Allow RSA 2047 bit keys (#20272) (#20396)

Backport #20272

Unfortunately it appears that 2048 bit RSA keys can occasionally be created in such
a way that they appear to have 2047 bit length. This PR simply changes our defaults to
allow these.

Fix #20249

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

custom/conf/app.example.ini

@@ -1245,7 +1245,7 @@ ROUTER = console
 ;; Define allowed algorithms and their minimum key length (use -1 to disable a type)
 ;ED25519 = 256
 ;ECDSA = 256
-;RSA = 2048
+;RSA = 2047 ; we allow 2047 here because an otherwise valid 2048 bit RSA key can be reported as having 2047 bit length
 ;DSA = -1 ; set to 1024 to switch on
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -620,7 +620,7 @@ Define allowed algorithms and their minimum key length (use -1 to disable a type
 
 - `ED25519`: **256**
 - `ECDSA`: **256**
-- `RSA`: **2048**
+- `RSA`: **2047**: We set 2047 here because an otherwise valid 2048 RSA key can be reported as 2047 length.
 - `DSA`: **-1**: DSA is now disabled by default. Set to **1024** to re-enable but ensure you may need to reconfigure your SSHD provider
 
 ## Webhook (`webhook`)

modules/setting/setting.go

@@ -170,7 +170,7 @@ var (
 		ServerMACs:                    []string{"hmac-sha2-256-etm@openssh.com", "hmac-sha2-256", "hmac-sha1"},
 		KeygenPath:                    "ssh-keygen",
 		MinimumKeySizeCheck:           true,
-		MinimumKeySizes:               map[string]int{"ed25519": 256, "ed25519-sk": 256, "ecdsa": 256, "ecdsa-sk": 256, "rsa": 2048},
+		MinimumKeySizes:               map[string]int{"ed25519": 256, "ed25519-sk": 256, "ecdsa": 256, "ecdsa-sk": 256, "rsa": 2047},
 		ServerHostKeys:                []string{"ssh/gitea.rsa", "ssh/gogs.rsa"},
 		AuthorizedKeysCommandTemplate: "{{.AppPath}} --config={{.CustomConf}} serv key-{{.Key.ID}}",
 		PerWriteTimeout:               PerWriteTimeout,