crypto: aes - Remove aes-fixed-time / CONFIG_CRYPTO_AES_TI

Remove aes-fixed-time, i.e. CONFIG_CRYPTO_AES_TI.  This was a wrapper
around the 256-byte-table-based AES implementation in lib/crypto/aes.c,
with extra code to enable and disable IRQs for constant-time hardening.

While nice in theory, in practice this had the following issues:

- For bulk en/decryption it was 2-4 times slower than aes-generic.  This
  resulted in aes-generic still being needed, creating fragmentation.

- Having both aes-generic and aes-fixed-time punted an AES
  implementation decision to distros and users who are generally
  unprepared to handle it.  In practice, whether aes-fixed-time gets
  used tends to be incidental and not match an explicit distro or user
  intent.  (While aes-fixed-time has a higher priority than aes-generic,
  whether it actually gets enabled, loaded, and used depends on the
  kconfig and whether a modprobe of "aes" happens to be done.  It also
  has a lower priority than aes-arm and aes-arm64.)

- My changes to the generic AES code (in other commits) significantly
  close the gap with aes-fixed-time anyway.  The table size is reduced
  from 8192 bytes to 1024 bytes, and prefetching is added.

- While AES code *should* be constant-time, the real solutions for that
  are AES instructions (which most CPUs have now) or bit-slicing.  arm
  and arm64 already have bit-sliced AES code for many modes; generic
  bit-sliced code could be written but would be very slow for single
  blocks.  Overall, I suggest that trying to write constant-time
  table-based AES code is a bit futile anyway, and in the rare cases
  where a proper AES implementation is still unavailable it's reasonable
  to compromise with an implementation that simply prefetches the table.

Thus, this commit removes aes-fixed-time and CONFIG_CRYPTO_AES_TI.  The
replacement is just the existing CONFIG_CRYPTO_AES, which for now maps
to the existing aes-generic code, but I'll soon be changing to use the
improved AES library code instead.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20260112192035.10427-9-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

1 files changed, 0 insertions, 21 deletions

diff --git a/crypto/Kconfig b/crypto/Kconfig
index 443fe8e016fd..db6b0c2fb50e 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -366,27 +366,6 @@ config CRYPTO_AES
 
 	  The AES specifies three key sizes: 128, 192 and 256 bits
 
-config CRYPTO_AES_TI
-	tristate "AES (Advanced Encryption Standard) (fixed time)"
-	select CRYPTO_ALGAPI
-	select CRYPTO_LIB_AES
-	help
-	  AES cipher algorithms (Rijndael)(FIPS-197, ISO/IEC 18033-3)
-
-	  This is a generic implementation of AES that attempts to eliminate
-	  data dependent latencies as much as possible without affecting
-	  performance too much. It is intended for use by the generic CCM
-	  and GCM drivers, and other CTR or CMAC/XCBC based modes that rely
-	  solely on encryption (although decryption is supported as well, but
-	  with a more dramatic performance hit)
-
-	  Instead of using 16 lookup tables of 1 KB each, (8 for encryption and
-	  8 for decryption), this implementation only uses just two S-boxes of
-	  256 bytes each, and attempts to eliminate data dependent latencies by
-	  prefetching the entire table into the cache at the start of each
-	  block. Interrupts are also disabled to avoid races where cachelines
-	  are evicted when the CPU is interrupted to do something else.
-
 config CRYPTO_ANUBIS
 	tristate "Anubis"
 	depends on CRYPTO_USER_API_ENABLE_OBSOLETE