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Diffstat (limited to 'internal/stupidgcm/doc.go')
| -rw-r--r-- | internal/stupidgcm/doc.go | 59 |
1 files changed, 59 insertions, 0 deletions
diff --git a/internal/stupidgcm/doc.go b/internal/stupidgcm/doc.go new file mode 100644 index 0000000..36c189b --- /dev/null +++ b/internal/stupidgcm/doc.go @@ -0,0 +1,59 @@ +// Package stupidgcm wraps OpenSSL to provide a cipher.AEAD interface for +// authenticated encryption algorithms. +// +// The supported algorithms are: +// +// (1) AES-GCM-256 (OpenSSL EVP_aes_256_gcm) +// +// (2) ChaCha20-Poly1305 (OpenSSL EVP_chacha20_poly1305) +// +// (3) XChaCha20-Poly1305 (OpenSSL EVP_chacha20_poly1305 + Go HChaCha20) +// +// 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. +// +// However, OpenSSL has optimized assembly for almost all platforms, which Go +// does not. Example for a 32-bit ARM device (Odroid XU4): +// +// $ gocrypts -speed +// gocryptfs v2.1-68-gedf9d4c.stupidchacha; go-fuse v2.1.1-0.20210825171523-3ab5d95a30ae; 2021-09-04 go1.16.7 linux/arm +// AES-GCM-256-OpenSSL 56.84 MB/s (selected in auto mode) +// AES-GCM-256-Go 16.61 MB/s +// AES-SIV-512-Go 16.49 MB/s +// XChaCha20-Poly1305-Go 39.08 MB/s (use via -xchacha flag) +// XChaCha20-Poly1305-OpenSSL 141.82 MB/s +// +// This package is "stupid" in the sense that it only supports a narrow set of +// key- and iv-lengths, and panics if it does not like what you pass it. +// See the constructor functions for which restrictions apply for each algorithm. +// Also, it is only tested for block lengths up to 5000 bytes, because this is +// what gocryptfs uses. +// +// Corrupt ciphertexts never cause a panic. Instead, ErrAuth is returned on +// decryption. +// +// XChaCha20-Poly1305 +// +// The XChaCha20-Poly1305 implementation is more complicated than the others, +// because OpenSSL does not support XChaCha20-Poly1305 directly. Follow +// https://github.com/openssl/openssl/issues/5523 to get notified when it is +// accepted into OpenSSL. +// +// Fortunately, XChaCha20-Poly1305 is just ChaCha20-Poly1305 with some key+iv +// mixing using HChaCha20 in front: +// +// key (32 bytes), iv (24 bytes) +// | +// v +// HChaCha20 (provided by golang.org/x/crypto/chacha20) +// | +// v +// key2 (32 bytes), iv2 (16 bytes) +// | +// v +// ChaCha20-Poly1305 (OpenSSL EVP_chacha20_poly1305) +// +// As HChaCha20 is very fast, XChaCha20-Poly1305 gets almost the same throughput +// as ChaCha20-Poly1305 (for 4kiB blocks). +package stupidgcm |