further clarifications

Author: gselander

draft-ietf-cose-cbor-encoded-cert.md

@@ -258,7 +258,7 @@ This document also specifies C509 Certification Requests; see {{CSR}}. It furthe
 
 {::boilerplate bcp14-tagged}
 
-This specification makes use of the terminology in {{RFC2986}}, {{RFC5280}}, {{RFC7228}}, {{RFC8610}}, and {{RFC8949}}. When referring to CBOR, this specification always refers to Deterministically Encoded CBOR as specified in {{Sections 4.2.1 and 4.2.2 of RFC8949}}.
+This specification makes use of the terminology in {{RFC2986}}, {{RFC5280}}, {{RFC7228}}, {{RFC8610}}, and {{RFC8949}}. Unless otherwise specified, when referring to CBOR this specification always refers to Deterministically Encoded CBOR as specified in {{Sections 4.2.1 and 4.2.2 of RFC8949}}.
 
 # C509 Certificate {#certificate}
 
@@ -332,11 +332,12 @@ C509 certificates are defined in terms of DER-encoded X.509 certificates {{RFC52
 
 The 'version' field is encoded in the 'c509CertificateType' CBOR int. The field 'c509CertificateType' also indicates the type of the C509 certificate. Two types are defined in this document: natively signed C509 certificates, following X.509 v3 (c509CertificateType = 2); and CBOR re-encoded X.509 v3 DER certificate (c509CertificateType = 3), see {{type}}. The number of elements in TBSCertificate is fixed and determined by the type. Additional types may be added in the future.
 
-### certificateSerialNumber
+### certificateSerialNumber {#csn}
 
-The 'certificateSerialNumber' INTEGER value field is encoded as the unwrapped CBOR unsigned bignum (~biguint) 'CertificateSerialNumber'. Any leading 0x00 byte is therefore omitted.
-(As the numbers encoded this way are unsigned, there is no need to prepend a 00 byte to avoid a leading one bit being interpreted as the sign of a negative number.)
-Serial numbers are always encoded as CBOR byte string even if the number can fit in a CBOR integer. This deviates from deterministic CBOR encoding {{RFC8949}}.
+The 'certificateSerialNumber' positive INTEGER value field is encoded as the unwrapped CBOR unsigned bignum (~biguint) 'CertificateSerialNumber'.
+Leading 0x00 bytes are omitted following the preferred serialization as specified in {{Section 3.4.3 of RFC8949}}.
+Serial numbers are always encoded as CBOR byte string even if the number can fit in an ordinary CBOR unsigned integer.
+This deviates from the preferred serialization of integers as specified in {{Section 3.4.3 of RFC8949}}, but provides an alternative deterministic encoding.
 When converting back to X.509 format, in case ofC509Certificate type 3 or C509CertificationRequest type 3, if the highest bit in first byte is set, the leading 0x00 byte is put back to prepend to the byte string.
 
 ### signature
@@ -760,6 +761,8 @@ Where both a specific and a generic CBOR encoding are supported, the specific CB
 
 Native C509 certificates MUST use only specific CBOR-encoded fields. However, when decoding non-native C509 certificates, the decoder may need to support, for example, the (extensionID: ~oid, extensionValue: bytes / [bytes]) encoding of an extension for which an (extensionID: int, extensionValue: Defined) encoding exists. One reason is that the certificate might have been issued before the specific CBOR extension was registered.
 
+One specific case of deterministic but non-preferred serialization is CertificateSerialNumber, where byte string encoding is used also for small unsigned integers, see {{csn}}.
+
 ## C509 Name in TLS and DTLS
 
 In TLS and DTLS, the subject of a trusted authority may be sent to the peer to help it select the certificate chain, as in the CertificateAuthoritiesExtension in {{RFC8446}}, in the certificate_authorities field of CertificateRequest in {{RFC5246}}, or in the TrustedAuthorities in {{RFC6066}}. For such usage in TLS and DTLS, the C509 name is wrapped in a distinguished name {{X.501}} with exactly one RelativeDistinguishedName, which in turn contains exactly one AttributeTypeAndValue with the attribute C509Name. The attribute value is the raw byte string of the encoded C509 Name as specified in {{subject}}.