ssh: add diffie-hellman-group-exchange-sha256
Add the diffie-hellman-group-exchange-sha256 defined in RFC 4419 to
the list of supported key exchange algorithms for ssh.
The server half is only a minimal implementation to satisfy the automated tests.
Fixes golang/go#17230
Change-Id: I25880a564347fd9b4738dd2ed1e347cd5d2e21bb
GitHub-Last-Rev: 9f0b8d02c0c96e9baf00cdf1cf063ff834245443
GitHub-Pull-Request: golang/crypto#87
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/174257
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
diff --git a/ssh/client_auth_test.go b/ssh/client_auth_test.go
index 9200cb3..63a8e22 100644
--- a/ssh/client_auth_test.go
+++ b/ssh/client_auth_test.go
@@ -316,7 +316,7 @@
PublicKeys(),
},
Config: Config{
- KeyExchanges: []string{"diffie-hellman-group-exchange-sha256"}, // not currently supported
+ KeyExchanges: []string{"non-existent-kex"},
},
HostKeyCallback: InsecureIgnoreHostKey(),
}
diff --git a/ssh/common.go b/ssh/common.go
index d97415d..e55fe0a 100644
--- a/ssh/common.go
+++ b/ssh/common.go
@@ -51,6 +51,13 @@
kexAlgoDH14SHA1, kexAlgoDH1SHA1,
}
+// serverForbiddenKexAlgos contains key exchange algorithms, that are forbidden
+// for the server half.
+var serverForbiddenKexAlgos = map[string]struct{}{
+ kexAlgoDHGEXSHA1: {}, // server half implementation is only minimal to satisfy the automated tests
+ kexAlgoDHGEXSHA256: {}, // server half implementation is only minimal to satisfy the automated tests
+}
+
// supportedHostKeyAlgos specifies the supported host-key algorithms (i.e. methods
// of authenticating servers) in preference order.
var supportedHostKeyAlgos = []string{
diff --git a/ssh/kex.go b/ssh/kex.go
index f34bcc0..1607200 100644
--- a/ssh/kex.go
+++ b/ssh/kex.go
@@ -10,7 +10,9 @@
"crypto/elliptic"
"crypto/rand"
"crypto/subtle"
+ "encoding/binary"
"errors"
+ "fmt"
"io"
"math/big"
@@ -24,6 +26,12 @@
kexAlgoECDH384 = "ecdh-sha2-nistp384"
kexAlgoECDH521 = "ecdh-sha2-nistp521"
kexAlgoCurve25519SHA256 = "curve25519-sha256@libssh.org"
+
+ // For the following kex only the client half contains a production
+ // ready implementation. The server half only consists of a minimal
+ // implementation to satisfy the automated tests.
+ kexAlgoDHGEXSHA1 = "diffie-hellman-group-exchange-sha1"
+ kexAlgoDHGEXSHA256 = "diffie-hellman-group-exchange-sha256"
)
// kexResult captures the outcome of a key exchange.
@@ -402,6 +410,8 @@
kexAlgoMap[kexAlgoECDH384] = &ecdh{elliptic.P384()}
kexAlgoMap[kexAlgoECDH256] = &ecdh{elliptic.P256()}
kexAlgoMap[kexAlgoCurve25519SHA256] = &curve25519sha256{}
+ kexAlgoMap[kexAlgoDHGEXSHA1] = &dhGEXSHA{hashFunc: crypto.SHA1}
+ kexAlgoMap[kexAlgoDHGEXSHA256] = &dhGEXSHA{hashFunc: crypto.SHA256}
}
// curve25519sha256 implements the curve25519-sha256@libssh.org key
@@ -538,3 +548,242 @@
Hash: crypto.SHA256,
}, nil
}
+
+// dhGEXSHA implements the diffie-hellman-group-exchange-sha1 and
+// diffie-hellman-group-exchange-sha256 key agreement protocols,
+// as described in RFC 4419
+type dhGEXSHA struct {
+ g, p *big.Int
+ hashFunc crypto.Hash
+}
+
+const numMRTests = 64
+
+const (
+ dhGroupExchangeMinimumBits = 2048
+ dhGroupExchangePreferredBits = 2048
+ dhGroupExchangeMaximumBits = 8192
+)
+
+func (gex *dhGEXSHA) diffieHellman(theirPublic, myPrivate *big.Int) (*big.Int, error) {
+ if theirPublic.Sign() <= 0 || theirPublic.Cmp(gex.p) >= 0 {
+ return nil, fmt.Errorf("ssh: DH parameter out of bounds")
+ }
+ return new(big.Int).Exp(theirPublic, myPrivate, gex.p), nil
+}
+
+func (gex *dhGEXSHA) Client(c packetConn, randSource io.Reader, magics *handshakeMagics) (*kexResult, error) {
+ // Send GexRequest
+ kexDHGexRequest := kexDHGexRequestMsg{
+ MinBits: dhGroupExchangeMinimumBits,
+ PreferedBits: dhGroupExchangePreferredBits,
+ MaxBits: dhGroupExchangeMaximumBits,
+ }
+ if err := c.writePacket(Marshal(&kexDHGexRequest)); err != nil {
+ return nil, err
+ }
+
+ // Receive GexGroup
+ packet, err := c.readPacket()
+ if err != nil {
+ return nil, err
+ }
+
+ var kexDHGexGroup kexDHGexGroupMsg
+ if err = Unmarshal(packet, &kexDHGexGroup); err != nil {
+ return nil, err
+ }
+
+ // reject if p's bit length < dhGroupExchangeMinimumBits or > dhGroupExchangeMaximumBits
+ if kexDHGexGroup.P.BitLen() < dhGroupExchangeMinimumBits || kexDHGexGroup.P.BitLen() > dhGroupExchangeMaximumBits {
+ return nil, fmt.Errorf("ssh: server-generated gex p is out of range (%d bits)", kexDHGexGroup.P.BitLen())
+ }
+
+ gex.p = kexDHGexGroup.P
+ gex.g = kexDHGexGroup.G
+
+ // Check if p is safe by verifing that p and (p-1)/2 are primes
+ one := big.NewInt(1)
+ var pHalf = &big.Int{}
+ pHalf.Rsh(gex.p, 1)
+ if !gex.p.ProbablyPrime(numMRTests) || !pHalf.ProbablyPrime(numMRTests) {
+ return nil, fmt.Errorf("ssh: server provided gex p is not safe")
+ }
+
+ // Check if g is safe by verifing that g > 1 and g < p - 1
+ var pMinusOne = &big.Int{}
+ pMinusOne.Sub(gex.p, one)
+ if gex.g.Cmp(one) != 1 && gex.g.Cmp(pMinusOne) != -1 {
+ return nil, fmt.Errorf("ssh: server provided gex g is not safe")
+ }
+
+ // Send GexInit
+ x, err := rand.Int(randSource, pHalf)
+ if err != nil {
+ return nil, err
+ }
+ X := new(big.Int).Exp(gex.g, x, gex.p)
+ kexDHGexInit := kexDHGexInitMsg{
+ X: X,
+ }
+ if err := c.writePacket(Marshal(&kexDHGexInit)); err != nil {
+ return nil, err
+ }
+
+ // Receive GexReply
+ packet, err = c.readPacket()
+ if err != nil {
+ return nil, err
+ }
+
+ var kexDHGexReply kexDHGexReplyMsg
+ if err = Unmarshal(packet, &kexDHGexReply); err != nil {
+ return nil, err
+ }
+
+ kInt, err := gex.diffieHellman(kexDHGexReply.Y, x)
+ if err != nil {
+ return nil, err
+ }
+
+ // Check if k is safe by verifing that k > 1 and k < p - 1
+ if kInt.Cmp(one) != 1 && kInt.Cmp(pMinusOne) != -1 {
+ return nil, fmt.Errorf("ssh: derived k is not safe")
+ }
+
+ h := gex.hashFunc.New()
+ magics.write(h)
+ writeString(h, kexDHGexReply.HostKey)
+ binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMinimumBits))
+ binary.Write(h, binary.BigEndian, uint32(dhGroupExchangePreferredBits))
+ binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMaximumBits))
+ writeInt(h, gex.p)
+ writeInt(h, gex.g)
+ writeInt(h, X)
+ writeInt(h, kexDHGexReply.Y)
+ K := make([]byte, intLength(kInt))
+ marshalInt(K, kInt)
+ h.Write(K)
+
+ return &kexResult{
+ H: h.Sum(nil),
+ K: K,
+ HostKey: kexDHGexReply.HostKey,
+ Signature: kexDHGexReply.Signature,
+ Hash: gex.hashFunc,
+ }, nil
+}
+
+// Server half implementation of the Diffie Hellman Key Exchange with SHA1 and SHA256.
+//
+// This is a minimal implementation to satisfy the automated tests.
+func (gex *dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshakeMagics, priv Signer) (result *kexResult, err error) {
+ // Receive GexRequest
+ packet, err := c.readPacket()
+ if err != nil {
+ return
+ }
+ var kexDHGexRequest kexDHGexRequestMsg
+ if err = Unmarshal(packet, &kexDHGexRequest); err != nil {
+ return
+ }
+
+ // smoosh the user's preferred size into our own limits
+ if kexDHGexRequest.PreferedBits > dhGroupExchangeMaximumBits {
+ kexDHGexRequest.PreferedBits = dhGroupExchangeMaximumBits
+ }
+ if kexDHGexRequest.PreferedBits < dhGroupExchangeMinimumBits {
+ kexDHGexRequest.PreferedBits = dhGroupExchangeMinimumBits
+ }
+ // fix min/max if they're inconsistent. technically, we could just pout
+ // and hang up, but there's no harm in giving them the benefit of the
+ // doubt and just picking a bitsize for them.
+ if kexDHGexRequest.MinBits > kexDHGexRequest.PreferedBits {
+ kexDHGexRequest.MinBits = kexDHGexRequest.PreferedBits
+ }
+ if kexDHGexRequest.MaxBits < kexDHGexRequest.PreferedBits {
+ kexDHGexRequest.MaxBits = kexDHGexRequest.PreferedBits
+ }
+
+ // Send GexGroup
+ // This is the group called diffie-hellman-group14-sha1 in RFC
+ // 4253 and Oakley Group 14 in RFC 3526.
+ p, _ := new(big.Int).SetString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
+ gex.p = p
+ gex.g = big.NewInt(2)
+
+ kexDHGexGroup := kexDHGexGroupMsg{
+ P: gex.p,
+ G: gex.g,
+ }
+ if err := c.writePacket(Marshal(&kexDHGexGroup)); err != nil {
+ return nil, err
+ }
+
+ // Receive GexInit
+ packet, err = c.readPacket()
+ if err != nil {
+ return
+ }
+ var kexDHGexInit kexDHGexInitMsg
+ if err = Unmarshal(packet, &kexDHGexInit); err != nil {
+ return
+ }
+
+ var pHalf = &big.Int{}
+ pHalf.Rsh(gex.p, 1)
+
+ y, err := rand.Int(randSource, pHalf)
+ if err != nil {
+ return
+ }
+
+ Y := new(big.Int).Exp(gex.g, y, gex.p)
+ kInt, err := gex.diffieHellman(kexDHGexInit.X, y)
+ if err != nil {
+ return nil, err
+ }
+
+ hostKeyBytes := priv.PublicKey().Marshal()
+
+ h := gex.hashFunc.New()
+ magics.write(h)
+ writeString(h, hostKeyBytes)
+ binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMinimumBits))
+ binary.Write(h, binary.BigEndian, uint32(dhGroupExchangePreferredBits))
+ binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMaximumBits))
+ writeInt(h, gex.p)
+ writeInt(h, gex.g)
+ writeInt(h, kexDHGexInit.X)
+ writeInt(h, Y)
+
+ K := make([]byte, intLength(kInt))
+ marshalInt(K, kInt)
+ h.Write(K)
+
+ H := h.Sum(nil)
+
+ // H is already a hash, but the hostkey signing will apply its
+ // own key-specific hash algorithm.
+ sig, err := signAndMarshal(priv, randSource, H)
+ if err != nil {
+ return nil, err
+ }
+
+ kexDHGexReply := kexDHGexReplyMsg{
+ HostKey: hostKeyBytes,
+ Y: Y,
+ Signature: sig,
+ }
+ packet = Marshal(&kexDHGexReply)
+
+ err = c.writePacket(packet)
+
+ return &kexResult{
+ H: H,
+ K: K,
+ HostKey: hostKeyBytes,
+ Signature: sig,
+ Hash: gex.hashFunc,
+ }, err
+}
diff --git a/ssh/messages.go b/ssh/messages.go
index db914d8..ac41a41 100644
--- a/ssh/messages.go
+++ b/ssh/messages.go
@@ -97,6 +97,36 @@
Signature []byte
}
+// See RFC 4419, section 5.
+const msgKexDHGexGroup = 31
+
+type kexDHGexGroupMsg struct {
+ P *big.Int `sshtype:"31"`
+ G *big.Int
+}
+
+const msgKexDHGexInit = 32
+
+type kexDHGexInitMsg struct {
+ X *big.Int `sshtype:"32"`
+}
+
+const msgKexDHGexReply = 33
+
+type kexDHGexReplyMsg struct {
+ HostKey []byte `sshtype:"33"`
+ Y *big.Int
+ Signature []byte
+}
+
+const msgKexDHGexRequest = 34
+
+type kexDHGexRequestMsg struct {
+ MinBits uint32 `sshtype:"34"`
+ PreferedBits uint32
+ MaxBits uint32
+}
+
// See RFC 4253, section 10.
const msgServiceRequest = 5
diff --git a/ssh/server.go b/ssh/server.go
index ac7f807..7a5a1d7 100644
--- a/ssh/server.go
+++ b/ssh/server.go
@@ -193,6 +193,12 @@
if fullConf.MaxAuthTries == 0 {
fullConf.MaxAuthTries = 6
}
+ // Check if the config contains any unsupported key exchanges
+ for _, kex := range fullConf.KeyExchanges {
+ if _, ok := serverForbiddenKexAlgos[kex]; ok {
+ return nil, nil, nil, fmt.Errorf("ssh: unsupported key exchange %s for server", kex)
+ }
+ }
s := &connection{
sshConn: sshConn{conn: c},
diff --git a/ssh/test/session_test.go b/ssh/test/session_test.go
index dcd2249..004b320 100644
--- a/ssh/test/session_test.go
+++ b/ssh/test/session_test.go
@@ -420,6 +420,11 @@
var config ssh.Config
config.SetDefaults()
kexOrder := config.KeyExchanges
+ // Based on the discussion in #17230, the key exchange algorithms
+ // diffie-hellman-group-exchange-sha1 and diffie-hellman-group-exchange-sha256
+ // are not included in the default list of supported kex so we have to add them
+ // here manually.
+ kexOrder = append(kexOrder, "diffie-hellman-group-exchange-sha1", "diffie-hellman-group-exchange-sha256")
for _, kex := range kexOrder {
t.Run(kex, func(t *testing.T) {
server := newServer(t)
