Stitched aes-128 and hmac-sha1 (encrypt-then-mac)

[Matt Caswell]

On 01/11/2019 11:59, pablo platt wrote:
> Thank you for the explanation.
> 
> The use case is a WebRTC server (SFU) that encrypts and authenticate
> SRTP packets.
> Encryption is a major part of CPU load on SFU servers. Reducing it by
> 50% will have a large impact.
> 
> Is it planned to add aes-128-hmac-sha1 encrypt-then-mac?

There are no current plans. You might investigate the impact of using
AEAD ciphers instead.

Matt

> 
> On Fri, Nov 1, 2019 at 1:32 PM Matt Caswell <matt at openssl.org
> <mailto:matt at openssl.org>> wrote:
> 
> 
> 
>     On 01/11/2019 07:56, pablo platt wrote:
>     > Hi,
>     >
>     > Stitching aes-cbc with sha1 can result with x2 performance [1].
>     > Is there support for stitched aes-128-hmac-sha1 encrypt-then-mac? This
>     > issue [2] says that only mac-then-encrypt is supported in OpenSSL.
> 
>     The issue is correct. Only mac-then-encrypt is supported. Furthermore
>     these stitched ciphers are specifically targeted at use by libssl and
>     are designed for use in SSL/TLS only. They are not general purpose
>     ciphers and should not be used directly unless you *really* know what
>     you are doing.
> 
>     Note that more modern TLS ciphersuites use AEAD modes such as GCM or CCM
>     so that mac-then-encrypt vs encrypt-then-mac and "stitched" ciphers are
>     irrelevant anyway.
> 
>     >
>     > Does this implement mac-then-encrypt and relevant [3]?
> 
>     [3] is the aesni assembler implementation used behind the
>     EVP_aes_128_cbc_hmac_sha1() and EVP_aes_256_cbc_hmac_sha1() ciphers,
>     i.e. all the same comments I made above apply here. It's
>     mac-then-encrypt, and specifically targeted for use in SSL/TLS by
>     libssl. It's not intended for general purpose use.
> 
>     The documentation says this about these ciphers:
> 
>     "EVP_aes_128_cbc_hmac_sha1(),
>     EVP_aes_256_cbc_hmac_sha1()
> 
>     Authenticated encryption with AES in CBC mode using SHA-1 as HMAC, with
>     keys of 128 and 256 bits length respectively. The authentication tag is
>     160 bits long.
> 
>     WARNING: this is not intended for usage outside of TLS and requires
>     calling of some undocumented ctrl functions. These ciphers do not
>     conform to the EVP AEAD interface."
> 
>     https://www.openssl.org/docs/man1.1.1/man3/EVP_aes_128_cbc_hmac_sha1.html
> 
> 
> 
>     > Is it possible to use the same code with just changing the order to
>     > achieve encrypt-then-mac?
> 
>     No.
> 
>     > How can I compile the Perl file to be used from a C program?
> 
>     This is an internal file not intended for use outside of OpenSSL and not
>     intended to be compiled separately. You might be able to extract it -
>     but if so, you're on your own.
> 
> 
>     Matt
>