83 lines
3.5 KiB
Markdown
83 lines
3.5 KiB
Markdown
# Cryptography & key management
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Keys that are used:
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- master secret key (for indexes)
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- curve25519 public/private key pair (for incoming mail)
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Keys that are stored in K2V under PK `keys`:
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- `public`: the public curve25519 key (plain text)
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- `salt`: the 32-byte salt `S` used to calculate digests that index keys below
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- if a password is used, `password:<truncated(128bit) argon2 digest of password using salt S>`:
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- a 32-byte salt `Skey`
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- followed a secret box
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- that is encrypted with a strong argon2 digest of the password (using the salt `Skey`) and a user secret (see below)
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- that contains the master secret key and the curve25519 private key
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User secret: an additionnal secret that is added to the password when deriving the encryption key for the secret box.
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This additionnal secret should not be stored in K2V/S3, so that just knowing a user's password isn't enough to be able
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to decrypt their mailbox (supposing the attacker has a dump of their K2V/S3 bucket).
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This user secret should typically be stored in the LDAP database or just in the configuration file when using
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the static login provider.
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Operations:
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- **Initialize**(`user_secret`, `password`):
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- if `"salt"` or `"public"` already exist, BAIL
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- generate salt `S` (32 random bytes)
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- generate `public`, `private` (curve25519 keypair)
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- generate `master` (secretbox secret key)
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- calculate `digest = argon2_S(password)`
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- generate salt `Skey` (32 random bytes)
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- calculate `key = argon2_Skey(user_secret + password)`
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- serialize `box_contents = (private, master)`
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- seal box `blob = seal_key(box_contents)`
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- write `S` at `"salt"`
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- write `concat(Skey, blob)` at `"password:{hex(digest[..16])}"`
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- write `public` at `"public"`
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- **InitializeWithoutPassword**(`private`, `master`):
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- if `"salt"` or `"public"` already exist, BAIL
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- generate salt `S` (32 random bytes)
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- write `S` at `"salt"`
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- calculate `public` the public key associated with `private`
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- write `public` at `"public"`
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- **Open**(`user_secret`, `password`):
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- load `S = read("salt")`
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- calculate `digest = argon2_S(password)`
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- load `blob = read("password:{hex(digest[..16])}")
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- set `Skey = blob[..32]`
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- calculate `key = argon2_Skey(user_secret + password)`
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- open secret box `box_contents = open_key(blob[32..])`
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- retrieve `master` and `private` from `box_contents`
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- retrieve `public = read("public")`
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- **OpenWithoutPassword**(`private`, `master`):
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- load `public = read("public")`
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- check that `public` is the correct public key associated with `private`
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- **AddPassword**(`user_secret`, `existing_password`, `new_password`):
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- load `S = read("salt")`
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- calculate `digest = argon2_S(existing_password)`
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- load `blob = read("existing_password:{hex(digest[..16])}")
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- set `Skey = blob[..32]`
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- calculate `key = argon2_Skey(user_secret + existing_password)`
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- open secret box `box_contents = open_key(blob[32..])`
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- retrieve `master` and `private` from `box_contents`
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- calculate `digest_new = argon2_S(new_password)`
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- generate salt `Skeynew` (32 random bytes)
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- calculate `key_new = argon2_Skeynew(user_secret + new_password)`
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- serialize `box_contents_new = (private, master)`
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- seal box `blob_new = seal_key_new(box_contents_new)`
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- write `concat(Skeynew, blob_new)` at `"new_password:{hex(digest_new[..16])}"`
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- **RemovePassword**(`password`):
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- load `S = read("salt")`
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- calculate `digest = argon2_S(existing_password)`
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- check that `"password:{hex(digest[..16])}"` exists
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- check that other passwords exist ?? (or not)
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- delete `"password:{hex(digest[..16])}"`
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