papers/lux-evm-precompile-engineering/sections/03-input-output-abi.tex

\section{Input/Output ABI}


\subsection{Design Principles}

\begin{enumerate}[nosep]
  \item \textbf{Fixed-size fields}: All inputs use fixed-width encoding (32 bytes for
    scalars, 48 bytes for G1 points, etc.). No length-prefixed variable fields.
  \item \textbf{Left-padded}: Scalars are left-padded with zeros to 32 bytes, matching
    Solidity ABI conventions.
  \item \textbf{Concatenated}: Multiple inputs are concatenated without separators. The
    precompile deduces structure from the total input length.
  \item \textbf{Error = revert}: Invalid inputs cause the precompile to return an error,
    which the EVM interprets as a revert consuming all provided gas.
\end{enumerate}

\subsection{Example: BLS12-381 Pairing Check}

\begin{lstlisting}[language=Go,caption={BLS12-381 pairing input/output format}]
// Input: k pairs of (G1, G2) points, concatenated
// Each pair: 128 bytes (48 for G1 compressed + 96 for G2 compressed,
//            padded to 128 for alignment)
// Total input: k * 128 bytes
//
// Output: 32 bytes
// 0x00...01 if pairing check passes (product of pairings == 1)
// 0x00...00 if pairing check fails

func (c *bls12381Pairing) Run(input []byte) ([]byte, error) {
    if len(input) == 0 || len(input)%128 != 0 {
        return nil, errInvalidInput
    }

    k := len(input) / 128
    pairs := make([]PairingPair, k)

    for i := 0; i < k; i++ {
        chunk := input[i*128 : (i+1)*128]

        g1, err := decodeG1(chunk[:48])
        if err != nil {
            return nil, err
        }
        if !g1.IsOnCurve() || !g1.IsInSubgroup() {
            return nil, errNotOnCurve
        }

        g2, err := decodeG2(chunk[48:128])
        if err != nil {
            return nil, err
        }
        if !g2.IsOnCurve() || !g2.IsInSubgroup() {
            return nil, errNotOnCurve
        }

        pairs[i] = PairingPair{G1: g1, G2: g2}
    }

    result := multiPairing(pairs)
    output := make([]byte, 32)
    if result.IsOne() {
        output[31] = 1
    }
    return output, nil
}
\end{lstlisting}

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