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73 lines
2.5 KiB
Go
73 lines
2.5 KiB
Go
package client
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import (
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"crypto"
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"crypto/rand"
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"crypto/sha256"
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"encoding/binary"
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"github.com/cbeuw/Cloak/internal/util"
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ecdh "github.com/cbeuw/go-ecdh"
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"io"
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)
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type keyPair struct {
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crypto.PrivateKey
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crypto.PublicKey
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}
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func MakeRandomField(sta *State) []byte {
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t := make([]byte, 8)
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binary.BigEndian.PutUint64(t, uint64(sta.Now().Unix()/(12*60*60)))
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rdm := make([]byte, 16)
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io.ReadFull(rand.Reader, rdm)
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preHash := make([]byte, 56)
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copy(preHash[0:32], sta.UID)
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copy(preHash[32:40], t)
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copy(preHash[40:56], rdm)
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h := sha256.New()
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h.Write(preHash)
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ret := make([]byte, 32)
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copy(ret[0:16], rdm)
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copy(ret[16:32], h.Sum(nil)[0:16])
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return ret
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}
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func MakeSessionTicket(sta *State) []byte {
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// sessionTicket: [marshalled ephemeral pub key 32 bytes][encrypted UID+sessionID 36 bytes][padding 124 bytes]
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// The first 16 bytes of the marshalled ephemeral public key is used as the IV
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// for encrypting the UID
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tthInterval := sta.Now().Unix() / int64(sta.TicketTimeHint)
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ec := ecdh.NewCurve25519ECDH()
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sta.keyPairsM.Lock()
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ephKP := sta.keyPairs[tthInterval]
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if ephKP == nil {
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ephPv, ephPub, _ := ec.GenerateKey(rand.Reader)
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ephKP = &keyPair{
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ephPv,
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ephPub,
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}
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sta.keyPairs[tthInterval] = ephKP
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}
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sta.keyPairsM.Unlock()
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ticket := make([]byte, 192)
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copy(ticket[0:32], ec.Marshal(ephKP.PublicKey))
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key, _ := ec.GenerateSharedSecret(ephKP.PrivateKey, sta.staticPub)
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plainUIDsID := make([]byte, 36)
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copy(plainUIDsID, sta.UID)
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binary.BigEndian.PutUint32(plainUIDsID[32:36], sta.sessionID)
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cipherUIDsID := util.AESEncrypt(ticket[0:16], key, plainUIDsID)
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copy(ticket[32:68], cipherUIDsID)
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// The purpose of adding sessionID is that, the generated padding of sessionTicket needs to be unpredictable.
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// As shown in auth.go, the padding is generated by a psudo random generator. The seed
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// needs to be the same for each TicketTimeHint interval. However the value of epoch/TicketTimeHint
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// is public knowledge, so is the psudo random algorithm used by math/rand. Therefore not only
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// can the firewall tell that the padding is generated in this specific way, this padding is identical
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// for all ckclients in the same TicketTimeHint interval. This will expose us.
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//
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// With the sessionID value generated at startup of ckclient and used as a part of the seed, the
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// sessionTicket is still identical for each TicketTimeHint interval, but others won't be able to know
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// how it was generated. It will also be different for each client.
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copy(ticket[68:192], util.PsudoRandBytes(124, tthInterval+int64(sta.sessionID)))
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return ticket
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}
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