He may simply encrypt the plaintext using every possible key, until the ciphertext matches, or decrypting the ciphertext to match the plaintext, whichever is faster.
Yeye anaweza tu encrypt maandishi wazi kwa kutumia kila ufunguo iwezekanavyo, mpaka mechi nakala ya siri, au decrypting nakala ya siri kwa mechi maandishi wazi, kwa namna yoyote ni kwa kasi zaidi.