variable flen in *_rsa_priv_enc

Carolin Latze carolin.latze at unifr.ch
Mon Feb 4 10:48:45 EST 2008



Jouni Malinen wrote:
> On Fri, Feb 01, 2008 at 04:41:12PM +0100, Carolin Latze wrote:
>
>   
>> I am still working on the integration of the TPM into wpa_supplicant. At
>> the moment, I am wondering, why flen in the *_rsa_priv_enc is always 36
>> (at least in my scenario). From what I see in cryptoapi_rsa_priv_enc in
>> tls_openssl.c, wpa_supplicant also expects it to be 36 as this is 16
>> (MD5) + 20 (SHA-1), but why?
>>     
>
> The length is required to be 36 since only the standard RSA signing for
> TLS is supported (see RFC 2246, Chap. 4.7).
>   
Oh yes, you are right.
>   
>> I need an SHA-1 hash to sign using the TPM.
>>     
>
> What exactly are you signing? Are you using DSS instead of RSA signing?
>   
No, but the TPM only supports signing of SHA-1 hashes till now (using
the RSA private key). From what I see in the RFC, the structure, that
will be signed consists of two 8 bytes fields and one 20 bytes SHA-1
hash. Furthermore, the RFC says "The contents of hash [the 20 bytes
SHA-1] are used as input for the signing algorithm, then the entire
structure is encrypted with a stream cipher.". So it should be possible
to sign the hash using the TPM and handle the rest outside the module.
Have to think of that...

Regards
Carolin

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