The attack would be one that is based on cipher text only (www.londoninternational.ac.uk). This means that only the cipher text is acquired but has no knowledge of the meaning of the message in plaintext. One can try to critically and statistically analyze the cipher text, in order to try and derive the meaning of the encrypted text or look for combinations of common letters or character strings that would aid in the decrypting and understanding the captured message.
Another such method, as stated as my peers, is to use the fact that symmetric encryption only uses a single secret key when encrypting a message. Thus, before capturing the message, the secret key can be covertly acquired through methods such as befriending an individual who has knowledge of the key or snooping to find the key. Thus once the encrypted message is obtained, the key will already be acquired so as to decipher the message.
Another method of deciphering a captured message is