Increase Salt randomness (#18179)

- The current implementation of `RandomString` doesn't give you a most-possible unique randomness. It gives you 6*`length` instead of the possible 8*`length` bits(or as `length`x bytes) randomness. This is because `RandomString` is being limited to a max value of 63, this in order to represent the random byte as a letter/digit.
- The recommendation of pbkdf2 is to use 64+ bit salt, which the `RandomString` doesn't give with a length of 10, instead of increasing 10 to a higher number, this patch adds a new function called `RandomBytes` which does give you the guarentee of 8*`length` randomness and thus corresponding of `length`x bytes randomness.
- Use hexadecimal to store the bytes value in the database, as mentioned, it doesn't play nice in order to convert it to a string. This will always be a length of 32(with `length` being 16).
- When we detect on `Authenticate`(source: db) that a user has the old format of salt, re-hash the password such that the user will have it's password hashed with increased salt.

Thanks to @zeripath for working out the rouge edges from my first commit 😄.

Co-authored-by: lafriks <lauris@nix.lv>
Co-authored-by: zeripath <art27@cantab.net>

models/migrations/migrations.go

@@ -363,6 +363,8 @@ var migrations = []Migration{
 	NewMigration("Add Sorting to ProjectIssue table", addProjectIssueSorting),
 	// v204 -> v205
 	NewMigration("Add key is verified to ssh key", addSSHKeyIsVerified),
+	// v205 -> v206
+	NewMigration("Migrate to higher varchar on user struct", migrateUserPasswordSalt),
 }
 
 // GetCurrentDBVersion returns the current db version

models/migrations/v205.go

@@ -0,0 +1,39 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"xorm.io/xorm"
+	"xorm.io/xorm/schemas"
+)
+
+func migrateUserPasswordSalt(x *xorm.Engine) error {
+	dbType := x.Dialect().URI().DBType
+	// For SQLITE, the max length doesn't matter.
+	if dbType == schemas.SQLITE {
+		return nil
+	}
+
+	if err := modifyColumn(x, "user", &schemas.Column{
+		Name: "rands",
+		SQLType: schemas.SQLType{
+			Name: "VARCHAR",
+		},
+		Length: 32,
+		// MySQL will like us again.
+		Nullable: true,
+	}); err != nil {
+		return err
+	}
+
+	return modifyColumn(x, "user", &schemas.Column{
+		Name: "salt",
+		SQLType: schemas.SQLType{
+			Name: "VARCHAR",
+		},
+		Length:   32,
+		Nullable: true,
+	})
+}