48 lines
1.5 KiB
Markdown
48 lines
1.5 KiB
Markdown
GoCryptFS Security
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==================
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Master Key Storage
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------------------
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The master key is used to perform content and file name encryption.
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It is stored in `gocryptfs.conf`, encrypted with AES-256-GCM using the
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Key Encryption Key (KEK).
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The KEK is generated from the user password using `scrypt`.
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File Contents
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-------------
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All file contents are encrypted using AES-256-GCM (Galois/Counter Mode).
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Files are segmented into 4KB blocks. Each block gets a fresh random
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96 bit IV each time it is modified. A 128-bit authentication tag (GHASH)
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protects each block from modifications.
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Each file has a header containing a random 128-bit file ID. The
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file ID and the block number are mixed into the GHASH as
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*additional authenticated data*. The prevents blocks from being copied
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between or within files.
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To support sparse files, all-zero blocks are accepted and passed through
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unchanged.
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File Names
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----------
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Every directory gets a 128-bit directory IV that is stored in each
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directory as `gocryptfs.diriv`.
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File names are encrypted using AES-256-EME (ECB-Mix-ECB wide-block encryption,
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see https://github.com/rfjakob/eme for details) with the directory IV
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as initialization vector. EME fixes the prefix leak that occours with CBC
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encryption.
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The Base64 encoding limits the usable filename length to 176 characters.
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