A Child’s Garden of Inter-Service Authentication Schemes is an opinionated overview of service to service authentication schemes from Latacora ( via , which has comments worth reading for once, including from various Latacora people). As with pretty much everything Latacora writes on their blog , it's not just informative, it's entertaining too. I find it well worth reading.

(Latacora people include Thomas Ptacek , who you may remember from Against DNSSEC .)