> Date: Fri, 21 Oct 2011 11:19:59 +0000 (UTC)
> From: Jonas <[email protected]>
>
> I have not a real problem, but I do not understand the result of
> RSA_pubblic_decrpyt.
>
> I made a 20 Byte long hash and signed it with the private key of the TPM
with
> TSPI_HASH_SIGN(). This signature I transfer to another computer which
has the
> matching public key. There I want to verify the signature and decrypt it
with
> RSA_PUBLIC_DECRYPT(). This works. But I'm not getting the 20 Byte hash
alone.
> The function returns a 35 Byte long decrypted hash. The last 20 Byte of
it is
> the hash value I signed before. And I do not know what are the 15 Bytes
in
> front. There is no sense for me. And I do not think, that this is right.
>
> Any ideas?
The 15 bytes are the OID (object identifier).
See the TPM spec Part 1 31.2.1 or the PKCS#1 standard for details.
Typically, one calls a signature verify function that does the decrypt,
then checks the padding, OID, and hash within the function. However,
the raw decrypt is very useful for debugging.
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