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Algorand Specifications

For a player with their participation keys registered on the Ledger (and hosted on an Algorand node), a set of ephemeral sub-keys is created.

An ephemeral sub-key is a key pair that produces one-time signature for messages. It MUST be deleted after use to ensure forward security. Algorand’s ephemeral subkeys use Ed25519 public-key signature system.

For further details, refer to the Cryptography primitives specification.

Algorand uses a two-level ephemeral signature scheme. Instead of signing voting messages directly, Algorand accounts use their registered voting keys to sign an intermediate ephemeral sub-key.

This intermediate ephemeral sub-key signs a batch of leaf-level ephemeral sub-keys. Hence, each intermediate ephemeral sub-key is associated with a batch number (BatchBatch), and each leaf ephemeral sub-key is associated with both a batch number (of its parent key) and an offset (OffsetOffset, denotes its offset within a BatchBatch). An agreement voting message is signed hierarchically:

Voting Keys Root Key
└── Batch Sub-Key
    └── Leaf Sub-Key
        └── Agreement Voting Message

Further details on this process in the One-time Signature subsection.

Each leaf-level ephemeral sub-key is used for voting on a single agreement round, and will be deleted afterward. Once a batch of leaf-level ephemeral sub-keys run out, a new batch is generated. Algorand allows users to set the number of leaf-level ephemeral sub-key per batch, KeyDilutionKeyDilution.

The default KeyDilutionKeyDilution value is 10,00010,000.

An Algorand account can change its KeyDilutionKeyDilution in the participation keys registration.

For further details about the structure of a participation keys registration (keyreg) transaction, refer to the Ledger specification.

The following diagram shows the tree structure of the voting signature scheme:

One-Time Signature container

Batch Signer 1

Batch Signer 2

...

Batch Signer N

Batch 1 Key 1

Batch 1 Key 2

...

Batch 1 Key N