file-format.md: document AES-SIV block layout
Fixes https://github.com/rfjakob/gocryptfs/issues/299
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@ -6,12 +6,18 @@ Header
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2 bytes header version (big endian uint16, currently 2)
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2 bytes header version (big endian uint16, currently 2)
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16 bytes file id
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16 bytes file id
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Data block
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Data block, default AES-GCM mode
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16 bytes GCM IV (nonce)
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16 bytes GCM IV (nonce)
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1-4096 bytes encrypted data
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1-4096 bytes encrypted data
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16 bytes GHASH
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16 bytes GHASH
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Data block, AES-SIV mode (used in reverse mode, or when explicitely enabled with `-init -aessiv`)
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16 bytes nonce
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16 bytes SIV
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1-4096 bytes encrypted data
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Full block overhead = 32/4096 = 1/128 = 0.78125 %
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Full block overhead = 32/4096 = 1/128 = 0.78125 %
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Example: 1-byte file
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Example: 1-byte file
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