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add more details on how encryption works
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@ -127,11 +127,28 @@ on the repository.
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Encryption
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Encryption
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----------
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AES_ is used with CTR mode of operation (so no need of padding). A 64
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AES_ is used with CTR mode of operation (so no need for padding). A 64
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bits initialization vector is used, a SHA256_ based HMAC_ is computed
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bits initialization vector is used, a SHA256_ based HMAC_ is computed
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on the encrypted chunk with a random 64 bits nonce and both are stored
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on the encrypted chunk with a random 64 bits nonce and both are stored
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in the chunk. The header of each chunk is actually : TYPE(1) +
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in the chunk. The header of each chunk is : ``TYPE(1)` +
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HMAC(32) + NONCE(8). Encryption and HMAC use two different keys.
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``HMAC(32)`` + ``NONCE(8)`` + ``CIPHERTEXT``. Encryption and HMAC use
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two different keys.
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In AES CTR mode you can think of the IV as the start value for the
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counter. The counter itself is incremented by one after each 16 byte
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block. The IV/counter is not required to be random but it must NEVER be
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reused. So to accomplish this Attic initializes the encryption counter
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to be higher than any previously used counter value before encrypting
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new data.
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To reduce payload size only 8 bytes of the 16 bytes nonce is saved in
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the payload, the first 8 bytes are always zeros. This does not affect
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security but limits the maximum repository capacity to only 295
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exabytes (2**64 * 16 bytes).
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Encryption keys are either a passphrase, passed through the
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``ATTIC_PASSPHRASE`` environment or prompted on the commandline, or
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stored in automatically generated key files.
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Key files
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Key files
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---------
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---------
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