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