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Security Fundamentals & Crypto · Intermediate

Authentication Header (AH)

IPsec protocol (IP protocol 51) that provides integrity, data origin authentication, and anti-replay but no encryption.

Definition

AH authenticates the payload and most of the IP header, which makes it incompatible with NAT because NAT changes header fields. It is rarely used today, since ESP can provide integrity and encryption together.

Example

An auditor finds AH in use and notes that traffic is integrity-protected but readable in a packet capture.

Exam tip

AH = integrity only, no confidentiality, breaks with NAT.

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