In cryptography, an unbroken algorithm is not necessarily an unbreakable one. There have been many cryptographic algorithms made and deployed in various situations throughout the world, some dating back from the time of Julius Caesar! More recent algorithms, AES Rijndael for example, are very strong, and have survived close scrutiny for many years and have remained secure. But, many other algorithms such as the Vigniere cipher were once believed to be totally unbreakable, but then all of a sudden, they may as well be written in plaintext. It was once thought that the simple XOR cipher could be the answer to an unbreakable algorithm, but new methods of cryptanalysis were born, and now, it can be cracked within moments.
Today's 'secure' ciphers such as AES and Twofish may be secure now, but in the future, with the advent of faster computers, better techniques and even quantum computing, these ciphers will only last so long.