Function x3dh_ke::x3dh_a

pub fn x3dh_a(
    signature: &Signature, 
    ika: &IdentityKey, 
    spkb: &SignedPreKey, 
    eka: &EphemeralKey, 
    ikb: &IdentityKey, 
    opkb: &OneTimePreKey
) -> Result<[u8; 32], &'static str>