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

[Matt Caswell]

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