tree 66d543b7aabfba692d15d141b7e3c0c1598de652
parent 78ee3da2576d9c84ac11fb9cee4d365e8f42e9dc
author David Benjamin <davidben@google.com> 1744038937 -0400
committer Boringssl LUCI CQ <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com> 1744663468 -0700

Implement functions to generate CMS external signatures

This implements just the small subset of OpenSSL's CMS API to support
the Linux kernel's sign-file.c tool. It is nowhere close to a full CMS
implementation and is not intended to become one. In particular, it does
not implement enough of CMS to support S/MIME. That requires much, much
more infrastructure than was implemented here.

CMS is, like PKCS#7, an over-engineered and cryptographically unsound
set of nestable combinators to support just about any configuration of
cryptographic operations. Profiling CMS down to a usable subset is, as a
result, more complicated, more risky, and less efficient than just
designing a bespoke structure for your use case. It is derived from
PKCS#7, and largely overlaps. However, both PKCS#7 and CMS use the v1
version number, but CMS made incompatible changes in some corner cases
that, so far, do not matter to us. (It is incompatible if you try to
layer SignedData atop another combinator, where the lack of proper
domain separation in this badly designed format is of extra risk.)

In the case of the kernel, sign-file.c wants an "external signature",
which is when the data to be signed lives elsewhere. This is, as a
result, a very, very inefficient way to concatenate an enum with a byte
string. But this is what the kernel chose, so here we are.

Because PKCS#7 and CMS are broadly the same structure, I've generalized
the internal PKCS#7 function rather than duplicating all this code. If
we ever hit the cases where PKCS#7 and CMS v1 are incompatible, plumbing
an extra boolean will be the least of our worries.

Test data was generated by compiling the actual sign-file.c against
OpenSSL and saving the output.

Change-Id: Idb0874d2b5294bfad564f3a00458c3fd044d9da5
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/78452
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>