The best description of cryptanalysis among the provided options is:
"Cryptanalysis is the statistical analysis of codes, ciphers, or encrypted text with the intent of decrypting encoded messages."
Cryptanalysis involves analyzing and breaking encryption protocols, while converting plaintext to ciphertext relates to encryption, and converting ciphertext to plaintext relates to decryption. The definition that emphasizes the statistical analysis aspect aimed at deciphering encrypted messages aligns with the purpose of cryptanalysis.