It looks good Nick,

My only question is md5.c and tool_metalink.c use the following pre-processor check:

#elif (defined(__MAC_OS_X_VERSION_MAX_ALLOWED) && \
              (__MAC_OS_X_VERSION_MAX_ALLOWED >= 1040)) || \
      (defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && \
              (__IPHONE_OS_VERSION_MAX_ALLOWED >= 20000))

whilst we use #elif defined(USE_SECTRANSP) here. Is there any benefit to either?

Before you send this upstream please make sure to rebase on upstream/master (ie no merge commits) and then amend the commit message to add a separate section with a reference to the PR and any issues it solves, for example

md4: use init/update/final functions in Secure Transport

Usually an explanation here.

Fixes #xxxx
Closes #4979

If you need a refresher please refer to push-access-guidelines

@captain-caveman2k, I'll add that preprocessor macro, though I'll be very surprised if anyone still targets macOS 10.3 or earlier anymore, since it's almost 17 years old at this point.

@jay, I'll edit that. Is it okay if I use the "rebase & merge" button on Github, or does that do something undesired? You can tell I don't do this often...

Is it okay if I use the "rebase & merge" button on Github,

Unfortunately no, this is covered in the guidelines, as far as I know the only way to get the correct attribution (ie committer remains committer) is by pushing from the command line. In this case that would look like:

git checkout nickzman/CC_MD4
git fetch upstream
git rebase upstream/master
git checkout master
git merge --ff-only nickzman/CC_MD4
gitk --all &    # check the final product
# amend if it isn't exactly what you expected, then check again
git push upstream

I'm assuming you use the name upstream for curl/curl, for example git remote -v may show

origin  git@github.com:nickzman/curl.git (fetch)
origin  git@github.com:nickzman/curl.git (push)
upstream        git@github.com:curl/curl.git (fetch)
upstream        git@github.com:curl/curl.git (push)

I'll add that preprocessor macro, though I'll be very surprised if anyone still targets macOS 10.3 or earlier anymore, since it's almost 17 years old at this point.

Do we still need the USE_SECTRANSP in there?

The main reason for asking is md5.c doesn't and I'm looking at your changes here for guidance in my sha256.c changes (over in #4956).

Actually, no, we really don't. It can run on macOS and iOS independently of the Security framework.

Okay, I'm still trying to merge this from the command line, and I'm not having much luck. Here's what I'm doing:

% git remote add upstream git@github.com:curl/curl.git
% git checkout nickzman/CC_MD4
% git fetch upstream
% git rebase upstream/master
% git checkout master
% git merge --ff-only nickzman/CC_MD4
(review the results in SourceTree, then...)
% git push upstream
ERROR: Permission to curl/curl.git denied to nickzman.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

I can type ssh git@github.com and get confirmation that my SSH key is working. Does some repository administrator need to add my key to the project independently of my settings? Because that's the only reason why this would fail, so far as I know...

That's weird and I don't know what that is. You have write access and 2FA enabled so there should not be anything stopping you.

Try git remote -v see what it shows, also try GIT_TRACE=1 git ...

Does some repository administrator need to add my key to the project independently of my settings?

Not as far as I know. The ssh key should be the ssh key in the settings on your account.

Sometimes github has intermittent issues.

I'll try again later, but everything looks correct to me...

% git remote -v
origin	git@github.com:nickzman/curl.git (fetch)
origin	git@github.com:nickzman/curl.git (push)
upstream	git@github.com:curl/curl.git (fetch)
upstream	git@github.com:curl/curl.git (push)
% setenv GIT_TRACE 1
% git push upstream
18:38:20.075280 exec-cmd.c:139          trace: resolved executable path from Darwin stack: /Applications/Xcode.app/Contents/Developer/usr/bin/git
18:38:20.075807 exec-cmd.c:236          trace: resolved executable dir: /Applications/Xcode.app/Contents/Developer/usr/bin
18:38:20.076242 git.c:419               trace: built-in: git push upstream
18:38:20.081102 run-command.c:643       trace: run_command: unset GIT_PREFIX; ssh git@github.com 'git-receive-pack '\''curl/curl.git'\'''
ERROR: Permission to curl/curl.git denied to nickzman.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
% ssh git@github.com
PTY allocation request failed on channel 0
Hi nickzman! You've successfully authenticated, but GitHub does not provide shell access.
Connection to github.com closed.

I pushed it!