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How to calculate the checksum in an XFA form

When you save an XFA form (XFA = XML Forms Architecture) using Adobe software, a checksum attribute is added to the form element. This checksum appears to be a SHA-1 digest, but it's unclear as to what is actually fed to the hash. Does anyone have any idea as to how this is generated? This value is ...

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Answered on 8/1/2025

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Adobe adds the checksum merely to be able to find out later whether a particular XFA packet has been changed.
It is not a security feature – it is only a “did-anything-inside-this-packet change?” flag – and therefore it is not described in the public XFA specification.

Which algorithm is used?
• It is SHA-1.
The 20-byte digest is written as 40 lower-case hexadecimal digits.
What is hashed?

• Exactly the byte sequence that is located between the start t...

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