// WritePrivKey will write the private key into the filename given
// It takes a suite in order to adequatly write the secret
// Returns an error if anything went wrong during file handling or writing key
func WritePrivKey(suite abstract.Suite, fileName string, priv abstract.Secret) error {
// Opening file
privFile, err := os.OpenFile(fileName, os.O_TRUNC|os.O_WRONLY|os.O_CREATE, 0744)
if err != nil {
return err
}
defer privFile.Close()
// Writing down !
err = suite.Write(privFile, priv)
if err != nil {
return err
}
privFile.WriteString("\n")
return nil
}
// This simplified implementation of Schnorr Signatures is based on
// crypto/anon/sig.go
// The ring structure is removed and
// The anonimity set is reduced to one public key = no anonimity
func SchnorrSign(suite abstract.Suite, random cipher.Stream, message []byte,
privateKey abstract.Scalar) []byte {
// Create random secret v and public point commitment T
v := suite.Scalar().Pick(random)
T := suite.Point().Mul(nil, v)
// Create challenge c based on message and T
c := hashSchnorr(suite, message, T)
// Compute response r = v - x*c
r := suite.Scalar()
r.Mul(privateKey, c).Sub(v, r)
// Return verifiable signature {c, r}
// Verifier will be able to compute v = r + x*c
// And check that hashElgamal for T and the message == c
buf := bytes.Buffer{}
sig := basicSig{c, r}
suite.Write(&buf, &sig)
return buf.Bytes()
}
// Generate a new public/private keypair with the given ciphersuite
// and Save it to the application's previously-loaded configuration.
func (f *File) GenKey(keys *Keys, suite abstract.Suite) (KeyPair, error) {
// Create the map if it doesn't exist
// if *keys == nil {
// *keys = make(map[string] KeyInfo)
// }
// Create a fresh public/private keypair
p := KeyPair{}
p.Gen(suite, random.Stream)
pubId := p.PubId()
// Write the private key file
secname := f.dirName + "/sec-" + pubId
r := util.Replacer{}
if err := r.Open(secname); err != nil {
return KeyPair{}, err
}
defer r.Abort()
// Write the secret key
if err := suite.Write(r.File, &p.Secret); err != nil {
return KeyPair{}, err
}
// Commit the secret key
if err := r.Commit(); err != nil {
return KeyPair{}, err
}
// Re-write the config file with the new public key
*keys = append(*keys, KeyInfo{suite.String(), pubId})
if err := f.Save(); err != nil {
return KeyPair{}, err
}
return p, nil
}
// Sign creates an optionally anonymous, optionally linkable
// signature on a given message.
//
// The caller supplies one or more public keys representing an anonymity set,
// and the private key corresponding to one of those public keys.
// The resulting signature proves to a verifier that the owner of
// one of these public keys signed the message,
// without revealing which key-holder signed the message,
// offering anonymity among the members of this explicit anonymity set.
// The other users whose keys are listed in the anonymity set need not consent
// or even be aware that they have been included in an anonymity set:
// anyone having a suitable public key may be "conscripted" into a set.
//
// If the provided anonymity set contains only one public key (the signer's),
// then this function produces a traditional non-anonymous signature,
// equivalent in both size and performance to a standard ElGamal signature.
//
// The caller may request either unlinkable or linkable anonymous signatures.
// If linkScope is nil, this function generates an unlinkable signature,
// which contains no information about which member signed the message.
// The anonymity provided by unlinkable signatures is forward-secure,
// in that a signature reveals nothing about which member generated it,
// even if all members' private keys are later released.
// For cryptographic background on unlinkable anonymity-set signatures -
// also known as ring signatures or ad-hoc group signatures -
// see Rivest, "How to Leak a Secret" at
// http://people.csail.mit.edu/rivest/RivestShamirTauman-HowToLeakASecret.pdf.
//
// If the caller passes a non-nil linkScope,
// the resulting anonymous signature will be linkable.
// This means that given two signatures produced using the same linkScope,
// a verifier will be able to tell whether
// the same or different anonymity set members produced those signatures.
// In particular, verifying a linkable signature yields a linkage tag.
// This linkage tag has a 1-to-1 correspondence with the signer's public key
// within a given linkScope, but is cryptographically unlinkable
// to either the signer's public key or to linkage tags in other scopes.
// The provided linkScope may be an arbitrary byte-string;
// the only significance these scopes have is whether they are equal or unequal.
// For details on the linkable signature algorithm this function implements,
// see Liu/Wei/Wong,
// "Linkable Spontaneous Anonymous Group Signature for Ad Hoc Groups" at
// http://www.cs.cityu.edu.hk/~duncan/papers/04liuetal_lsag.pdf.
//
// Linkage tags may be used to protect against sock-puppetry or Sybil attacks
// in situations where a verifier needs to know how many distinct members
// of an anonymity set are present or signed messages in a given context.
// It is cryptographically hard for one anonymity set member
// to produce signatures with different linkage tags in the same scope.
// An important and fundamental downside, however, is that
// linkable signatures do NOT offer forward-secure anonymity.
// If an anonymity set member's private key is later released,
// it is trivial to check whether or not that member produced a given signature.
// Also, anonymity set members who did NOT sign a message could
// (voluntarily or under coercion) prove that they did not sign it,
// e.g., simply by signing some other message in that linkage context
// and noting that the resulting linkage tag comes out different.
// Thus, linkable anonymous signatures are not appropriate to use
// in situations where there may be significant risk
// that members' private keys may later be compromised,
// or that members may be persuaded or coerced into revealing whether or not
// they produced a signature of interest.
//
func Sign(suite abstract.Suite, random cipher.Stream, message []byte,
anonymitySet Set, linkScope []byte, mine int, privateKey abstract.Secret) []byte {
// Note that Rivest's original ring construction directly supports
// heterogeneous rings containing public keys of different types -
// e.g., a mixture of RSA keys and DSA keys with varying parameters.
// Our ring signature construction currently supports
// only homogeneous rings containing compatible keys
// drawn from the cipher suite (e.g., the same elliptic curve).
// The upside to this constrint is greater flexibility:
// e.g., we also easily obtain linkable ring signatures,
// which are not readily feasible with the original ring construction.
n := len(anonymitySet) // anonymity set size
L := []abstract.Point(anonymitySet) // public keys in anonymity set
pi := mine
// If we want a linkable ring signature, produce correct linkage tag,
// as a pseudorandom base point multiplied by our private key.
// Liu's scheme specifies the linkScope as a hash of the ring;
// this is one reasonable choice of linkage scope,
// but there are others, so we parameterize this choice.
var linkBase, linkTag abstract.Point
if linkScope != nil {
linkStream := suite.Cipher(linkScope)
linkBase, _ = suite.Point().Pick(nil, linkStream)
linkTag = suite.Point().Mul(linkBase, privateKey)
}
// First pre-hash the parameters to H1
// that are invariant for different ring positions,
// so that we don't have to hash them many times.
H1pre := signH1pre(suite, linkScope, linkTag, message)
// Pick a random commit for my ring position
u := suite.Secret().Pick(random)
var UB, UL abstract.Point
UB = suite.Point().Mul(nil, u)
if linkScope != nil {
UL = suite.Point().Mul(linkBase, u)
}
// Build the challenge ring
s := make([]abstract.Secret, n)
c := make([]abstract.Secret, n)
c[(pi+1)%n] = signH1(suite, H1pre, UB, UL)
var P, PG, PH abstract.Point
P = suite.Point()
PG = suite.Point()
if linkScope != nil {
PH = suite.Point()
}
for i := (pi + 1) % n; i != pi; i = (i + 1) % n {
s[i] = suite.Secret().Pick(random)
PG.Add(PG.Mul(nil, s[i]), P.Mul(L[i], c[i]))
if linkScope != nil {
PH.Add(PH.Mul(linkBase, s[i]), P.Mul(linkTag, c[i]))
}
c[(i+1)%n] = signH1(suite, H1pre, PG, PH)
//fmt.Printf("s%d %s\n",i,s[i].String())
//fmt.Printf("c%d %s\n",(i+1)%n,c[(i+1)%n].String())
}
s[pi] = suite.Secret()
s[pi].Mul(privateKey, c[pi]).Sub(u, s[pi]) // s_pi = u - x_pi c_pi
// Encode and return the signature
buf := bytes.Buffer{}
if linkScope != nil { // linkable ring signature
sig := lSig{uSig{c[0], s}, linkTag}
suite.Write(&buf, &sig)
} else { // unlinkable ring signature
sig := uSig{c[0], s}
suite.Write(&buf, &sig)
}
return buf.Bytes()
}
func write64(suite abstract.Suite, wc io.WriteCloser, data ...interface{}) error {
if err := suite.Write(wc, data); err != nil {
return err
}
return wc.Close()
}
func WriteSecret64(suite abstract.Suite, w io.Writer, secret abstract.Secret) error {
enc := base64.NewEncoder(base64.StdEncoding, w)
err := suite.Write(enc, secret)
enc.Close()
return err
}
// Write a public point to a base64 representation
func WritePub64(suite abstract.Suite, w io.Writer, point abstract.Point) error {
enc := base64.NewEncoder(base64.StdEncoding, w)
err := suite.Write(enc, point)
enc.Close()
return err
}
