bbssig

Documentation

Overview

Package bbssig implements the BBS, short group signatures scheme as detailed in http://crypto.stanford.edu/~dabo/papers/groupsigs.pdf.

Group signatures are a variation on traditional public-key signatures in that there are many different private-keys that can create valid signatures. Holders of these private keys are called members, and a signature from any member is indistinguishable from any other, from the point of view of the public key.

However, there is also a group private key, which differs from the member private keys. The group private key can create new member private keys, and can open signatures and discover which member created the signature.

This implementation of group signatures also supports revocation of member private keys. The group private key can produce a public 'revocation' of a member private key. A revocation can be combined with the group, and with each member private key to produce an updated group and updated private keys. Signatures under the old keys are invalid under the new but, critically, the revoked private key cannot be updated due to a divisor becoming zero.

This form of revocation is complicated, but avoids deanonymising all previous signatures from the revoked member.

Index

• Constants
• type Group
  • func (g *Group) Marshal() []byte
  • func (g *Group) Unmarshal(b []byte) (*Group, bool)
  • func (g *Group) Update(r *Revocation)
  • func (g *Group) Verify(digest []byte, hashFunc hash.Hash, sig []byte) bool
• type MemberKey
  • func (mem *MemberKey) Marshal() []byte
  • func (mem *MemberKey) Sign(r io.Reader, digest []byte, hashFunc hash.Hash) ([]byte, error)
  • func (mem *MemberKey) Tag() []byte
  • func (mem *MemberKey) Unmarshal(g *Group, b []byte) (*MemberKey, bool)
  • func (mem *MemberKey) Update(r *Revocation) bool
• type PrivateKey
  • func GenerateGroup(r io.Reader) (*PrivateKey, error)
  • func (priv *PrivateKey) GenerateRevocation(mem *MemberKey) *Revocation
  • func (priv *PrivateKey) Marshal() []byte
  • func (priv *PrivateKey) NewMember(r io.Reader) (*MemberKey, error)
  • func (priv *PrivateKey) Open(sig []byte) ([]byte, bool)
  • func (priv *PrivateKey) Unmarshal(g *Group, b []byte) (*PrivateKey, bool)
• type Revocation
  • func (r *Revocation) Marshal() []byte
  • func (r *Revocation) Unmarshal(b []byte) (*Revocation, bool)

Constants

const SignatureSize = 12 * 32

SignatureSize is the size, in bytes, of the signatures produced by this package. (3072 bits.)

Types

type Group

type Group struct {
	// contains filtered or unexported fields
}

Group represents a public key in the group signature scheme. Signatures by the group members can be verified given the Group.

func (*Group) Marshal

func (g *Group) Marshal() []byte

Marshal serializes g to a slice of bytes, suitable for Unmarshal.

func (*Group) Unmarshal

func (g *Group) Unmarshal(b []byte) (*Group, bool)

Unmarshal sets g to the result of unmarshaling b and returns both g and a bool that is true on success. Since Group contains some precomputed values that aren't included in the serialisation, Unmarshal does significant computation.

func (*Group) Update

func (g *Group) Update(r *Revocation)

Update alters g to create a new Group that includes all previous members, save a specifically revoked member.

func (*Group) Verify

func (g *Group) Verify(digest []byte, hashFunc hash.Hash, sig []byte) bool

Verify verifies that sig is a valid signature of digest using the given hash function.

type MemberKey

type MemberKey struct {
	*Group
	// contains filtered or unexported fields
}

MemberKey represents a member private key. It is capable of signing messages such that nobody, save the holder of the group private key, can determine which member of the group made the signature.

func (*MemberKey) Marshal

func (mem *MemberKey) Marshal() []byte

Marshal serializes mem to a slice of bytes, suitable for Unmarshal.

func (*MemberKey) Sign

func (mem *MemberKey) Sign(r io.Reader, digest []byte, hashFunc hash.Hash) ([]byte, error)

Sign computes a group signature of digest using the given hash function.

func (*MemberKey) Tag

func (mem *MemberKey) Tag() []byte

Tag returns an opaque byte slice that identifies the member private key for the purposes of comparing against the result of Open.

func (*MemberKey) Unmarshal

func (mem *MemberKey) Unmarshal(g *Group, b []byte) (*MemberKey, bool)

Unmarshal sets mem to the result of unmarshaling b and returns both mem and a bool that is true on success.

func (*MemberKey) Update

func (mem *MemberKey) Update(r *Revocation) bool

Update alters mem to create a member private key for an updated Group. (Note that the Group of mem must also be updated.) This functions returns false if mem is the member private key that has been revoked.

type PrivateKey

type PrivateKey struct {
	*Group
	// contains filtered or unexported fields
}

PrivateKey represents a group private key. The holder of the private key can create new group members and can reveal which member created a given signature.

func GenerateGroup

func GenerateGroup(r io.Reader) (*PrivateKey, error)

GenerateGroup generates a new group and group private key.

func (*PrivateKey) GenerateRevocation

func (priv *PrivateKey) GenerateRevocation(mem *MemberKey) *Revocation

GenerateRevocation creates a Revocation that revokes the given member private key.

func (*PrivateKey) Marshal

func (priv *PrivateKey) Marshal() []byte

Marshal serializes priv to a slice of bytes, suitable for Unmarshal.

func (*PrivateKey) NewMember

func (priv *PrivateKey) NewMember(r io.Reader) (*MemberKey, error)

NewMember creates a new member private key for the group.

func (*PrivateKey) Open

func (priv *PrivateKey) Open(sig []byte) ([]byte, bool)

Open reveals which member private key made the given signature. The return value will match the result of calling Tag on the member private key in question.

func (*PrivateKey) Unmarshal

func (priv *PrivateKey) Unmarshal(g *Group, b []byte) (*PrivateKey, bool)

Unmarshal sets priv to the result of unmarshaling b and returns both priv and a bool that is true on success.

type Revocation

type Revocation struct {
	// contains filtered or unexported fields
}

Revocation represents a revocation of a member private key. A Revocation can be applied to update a member private key and also to a group to create a new group that does not include the revoked member.

func (*Revocation) Marshal

func (r *Revocation) Marshal() []byte

Marshal serializes r to a slice of bytes, suitable for Unmarshal.

func (*Revocation) Unmarshal

func (r *Revocation) Unmarshal(b []byte) (*Revocation, bool)