Decrypting a Wireshark capture without the private key

The idea of encryption is to keep data secure and "hideen" and that only those who own the key are able to decrypt the data set. So, a work assignment as described above ("Decrypting a capture without the private key"), does not make any sense, unless you omited the relevant parts in your question (see comment of @ArdenUK about caesar cipher).

So, I see the following possible reasons for such a 'work assignment'.

• The person who asked for it, tries to probe you to see if you understand encryption
• You misunderstood the work assignment and in reality it's about decoding a pcap file that contains SMTP traffic and/or POP3/IMAP, and not decrypting data
• The person who asked for it has zero knowledge of encryption and does not understand that he/she is asking for the impossible
• You are working for an ultra secure TLA (three letter agency) and you are trained with alien technology to crack even the hardest encryption algorithms. In that case, you failed miserably in several ways (Nr #1: not being able to do what you have been trained for, Nr #2: Posting about this in a public Q&A site, Nr #3: not creating your own business with such a skill set).

Please choose the most likely reason for you.

If you'd ask me, I's say item #1 and/or item #2.

@ArdenUK: similar for you, although you actually mentioned a real encryption system (caesar cipher). However: Without any information about the work assignment, how are we supposed to help or even comment on your question/problem?

Regards
Kurt