1. RFC documents
2. SCEP operations
    - 
    PKIOperation:       
        - 
        Certificate Enrollment - request: PKCSReq, response: PENDING, FAILURE, SUCCESS 
- 
        Poll for Requester Initial Certificate - request: GetCertInitial, response: same as for PKCSReq 
- 
        Certificate Access - request: GetCert, response: SUCCESS, FAILURE 
- 
        CRL Access - request: GetCRL, response: raw DER encoded CRL 
    - Non-PKIOperation: clear HTTP Get 
 
        - 
        Get Certificate Authority Certificate - GetCACert, GetNextCACert, GetCACaps 
- 
        Get Certificate Authority Certificate Chain - GetCACertChain 
3. Request message formats for PKIOperation
    - Common fields in all PKIOperation messages:
        - 
        senderNonce 
- 
        transactionID 
- the SCEP message being transported(SCEP messages) -> encrypted using the public key of the recipient(Enveloped-data)
 -> signed by one of certificates(Signed-data): the requester can generate a self-signed certificate, or the requester can use
 a previously issued certificate, if the RA/CA supports the RENEWAL option.
 
    - SCEP messages:
        - 
        PKCSReq: PKCS#10 
- GetCertInitial: messages for old versions of scep clients such as Sscep, AutoSscep, and Openscep, are different with draft-18
 issuerAndSubject ::= SEQUENCE {
 issuer Name,
 subject Name
 }
    
        - 
        GetCert: an ASN.1 IssuerAndSerialNumber type, as specified in PKCS#7 Section 6.7 
- 
        GetCRL: an ASN.1 IssuerAndSerialNumber type, as defined in PKCS#7 Section 6.7