Abstract. In this paper, we describe and analyze the security of the AES-GCM-SIV mode of operation, as defined in the CFRG specification . This mode differs from the original GCM-SIV mode that was designed in  in two main aspects. First, the CTR encryption uses a 127-bit pseudo-random counter instead of a 95-bit pseudo-random value concatenated with a 32-bit counter. This construction leads to improved security bounds when encrypting short messages. In addition, a new key derivation function is used for deriving a fresh set of keys for each nonce. This addition allows for encrypting up to 250 messages with the same key, compared to the significant limitation of only 232 messages that were allowed with GCM-SIV (which inherited this same limit from AES-GCM). As a result, the new construction is well suited for real world applications that need a nonce-misuse resistant Authenticated Encryption scheme. We explain the limitations of GCM-SIV, which motivate the new construction, prove the security properties of AES-GCM-SIV, and show how these properties support real usages. Implementations are publicly available in . We remark that AES-GCM-SIV is already integrated into Google’s BoringSSL library , and its deployment for ticket encryption in QUIC  is underway.