TLS Pinning

TLS certificate pinning embeds the expected certificate public key or hash in the client and rejects all other certificates, preventing man-in-the-middle attacks (including forged certificates issued by legitimate CAs).

Purpose

To prevent man-in-the-middle attacks by adversaries holding legitimate CA certificates (e.g., governments or compromised CAs).

Mechanism

The client preconfigures the SPKI hash (pin) of the server certificate. During the TLS handshake it compares the server certificate with the pin and refuses the connection on mismatch. Types: SPKI pin, certificate-chain pin. Common in mobile apps; browsers rarely pin because certificate rotation is hard.

Use cases

Mobile app API MITM defense
IoT device communication
High-security API clients
Inter-service communication

Examples

Input

pin: sha256/SPKI=abc...

Output

Reject on certificate mismatch

Public key pinning

Common errors

Expired pins without updates cause connection failures: Provide backup pins and a rotation policy

Related terms

TLS (Transport Layer Security) provides encryption, integrity, and authentication at the transport layer; it replaced SSL and underpins HTTPS, SMTPS, and more.

X.509 Certificate

An X.509 certificate binds a public key to an identity, signed by a CA for verification, containing validity, issuer, subject, and extension fields.

A CA (Certificate Authority) is a trusted third party that issues digital certificates binding public keys to identities, serving as the trust root of PKI.

References