Multiple signatures

In most cases multiple signatures need to be created (e.g. a contract signing by multiple parties). In such cases, it is useful to note that multiple signatures can be created in parallel or in a sequential order.

Parallel signatures

Parallel signatures are stand-alone, mutually independent signatures where the ordering of the signatures is not important. All the involved parties can receive the data at the same time and sign in any order. The computation of these signatures is performed on exactly the same hash data but using different private keys associated to the different signers. Parallel signatures can be validated independently to verify whether the associated data is validly signed.

The following schema illustrates the creation of parallel signatures:

Sequential signatures

Sequential signatures are mutually dependent signatures where the ordering of the signatures is important. A fixed signing order is defined and the next signer in the chain shall not sign before the preceding signers have signed the data. The computation of these signatures is not performed on the same data. A signer that is further in the signing chain will sign the initial data previously signed by the signers preceding him in the chain. Each signer uses his own private key to sign.

The following schema illustrates the creation of sequential signatures:

Counter signatures

A counter signature is an additional signature applied on data that has already been signed previously. This type of signature is used to show approval of the data and signature, to confirm the authenticity of the data. The computation of a counter signature is performed on the signed data, and it is added to the signature as an unsigned attribute, i.e. after initial signature creation.

Counter signatures are often created by trustworthy entities such as notaries, doctors or attorneys. Possible use cases are rental and mortgage applications, health documents, passports and visas.

The following schema illustrates the creation of counter signatures: