> From: owner-openssl-us...@openssl.org On Behalf Of Akos Vandra
> Sent: Tuesday, 14 July, 2009 13:34
> I am trying to decode the CertificateVerify structure, but have thus
far failed.
> I have access to both client and server keys, and have sniffed their
communication,
> what I came up with (along the stream) is this CertificateVerify packet
<snip>
> As far as I know, this means <snip>
> TLSPlainText.Handshake.CertificateVerify
> Length = 0x00, 0x40, <-- The RFC doesn't say that there
should be
> a length info here, but this certainly is the remaining part's length.
> Data = 0xe4, 0xe9, <snip>
Yes it does say so. 4346 7.4.3 says Signature is (assuming RSA)
digitally-signed struct { opaque md5_hash[16]; opaque sha_hash[20]; }
and 4.7 says digitally-signed "is encoded as an opaque vector <0..2^16-1>
where the length is specified by the signing algorithm and key."
4.3 says vector has a prefix length, which for that range is 2 octets.
The size of an RSA signature is the same as the modulus, which
for your case obviously is 64 bytes = 512 bits. (You do know RSA-512
has been factorable = insecure for years now, right? Of course
if this is only test/debug data its security doesn't matter.)
> I tried decoding this signed data with openssl (successfully), it
yielded:
> $openssl rsautl -verify -inkey clientkey.pem -in sign.bin -hexdump
> 0000 - 47 f6 a5 1b a9 cb 4a a6-90 63 2c 65 ec 6f 6d 20
> 0010 - 10 af a8 f0 f0 80 0d 99-a3 22 cf 2b 07 b0 a4 c8
> 0020 - c7 ec 1d 33
> My question is how to interpret this data? From the rfc I understood
that
> there should be a
> struct {
> SignatureAndHashAlgorithm algorithm;
>
> opaque signature<0..2^16-1>;
> } DigitallySigned;
Not in TLS1.0 or 1.1 for sure. It should be two (fixed-length) hashes
as above, and it is (36 bytes = 16 + 20). I don't know if 1.2 changed.
> I am not sure what is the hash that should be calculated, so I am
not
> sending that along, because I don't think it is correct.
Per 7.4.8, the hash(es) should be of all the handshake messages so far.
This is the same idea as used (later) for Finished.
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