The Dolev-Yao adversary model is a formal mathematical model used to demonstrate the soundness of cryptographic protocols. First published in 1983 [1], this model assumes a very powerful man-in-the-middle, who can attack message traffic in profound ways. Some of these ways include:
The point of the Dolev-Yao adversary assumptions is to force cryptography to rely on its own strengths and not to assume favorable treatment of the ciphertext in transit.
References:
[1] http://www.cs.huji.ac.il/~dolev/pubs/dolev-yao-ieee-01056650.pdf
http://csis.bits-pilani.ac.in/faculty/murali/netsec-09/lectures/threat-model.pdf