Deserialization of pk fails Plonk Prover

Serialization and Deserialization of the prover key leads to the an error in the Plonk Prover. For recreation of the issue, please find the following complete example:

package main

import (
    "os"

    "github.com/consensys/gnark-crypto/ecc/bn254"
    "github.com/consensys/gnark/backend/plonk"
    "github.com/consensys/gnark/frontend"
    "github.com/consensys/gnark/frontend/cs/scs"
    "github.com/consensys/gnark/test"
)

// Circuit defines a simple circuit
// x**3 + x + 5 == y
type CubicCircuit struct {
    // struct tags on a variable is optional
    // default uses variable name and secret visibility.
    X frontend.Variable `gnark:"x"`
    Y frontend.Variable `gnark:",public"`
}

// Define declares the circuit constraints
// x**3 + x + 5 == y
func (circuit *CubicCircuit) Define(api frontend.API) error {
    x3 := api.Mul(circuit.X, circuit.X, circuit.X)
    api.AssertIsEqual(circuit.Y, api.Add(x3, circuit.X, 5))
    return nil
}

func main() {
    ccs, err := frontend.Compile(bn254.ID.ScalarField(), scs.NewBuilder, &CubicCircuit{})
    if err != nil {
        panic("Error in compilation of constraint system!")
    }

    srs, err := test.NewKZGSRS(ccs)
    if err != nil {
        panic("Error in KZG")
    }

    pk, _, err := plonk.Setup(ccs, srs)
    if err != nil {
        panic("Error in Setup")
    }

    // SERIALIZATION + DESERIALIZATION BEGIN

    // Open the file in write mode for pk
    fPK, err := os.OpenFile("pk.dat", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0666)
    if err != nil {
        panic("Failed to open file for writing: " + err.Error())
    }
    defer fPK.Close()

    _, err = pk.WriteTo(fPK)
    if err != nil {
        panic("Failed to write pk to file: " + err.Error())
    }

    pk_deserialized := plonk.NewProvingKey(bn254.ID)

    // Read PK
    f, err := os.Open("pk.dat")
    if err != nil {
        panic("Failed to open file: " + err.Error())
    }

    _, err = pk_deserialized.ReadFrom(f)
    if err != nil {
        panic("Failed to read from file: " + err.Error())
    }

    // SERIALIZATION + DESERIALIZATION   END

    witness := CubicCircuit{}
    witness.X = "3"
    witness.Y = "35"

    w, err := frontend.NewWitness(&witness, bn254.ID.ScalarField())

    _, err = plonk.Prove(ccs, pk_deserialized, w)

    if err != nil {
        panic("Proving Fails! " + err.Error())
    }

}

If executed with pk, it succeeds, if executed with pk_deserialized, it fails.

For version

github.com/consensys/gnark v0.8.0 github.com/consensys/gnark-crypto v0.9.1

the following error is returned:

goroutine 964 [running]: github.com/consensys/gnark-crypto/ecc/bn254/fr/kzg.Commit({0xc00029e400?, 0x17e6b70?, 0xc000121f70?}, 0x1060ff9?, {0xc000121f70?, 0xc000121f00?, 0x0?}) /Users/serious/golang/pkg/mod/github.com/consensys/gnark-crypto@v0.9.1/ecc/bn254/fr/kzg/kzg.go:119 +0x60 github.com/consensys/gnark/internal/backend/bn254/plonk.commitToLRO.func1() /Users/serious/golang/pkg/mod/github.com/consensys/gnark@v0.8.0/internal/backend/bn254/plonk/prove.go:455 +0x7f created by github.com/consensys/gnark/internal/backend/bn254/plonk.commitToLRO /Users/serious/golang/pkg/mod/github.com/consensys/gnark@v0.8.0/internal/backend/bn254/plonk/prove.go:454 +0x1e5 panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1235280]

goroutine 1 [running]: github.com/consensys/gnark-crypto/ecc/bn254/fr/kzg.Commit({0xc00029ec00?, 0x20?, 0xc000612260?}, 0xc00059cc10?, {0xc00059cba8?, 0xc00059cbd8?, 0xc00059cbb8?}) /Users/serious/golang/pkg/mod/github.com/consensys/gnark-crypto@v0.9.1/ecc/bn254/fr/kzg/kzg.go:119 +0x60 github.com/consensys/gnark/internal/backend/bn254/plonk.commitToLRO({0xc00029e400, 0xa, 0x20}, {0xc00029e800, 0xa, 0x20}, {0xc00029ec00, 0xa, 0x20}, 0xc000242000, ...) /Users/serious/golang/pkg/mod/github.com/consensys/gnark@v0.8.0/internal/backend/bn254/plonk/prove.go:462 +0x31b github.com/consensys/gnark/internal/backend/bn254/plonk.Prove(0xc0001f4780, 0xc000604480, {0xc000294140?, 0x2, 0x2}, {0x0, 0xc00058a330, {{0x1596a10, 0xc000017bd0}, 0xff, ...}}) /Users/serious/golang/pkg/mod/github.com/consensys/gnark@v0.8.0/internal/backend/bn254/plonk/prove.go:111 +0xc74 github.com/consensys/gnark/backend/plonk.Prove({0x159b130?, 0xc0001f4780?}, {0x15976c8?, 0xc000604480?}, {0x1599778, 0xc000592018}, {0x0?, 0xc0000061a0?, 0x200000003?})

For the develop branch, i.e. the following version,

github.com/consensys/gnark v0.7.2-0.20230605143745-6ec0af711b06 github.com/consensys/gnark-crypto v0.11.1-0.20230508024855-0cd4994b7f0b

the error returned is

09:41:14 DBG constraint system solver done nbConstraints=4 took=0.092125 panic: Proving Fails! invalid polynomial size (larger than SRS or == 0)

I suppose this is related to the logic for compressed/uncompressed serialization and deserialization. I noticed that for groth16, the API exposes both WriteTo and WriteRawTo. With uncompressed serialization for groth16 (WriteRawTo) the above example works fine.

Consensys / gnark

Deserialization of pk fails Plonk Prover #720