103 lines
4.3 KiB
Markdown
103 lines
4.3 KiB
Markdown
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title = "Cryptography & key management"
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weight = 20
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## Key types
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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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## Stored keys
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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
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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 pseudo-code
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We resume here the key cryptography logic for various operations on the mailbox
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### Creating a user account
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This logic is run when the mailbox is created.
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Two modes are supported: password and certificate.
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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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INITIALIZE_WITHOUT_PASSWORD(`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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### Opening the user's mailboxes (upon login)
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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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OPEN_WITHOUT_PASSWORD(`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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### Account maintenance
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ADD_PASSWORD(`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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REMOVE_PASSWORD(`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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