* cipher/Makefile.am: Add 'cipher-siv.c'.
* cipher/cipher-ctr.c (_gcry_cipher_ctr_encrypt): Rename to
_gcry_cipher_ctr_encrypt_ctx and add algo context parameter.
(_gcry_cipher_ctr_encrypt): New using _gcry_cipher_ctr_encrypt_ctx.
* cipher/cipher-internal.h (gcry_cipher_handle): Add 'u_mode.siv'.
(_gcry_cipher_ctr_encrypt_ctx, _gcry_cipher_siv_encrypt)
(_gcry_cipher_siv_decrypt, _gcry_cipher_siv_set_nonce)
(_gcry_cipher_siv_authenticate, _gcry_cipher_siv_set_decryption_tag)
(_gcry_cipher_siv_get_tag, _gcry_cipher_siv_check_tag)
(_gcry_cipher_siv_setkey): New.
* cipher/cipher-siv.c: New.
* cipher/cipher.c (_gcry_cipher_open_internal, cipher_setkey)
(cipher_reset, _gcry_cipher_setup_mode_ops, _gcry_cipher_info): Add
GCRY_CIPHER_MODE_SIV handling.
(_gcry_cipher_ctl): Add GCRYCTL_SET_DECRYPTION_TAG handling.
* doc/gcrypt.texi: Add documentation for SIV mode.
* src/gcrypt.h.in (GCRYCTL_SET_DECRYPTION_TAG): New.
(GCRY_CIPHER_MODE_SIV): New.
(gcry_cipher_set_decryption_tag): New.
* tests/basic.c (check_siv_cipher): New.
(check_cipher_modes): Add call for 'check_siv_cipher'.
* tests/bench-slope.c (bench_encrypt_init): Use double size key for
SIV mode.
(bench_aead_encrypt_do_bench, bench_aead_decrypt_do_bench)
(bench_aead_authenticate_do_bench): Reset cipher context on each run.
(bench_aead_authenticate_do_bench): Support nonce-less operation.
(bench_siv_encrypt_do_bench, bench_siv_decrypt_do_bench)
(bench_siv_authenticate_do_bench, siv_encrypt_ops)
(siv_decrypt_ops, siv_authenticate_ops): New.
(cipher_modes): Add SIV mode benchmarks.
(cipher_bench_one): Restrict SIV mode testing to 16 byte block-size.