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CKKS’s ciphertext-to-ciphertext addition scheme is exactly the same as BFV’s ciphertext-to-ciphertext addition scheme (Summary D-2.4 in §D-2.4).
⟨Summary D-3.3⟩ CKKS Ciphertext-to-Ciphertext Addition
𝖱𝖫𝖶𝖤S,σ(ΔM⟨1⟩) + 𝖱𝖫𝖶𝖤S,σ(ΔM⟨2⟩)
= (A⟨1⟩, B⟨1⟩) + (A⟨2⟩, B⟨2⟩)
= (A⟨1⟩+ A⟨2⟩, B⟨1⟩+ B⟨2⟩)
= 𝖱𝖫𝖶𝖤S,σ(Δ(M⟨1⟩+ M⟨2⟩))