Re: [TLS] Cached-info substitution

[Stefan Santesson <[e-mail]>]

On 10-02-19 10:48 PM, "Wan-Teh Chang" <[e-mail]> wrote:

> On Fri, Feb 19, 2010 at 7:01 AM, Stefan Santesson <[e-mail]> wrote:
>> 
>> As I see it we have two choices.
>> 
>> 1) To define a format for the handshake message which include the hash
>> substitution using a syntax that alters the defined syntax of the handshake
>> message. (current draft approach)
>> 
>> 2) To define a format which preserves the original handshake message syntax.
>> 
>> 
>> In practice this would look something like this:
>> 
>> 
>> Certificate handshake message:
>> 
>> Original syntax:
>>      opaque ASN.1Cert<1..2^24-1>;
>> 
>>      struct {
>>          ASN.1Cert certificate_list<0..2^24-1>;
>>      } Certificate;
>> 
>> 
>> Substitution syntax, choice 1 (current draft):
>>      struct {
>>          CachedInformationHash certificate_list<0..2^24-1>;
>>      } Certificate;
>> 
>> 
>> Substitution syntax, choice 2 (preserving original syntax):
>>      CachedInformationHash CachedCertList<1..2^24-1>;
>> 
>>      struct {
>>          CachedCertList certificate_list<0..2^24-1>;
>>      } Certificate;
> 
> In choice 2, did you mean CachedCertList or CachedCert?

It's CachedCertList in the sense that it contains a hash of a
certificate_list vector.


> With CachedCertList, the Certificate message would contain
> a list of lists of certificates.
> 
It would use the same structure as the original handshake message. That is,
a 3 byte total length followed by a number of 3byte length vectors.

The only difference is that in the original syntax the inner vectors are
opaque and in the substitution syntax they have the structure of a
CachedInformationHash.



> In choice 2, how does a decoder distinguish between 'opaque' and
> CachedInformationHash?  Is it supposed to "sniff" the first few
> bytes to see whether it looks like an ASN.1 DER encoded cert?
> 

The client is supposed to compare the content against the
CachedInformationHash values sent to the server in the Client Hello
extension. If both length and content matches a value sent to the server,
there is a match.

/Stefan 


> Wan-Teh