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I'm currently using strongSwan as a peer for the IKEv2 protocol. I've configured it to use HMAC SHA-256 for its pseudorandom function, and AES-256-GCM-16 for its encryption and integrity algorithm.

During the initial exchanges where strongSwan generates SKEYSEED and derives keying material, each of SK_ei and SK_er get outputted with a strange length of 36 bytes.

If I change AES-256-GCM-16 to AES-128-GCM-16, each of SK_ei and SK_er instead become 20 bytes long. With this in mind, it seems like there's an extra 4 bytes being appended to each of these derived keys.

What are the extra 4 bytes for? At first I thought it may be an initialization vector, but I hear for AES-GCM, IVs are typically 12 bytes long. According to the RFC, SK_ei/SK_er are supposed to be used as input for the encryption algorithm, but as-is, they aren't the correct length with the 4 extra bytes, and this case doesn't seem to be explained anywhere.

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1 Answer 1

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As specified in RFC 5282, section 7.1 (and RFC 4106, section 8.1 for ESP), these additional four bytes are allocated as salt value that AES-GCM uses in the nonce.

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