Kerchkoff's principle is similar to the principle of TCB's in high-assurance security. You want your main trust to be something small and highly vetted. The openness part introduces extra attack or defense potential to that. This can only help if significantly more effort is going into bug-fixing the open tool than exploiting it. Currently, that's backwards for software that's not incredibly popular.

It should be straight-forward to test our hypotheses with a few examples plus a highly-simplified deployment. Scenario is private information served via web server over TLS with people only seeing what their credentials authorized them to. NIDS looks for baseline activity and attack profiles. Attacker wants all of it. Attacker has one month to get in.

Option 1: Regular http server on Linux leveraging OpenSSL for protection of secrets. All configuration data except passwords or private keys are published. This includes how traffic looks.

Option 2: Unknown, non-mainstream OS w/ decent quality & defaults running unknown server with unknown crypto & compiler-assisted protections in unknown configuration with unknown traffic patterns.

Which do you think will succeed easiest? Your interpretation of Kerckhoff suggests No 1 is going to be hardest for attacker. Mine says No 2 is going to give them a lot of detective work that's also likely to set off the NIDS. We do know No 1 gets smashed regularly and with low effort. Evidence leans in my favor so far.

Option 3: Runs OpenBSD on POWER processors with HSM doing the crypto. The HSM is black box w/ tamper resistance that's also a black box. It cost a fortune like the POWER server itself. Reverse engineering the HSM's, esp if each attack bricks it, can take tons of time and money. The server is advertised as an Intel server running FreeBSD or Linux with options turned off. Think your attacker will hack it since obfuscation = obscurity = no security? And if they could, how many are even able to try to given economic cost and skills required?

Option 4: Runs on Boeing SNS server w/ HSM. That's one of earliest systems in high-assurance security certified through NSA pentesting in early 1990's. No reported hacks to this day (20+ years) although undoubtedly something to hit in there. Also unavailable for purchase outside defense. If available, you're probably spending $60-150k a unit if XTS-400 is any indicator of cost of low-volume, high-security servers. Docs say it has Xeon CPU's, custom firmware, a "transactional" kernel (also tiny), and MLS policy. Think your attacker will do better than the others did over two decades?

Option 5: Uses LOCK platform. Ancestor of SELinux made by Secure Computing Corporation. Did Type Enforcement at the level of CPU & memory interactions with security kernel on software layer. Built-in crypto-processor called SIDEARM. UNIX layer running deprivileged for the server-side app. Security-critical components developed & reviewed in rigorous way. Although no longer available, this organization still has the installation media that it uses on obsolete computers it buys off the Internet. The untrusted, networking interface just says it's a BSD. So, it's a high-security product that's not available for sell or on eBay that looks like an old UNIX box on outside. How you think the remote attacker will get in?

I hope I've amply demonstrated Kerckhoff's principle or the interpretation you're bringing are incorrect. The best approach is a combo of solid, vetted security with obfuscation. Some obfuscations can even make it impossible for vast majority of attackers to hack the system. They'll go for supply chain poisoning or infiltration before trying to hack SNS or LOCK. If physical and personnel security are good, then that obfuscation just bought you a lot.