in the lock to open it, and, if desired, recover its combination for future use. Similar damage-free bypass can also be achieved by using a computerized auto-dialer or manipulation robot in a so-called brute-force attack. These auto-dialer machines may take 24 hours or more to reach the correct combination, although modern devices with advanced software may do so faster.

Mechanical safe locks are manipulated primarily by feel and vision, with sound sometimes supplementing the process. To find the combination the operator uses the lock against itself by measuring internal movements with the dial numbers. More sophisticated locks use advanced mechanics to reduce any feedback available to a technician in identifying a combination. These group 1 locks were developed in response to group 2 lock manipulation. Wheels made from lightweight materials will reduce valuable sensory feedback, but are mainly used for improved resistance against radiographic attacks. Manipulation is often the preferred choice in lost-combination lockouts, since it requires no repairs or damage, but can be time consuming for an operator, with the specific difficulty depending on the unique wheel shapes and where the gates rest in relation to them. A novice's opening time will be governed by these random inconsistencies, while some leading champions of this art show consistency. There are also a number of tools on the market to assist safe engineers in manipulating a combination lock open in the field.

Nearly all combination locks allow some "slop", or deviation, while entering a combination on the dial. On average, 1% radial rotation in either direction from the center of the true combination number allows the fence to fall despite slight deviation, so that for a given safe, it may be necessary only to try a subset of possible combinations. Such "slops" may allow for a margin of error of plus or minus two digits, which means that trying multiples of five would be sufficient in this case. This drastically reduces the time required to exhaust the number of meaningful combinations. A further reduction in solving time is obtained by trying all possible settings for the last wheel for a given setting of the first wheels before nudging the next-to-last wheel to its next meaningful setting, instead of zeroing the lock each time with a number of turns in one direction.

A safe may be compromised by using a manufacturer-set combination. Known as try-out combinations, these allow an owner initial access to their safe in order to set a new unique one. Sources of try-out combinations exist by manufacturer.

Other easy-to-guess combinations include a birthdate, street address, or driver's license number.