canopy-network/canopy · error

invalid empty class-group coefficient

Error message

invalid empty class-group coefficient

What it means

Raised in ClassGroup.Decode when the decoded ProtoClassGroup has an empty A or B coefficient byte slice. A class group form requires both coefficients to be present; empty coefficients indicate a corrupted or truncated VDF class-group encoding that cannot reconstruct a valid group.

Source

Thrown at lib/crypto/classgroup.go:336

// Encode() converts the ClassGroup into bytes
func (group *ClassGroup) Encode() (bz []byte) {
	r := group.Reduced()
	bz, _ = cdc.Marshal(&ProtoClassGroup{
		A: encodeBigInt(r.a),
		B: encodeBigInt(r.b),
	})
	return
}

// Encode() converts the bytes into ClassGroup
func (group *ClassGroup) Decode(bz []byte, discriminant *big.Int) (err error) {
	classGroup := new(ProtoClassGroup)
	if err = cdc.Unmarshal(bz, classGroup); err != nil {
		return err
	}
	if len(classGroup.A) == 0 || len(classGroup.B) == 0 {
		return fmt.Errorf("invalid empty class-group coefficient")
	}
	// convert the byte slices to big.Int values
	a := decodeBigInt(classGroup.A)
	b := decodeBigInt(classGroup.B)
	*group = *newClassGroup(a, b, discriminant)
	return
}

// Equal() compares two ClassGroups using their underlying values and returns true if they are equivalent
func (group *ClassGroup) Equal(other *ClassGroup) bool {
	// reduce both to their simplest form
	g := group.Reduced()
	o := other.Reduced()
	// compare a,b,c
	return g.a.Cmp(o.a) == 0 && g.b.Cmp(o.b) == 0 && g.c.Cmp(o.c) == 0
}

// Discard() recycles the big ints used for the class group

View on GitHub (pinned to ee8197d91d)

Solutions

1. Re-encode the ClassGroup via Encode() at the source; the producing side serialized zeroed coefficients.
2. Validate discriminant/parameters used in the VDF proof before decode so the right group is reconstructed.
3. Check for data truncation in storage/transport that zeroed the coefficient bytes.

When it happens

Trigger: Thrown at lib/crypto/classgroup.go:336 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of canopy-network/canopy@ee8197d91d (2026-09-06). Data as JSON: /api/errors/4d8f81c56d9cb768. Report an issue: GitHub.