HKDF for a Java userspace implementation?

[Jason A. Donenfeld]

Hey Christian,

If you're already using noise-java, then that library should take care
of all the HMAC/HKDF stuff for you. WireGuard builds upon the NoseIK
handshake, and the aspects that WireGuard adds on top of Noise not
require HKDF or HMAC. You should not be implementing the internal
mechanisms of the Noise handshake yourself, if you're using the
noise-java library. Instead you should be able to specify to it, "I
would like to use NoiseIK", and then you'll get functions for
generating and receiving the first handshake message and the second
handshake message. Specifically, the two WireGuard handshake messages
are:

msg1 = handshake_initiation {
    u8 message_type
    u8 reserved_zero[3]
    u32 sender_index
    u8 unencryped_ephemeral[32]
    u8 encrypted_static[AEAD_LEN(32)]
    u8 encrypted_timestamp[AEAD_LEN(12)]
    u8 mac1[16]
    u8 mac2[16]
}

msg2 = handshake_response {
    u8 message_type
    u8 reserved_zero[3]
    u32 sender_index
    u32 receiver_index
    u8 unencrypted_ephemeral[32]
    u8 encrypted_nothing[AEAD_LEN(0)]
    u8 mac1[16]
    u8 mac2[16]
}

In these you use the noise-java library generate the values
{unencryped_ephemeral, encrypted_static, encrypted_timestamp} and
{unencrypted_ephemeral, encrypted_nothing}, likely as one solid
contiguous byte[] blob, where encrypted_timestamp is that handshake
message's payload containing the TAI64N 12 byte timestamp (you pass
the timestamp to the 'generate' function as the 'payload'), and where
encrypted_nothing is that handshake message's payload containing
nothing (but still with the result containing the noise-java generated
auth tag).

The wireguard.io/protocol/ page and the white paper try to describe
the WireGuard protocol from the fundamentals. In the coming days, I
think I'll add some documentation for building a WireGuard protocol
implementation out of an existing Noise implementation.

Feel free to find me on Freenode -- I'm zx2c4 -- and I'm happy to give
you some pointers or walk you through the implementation.

Looking forward to seeing what you come up with!

Regards,
Jason