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<div class="">There’s no need to collide SHA2 with this scheme.</div>
<div class="">The attacker can know in advance what the serial number will be; it may not be sequential, but is nevertheless predictable. So the attacker knows in advance what the final tbsCertificate will be. This is everything the attacker needs to mount
a chosen-prefix collision.</div>
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<div class="">Cordialement,</div>
<div class="">Erwann Abalea</div>
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<blockquote type="cite" class="">
<div class="">Le 19 juil. 2016 à 16:22, Rob Stradling <<a href="mailto:rob.stradling@comodo.com" class="">rob.stradling@comodo.com</a>> a écrit :</div>
<br class="Apple-interchange-newline">
<div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">On
19/07/16 15:15, Erwann Abalea wrote:</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
<blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
There’s a disadvantage with this approach, in that now, there’s no random information inserted by the CA to raise the attack cost.<br class="">
Back to chosen-prefix collision, from random-prefix collision.<br class="">
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<br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">With
Phill's scheme, if you tweak some part of the TBSCertificate (e.g. the public key) in an attempt to find a collision, it affects the rigidly constructed serial number.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
<br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">So
you'd need to simultaneously collide SHA-1 and (at least 16 bytes of) SHA-2-512, right?</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
<br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">Is
that any less hard than guessing "at least 64 bits of output from a CSPRNG" ?</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
<br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
<blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
Cordialement,<br class="">
Erwann Abalea<br class="">
<br class="">
<blockquote type="cite" class="">Le 18 juil. 2016 à 19:36, <a href="mailto:philliph@comodo.com" class="">
philliph@comodo.com</a> a écrit :<br class="">
<br class="">
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.<br class="">
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So lets go to the solution we chose for EC - rigid construction. This can be made to be auditable.<br class="">
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I propose that the way be generate SHA-1 certs is as follows.<br class="">
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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'<br class="">
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2) Generate the SHA-2-512 hash of the tbsCertificate structure<br class="">
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3) Truncate the result of (2) to the length of the desired Serial number and populate. The result is the final tbsCertificate value.<br class="">
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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.<br class="">
<br class="">
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.<br class="">
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<span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">--<span class="Apple-converted-space"> </span></span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">
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