The public key of ML-KEM-768 is 1,184 bytes and its ciphertext 1,088 bytes, while the public value of the elliptic-curve key exchange X25519 is 32 bytes. How many times larger than the X25519 public value is the ML-KEM-768 public key?

Answer: A. 37 times

1,184 ÷ 32 = 37, so it is 37 times larger. A key exchange also sends the 1,088-byte ciphertext, so far more data are exchanged than in an elliptic-curve key exchange that swaps 32-byte public values. On smart cards and small microcontrollers, the sizes of the RAM and buffers that hold keys and intermediate values, the maximum size of data sent at once and so on have to be revisited, which affects hardware design. For signatures too, an ML-DSA-65 signature is 3,309 bytes, much larger than an elliptic-curve signature.

• A. Correct. 1,184 ÷ 32 = 37.
• B. This is (1,184 + 1,088) ÷ 32 = 71, adding the public key and the ciphertext, not the ratio for the public key alone.
• C. This divides 9,472, the 1,184 bytes converted to bits, by 32 still in bytes.