Null content digitally signed message

Question

IA technical specification I'm following requires me to send:

The Digital Signature must be in the form of a null content Digitally Signed message (i.e. a PKCS#7 object containing the signature but not the data that was signed).

To my understanding, this means a detached PKCS#7 signature, is that correct?

I couldn't find any definition of what a "Null Content Digitally Signed message" is. So a PKCS#7 detached seems the closest match..

Answer 1

Yes. At the ASN.1 level, it means that the OCTET STRING field (EncapsulatedContentInfo.eContent) is omitted (it is marked OPTIONAL precisely to allow for detached signatures -- the expression "null content" in that context is new to me, the usual term being "detached signature" as you correctly infer).

Answer 2

(i.e. a PKCS#7 object containing the signature but not the data that was signed)

That describes exactly what a detached signature is, a signature separated from the data which was signed.