  We define stealth Man-in-the-Middle adversaries, and analyse their ability to
launch denial and degradation of service (DoS) attacks on secure channels. We
show realistic attacks, disrupting TCP communication over secure VPNs using
IPsec. We present:
  First amplifying DoS attack on IPsec, when deployed without anti-replay
window.
  First amplifying attack on IPsec, when deployed with a `small' anti-replay
window, and analysis of `sufficient' window size.
  First amplifying attack on IPsec, when deployed with `sufficient' window
size. This attack (as the previous) is realistic: attacker needs only to
duplicate and speed-up few packets.
  We also suggest a solution designed to prevent the presented attacks, and to
provide secure channel immune to degradation and other DoS attacks. Our
solution involves changes (only) to the two gateway machines running IPsec.
  In addition to their practical importance, our results also raise the
challenge of formally defining secure channels immune to DoS and degradation
attacks, and providing provably-secure implementations.
