DocumentID: ECMA-376/Part2/12.5
Title: ECMA-376, Part2: 12.5 Validating Signatures
Extracted-From: ECMA-376 Office Open XML File Formats, 1st Edition / December 2006
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12.5 Validating Signatures

Consumers validate signatures following the steps described in §3.2 of the W3C Recommendation "XML-Signature Syntax and Processing." When validating digital signatures, consumers shall verify the content type and the digest contained in each Reference descendant element of the SignedInfo element, and validate the signature calculated using the SignedInfo element. [M6.29]

The steps below might not be sufficient to validate signatures that contain application-specific Object elements. Format designers that utilize application-specific Object elements shall also define the additional steps that shall be performed to validate the application-specific Object elements.

To validate references:

  1. The package implementer shall canonicalize the SignedInfo element based on the CanonicalizationMethod element specified in the SignedInfo element.
  2. For each Reference element in the SignedInfo element:
  • The package implementer shall o btain the Object element to be digested.
  • For the package-specific Object element, the package implementer shall validate references to signed parts stored in the Manifest element. The package implementer shall consider references invalid if there is a missing part. [M6.9] [Note: If a relationships transform is specified for a signed Relationships part, only the specified subset of relationships within the entire Relationships part are validated. end note]
  • For the package-specific Object element, validation of Reference elements includes verifying the content type of the referenced part and the content type specified in the reference query component. Package implementers shall consider references invalid if these two values are different. The string comparison shall be case-sensitive and locale-invariant. [M6.11]
  • The package implementer shall d igest the obtained Object element using the DigestMethod element specified in the Reference element.
  • The package implementer shall c ompare the generated digest value against the DigestValue element in the Reference element of the SignedInfo element. Package implementers shall consider references invalid if there is any mismatch. [M6.30]

    • To validate signatures:

    1. The package implementer shall obtain the public key information from the KeyInfo element or from an external source.
    2. The package implementer shall obtain the canonical form of the SignatureMethod element using the CanonicalizationMethod element. The package implementer shall use the result and the previously obtained KeyInfo element to confirm the SignatureValue element stored in the SignedInfo element. The package implementer shall decrypt the SignatureValueelement using the public key prior to comparison.
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