[openssl-users] FW: Help to understand WPACKET API

Ananthaneni Saitejachowdary ananthaneni.saitejachowdary at citrix.com
Tue Dec 26 04:34:24 UTC 2017

Thank you Matt, 

I understand that _u8,_u16. _u24 are the variants that helps us set the size of length field, that precedes the data.


-----Original Message-----
From: openssl-users [mailto:openssl-users-bounces at openssl.org] On Behalf Of Matt Caswell
Sent: Friday, December 22, 2017 4:23 PM
To: openssl-users at openssl.org
Subject: Re: [openssl-users] FW: Help to understand WPACKET API

On 19/12/17 13:43, Sai Teja Chowdary wrote:
> After going through the code I understand the new tls1.3 implemented 
> OpenSSL code is using WPACKET API to frame the records. I need help in 
> understanding what the following functions do. I am new to the mailing 
> list, thanks in advance.
> WPACKET_start_sub_packet_
> WPACKET_allocate_bytes
> WPACKET_sub_allocate_bytes__
> WPACKET_reserve_bytes
> WPACKET_start_sub_packet_len__
> WPACKET_sub_memcpy__
> Basically what does the word _sub_ mean in these functions and how do 
> that differ from the non sub function say
> Difference between WPACKET_sub_memcpy__ and WPACKET_memcpy__

It is common in TLS for a block of data to be preceded by a length.
Depending on the context the number of length bytes might typically be 1, 2, 3 or 4. Such a block of data is considered a "sub" packet in the WPACKET API. So:

int WPACKET_memcpy(WPACKET *pkt, const void *src, size_t len);

Copies len bytes of data from src into the pkt. Compare with:

#define WPACKET_sub_memcpy_u8(pkt, src, len) \
    WPACKET_sub_memcpy__((pkt), (src), (len), 1)

So WPACKET_sub_memcpy_u8 will also copy len bytes of data from src into pkt but will precede it with 1 byte containing the value of len.

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