Add optional support for AEGIS encryption

AEGIS is a new family of authenticated encryption algorithms that offers
stronger security, higher usage limits, and better performance than AES-GCM.

This pull request adds support for a new `-aegis` command-line flag, allowing
AEGIS-128X2 to be used as an alternative to AES-GCM on CPUs with AES acceleration.

It also introduces the ability to use ciphers with different key sizes.

More information on AEGIS is available here:
- https://cfrg.github.io/draft-irtf-cfrg-aegis-aead/draft-irtf-cfrg-aegis-aead.html
- https://github.com/cfrg/draft-irtf-cfrg-aegis-aead

gocryptfs -speed speed on Apple M1:

AES-GCM-256-OpenSSL              3718.79 MB/s
AES-GCM-256-Go                   5083.43 MB/s   (selected in auto mode)
AES-SIV-512-Go                    625.20 MB/s
XChaCha20-Poly1305-OpenSSL       1358.63 MB/s   (selected in auto mode)
XChaCha20-Poly1305-Go             832.11 MB/s
Aegis128X2-Go                   11818.73 MB/s

gocryptfs -speed speed on AMD Zen 4:

AES-GCM-256-OpenSSL              5215.86 MB/s
AES-GCM-256-Go                   6918.01 MB/s   (selected in auto mode)
AES-SIV-512-Go                    449.61 MB/s
XChaCha20-Poly1305-OpenSSL       2643.48 MB/s
XChaCha20-Poly1305-Go            3727.46 MB/s   (selected in auto mode)
Aegis128X2-Go                   28109.92 MB/s

1 files changed, 4 insertions, 2 deletions

diff --git a/internal/stupidgcm/doc.go b/internal/stupidgcm/doc.go
index dce82ae..f5d4177 100644
--- a/internal/stupidgcm/doc.go
+++ b/internal/stupidgcm/doc.go
@@ -1,5 +1,5 @@
-// Package stupidgcm wraps OpenSSL to provide a cipher.AEAD interface for
-// authenticated encryption algorithms.
+// Package stupidgcm wraps OpenSSL and libaegis to provide a cipher.AEAD
+// interface for authenticated encryption algorithms.
 //
 // The supported algorithms are:
 //
@@ -9,6 +9,8 @@
 //
 // (3) XChaCha20-Poly1305 (OpenSSL EVP_chacha20_poly1305 + Go HChaCha20)
 //
+// (4) AEGIS (go-libaegis)
+//
 // The golang.org/x/crypto libraries provides implementations for all algorithms,
 // and the test suite verifies that the implementation in this package gives
 // the exact same results.