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Clarify use of poly1305

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Alexander Neumann 2015-03-22 21:26:14 +01:00
parent 53ad706c6d
commit 02282a4fff
1 changed files with 12 additions and 3 deletions
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doc/Design.md
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@ -72,6 +72,16 @@ A repository can be initialized with the `restic init` command, e.g.:
Keys, Encryption and MAC
------------------------
All data stored by restic in the repository is encrypted with AES-256 in
counter mode and signed with Poly1305-AES. For encrypting new data first 16
bytes are read from a cryptographically secure pseudorandom number generator as
a random nonce. This is used both as the IV for counter mode and the nonce for
Poly1305. This operation needs three keys: A 32 byte for AES-256 for
encryption, a 16 byte AES key and a 16 byte key for Poly1305. For details see
the original paper[The Poly1305-AES message-authentication
code](http://cr.yp.to/mac/poly1305-20050329.pdf) by Dan Bernstein. The
ciphertext is stored as IV || CIPHERTEXT || MAC.
The directory `keys` contains key files. These are simple JSON documents which
contain all data that is needed to derive the repository's master signing and
encryption keys from a user's password. The JSON document from the repository
@ -97,9 +107,8 @@ repository password. This is then used with `scrypt`, a key derivation function
bytes. The first 32 bytes are used as the encryption key (for AES-256) and the
last 32 bytes are used as the signing key (for Poly1305-AES). These last 32
bytes are divided into a 16 byte AES key `k` followed by 16 bytes of secret key
`r`. They key `r` is then masked for use with Poly1305. For details see the
original paper [The Poly1305-AES message-authentication
code](http://cr.yp.to/mac/poly1305-20050329.pdf) by Dan Bernstein.
`r`. They key `r` is then masked for use with Poly1305 (see the paper for
details).
This signing key is used to compute a MAC over the bytes contained in the
JSON field `data` (after removing the Base64 encoding and not including the