Why do digital signatures and PKI make SAML demanding to operate, especially for legacy systems?

Trust between IdP and SPs rests entirely on creating and verifying XML digital signatures, so every party needs enough CPU for the cryptography and working key management (PKI); older legacy systems may lack either.

Each login involves at least one signature by the IdP and its verification by the SP, often an XML encryption of the assertion as well. XML signatures are more expensive than they look: the XML must be canonicalised (brought into one exact byte form) and parsed before the hash can be checked. For high-volume SPs and old hardware, that cost is noticeable.

Key management is the larger burden: signing and encryption certificates expire and must be rolled over in coordination with every partner through metadata; a compromised IdP key must be revoked federation-wide. Many SAML outages are simply an expired certificate on one side. Legacy applications that cannot load a modern XML security library or speak the required algorithms often need a proxy in front of them.

Go deeper:

• OWASP SAML Security Cheat Sheet: X.509 certificate considerations — certificate lifetimes, signing keys and rollover in SAML.
• XML Signature (Wikipedia) — why canonicalisation makes XML signatures complex.