Do Tezos support multisignature transaction?

Question

If it does, where's the documentation? Are there any additional fee for multisig tx?

Answer 1

Yes it has been supported since the launch of tezos however the contract was not publicly advertised in the beginning.

Now @arthurb has published an implementation.

Quoting his announcement on reddit

It is supported. You can deploy https://github.com/murbard/smart-contracts/blob/master/multisig/michelson/multisig.tz for instance.* More user friendly CLI support is coming but you can already use this. (*) It's implemented in a straightforward way, and its properties (no replay, need the sigs etc) have been formally verifed, nevertheless please read the license https://github.com/murbard/smart-contracts/blob/master/LICENSE

Answer 2

The Tezos command-line client features a few commands for interacting with the Michelson multisig smart contract.

These commands are listed in the section named "Commands for managing a multisig smart contract" of the client manual (https://tezos.gitlab.io/master/api/cli-commands.html).

More gas is required than for a direct transaction, the precise amount of extra gas depends on the number of signers.