Add documentation for SCT formats (#718)

Fixes #539

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

fulcio.proto

@@ -159,9 +159,12 @@ message SigningCertificateDetachedSCT {
      */
     CertificateChain chain = 1;
     /*
-     * The signed certificate timestamp is a promise for including the certificate in
+     * The Signed Certificate Timestamp (SCT) is a promise for including the certificate in
      * a certificate transparency log. It can be "stapled" to verify the inclusion of
      * a certificate in the log in an offline fashion.
+     *
+     * The SCT format is an AddChainResponse struct, defined in
+     * https://github.com/google/certificate-transparency-go
      */
     bytes signed_certificate_timestamp = 2;
 }
@@ -172,6 +175,10 @@ message SigningCertificateEmbeddedSCT {
      * by all intermediate certificates (if present), finishing with the root certificate.
      *
      * All values are PEM-encoded certificates.
+     *
+     * The leaf certificate contains an embedded Signed Certificate Timestamp (SCT) to
+     * verify inclusion of the certificate in a log. The SCT format is a SignedCertificateTimestampList,
+     * as defined in https://datatracker.ietf.org/doc/html/rfc6962#section-3.3
      */
     CertificateChain chain = 1;
 }

fulcio.swagger.json

@@ -295,7 +295,7 @@
         "signedCertificateTimestamp": {
           "type": "string",
           "format": "byte",
-          "description": "The signed certificate timestamp is a promise for including the certificate in\na certificate transparency log. It can be \"stapled\" to verify the inclusion of\na certificate in the log in an offline fashion."
+          "description": "The Signed Certificate Timestamp (SCT) is a promise for including the certificate in\na certificate transparency log. It can be \"stapled\" to verify the inclusion of\na certificate in the log in an offline fashion.\n\nThe SCT format is an AddChainResponse struct, defined in\nhttps://github.com/google/certificate-transparency-go"
         }
       },
       "title": "(-- api-linter: core::0142::time-field-type=disabled\n    aip.dev/not-precedent: SCT is defined in RFC6962 and we keep the name consistent for easier understanding. --)"
@@ -305,7 +305,7 @@
       "properties": {
         "chain": {
           "$ref": "#/definitions/v2CertificateChain",
-          "description": "The certificate chain serialized with the leaf certificate first, followed\nby all intermediate certificates (if present), finishing with the root certificate.\n\nAll values are PEM-encoded certificates."
+          "description": "The certificate chain serialized with the leaf certificate first, followed\nby all intermediate certificates (if present), finishing with the root certificate.\n\nAll values are PEM-encoded certificates.\n\nThe leaf certificate contains an embedded Signed Certificate Timestamp (SCT) to\nverify inclusion of the certificate in a log. The SCT format is a SignedCertificateTimestampList,\nas defined in https://datatracker.ietf.org/doc/html/rfc6962#section-3.3"
         }
       }
     },