models/ssh_key: don't run ssh-keygen check if disabled (#4519)

* don't load key when key size should not be checked

This makes it possible for users with incompatible ssh-keygen versions
to just disable the check completely without running into errors.

* add proper ed25519 constant

The x/crypto/ssh library has support for ed25519 now, so it is possible
to support it.

models/ssh_key.go

@@ -23,9 +23,9 @@ import (
 	"golang.org/x/crypto/ssh"
 	log "gopkg.in/clog.v1"
 
-	"github.com/gogits/gogs/pkg/tool"
 	"github.com/gogits/gogs/pkg/process"
 	"github.com/gogits/gogs/pkg/setting"
+	"github.com/gogits/gogs/pkg/tool"
 )
 
 const (
@@ -223,7 +223,6 @@ func SSHKeyGenParsePublicKey(key string) (string, int, error) {
 }
 
 // SSHNativeParsePublicKey extracts the key type and length using the golang SSH library.
-// NOTE: ed25519 is not supported.
 func SSHNativeParsePublicKey(keyLine string) (string, int, error) {
 	fields := strings.Fields(keyLine)
 	if len(fields) < 2 {
@@ -272,7 +271,7 @@ func SSHNativeParsePublicKey(keyLine string) (string, int, error) {
 		return "ecdsa", 384, nil
 	case ssh.KeyAlgoECDSA521:
 		return "ecdsa", 521, nil
-	case "ssh-ed25519": // TODO: replace with ssh constant when available
+	case ssh.KeyAlgoED25519:
 		return "ed25519", 256, nil
 	}
 	return "", 0, fmt.Errorf("unsupported key length detection for type: %s", pkey.Type())
@@ -298,6 +297,10 @@ func CheckPublicKeyString(content string) (_ string, err error) {
 	// remove any unnecessary whitespace now
 	content = strings.TrimSpace(content)
 
+	if !setting.SSH.MinimumKeySizeCheck {
+		return content, nil
+	}
+
 	var (
 		fnName  string
 		keyType string
@@ -315,9 +318,6 @@ func CheckPublicKeyString(content string) (_ string, err error) {
 	}
 	log.Trace("Key info [native: %v]: %s-%d", setting.SSH.StartBuiltinServer, keyType, length)
 
-	if !setting.SSH.MinimumKeySizeCheck {
-		return content, nil
-	}
 	if minLen, found := setting.SSH.MinimumKeySizes[keyType]; found && length >= minLen {
 		return content, nil
 	} else if found && length < minLen {