Stitched vs non-Stitched Ciphersuites

[Matt Caswell]

On 26/02/2019 15:03, Short, Todd via openssl-users wrote:
> The latest security advisory:
> 
> https://www.openssl.org/news/secadv/20190226.txt
> 
> mentions stitched vs. non-stitched ciphersuites, but doesn’t really elaborate on
> which ciphersuites are stitched and non-stitched.

The actual list in use is platform specific - the stitched ciphers are based on
asm implementations. Libssl in 1.0.2 knows about these stitched ciphers:

https://github.com/openssl/openssl/blob/56ff0f643482b19f7b2d7ed532dfb94ed3a4e294/ssl/ssl_ciph.c#L651-L671

Any TLS ciphersuite based on the above ciphers will use the stitched
implementation if it is available on that platform.

So, for example, if a stitched implementation of AES-128-CBC-HMAC-SHA1 is
available on your platform then it will be used if you negotiate the AES128-SHA
ciphersuite (aka TLS_RSA_WITH_AES_128_CBC_SHA). Similarly it will be used if you
negotiate DH-RSA-AES128-SHA (aka TLS_DH_RSA_WITH_AES_128_CBC_SHA) The combined
encrypt and mac operation will be performed in one go by the stitched
implementation. If you don't have a stitched implementation then the encrypt and
mac operations are performed individually.

Matt


> 
>> "In order for this to be exploitable "non-stitched" ciphersuites must be in
>> use. Stitched ciphersuites are optimised implementations of certain commonly
>> used ciphersuites."
> 
> Can someone give some examples of both?
> 
> --
> -Todd Short
> // tshort at akamai.com <mailto:tshort at akamai.com>
> // "One if by land, two if by sea, three if by the Internet."
>