Endorsement policies

Endorsement policies are used to instruct a peer on how to decide whether a transaction is properly endorsed. When a peer receives a transaction, it invokes the VSCC (Validation System Chaincode) associated with the transaction’s Chaincode as part of the transaction validation flow to determine the validity of the transaction. Recall that a transaction contains one or more endorsement from as many endorsing peers. VSCC is tasked to make the following determinations: - all endorsements are valid (i.e. they are valid signatures from valid certificates over the expected message) - there is an appropriate number of endorsements - endorsements come from the expected source(s)

Endorsement policies are a way of specifying the second and third points.

Endorsement policy design

Endorsement policies have two main components: - a principal - a threshold gate

A principal P identifies the entity whose signature is expected.

A threshold gate T takes two inputs: an integer t (the threshold) and a list of n principals or gates; this gate essentially captures the expectation that out of those n principals or gates, t are requested to be satisfied.

For example: - T(2, 'A', 'B', 'C') requests a signature from any 2 principals out of ‘A’, ‘B’ or ‘C’; - T(1, 'A', T(2, 'B', 'C')) requests either one signature from principal A or 1 signature from B and C each.

Endorsement policy syntax in the CLI

In the CLI, a simple language is used to express policies in terms of boolean expressions over principals.

A principal is described in terms of the MSP that is tasked to validate the identity of the signer and of the role that the signer has within that MSP. Currently, two roles are supported: member and admin. Principals are described as MSP.ROLE, where MSP is the MSP ID that is required, and ROLE is either one of the two strings member and admin. Examples of valid principals are 'Org0.admin' (any administrator of the Org0 MSP) or 'Org1.member' (any member of the Org1 MSP).

The syntax of the language is:

EXPR(E[, E...])

where EXPR is either AND or OR, representing the two boolean expressions and E is either a principal (with the syntax described above) or another nested call to EXPR.

For example: - AND('Org1.member', 'Org2.member', 'Org3.member') requests 1 signature from each of the three principals - OR('Org1.member', 'Org2.member') requests 1 signature from either one of the two principals - OR('Org1.member', AND('Org2.member', 'Org3.member')) requests either one signature from a member of the Org1 MSP or 1 signature from a member of the Org2 MSP and 1 signature from a member of the Org3 MSP.

Specifying endorsement policies for a chaincode

Using this language, a chaincode deployer can request that the endorsements for a chaincode be validated against the specified policy. NOTE - the default policy requires one signature from a member of the DEFAULT MSP). This is used if a policy is not specified in the CLI.

The policy can be specified at deploy time using the -P switch, followed by the policy.

For example:

peer chaincode deploy -C testchainid -n mycc -p github.com/hyperledger/fabric/examples/chaincode/go/chaincode_example02 -c '{"Args":["init","a","100","b","200"]}' -P "AND('Org1.member', 'Org2.member')"

This command deploys chaincode mycc on chain testchainid with the policy AND('Org1.member', 'Org2.member').

Future enhancements

In this section we list future enhancements for endorsement policies: - alongside the existing way of identifying principals by their relationship with an MSP, we plan to identify principals in terms of the Organization Unit (OU) expected in their certificates; this is useful to express policies where we request signatures from any identity displaying a valid certificate with an OU matching the one requested in the definition of the principal. - instead of the syntax AND(., .) we plan to move to a more intuitive syntax . AND . - we plan to expose generalized threshold gates in the language as well alongside AND (which is the special n-out-of-n gate) and OR (which is the special 1-out-of-n gate)