Ciphering method where N out of M keys are required for decryption

Question

Which ciphering method allows to decrypt data with N out of M keys?

Example: There are 5 persons, each of them gets an individual key. They should be able to decrypt encrypted data, but at least 3 of them (any combination) should be required to decrypt it.

Answer 1

The technical name for what you want is a threshold scheme. There are several different varieties; Google will point you to them. If you are interested, I have an implementation of Adi Shamir's threshold system at my blog.