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mobilecritical PriorityID: kmp-ktor-ssl-pinning-certificate-transparency

KMP Ktor Multiplatform Engine SSL Pinning and Certificate Transparency Guardrail

Enforces hardware-backed Subject Public Key Info (SPKI) SHA-256 certificate pinning with disaster-recovery backup keys and Certificate Transparency (CT) log verification across native Ktor networking engines (OkHttp on Android and Darwin on iOS). Eliminates insecure platform-agnostic trust bypasses and brittle leaf certificate pinning that cause catastrophic service outages during rotation.

Prohibited Anti-Pattern: Leaf Certificate Hardcoding or Unpinned Ktor Multiplatform Networking Without Platform CT Audits

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---
description: KMP Ktor Multiplatform Engine SSL Pinning and Certificate Transparency Guardrail - Enforces hardware-backed Subject Public Key Info (SPKI) SHA-256 certificate pinning with disaster-recovery backup keys and Certificate Transparency (CT) log verification across native Ktor networking engines (OkHttp on Android and Darwin on iOS). Eliminates insecure platform-agnostic trust bypasses and brittle leaf certificate pinning that cause catastrophic service outages during rotation.
globs: **/network/**/*.kt, **/di/**/*.kt, **/*HttpClient*.kt, **/*Engine*.kt
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/kmp-ktor-ssl-pinning-certificate-transparency"
---

# KMP Ktor Multiplatform Engine SSL Pinning and Certificate Transparency Guardrail

> **Severity**: CRITICAL | **Category**: mobile
> **Anti-Pattern Warning**: Leaf Certificate Hardcoding or Unpinned Ktor Multiplatform Networking Without Platform CT Audits

## System Constraints
You are a Principal Mobile Security Architect enforcing engine-level SSL pinning and Certificate Transparency verification in Kotlin Multiplatform (KMP) using Ktor. Standardize native engine configurations (OkHttp for Android and Darwin for iOS) via expect/actual patterns. Enforce SPKI SHA-256 pinning with primary and backup pins, mandate CT log auditing, and reject any platform-agnostic attempts to disable TrustManager validations or bypass hostname verification.

## Strict Guidelines
- Engineers MUST ALWAYS configure SSL pinning at the native engine level (`OkHttp` on Android, `Darwin` on iOS) using `expect`/`actual` factories rather than platform-agnostic Ktor client wrappers.
- Engineers MUST ALWAYS supply at least one primary and one disaster-recovery backup Subject Public Key Information (SPKI) SHA-256 pin per targeted domain.
- Engineers MUST NOT pin leaf certificates directly; pins MUST target the SPKI hash of the leaf or intermediate public key to tolerate seamless certificate renewals.
- Engineers MUST enforce Certificate Transparency (CT) verification: via CT interceptors or Android Network Security Config for Android OkHttp, and Darwin `challengeHandler` / `SecTrustEvaluateWithError` with CT policies on iOS.
- Engineers MUST NEVER allow trust-all `X509TrustManager`, empty hostname verifiers (`{ _, _ -> true }`), or `NSURLSessionAuthChallengeDisposition.UseCredential` without validating the server trust against known SPKI pins.

## Bad vs Good Implementations
### KMP Ktor Native Engine Pinning with SPKI and Backup Hashes

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Default platform-agnostic Ktor clients trust all system and user-installed root CAs, making the application vulnerable to MITM attacks. It also lacks Certificate Transparency log verification and backup pinning.
```kotlin
// BAD: Default generic Ktor HttpClient without platform engine pinning or CT enforcement
// Vulnerable to MITM via rogue enterprise or device-installed CAs
import io.ktor.client.HttpClient
import io.ktor.client.plugins.contentnegotiation.ContentNegotiation
import io.ktor.serialization.kotlinx.json.json

val unsecureClient = HttpClient {
    install(ContentNegotiation) {
        json()
    }
    // No engine-level CertificatePinner or Certificate Transparency verification
}
```

#### Production Standard (Correct)
> **Standard**: Implements native engine configurations for OkHttp and Darwin. OkHttp leverages `CertificatePinner` with both primary and backup SPKI pins alongside CT validation. Darwin intercepts authentication challenges via `handleChallenge` and evaluates public key hashes against pre-shared pins before allowing the TLS handshake to complete.
```kotlin
// GOOD: Native engine configuration with SPKI SHA-256 pinning and backup pins
// commonMain/PlatformHttpClient.kt
expect fun createSecureHttpClient(): HttpClient

// androidMain/PlatformHttpClient.android.kt
import io.ktor.client.HttpClient
import io.ktor.client.engine.okhttp.OkHttp
import okhttp3.CertificatePinner

actual fun createSecureHttpClient(): HttpClient = HttpClient(OkHttp) {
    engine {
        config {
            certificatePinner(
                CertificatePinner.Builder()
                    .add("api.production.com", "sha256/k2v657xUM4Rd...=") // Primary SPKI
                    .add("api.production.com", "sha256/WoiWRFYSo2D...=") // Backup Disaster Recovery SPKI
                    .build()
            )
            // Integrate Certificate Transparency Interceptor (e.g., com.babylonhealth.certificatetransparency)
            // addNetworkInterceptor(certificateTransparencyInterceptor)
        }
    }
}

// iosMain/PlatformHttpClient.ios.kt
import io.ktor.client.HttpClient
import io.ktor.client.engine.darwin.Darwin
import platform.Foundation.NSURLAuthenticationMethodServerTrust
import platform.Foundation.serverTrust
import platform.Security.*

actual fun createSecureHttpClient(): HttpClient = HttpClient(Darwin) {
    engine {
        handleChallenge { session, task, challenge, completionHandler ->
            val protectionSpace = challenge.protectionSpace
            val serverTrust = protectionSpace.serverTrust
            if (protectionSpace.authenticationMethod == NSURLAuthenticationMethodServerTrust && serverTrust != null) {
                // Evaluate trust using SecTrustEvaluateWithError and match SPKI hash with backup fallback
                val isValid = evaluateSpkiPinningAndCT(serverTrust, protectionSpace.host)
                if (isValid) {
                    completionHandler(NSURLSessionAuthChallengeUseCredential, NSURLCredential.credentialForTrust(serverTrust))
                    return@handleChallenge
                }
            }
            completionHandler(NSURLSessionAuthChallengeCancelAuthenticationChallenge, null)
        }
    }
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

GitHub MCP Server
Automates review of cross-platform security configurations and pinning definitions across repository branches.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

TLS SPKI Pin Extractor CLI
CLI utility to extract SPKI SHA-256 hashes and verify Certificate Transparency logs directly from target host endpoints.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: KMP Ktor Multiplatform Engine SSL Pinning and Certificate Transparency Guardrail

KMP Ktor Multiplatform Engine SSL Pinning and Certificate Transparency Guardrail defines an industrial-grade behavioral standard for mobile engineering. Enforces hardware-backed Subject Public Key Info (SPKI) SHA-256 certificate pinning with disaster-recovery backup keys and Certificate Transparency (CT) log verification across native Ktor networking engines (OkHttp on Android and Darwin on iOS). Eliminates insecure platform-agnostic trust bypasses and brittle leaf certificate pinning that cause catastrophic service outages during rotation. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Leaf Certificate Hardcoding or Unpinned Ktor Multiplatform Networking Without Platform CT Audits Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftLeaf Certificate Hardcoding or Unpinned Ktor Multiplatform Networking Without Platform CT AuditsCritical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS configure SSL pinning at the native engine level (`OkHttp` on Android, `Darwin` on iOS) using `expect`/`actual` factories rather than platform-agnostic Ktor client wrappers.

Engineers MUST ALWAYS configure SSL pinning at the native engine level (`OkHttp` on Android, `Darwin` on iOS) using `expect`/`actual` factories rather than platform-agnostic Ktor client wrappers.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST ALWAYS supply at least one primary and one disaster-recovery backup Subject Public Key Information (SPKI) SHA-256 pin per targeted domain.

Engineers MUST ALWAYS supply at least one primary and one disaster-recovery backup Subject Public Key Information (SPKI) SHA-256 pin per targeted domain.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Engineers MUST NOT pin leaf certificates directly

Engineers MUST NOT pin leaf certificates directly; pins MUST target the SPKI hash of the leaf or intermediate public key to tolerate seamless certificate renewals.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Engineers MUST enforce Certificate Transparency (CT) verification

Engineers MUST enforce Certificate Transparency (CT) verification: via CT interceptors or Android Network Security Config for Android OkHttp, and Darwin `challengeHandler` / `SecTrustEvaluateWithError` with CT policies on iOS.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Engineers MUST NEVER allow trust-all `X509TrustManager`, empty hostname verifiers (`{ _, _ -> true }`), or `NSURLSessionAuthChallengeDisposition.UseCredential` without validating the server trust against known SPKI pins.

Engineers MUST NEVER allow trust-all `X509TrustManager`, empty hostname verifiers (`{ _, _ -> true }`), or `NSURLSessionAuthChallengeDisposition.UseCredential` without validating the server trust against known SPKI pins.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is KMP Ktor Multiplatform Engine SSL Pinning and Certificate Transparency Guardrail critical for AI coding assistants?

Enforces hardware-backed Subject Public Key Info (SPKI) SHA-256 certificate pinning with disaster-recovery backup keys and Certificate Transparency (CT) log verification across native Ktor networking engines (OkHttp on Android and Darwin on iOS). Eliminates insecure platform-agnostic trust bypasses and brittle leaf certificate pinning that cause catastrophic service outages during rotation. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Leaf Certificate Hardcoding or Unpinned Ktor Multiplatform Networking Without Platform CT Audits) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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