--- /dev/null
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements the Socialist Millionaires Protocol as described in
+// http://www.cypherpunks.ca/otr/Protocol-v2-3.1.0.html. The protocol
+// specification is required in order to understand this code and, where
+// possible, the variable names in the code match up with the spec.
+
+package otr
+
+import (
+ "bytes"
+ "crypto/sha256"
+ "errors"
+ "hash"
+ "math/big"
+)
+
+type smpFailure string
+
+func (s smpFailure) Error() string {
+ return string(s)
+}
+
+var smpFailureError = smpFailure("otr: SMP protocol failed")
+var smpSecretMissingError = smpFailure("otr: mutual secret needed")
+
+const smpVersion = 1
+
+const (
+ smpState1 = iota
+ smpState2
+ smpState3
+ smpState4
+)
+
+type smpState struct {
+ state int
+ a2, a3, b2, b3, pb, qb *big.Int
+ g2a, g3a *big.Int
+ g2, g3 *big.Int
+ g3b, papb, qaqb, ra *big.Int
+ saved *tlv
+ secret *big.Int
+ question string
+}
+
+func (c *Conversation) startSMP(question string) (tlvs []tlv) {
+ if c.smp.state != smpState1 {
+ tlvs = append(tlvs, c.generateSMPAbort())
+ }
+ tlvs = append(tlvs, c.generateSMP1(question))
+ c.smp.question = ""
+ c.smp.state = smpState2
+ return
+}
+
+func (c *Conversation) resetSMP() {
+ c.smp.state = smpState1
+ c.smp.secret = nil
+ c.smp.question = ""
+}
+
+func (c *Conversation) processSMP(in tlv) (out tlv, complete bool, err error) {
+ data := in.data
+
+ switch in.typ {
+ case tlvTypeSMPAbort:
+ if c.smp.state != smpState1 {
+ err = smpFailureError
+ }
+ c.resetSMP()
+ return
+ case tlvTypeSMP1WithQuestion:
+ // We preprocess this into a SMP1 message.
+ nulPos := bytes.IndexByte(data, 0)
+ if nulPos == -1 {
+ err = errors.New("otr: SMP message with question didn't contain a NUL byte")
+ return
+ }
+ c.smp.question = string(data[:nulPos])
+ data = data[nulPos+1:]
+ }
+
+ numMPIs, data, ok := getU32(data)
+ if !ok || numMPIs > 20 {
+ err = errors.New("otr: corrupt SMP message")
+ return
+ }
+
+ mpis := make([]*big.Int, numMPIs)
+ for i := range mpis {
+ var ok bool
+ mpis[i], data, ok = getMPI(data)
+ if !ok {
+ err = errors.New("otr: corrupt SMP message")
+ return
+ }
+ }
+
+ switch in.typ {
+ case tlvTypeSMP1, tlvTypeSMP1WithQuestion:
+ if c.smp.state != smpState1 {
+ c.resetSMP()
+ out = c.generateSMPAbort()
+ return
+ }
+ if c.smp.secret == nil {
+ err = smpSecretMissingError
+ return
+ }
+ if err = c.processSMP1(mpis); err != nil {
+ return
+ }
+ c.smp.state = smpState3
+ out = c.generateSMP2()
+ case tlvTypeSMP2:
+ if c.smp.state != smpState2 {
+ c.resetSMP()
+ out = c.generateSMPAbort()
+ return
+ }
+ if out, err = c.processSMP2(mpis); err != nil {
+ out = c.generateSMPAbort()
+ return
+ }
+ c.smp.state = smpState4
+ case tlvTypeSMP3:
+ if c.smp.state != smpState3 {
+ c.resetSMP()
+ out = c.generateSMPAbort()
+ return
+ }
+ if out, err = c.processSMP3(mpis); err != nil {
+ return
+ }
+ c.smp.state = smpState1
+ c.smp.secret = nil
+ complete = true
+ case tlvTypeSMP4:
+ if c.smp.state != smpState4 {
+ c.resetSMP()
+ out = c.generateSMPAbort()
+ return
+ }
+ if err = c.processSMP4(mpis); err != nil {
+ out = c.generateSMPAbort()
+ return
+ }
+ c.smp.state = smpState1
+ c.smp.secret = nil
+ complete = true
+ default:
+ panic("unknown SMP message")
+ }
+
+ return
+}
+
+func (c *Conversation) calcSMPSecret(mutualSecret []byte, weStarted bool) {
+ h := sha256.New()
+ h.Write([]byte{smpVersion})
+ if weStarted {
+ h.Write(c.PrivateKey.PublicKey.Fingerprint())
+ h.Write(c.TheirPublicKey.Fingerprint())
+ } else {
+ h.Write(c.TheirPublicKey.Fingerprint())
+ h.Write(c.PrivateKey.PublicKey.Fingerprint())
+ }
+ h.Write(c.SSID[:])
+ h.Write(mutualSecret)
+ c.smp.secret = new(big.Int).SetBytes(h.Sum(nil))
+}
+
+func (c *Conversation) generateSMP1(question string) tlv {
+ var randBuf [16]byte
+ c.smp.a2 = c.randMPI(randBuf[:])
+ c.smp.a3 = c.randMPI(randBuf[:])
+ g2a := new(big.Int).Exp(g, c.smp.a2, p)
+ g3a := new(big.Int).Exp(g, c.smp.a3, p)
+ h := sha256.New()
+
+ r2 := c.randMPI(randBuf[:])
+ r := new(big.Int).Exp(g, r2, p)
+ c2 := new(big.Int).SetBytes(hashMPIs(h, 1, r))
+ d2 := new(big.Int).Mul(c.smp.a2, c2)
+ d2.Sub(r2, d2)
+ d2.Mod(d2, q)
+ if d2.Sign() < 0 {
+ d2.Add(d2, q)
+ }
+
+ r3 := c.randMPI(randBuf[:])
+ r.Exp(g, r3, p)
+ c3 := new(big.Int).SetBytes(hashMPIs(h, 2, r))
+ d3 := new(big.Int).Mul(c.smp.a3, c3)
+ d3.Sub(r3, d3)
+ d3.Mod(d3, q)
+ if d3.Sign() < 0 {
+ d3.Add(d3, q)
+ }
+
+ var ret tlv
+ if len(question) > 0 {
+ ret.typ = tlvTypeSMP1WithQuestion
+ ret.data = append(ret.data, question...)
+ ret.data = append(ret.data, 0)
+ } else {
+ ret.typ = tlvTypeSMP1
+ }
+ ret.data = appendU32(ret.data, 6)
+ ret.data = appendMPIs(ret.data, g2a, c2, d2, g3a, c3, d3)
+ return ret
+}
+
+func (c *Conversation) processSMP1(mpis []*big.Int) error {
+ if len(mpis) != 6 {
+ return errors.New("otr: incorrect number of arguments in SMP1 message")
+ }
+ g2a := mpis[0]
+ c2 := mpis[1]
+ d2 := mpis[2]
+ g3a := mpis[3]
+ c3 := mpis[4]
+ d3 := mpis[5]
+ h := sha256.New()
+
+ r := new(big.Int).Exp(g, d2, p)
+ s := new(big.Int).Exp(g2a, c2, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+ t := new(big.Int).SetBytes(hashMPIs(h, 1, r))
+ if c2.Cmp(t) != 0 {
+ return errors.New("otr: ZKP c2 incorrect in SMP1 message")
+ }
+ r.Exp(g, d3, p)
+ s.Exp(g3a, c3, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+ t.SetBytes(hashMPIs(h, 2, r))
+ if c3.Cmp(t) != 0 {
+ return errors.New("otr: ZKP c3 incorrect in SMP1 message")
+ }
+
+ c.smp.g2a = g2a
+ c.smp.g3a = g3a
+ return nil
+}
+
+func (c *Conversation) generateSMP2() tlv {
+ var randBuf [16]byte
+ b2 := c.randMPI(randBuf[:])
+ c.smp.b3 = c.randMPI(randBuf[:])
+ r2 := c.randMPI(randBuf[:])
+ r3 := c.randMPI(randBuf[:])
+ r4 := c.randMPI(randBuf[:])
+ r5 := c.randMPI(randBuf[:])
+ r6 := c.randMPI(randBuf[:])
+
+ g2b := new(big.Int).Exp(g, b2, p)
+ g3b := new(big.Int).Exp(g, c.smp.b3, p)
+
+ r := new(big.Int).Exp(g, r2, p)
+ h := sha256.New()
+ c2 := new(big.Int).SetBytes(hashMPIs(h, 3, r))
+ d2 := new(big.Int).Mul(b2, c2)
+ d2.Sub(r2, d2)
+ d2.Mod(d2, q)
+ if d2.Sign() < 0 {
+ d2.Add(d2, q)
+ }
+
+ r.Exp(g, r3, p)
+ c3 := new(big.Int).SetBytes(hashMPIs(h, 4, r))
+ d3 := new(big.Int).Mul(c.smp.b3, c3)
+ d3.Sub(r3, d3)
+ d3.Mod(d3, q)
+ if d3.Sign() < 0 {
+ d3.Add(d3, q)
+ }
+
+ c.smp.g2 = new(big.Int).Exp(c.smp.g2a, b2, p)
+ c.smp.g3 = new(big.Int).Exp(c.smp.g3a, c.smp.b3, p)
+ c.smp.pb = new(big.Int).Exp(c.smp.g3, r4, p)
+ c.smp.qb = new(big.Int).Exp(g, r4, p)
+ r.Exp(c.smp.g2, c.smp.secret, p)
+ c.smp.qb.Mul(c.smp.qb, r)
+ c.smp.qb.Mod(c.smp.qb, p)
+
+ s := new(big.Int)
+ s.Exp(c.smp.g2, r6, p)
+ r.Exp(g, r5, p)
+ s.Mul(r, s)
+ s.Mod(s, p)
+ r.Exp(c.smp.g3, r5, p)
+ cp := new(big.Int).SetBytes(hashMPIs(h, 5, r, s))
+
+ // D5 = r5 - r4 cP mod q and D6 = r6 - y cP mod q
+
+ s.Mul(r4, cp)
+ r.Sub(r5, s)
+ d5 := new(big.Int).Mod(r, q)
+ if d5.Sign() < 0 {
+ d5.Add(d5, q)
+ }
+
+ s.Mul(c.smp.secret, cp)
+ r.Sub(r6, s)
+ d6 := new(big.Int).Mod(r, q)
+ if d6.Sign() < 0 {
+ d6.Add(d6, q)
+ }
+
+ var ret tlv
+ ret.typ = tlvTypeSMP2
+ ret.data = appendU32(ret.data, 11)
+ ret.data = appendMPIs(ret.data, g2b, c2, d2, g3b, c3, d3, c.smp.pb, c.smp.qb, cp, d5, d6)
+ return ret
+}
+
+func (c *Conversation) processSMP2(mpis []*big.Int) (out tlv, err error) {
+ if len(mpis) != 11 {
+ err = errors.New("otr: incorrect number of arguments in SMP2 message")
+ return
+ }
+ g2b := mpis[0]
+ c2 := mpis[1]
+ d2 := mpis[2]
+ g3b := mpis[3]
+ c3 := mpis[4]
+ d3 := mpis[5]
+ pb := mpis[6]
+ qb := mpis[7]
+ cp := mpis[8]
+ d5 := mpis[9]
+ d6 := mpis[10]
+ h := sha256.New()
+
+ r := new(big.Int).Exp(g, d2, p)
+ s := new(big.Int).Exp(g2b, c2, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+ s.SetBytes(hashMPIs(h, 3, r))
+ if c2.Cmp(s) != 0 {
+ err = errors.New("otr: ZKP c2 failed in SMP2 message")
+ return
+ }
+
+ r.Exp(g, d3, p)
+ s.Exp(g3b, c3, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+ s.SetBytes(hashMPIs(h, 4, r))
+ if c3.Cmp(s) != 0 {
+ err = errors.New("otr: ZKP c3 failed in SMP2 message")
+ return
+ }
+
+ c.smp.g2 = new(big.Int).Exp(g2b, c.smp.a2, p)
+ c.smp.g3 = new(big.Int).Exp(g3b, c.smp.a3, p)
+
+ r.Exp(g, d5, p)
+ s.Exp(c.smp.g2, d6, p)
+ r.Mul(r, s)
+ s.Exp(qb, cp, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+
+ s.Exp(c.smp.g3, d5, p)
+ t := new(big.Int).Exp(pb, cp, p)
+ s.Mul(s, t)
+ s.Mod(s, p)
+ t.SetBytes(hashMPIs(h, 5, s, r))
+ if cp.Cmp(t) != 0 {
+ err = errors.New("otr: ZKP cP failed in SMP2 message")
+ return
+ }
+
+ var randBuf [16]byte
+ r4 := c.randMPI(randBuf[:])
+ r5 := c.randMPI(randBuf[:])
+ r6 := c.randMPI(randBuf[:])
+ r7 := c.randMPI(randBuf[:])
+
+ pa := new(big.Int).Exp(c.smp.g3, r4, p)
+ r.Exp(c.smp.g2, c.smp.secret, p)
+ qa := new(big.Int).Exp(g, r4, p)
+ qa.Mul(qa, r)
+ qa.Mod(qa, p)
+
+ r.Exp(g, r5, p)
+ s.Exp(c.smp.g2, r6, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+
+ s.Exp(c.smp.g3, r5, p)
+ cp.SetBytes(hashMPIs(h, 6, s, r))
+
+ r.Mul(r4, cp)
+ d5 = new(big.Int).Sub(r5, r)
+ d5.Mod(d5, q)
+ if d5.Sign() < 0 {
+ d5.Add(d5, q)
+ }
+
+ r.Mul(c.smp.secret, cp)
+ d6 = new(big.Int).Sub(r6, r)
+ d6.Mod(d6, q)
+ if d6.Sign() < 0 {
+ d6.Add(d6, q)
+ }
+
+ r.ModInverse(qb, p)
+ qaqb := new(big.Int).Mul(qa, r)
+ qaqb.Mod(qaqb, p)
+
+ ra := new(big.Int).Exp(qaqb, c.smp.a3, p)
+ r.Exp(qaqb, r7, p)
+ s.Exp(g, r7, p)
+ cr := new(big.Int).SetBytes(hashMPIs(h, 7, s, r))
+
+ r.Mul(c.smp.a3, cr)
+ d7 := new(big.Int).Sub(r7, r)
+ d7.Mod(d7, q)
+ if d7.Sign() < 0 {
+ d7.Add(d7, q)
+ }
+
+ c.smp.g3b = g3b
+ c.smp.qaqb = qaqb
+
+ r.ModInverse(pb, p)
+ c.smp.papb = new(big.Int).Mul(pa, r)
+ c.smp.papb.Mod(c.smp.papb, p)
+ c.smp.ra = ra
+
+ out.typ = tlvTypeSMP3
+ out.data = appendU32(out.data, 8)
+ out.data = appendMPIs(out.data, pa, qa, cp, d5, d6, ra, cr, d7)
+ return
+}
+
+func (c *Conversation) processSMP3(mpis []*big.Int) (out tlv, err error) {
+ if len(mpis) != 8 {
+ err = errors.New("otr: incorrect number of arguments in SMP3 message")
+ return
+ }
+ pa := mpis[0]
+ qa := mpis[1]
+ cp := mpis[2]
+ d5 := mpis[3]
+ d6 := mpis[4]
+ ra := mpis[5]
+ cr := mpis[6]
+ d7 := mpis[7]
+ h := sha256.New()
+
+ r := new(big.Int).Exp(g, d5, p)
+ s := new(big.Int).Exp(c.smp.g2, d6, p)
+ r.Mul(r, s)
+ s.Exp(qa, cp, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+
+ s.Exp(c.smp.g3, d5, p)
+ t := new(big.Int).Exp(pa, cp, p)
+ s.Mul(s, t)
+ s.Mod(s, p)
+ t.SetBytes(hashMPIs(h, 6, s, r))
+ if t.Cmp(cp) != 0 {
+ err = errors.New("otr: ZKP cP failed in SMP3 message")
+ return
+ }
+
+ r.ModInverse(c.smp.qb, p)
+ qaqb := new(big.Int).Mul(qa, r)
+ qaqb.Mod(qaqb, p)
+
+ r.Exp(qaqb, d7, p)
+ s.Exp(ra, cr, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+
+ s.Exp(g, d7, p)
+ t.Exp(c.smp.g3a, cr, p)
+ s.Mul(s, t)
+ s.Mod(s, p)
+ t.SetBytes(hashMPIs(h, 7, s, r))
+ if t.Cmp(cr) != 0 {
+ err = errors.New("otr: ZKP cR failed in SMP3 message")
+ return
+ }
+
+ var randBuf [16]byte
+ r7 := c.randMPI(randBuf[:])
+ rb := new(big.Int).Exp(qaqb, c.smp.b3, p)
+
+ r.Exp(qaqb, r7, p)
+ s.Exp(g, r7, p)
+ cr = new(big.Int).SetBytes(hashMPIs(h, 8, s, r))
+
+ r.Mul(c.smp.b3, cr)
+ d7 = new(big.Int).Sub(r7, r)
+ d7.Mod(d7, q)
+ if d7.Sign() < 0 {
+ d7.Add(d7, q)
+ }
+
+ out.typ = tlvTypeSMP4
+ out.data = appendU32(out.data, 3)
+ out.data = appendMPIs(out.data, rb, cr, d7)
+
+ r.ModInverse(c.smp.pb, p)
+ r.Mul(pa, r)
+ r.Mod(r, p)
+ s.Exp(ra, c.smp.b3, p)
+ if r.Cmp(s) != 0 {
+ err = smpFailureError
+ }
+
+ return
+}
+
+func (c *Conversation) processSMP4(mpis []*big.Int) error {
+ if len(mpis) != 3 {
+ return errors.New("otr: incorrect number of arguments in SMP4 message")
+ }
+ rb := mpis[0]
+ cr := mpis[1]
+ d7 := mpis[2]
+ h := sha256.New()
+
+ r := new(big.Int).Exp(c.smp.qaqb, d7, p)
+ s := new(big.Int).Exp(rb, cr, p)
+ r.Mul(r, s)
+ r.Mod(r, p)
+
+ s.Exp(g, d7, p)
+ t := new(big.Int).Exp(c.smp.g3b, cr, p)
+ s.Mul(s, t)
+ s.Mod(s, p)
+ t.SetBytes(hashMPIs(h, 8, s, r))
+ if t.Cmp(cr) != 0 {
+ return errors.New("otr: ZKP cR failed in SMP4 message")
+ }
+
+ r.Exp(rb, c.smp.a3, p)
+ if r.Cmp(c.smp.papb) != 0 {
+ return smpFailureError
+ }
+
+ return nil
+}
+
+func (c *Conversation) generateSMPAbort() tlv {
+ return tlv{typ: tlvTypeSMPAbort}
+}
+
+func hashMPIs(h hash.Hash, magic byte, mpis ...*big.Int) []byte {
+ if h != nil {
+ h.Reset()
+ } else {
+ h = sha256.New()
+ }
+
+ h.Write([]byte{magic})
+ for _, mpi := range mpis {
+ h.Write(appendMPI(nil, mpi))
+ }
+ return h.Sum(nil)
+}
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