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So say I want to encrypt a file and the only way I want it to be read is if two different people enter their keys. So, for instance there are four keys:
key1, key2, key3, key4.
If I encrypt it with key1 then the following combinations will decypt it:
Is this possible using a standard method?
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Asymmetric, or public/private encryption, uses a pair of keys. Data encrypted with one key are decrypted only with the other key in the public/private key pair.
There are four basic type of encryption keys: symmetric, asymmetric, public, and private. The first two describe where the keys are used in the encryption process, and the last two describe who has access to the keys.
Asymmetric cryptography involves a pair of keys to encrypt and decrypt data. The two participants in the asymmetric encryption workflow are the sender and the receiver. Each has its own pair of public and private keys. First, the sender obtains the receiver's public key.
Generate a unique content key to encrypt the message (this is common to many message encryption standards), then apply an erasure code scheme such as Reed-Solomon coding against that content key concatenated with enough additional random data to ensure that any m of n "shards" of the key can be put together to create the final key. Shards are only given out from the random data portion so that none of the shards given out contain actual bits from the content key. This way, any number of collected shards short of m does not give any useful information about the key itself.
EDIT: Reed-Solomon to generate key shards appears to be identical to Shamir's secret-sharing, first published in 1979; thanks to @caf for pointing out the article.
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