In IOTA, nodes use the milestones issued by the Coordinator to reach a consensus on which transactions are confirmed. This TIP proposes a milestone payload for the blocks described in the IOTA protocol TIP-24. It uses Edwards-curve Digital Signature Algorithm (EdDSA) to authenticate the milestones.
In order to integrate the concept of milestones consistently into Tangle blocks, this TIP describes a dedicated payload type for milestones. In this context, the document also describes how Ed25519 signatures are used to assure authenticity of the issued milestones. In order to make the management and security of the used private keys easier, simple multisignature features with support for key rotation have been added.
The BLAKE2b-256 hash of the Milestone Essence, consisting of the actual milestone information (like its index number or position in the tangle), is signed using the Ed25519 signature scheme as described in the IRTF RFC 8032.
To increase the security of the design, a milestone can (optionally) be independently signed by multiple keys at once. These keys should be operated by detached signature provider services running on independent infrastructure elements. This assists in mitigating the risk of an attacker having access to all the key material necessary for forging milestones. While the Coordinator takes responsibility for forming Milestone Payload Blocks, it delegates signing in to these providers through an ad-hoc RPC connector. Mutual authentication should be enforced between the Coordinator and the signature providers: a client-authenticated TLS handshake scheme is advisable. To increase the flexibility of the mechanism, nodes can be configured to require a quorum of valid signatures to consider a milestone as genuine.
In addition, a key rotation policy can also be enforced by limiting key validity to certain milestone intervals. Accordingly, nodes need to know which public keys are applicable for which milestone index. This can be provided by configuring a list of entries consisting of the following fields:
- Index Range providing the interval of milestone indices for which this entry is valid. The interval must not overlap with any other entry.
- Applicable Public Keys defining the set of valid public keys.
- Signature Threshold specifying the minimum number of valid signatures. Must be at least one and not greater than the number of Applicable Public Keys.
The Milestone ID is the BLAKE2b-256 hash of the serialized Milestone Essence. It is important to note that the signatures do not impact the Milestone ID.
All values are serialized in little-endian encoding. The serialized form of the milestone is deterministic, meaning the same logical milestone always results in the same serialized byte sequence.
The following table describes the entirety of a Milestone Payload in its serialized form following the notation from TIP-21:
|Payload Type||uint32||Set to value 7 to denote a Milestone Payload.|
Describes the signed part of a Milestone Payload.
|Signatures Count||uint8||The number of signature entries following.|
¹: See TIP-4.
Options field holds additional data authenticated by the milestone.
The following table lists all the
Milestone Option Type that are currently supported as well as links to the corresponding specification:
|Payload Name||Type Value||TIP|
Protocol Parameters Milestone Option
Milestone Option is used to signal to nodes the commencing of new protocol parameters, including new protocol
version or PoW difficulty.
Protocol Parameters Milestone Option
Defines changing protocol parameters.
|Milestone Option Type||uint8||Set to value 1 to denote a Protocol Parameters Milestone Option.|
|Target Milestone Index||uint32||The milestone index at which these protocol parameters become active.|
|Protocol Version||uint8||The to be applied protocol version.|
|Parameters||(uint16)ByteArray||The protocol parameters in binary, serialized form.|
Target Milestone Indexmust be greater than
Index Numberof the milestone it is contained in.
Target Milestone Indexmust be less than or equal to
Index Number+ 30. (This value is fixed and technically not a protocol parameter as defined in TIP-22, as it should not be subject to protocol parameter changes induced by this option.)
length(Parameters)must not exceed
Max Metadata Length.
Similar to transaction validation, milestone validation has been separated into two classes. For a milestone to be valid, both of them need to be true.
Syntactic validation can be checked from the Milestone Essence plus the blocks in the past cone referenced by it.
Index Numbermust not be smaller than
First Milestone Index.
First Milestone Index, the following fields must be zeroed out:
Previous Milestone ID
Inclusion Merkle Root
Applied Merkle Root
Index Numberis greater than
First Milestone Index, the milestone must reference (i.e. one of the
Parentsmust contain or reference) another syntactically valid milestone whose Milestone ID matches
Previous Milestone ID. With respect to that referenced milestone, the following must hold:
Index Numbermust increment by
Timestampmust be strictly larger (i.e. at least one second later).
Inlcusion Merkle Rootmust match the Merkle tree hash of the IDs of all blocks in White Flag Ordering (as described in TIP-2) that are newly referenced. (This always includes at least one valid milestone block with
Previous Milestone ID.)
Applied Merkle Rootmust match the Merkle tree hash of the not-ignored state-mutating transactions that are newly referenced (see TIP-2).
Parentsmust match the
Parentsfield of the encapsulating Block, i.e. the Block that contains the Milestone Payload.
length(Metadata)must not exceed
Max Metadata Length.
Milestone Option Typemust match one of the values described under Milestone Options.
- The option itself must pass syntactic validation.
- The options must be sorted in ascending order based on their
Milestone Option Type.
Signatures Countmust be at least the Signature Threshold and at most the number of Applicable Public Keys for the current milestone index.
- For each signature block the following must be true:
Signature Typevalue must denote an
Public Keymust be contained in Applicable Public Keys for the current milestone index.
Signaturemust contain a valid signature for
- The signature blocks must be sorted with respect to their
Public Keyin lexicographical order.
Public Keymust be unique.
- Given the type and length information, the Milestone Payload must consume the entire byte array of the
Payloadfield of the Block.
Semantic validation is defined in the context of all available blocks.
- The milestone chain must not fork, i.e. there must not be two different, syntactically valid milestones with the same
Index Number. In case of a fork, the correct state of the ledger cannot be derived from the milestones alone and usually the node implementation should alert the user and halt.
- Due to the layered design of blocks and payloads, it is practically not possible to prevent reattachments of Milestone Payloads. Hence, this payload has been designed in a way to be independent from the block it is contained in. A milestone should be considered as a virtual marker (referencing
Parents) rather than an actual block in the Tangle. This concept is compatible with reattachments and supports a cleaner separation of the block layers.
- Forcing matching
Parentsin the Milestone Payload and its block makes it impossible to reattach the same payload at different positions in the Tangle. Strictly speaking, this violates the separation of payload and block. However, it simplifies milestone processing as the position of the block will be the same as the position encoded in the Milestone Payload. Having these clear structural properties seems to be more desirable than a strict separation of layers.
- While it is always possible to cryptographically prove that a block was confirmed by a given milestone by supplying all the blocks of a path from the milestone to the block, such a proof can become rather large (depending on the blocks). To simplify such proof-of-inclusions, the
Inclusion Merkle Rootof all the included blocks has been added.
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