When someone talks about "cracking" or "breaking" an encryption algorithm, they always seem to mean this: the "attacker" decrypted a message by guessing the secret key that was used for the encryption. This is not breaking or cracking a particular algorithm. But it does demonstrate the importance of key size. The key size -- the number of bits used to store the key, which is an integer number -- determines the size of the key space, the number of possible keys that can be used. If you knew that to decrypt a message you needed to guess a number between 1 and 10, would you feel challenged? How about between 1 and 1000? How about 1 and 1^38 (1 followed by 38 zeros). That is (roughly) the key space using a 128-bit key. For comparison purposes, let’s use a (so far) non-existent computer that can guess 1 trillion (1 followed by 12 zeroes) keys a second. On average, it would take around 2 million million million (2 followed by 18 zeroes) years to guess the key.