I understand that a bundle consists of input and output txs like in this example.

  • Does a signature always need two txs and why? Because it's too big?
  • Which parts of a "value transacting bundle" are signed (e.g. addresses of all txs, values of all txs, etc.)

1 Answer 1


A signature may even need more than two transactions, e. g. if it is a multisig signature.

Yes, the reason is that Winternitz signatures are large (for every tryte you are signing and each key that is part of the multisig you need a full hash length, i. e. 81 trytes in case of KERL, of signature data).

All spending transactions of a bundle sign the same information, the bundle hash.

The bundle hash is computed of the following fields of all transactions of the bundle:

  • Address
  • Value
  • (Obsolete) Tag
  • Time Stamp
  • Current Index
  • Last Index
  • In the example I linked, why couldn't the second half of Alice's signature be stored in one of the output txs?
    – Zauz
    Commented Dec 2, 2017 at 23:56
  • 1
    Convention. Messages of output transactions are intended to give data/messages to the recipient. And currently iri will only combine signature fragments if the address is the same and the value of the other segments is zero. But it would be possible to change these conventions if everyone agrees (and if not, you know what can happen :D)
    – mihi
    Commented Dec 3, 2017 at 0:06
  • Touché! Yes that would be one reason for this to happen.
    – Zauz
    Commented Dec 3, 2017 at 0:14

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