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XEX encryption mode with tweak and ciphertext stealing (XTS) is
standardized in IEEE 1619 and generally used for storage devices.
Signed-off-by: Simo Sorce <simo@redhat.com>

On BCM2837B0 (Cortex-A53) @1.4GHz (Raspberry Pi 3B+),
Before:
`gnutls-cli --benchmark-ciphers`
       CHACHA20-POLY1305 (16384) 51.54 MB/sec
`gnutls-cli --benchmark-tls-ciphers`:
       ECDHE_RSA_CHACHA20_POLY1305 (payload 1400)  21.31 MB/sec
       ECDHE_RSA_CHACHA20_POLY1305 (payload 15360)  24.60 MB/sec
`nettle-benchmark`
		 chacha     encrypt   71.90
		 chacha     decrypt   71.89
	chacha_poly1305     encrypt   48.17
	chacha_poly1305     decrypt   48.17
	chacha_poly1305      update  146.03

After:
`gnutls-cli --benchmark-ciphers`
       CHACHA20-POLY1305 (16384) 68.44 MB/sec
`gnutls-cli --benchmark-tls-ciphers`:
       ECDHE_RSA_CHACHA20_POLY1305 (payload 1400) 27.25 MB/sec
       ECDHE_RSA_CHACHA20_POLY1305 (payload 15360) 32.41 MB/sec
`nettle-benchmark`
		 chacha     encrypt  106.00
		 chacha     decrypt  105.94
	chacha_poly1305     encrypt   65.94
	chacha_poly1305     decrypt   65.96
	chacha_poly1305      update  175.24

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>

(cherry picked from commit 73d3c6d5)

Intended to silence warning from the clang static analyzer.

* rsa-sign-tr.c (sec_equal): Fix accidental use of C99 for loop.
Reported by Andreas Gustafsson.
* testsuite/rsa-sec-decrypt-test.c (test_main): Likewise.

Mention dependency on GMP-6, and RSA performance regression.

* testsuite/rsa-encrypt-test.c (test_main): Fix allocation of
decrypted storage. Update test of rsa_decrypt, to allow clobbering
of all of the passed in message area.

Patch from Simo Sorce.

Use new local helper functions, with their own itch functions.

Also renamed with leading underscore, and updated all callers.