AS ABAP 7.02 (and latest SPs of 7.01) contain the class CL_SEC_SXML_DSIGNATURE to create XML signatures. This is the implmentation used for XML signing to goverment offices (nota fiscale Brasil, elster Gemany) and also by SAML2.
In ABAP digital signature are handled by different components:
1) Canonicalization (C14N) is handled by the XML parser in the ABAP kernel
2) Logic inside ABAP
3) RSASHA1 signature calculation by the SAPCRYPTOLIB
Please check the class documentation. It references a report for testing the functionality, similar to SSF02.