Re: [TrouSerS-users] Verifying Signature of TCPA_CERTIFY_INFO Structure

[Carolin L. <car...@un...>]

Hi Hal,

thanks for the answer. I hoped it would be a different one, since the 20 
bytes do not match my signature ;-) I think I have to go over the code 
again...

BTW: Is there already any Open Source implementation of SKAE available?

Regards
Carolin

Hal Finney wrote:
> HI Carolin - The 35 bytes are a concatenation of a byte string
> representing the hash algorithm, and the hash itself. The byte string
> is what is called an OID, or Object ID, in the ASN.1 DER encoding. A
> good introduction to ASN.1 and the byte encodings is the "Layman's
> Guide", found for example at
> http://luca.ntop.org/Teaching/Appunti/asn1.html .
>
> In the case of SHA-1, the 15 byte prefix should be:
>
> 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14
>
> (Googled "sha1 oid" for finding this.)
>
> So you should see this byte string, followed by your 20-byte sha-1
> hash, when you do the RSA decrypt (which strips off the PKCS-1
> padding).
>
> Hal
>
> On Fri, Aug 15, 2008 at 7:18 AM, Carolin Latze <car...@un...> wrote:
>   
>> Hi everybody,
>>
>> I started again working on my TPM project after a long unplanned break.
>> I have some problems that I already solved in the past, but unfortunatly
>> I cannot remember their solution :( Perhaps, somebody on the list can
>> help me:
>>
>> I have a key, that is certified using another key using the
>> Tspi_Key_CertifyKey method. Now I want to verify the signature of the
>> TCPA_CERTIFY_INFO structure using OpenSSL. In order to do so, I
>> decrypted the 256 bytes long signature using RSA_public_decrypt, which
>> results in a 35 bytes long hash?! I expected 20 bytes SHA-1. I thought,
>> that the TCPA_CERTIFY_INFO structure gets hashed using SHA-1 and then
>> signed. Are the 35 bytes a concatenation of something?
>>
>> Thanks in Advance
>> Carolin
>>