Encodings
Details about encodings used to represent core data structures using both field elements and bits.
Compressed Point Encoding
We often represent Baby Jubjub curve points in a "compressed" 255-bit encoding to save on hashing, bandwidth, and calldata. Our encoding scheme is identical to the scheme used in circomlib's PointBits
templates and circomlibjs's packPoint
and unpackPoint
methods.
For a given point , the compressed encoding is the pair , where is a single bit representing the "sign" ( means negative) of , and a field element is considered "negative" if .
To decompress a pair , we do the following:
check that and are well-formed.
compute , where and are Baby Jubjub's curve parameters
check that the square root exists. If it doesn't, the pair does not represent a valid curve point.
if the square root is , assert that is also . Otherwise, the pair is an invalid encoding
otherwise, there will be two square roots - return the one whose "sign" matches :
compute one of them and call it .
compute the sign of by comparing with
if , return . Otherwise, return .
We encode pairs into a 255-bit value as the sign bit followed by the binary representation of , which is 254-bits. In code:
function encodePair(signBit: bool, y: uint256) returns (uint256) {
return uint256(signBit) << 254 | y
}
Note that the compressed encoding does not fit in a field element, even though it does fit in a uint256
. Therefore it is (for the most part) not used in-circuit.
Asset Encoding
We define an Asset
by the following struct:
struct Asset {
// an enum representing the type of the asset
AssetType assetType,
// a 160-bit address
address assetAddr,
// the "ID" of the asset. This is only relevant for ERC721 and ERC1155
uint256 assetId,
}
enum AssetType {
// maps to integer value `0`
ERC20,
// maps to integer value `1`
ERC721,
// maps to integer value `2`
ERC1155
}
To represent the asset using only valid elements of , we transform it into the following form:
struct EncodedAsset {
// the asset address with the top-3 bits of `assetId`
// and `assetType` packed into it
uint254 encodedAssetAddr;
// the bottom 253 bits of `assetId`
uint254 encodedAssetId;
}
encodedAssetId
is the number represented by the 253 least-significant bits ofassetId
.encodedAssetAddr
is defined as the number represented by the following bits concatenated together, read from most-significant to least-significant (i.e. in big-endian order):3
0
bitsthe 3 most-significant bits of
assetId
88 bits that are left unspecified (currently they are ignored)
2 bits representing
assetType
-00
forERC20
,01
forERC721
,10
forERC1155
.160 bits representing
assetAddr
.
Note Encoding
We define the Note
and EncodedNote
structs as follows:
struct Note {
// an anonymous stealth address for the note's owner
CompressedStealthAddress owner;
// a nonce that must be a valid element of the BN254 Scalar field
uint256 nonce;
// the asset the note is for
Asset asset;
// the amount of value in `asset` the note "holds"
// this must be less than 2^252
uint256 value;
}
struct CompressedStealthAddress {
// the compressed encoding of the first component of the stealth address
uint256 h1;
// the compressed encoding of the second component of the stealth address
uint256 h2;
}
struct StealthAddress {
// the X coordinate of the first component of the stealth address
uint256 h1X;
// the Y coordinate of the first component of the stealth address
uint256 h1Y;
// the X coordinate of the second component of the stealth address
uint256 h2X;
// the Y coordinate of the second component of the stealth address
uint256 h2Y
}
struct EncodedNote {
StealthAddress owner;
// same as above
uint256 nonce;
// pull out from EncodedAsset
uint256 encodedAssetAddr;
// pull out from EncodedAsset
uint256 encodedAssetId;
// same as above
uint256 value;
}
Within Nocturne, all amounts and balances are forced by the contracts and circuits to be 252-bit integers. This ensures it's impossible to overflow the field.
The StealthAddress
struct is a "flattened" form of a stealth address. If we recall that a stealth address is pair of Baby Jubjub curve elements , h1X
, h1Y
, h2X
, and h2Y
are the and coordinates of and respectively.
The CompressedStealthAddress
struct is the "compressed" form of a stealth address, where the two components are represented using compressed point encoding (see above).
The encoding process for a Note
is:
encode the
asset
decompress the
stealthAddress
fieldpull out the
encodedAssetAddr
andencodedAssetId
fields.
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