Implement /_synapse/admin/v1/register (#1911)

* Implement /_synapse/admin/v1/register

This is implemented identically to Synapse, so scripts which work
with Synapse should work with Dendrite.

```
    Test 27 POST /_synapse/admin/v1/register with shared secret... OK
    Test 28 POST /_synapse/admin/v1/register admin with shared secret... OK
    Test 29 POST /_synapse/admin/v1/register with shared secret downcases capitals... OK
    Test 30 POST /_synapse/admin/v1/register with shared secret disallows symbols... OK
```

Sytest however has `implementation_specific => "synapse"` which stops these
tests from running.

* Add missing muxes to gobind

* Linting

clientapi/routing/register_secret.go

@@ -0,0 +1,99 @@
+package routing
+
+import (
+	"context"
+	"crypto/hmac"
+	"crypto/sha1"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+	"time"
+
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/util"
+	cache "github.com/patrickmn/go-cache"
+)
+
+type SharedSecretRegistrationRequest struct {
+	User     string `json:"username"`
+	Password string `json:"password"`
+	Nonce    string `json:"nonce"`
+	MacBytes []byte
+	MacStr   string `json:"mac"`
+	Admin    bool   `json:"admin"`
+}
+
+func NewSharedSecretRegistrationRequest(reader io.ReadCloser) (*SharedSecretRegistrationRequest, error) {
+	defer internal.CloseAndLogIfError(context.Background(), reader, "NewSharedSecretRegistrationRequest: failed to close request body")
+	var ssrr SharedSecretRegistrationRequest
+	err := json.NewDecoder(reader).Decode(&ssrr)
+	if err != nil {
+		return nil, err
+	}
+	ssrr.MacBytes, err = hex.DecodeString(ssrr.MacStr)
+	return &ssrr, err
+}
+
+type SharedSecretRegistration struct {
+	sharedSecret string
+	nonces       *cache.Cache
+}
+
+func NewSharedSecretRegistration(sharedSecret string) *SharedSecretRegistration {
+	return &SharedSecretRegistration{
+		sharedSecret: sharedSecret,
+		// nonces live for 5mins, purge every 10mins
+		nonces: cache.New(5*time.Minute, 10*time.Minute),
+	}
+}
+
+func (r *SharedSecretRegistration) GenerateNonce() string {
+	nonce := util.RandomString(16)
+	r.nonces.Set(nonce, true, cache.DefaultExpiration)
+	return nonce
+}
+
+func (r *SharedSecretRegistration) validNonce(nonce string) bool {
+	_, exists := r.nonces.Get(nonce)
+	return exists
+}
+
+func (r *SharedSecretRegistration) IsValidMacLogin(
+	nonce, username, password string,
+	isAdmin bool,
+	givenMac []byte,
+) (bool, error) {
+	// Check that shared secret registration isn't disabled.
+	if r.sharedSecret == "" {
+		return false, errors.New("Shared secret registration is disabled")
+	}
+	if !r.validNonce(nonce) {
+		return false, fmt.Errorf("Incorrect or expired nonce: %s", nonce)
+	}
+
+	// Check that username/password don't contain the HMAC delimiters.
+	if strings.Contains(username, "\x00") {
+		return false, errors.New("Username contains invalid character")
+	}
+	if strings.Contains(password, "\x00") {
+		return false, errors.New("Password contains invalid character")
+	}
+
+	adminString := "notadmin"
+	if isAdmin {
+		adminString = "admin"
+	}
+	joined := strings.Join([]string{nonce, username, password, adminString}, "\x00")
+
+	mac := hmac.New(sha1.New, []byte(r.sharedSecret))
+	_, err := mac.Write([]byte(joined))
+	if err != nil {
+		return false, err
+	}
+	expectedMAC := mac.Sum(nil)
+
+	return hmac.Equal(givenMac, expectedMAC), nil
+}