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Multi-signature is a technology that allows multiple public keys to jointly sign a bitcoin transaction. For example, using multi-signature technology, Alice, Bob, and Charlie can jointly supervise a bitcoin. In this way, to use the bitcoin, at least two of them must agree.
At the beginning of Bitcoin's release, coins can only be stored with a public key. This means that no matter who it is, as long as he learns the private key that matches the public key, he can dominate the bitcoins under the public key.
For a reliable and safe system, immunity to a single point of failure is a basic principle (a single point of failure means that if a failure occurs here, the whole system will stop operating). Here, this principle is not respected, because as long as the private key is lost or leaked, it means losing the identity of the rightful owner of Bitcoin.
The earliest solution to this was a cryptographic method called secret fragments. It decomposes a private key into several independent parts (called fragments), and a certain number of fragments (less than the total) can be combined to reconstruct the original private key. In this way, the loss or disclosure of a private key fragment will not endanger the security of Bitcoin.
However, this method is not very compatible with Bitcoin software: you must use additional tools to create or merge private key fragments. Moreover, in order to use Bitcoin, you have to put together a large number of fragments, which means that the original intention of immunity to single points of failure has not been well realized.
In fact, the solution has always been included in the Bitcoin core code. It has existed from the beginning, but it has not been standardized: there are two script operations that allow multi-signatures of ordinary Bitcoin public keys. In this way, the different private keys that need to be verified in a multi-signature transaction do not need to be in the same space. Compared with the method of using a single private key or encrypted fragment, the security of the system can be greatly improved.
A Bitcoin Improvement Protocol (BIP11) standardizes this type of transaction and limits the maximum number of secret keys to three. On December 20, 2011, BIP11 was incorporated into the Bitcoin core code. In late January 2012, the first BIP11 type transaction appeared on the blockchain.