[cabfpub] A better way to do SHA-1 legacy

philliph at comodo.com philliph at comodo.com
Mon Jul 18 10:36:44 MST 2016

Looking at the recent SHA-1 muck up, I am not confident that the current approach works. It fails for the same reason that random Elliptic Curve parameters fails, there is no mechanism that allows a process for generating random numbers to be audited.

So lets go to the solution we chose for EC - rigid construction. This can be made to be auditable.

I propose that the way be generate SHA-1 certs is as follows.

1) Generate the tbsCertificate with the Serial number field containing the bytes [0x01 … 0x01], minimum of 16 bytes. This is just a fixed value placeholder. Also add an extension OID for ‘phb-sha1-hack'

2) Generate the SHA-2-512 hash of the tbsCertificate structure

3) Truncate the result of (2) to the length of the desired Serial number and populate. The result is the final tbsCertificate value.

The advantage of this approach is that it is rigid and auditable. CertSentry can look for SHA-1 certs and check to see that the serial number format is compatible. If it is not compatible, the requisite consequences can be delivered to the guilty party automatically.

Yes, there are other ways to strengthen the process with commitments and such. But this approach guarantees that the cert has a work factor of 2^128 even if a second preimage attack on SHA-1 becomes feasible.

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