What is Message Integrity Check?

Understanding Message Integrity Check

A Message Integrity Check (MIC) is a cryptographic value computed over a message and sent alongside it so the receiver can verify the data was not altered in transit. The receiver recomputes the value from the received message and compares; a mismatch reveals tampering or corruption. MIC provides the integrity service, ensuring data arrives exactly as sent.

Mechanically, a MIC is usually a keyed value such as a Message Authentication Code (MAC), commonly an HMAC (e.g., HMAC-SHA256) or a cipher-based MAC. The sender computes it using a shared secret key over the message contents; the receiver, holding the same key, recomputes and compares. Because the key is secret, an attacker who modifies the message cannot forge a matching MIC. A plain checksum or CRC, by contrast, detects accidental errors but not deliberate tampering, since anyone can recompute it. The term MIC is used specifically in 802.11 (WPA2/WPA3), where it protects each frame and the four-way handshake.

Message integrity matters because confidentiality alone is not enough: encrypted data can still be altered, replayed, or corrupted in ways that change meaning. A MIC lets the receiver reject any modified packet, defeating man-in-the-middle tampering, bit-flipping attacks, and injection. It underpins secure protocols including IPsec (with its Integrity Check Value), TLS record MACs, and Wi-Fi, and is a core element of authenticated encryption modes like AES-GCM.

For example, in WPA2, every data frame includes a MIC generated with a key derived during the handshake. If an attacker intercepts a frame and flips bits to inject malicious content, the access point recomputes the MIC, finds it does not match, and silently drops the frame. The historic KRACK attack and the older WEP/TKIP weaknesses showed exactly why a strong, replay-resistant MIC is essential to wireless security.