The 5C Nano FIPS is the certified version of YubiKey's smallest USB-C key. The hardware is the same as the standard YubiKey 5 Series, with the same protocols and the same firmware 5.7, but this one carries FIPS 140-3 validation, the current US federal standard that replaces the 140-2 generation now being retired. If you don't have a compliance reason to need that certificate, the ordinary 5C Nano does exactly the same job. The FIPS validation is the whole point of paying for this one.
That certification is what regulated and government buyers put on the paperwork. If a procurement requirement, a security control, or an auditor asks for a FIPS 140-3 validated authenticator, this satisfies it where the standard key, functionally identical, will not. For everyone else it is an unnecessary line item.
Underneath, it is a full multi-protocol security key. It handles FIDO2/WebAuthn passkeys and FIDO U2F for phishing-resistant sign-in, PIV smart-card certificates, OATH-TOTP and OATH-HOTP one-time codes, OpenPGP, YubiKey OTP, a static password slot, and HMAC-SHA1 challenge-response. That covers passwordless and phishing-resistant MFA into Microsoft 365 and Entra ID, and the strong-authentication expectations in the Essential Eight.
The nano form factor is the deciding factor. It sits almost flush in a USB-C port and is meant to be left in permanently, which suits a fixed workstation or a laptop that lives on one desk. There is no NFC, so you cannot tap it against a phone, and prising a near-flush key in and out of a shared machine is awkward. It has no battery and no moving parts, and is crush- and water-resistant.
Buy it where the FIPS requirement is real and the key stays put. If you move between devices, need NFC, or have no certification mandate, a different key will serve you better for the same money.
