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# Algorand Specifications

Informally, the protocol interleaves δsδs seeds in an alternating
sequence. Each seed is derived from a seed δsδs rounds in the past through
either a hash function or through a VRFVRF, keyed on the entry
proposer. Additionally, every δsδrδsδr rounds, the digest of a previous
entry (specifically, from round r−δsδrr−δsδr) is hashed into the result.
The seed proof is the corresponding VRF proof, or 0 if the VRFVRF was not used.

More formally, suppose II is a correct proposer in round rr and period
pp.

Let

- (pk,B,rfirst,rlast)=Record(L,r−δb,I)(pk,B,rfirst,rlast)=Record(L,r−δb,I),
- sksk be the secret key corresponding to pkpk,
- αα be a 256-bit integer.

Then II computes the seed proof yy for a new entry as follows:

- If p=0p=0:
  - y=VRF.Prove(Seed(L,r−δs),sk)y=VRF.Prove(Seed(L,r−δs),sk),
  - α=Hash(VRF.ProofToHash(y),I)α=Hash(VRF.ProofToHash(y),I).
- If p≠0p≠0:
  - y=0y=0,
  - α=Hash(Seed(L,r−δs))α=Hash(Seed(L,r−δs)).

Now II computes the seed QQ as follows:

Q={H(α,DigestLookup(L,r−δsδr))H(α):(rmodδsδr)<δs:otherwiseQ={H(α,DigestLookup(L,r−δsδr)):(rmodδsδr)<δsH(α):otherwise

> Important
>
> **IMPLEMENTATION:**
>
> Seed computation [reference implementation](https://github.com/algorand/go-algorand/blob/b6e5bcadf0ad3861d4805c51cbf3f695c38a93b7/agreement/proposal.go#L155).

The seed is valid if the following verification procedure succeeds:

1. Let (pk,B,rfirst,rlast)=Record(L,r−δb,I)(pk,B,rfirst,rlast)=Record(L,r−δb,I);
let q0=Seed(L,r−δs)q0=Seed(L,r−δs).

2. If p=0p=0, check VRF.Verify(y,q0,pk)VRF.Verify(y,q0,pk), immediately
returning failure if verification fails. Let q1=Hash(VRF.ProofToHash(y),I)q1=Hash(VRF.ProofToHash(y),I)
and continue to step 4.

3. If p≠0p≠0, let q1=Hash(q0)q1=Hash(q0). Continue.

4. If r≡(rmodδs)modδrδsr≡(rmodδs)modδrδs, then check
Q=Hash(q1\|\|DigestLookup(L,r−δsδr))Q=Hash(q1\|\|DigestLookup(L,r−δsδr)). Otherwise,
check Q=q1Q=q1.

> Round rr leader selection and committee selection both use the seed from
> r−δsr−δs and the balances / public keys from r−δbr−δb.

> For re-proposals, the period pp used in this section is the _original_
> period, not the reproposal period.

> For a detailed overview of the seed computation algorithm and some explanatory
> examples, refer to the Algorand ABFT [non-normative specification](https://specs.algorand.co/abft/non-normative/abft-nn).