Denis intercepts an encrypted message and wants to determine what type of algorithm was used to create the message. He first performs a frequency analysis and notes that the frequency of letters in the message closely matches the distribution of letters in the English language. What type of cipher was most likely used to create this message?

This message was most likely encrypted with a transposition cipher. The use of a substitution cipher, a category that includes AES and 3DES, would change the frequency distribution so that it did not mirror that of the English language. This type of attack, where the attacker only has access to an encrypted message, is also known as a ciphertext-only attack.