1.RSA Encryption
https://blogs.sap.com/2021/04/13/rsa-encryption-in-abap/
2.Connect from AS ABAP to Google Cloud Platform App-Engine resource secured with Google Identity-Aware Proxy
测试程序：SSF01
you are lucky guy. ABAP AS has limited support for crypto. Basically, it supports only PKCS#7. Fortunately, that's what you need.
Anyway, all stuff is performed within kernel but there are ABAP function modules which call C routines. You need to look for FMs with name like SSF_*. As Julius mentioned there is a nice documentation for this API. Basically, the easiest way is to use FM SSFV_KRN_ENVELOPE. It has a simple interface, you pass application ID and string of bytes. You need to manually add new application code to table SSFAPPLIC and then you can use transaction SSFA to configure it. Here you need can define additional parameters like hashing algorithm (in your case SHA1) or PSE location. Your new application will use separate certificate which you need to import using transaction STRUST. Your client has already given you public key which you need to use for encryption. Just import .cer file into corresponding PSE. For this scenario you don't need a private key.