CMS Detached Signatures

Detached signatures allow the signature to be placed in a separate file next to the original file, and thus the original file does not have to be updated. With CMS signatures, what is covered by the signature is a set of attributes (unless it is a direct signature) where one of the attributes is called messageDigest, which is the hash of the original file. This means that in order to create a CMS detached signature, it is sufficient to get the hash of the original file as input.

The CMS Signer is for instance configured to accept a hash as input by setting CLIENTSIDEHASHING=TRUE, or by allowing the client to specify if the input is the original file or a hash of it, by configuring ALLOW_CLIENTSIDEHASHING_OVERRIDE=TRUE.

When the client sends the request, the following properties need to be provided as request metadata:

• USING_CLIENTSUPPLIED_HASH=true

• CLIENTSIDE_HASHDIGESTALGORITHM=SHA-256

where SHA-256 should be replaced with the name of the hash algorithm used to digest the data. The algorithm must be one of those configured in signer with the ACCEPTED_HASH_DIGEST_ALGORITHM property.

The following commands can be used to create a detached CMS signature using this mode with a CMSSigner set up to default to consider requests as client-side computed hash (using the CLIENTSIDEHASHING property) (assuming a GNU/Linux, or Unix-like system is used):

Plain Signatures

For plain signatures (as produced by the Plain Signer), generally the whole file is sent to the signer and returned is the small signature. However, as the plain signature schemes involves a hash operation, it is be possible to perform that on the client side.

For the Plain Signer, this can be achieved by configuring the signer with a signature algorithm with the word NONE instead of the name of the hash. For example: "NONEwithRSA" or "NONEwithECDSA". Note that the input to the signer has to be in a particular format when using the "NONEwithRSA" algorithm for the final signature to be correct. See RFC#3447 for details.

Embedded Signature Formats

ENTERPRISE This is a SignServer Enterprise feature.

For signature formats where the signature is to be placed within the original document, additional logic has to be implemented on the client-side in order to, typically, first prepare the document for signing, compute the digest and send it to the server, and then finally embed the signature within the file.

On the client-side, support has been implemented for "client-side hashing and construction" for Authenticode signing of Portable Executable (PE) files (i.e. EXE, .DLL,..), Windows Installer files (.MSI), and for Java Archives (.JAR,.APK,..) in the SignClient signdocument command. Support for other file types such as PDF may also be implemented.

On the server-side, typically a CMS Signer is needed but since Authenticode and JAR signing are producing slightly different variants of the CMS signatures, different implementations are available for use. For Authenticode, there is the MS Authenticode CMS Signer, and for JAR signing the JArchive CMS Signer.

OpenPGP

When using the client-side hashing and construction with OpenPGP using the OpenPGPPlainSigner, the parameters key ID and key algorithm (when using a key with an algorithm other than RSA) need to be specified with the request using the -extraoption options.

Debian Dpkg-sig

When using the client-side hashing and construction with Dpkg-sig using the OpenPGPPlainSigner, the key ID, key fingerprint, and key algorithm (when using a key with an algorithm other than RSA) need to be specified with the request using the -extraoption options.