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