Encryption is a component of Ipsec framework, your question is very broad which can cover many componets and under different kinds of circumstances such as RA vpn, l2l vpn etc.., however, I will try to help in providing a link and some basic examples.
The level of encryption depends on the type of encryption algorythm is used in the transform set configuration given on any device or firewall supporting Ipsec standard. A transform set could have AES algorythm providing 128 or 256 bits CBC, or DES 56bit encryption algo, or 3DES 168bit, look at the transform set if any configured in your firewall.
Reference this link for understanding Ipsec and encryption basics and important terminology,knowing these you can identify how strong encryption is implemented across any platform that support Ipsec standard and is configured with as such, not just asa5505.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...