[openssl-dev] [openssl.org #4064] Re: Client Hello longer than 2^14 bytes are rejected

[Matt Caswell via RT]

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On 25/09/15 14:19, Hubert Kario wrote:
> Current OpenSSL-1.0.1, 1.0.2 as well as state-machine-rewrite
> branches reject Client Hello messages bigger than 2^14+4 bytes.


Right. The reason for that is that there is an explicit (deliberate)
check for it. Each message in its call to ssl_get_message specifies
the max size. For ClientHello:

    n = s->method->ssl_get_message(s,
                                   SSL3_ST_SR_CLNT_HELLO_B,
                                   SSL3_ST_SR_CLNT_HELLO_C,
                                   SSL3_MT_CLIENT_HELLO,
                                   SSL3_RT_MAX_PLAIN_LENGTH, &ok);

The max size here is the fifth param, i.e. SSL3_RT_MAX_PLAIN_LENGTH
(=16384, or the max possible size that can fit into a single record).
Every message has one of these defined. Some of them are quite
arbitrary values.

E.g. for ServerHello
    n = s->method->ssl_get_message(s,
                                   SSL3_ST_CR_SRVR_HELLO_A,
                                   SSL3_ST_CR_SRVR_HELLO_B, -1, 20000,
&ok);

Why 20000? No idea.

The same restriction exists in the state-machine-rewrite branches
because I'm ultra-cautious. I am reluctant to remove an explicit check
like that without understanding why it's there in the first place.
Especially if its not breaking anything. Are we ever likely to
encounter a ClientHello > 16384 bytes?

Matt
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