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Microsoft publishes their untrusted certificates as disallowedcerts.stl Warning: direct download.

It is ASN.1 encoded. After decoding it there is a list of sequences like so:

SEQUENCE {
                    OCTETSTRING dfbdd72f99c3b64a797e5ac96d59be56
                 }
SEQUENCE {
                    OCTETSTRING c668154be95e16adbc321abc316e384a
                 }

How does this relate to the revoked certificates? My first guess was serial number or other identifiers of a certificate, but that doesn't appear to be the case.

Any insight on this?

  • these are MD5 hashes of revoked certificates. – Crypt32 Nov 8 '18 at 18:57
  • That was one of my guesses as well, but neither crt.sh nor censys.io were able to find them so I figured that can't be it. Do you have any information what the input to the hash function is to get these values? – maax Nov 9 '18 at 8:25
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How does this relate to the revoked certificates?

these are MD5 hashes (thumbprints) of explicitly untrusted certificates.

neither crt.sh nor censys.io were able to find them

and there are two reasons:

  1. (main) they never were added to CT logs, because they were revoked long prior to when CT was adopted by CAB Forum.

  2. CT logs use SHA-1 or SHA-256 hashes and don't store calculated MD5 hashes. You may need to traverse over entire CT Log tree and calculate MD5 for each certificate and determine if it matches the value in CTL.

Do you have any information what the input to the hash function is to get these values?

raw binary certificate.

  • Thanks! Just one note for completeness: censys.io does provide md5 fingerprints of certificates but makes sense that they don't appear if they are not in any log. – maax Nov 9 '18 at 8:59
  • Ok. Afaik crt.sh doesn't search by MD5. But anyway, the oldest revoked certificate in CTL was revoked about 17 years ago, so there is no chance for it to be added to CT logs. Also, there are certificates that were revoked from MSFT CAs (used for servicing purposes) that doesn't participate in CT Logs. You can get full copies of such certificates on pre-Windows 8 machines. – Crypt32 Nov 9 '18 at 9:09
  • Yes, crt.sh isn't searchable by md5. – maax Nov 9 '18 at 9:17

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