SAML Signing & Encryption (XML Signature, XML Encryption)

#The Cryptographic Seal of XML#link

In a SAML flow, the assertion is passed through the user's browser. Because the browser is an untrusted environment, the SP must have a way to guarantee that the assertion was not modified. This is achieved through XML-DSig (Digital Signatures) and XML-Enc (Encryption).

XML Digital Signatures (XML-DSig)

Unlike a simple HMAC, XML-DSig allows for 'partial' signing. The IdP can sign the entire assertion, or just specific elements. The SP uses the IdP's public key (from the metadata) to verify the signature.

<ds:Signature xmlns:ds="http://www.w3.org/2000/09-10/xmldsig#">
  <ds:SignedInfo>
    <ds:CanonicalizationMethod Algorithm="...C14N" />
    <ds:SignatureMethod Algorithm="...RSA-SHA256" />
    <ds:Reference URI="#_assertion_id">
      <ds:DigestMethod Algorithm="...SHA256" />
      <ds:DigestValue>aB3...xY9</ds:DigestValue>
    </ds:Reference>
  </ds:SignedInfo>
  <ds:SignatureValue>M1z...pL0</ds:SignatureValue>
</ds:Signature>

Canonicalization (C14N) is critical. Since XML can have different spacing or attribute orders but remain logically identical, C14N 'standardizes' the XML before hashing it, ensuring the signature doesn't break due to a stray newline.

Assertion Encryption (XML-Enc)

When an assertion is encrypted, the IdP uses the SP's public key to encrypt the data. Only the SP, possessing the corresponding private key, can decrypt and read the user's attributes.

The Implementation Trap

• ▪Ensure the signature is wrapped around the entire SAML Response
• ▪Use strong algorithms (RSA-SHA256 or better)
• ▪Strictly enforce C14N standards
• ▪Rotate signing keys every 12-24 months