tree 62263fb3bb6eac2192ffaa0a102ed77375d007f8
parent 27a67557405dcf18567f4528aa537da25f0a34e2
author David Benjamin <davidben@google.com> 1637175671 -0500
committer Adam Langley <agl@google.com> 1637268523 +0000

Add various OpenSSL compatibility functions.

The non-_ex EVP_CIPHER_CTX Final functions are a bit interesting. Unlike
EVP_DigestFinal(_ex), where the non-_ex version calls EVP_MD_CTX_cleanup
for you, the EVP_CIPHER_CTX ones do not automatically cleanup.
EVP_CipherFinal and EVP_CipherFinal_ex are identical in all releases
where they exist.

This appears to date to OpenSSL 0.9.7:

Prior to OpenSSL 0.9.7, EVP_MD_CTX and EVP_CIPHER_CTX did not use void*
data fields. Instead, they just had a union of context structures for
every algorithm OpenSSL implemented.

EVP_MD_CTX was truly cleanup-less. There were no EVP_MD_CTX_init or
EVP_MD_CTX_cleanup functions at all. EVP_DigestInit filled things in
without reference to the previous state. EVP_DigestFinal didn't cleanup
because there was nothing to cleanup.

EVP_CIPHER_CTX was also a union, but for some reason did include
EVP_CIPHER_CTX_init and EVP_CIPHER_CTX_cleanup. EVP_CIPHER_CTX_init
seemed to be optional: EVP_CipherInit with non-NULL EVP_CIPHER similarly
didn't reference the previous state. EVP_CipherFinal did not call
EVP_CIPHER_CTX_cleanup, but EVP_CIPHER_CTX_cleanup didn't do anything.
It called an optional cleanup hook on the EVP_CIPHER, but as far as I
can tell, no EVP_CIPHER implemented it.

Then OpenSSL 0.9.7 introduced ENGINE. The union didn't work anymore, so
EVP_MD_CTX and EVP_CIPHER_CTX contained void* with allocated
type-specific data. The introduced EVP_MD_CTX_init and
EVP_MD_CTX_cleanup. For (imperfect!) backwards compatibility,
EVP_DigestInit and EVP_DigestFinal transparently called init/cleanup for
you. EVP_DigestInit_ex and EVP_DigestFinal_ex became the more flexible
versions that left init/cleanup to the caller.

EVP_CIPHER_CTX got the same treatment with
EVP_CipherInit/EVP_CipherInit_ex, but *not*
EVP_CipherFinal/EVP_CipherFinal_ex. The latter did the same thing. The
history seems to be that 581f1c84940d77451c2592e9fa470893f6c3c3eb
introduced the Final/Final_ex split, with the former doing an
auto-cleanup, then 544a2aea4ba1fad76f0802fb70d92a5a8e6ad85a undid it.
Looks like the motivation is that EVP_CIPHER_CTX objects are often
reused to do multiple operations with a single key. But they missed that
the split functions are now unnecessary.

Amusingly, OpenSSL's documentation incorrectly said that EVP_CipherFinal
cleaned up after the call until it was fixed in
538860a3ce0b9fd142a7f1a62e597cccb74475d3. The fix says that some
releases cleaned up, but there were, as far as I can tell, no actual
releases with that behavior.

I've put the new Final functions in the deprecated section, purely
because there is no sense in recommending two different versions of the
same function to users, and Final_ex seems to be more popular. But there
isn't actually anything wrong with plain Final.

Change-Id: Ic2bfda48fdcf30f292141add8c5f745348036852
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/50485
Reviewed-by: Adam Langley <agl@google.com>