libsodium-compatible sealed boxes
Sealed boxes are designed to anonymously send messages to a recipient given their public key. Only the recipient can decrypt these messages, using their private key. While the recipient can verify the integrity of the message, it cannot verify the identity of the sender.
A message is encrypted using an ephemeral key pair, whose secret part is destroyed right after the encryption process.
pub const SealedBox = structpub const SealedBox = struct {
pub const public_length = Box.public_length;
pub const secret_length = Box.secret_length;
pub const seed_length = Box.seed_length;
pub const seal_length = Box.public_length + Box.tag_length;
/// A key pair.
pub const KeyPair = Box.KeyPair;
fn createNonce(pk1: [public_length]u8, pk2: [public_length]u8) [Box.nonce_length]u8 {
var hasher = Blake2b(Box.nonce_length * 8).init(.{});
hasher.update(&pk1);
hasher.update(&pk2);
var nonce: [Box.nonce_length]u8 = undefined;
hasher.final(&nonce);
return nonce;
}
/// Encrypt a message `m` for a recipient whose public key is `public_key`.
/// `c` must be `seal_length` bytes larger than `m`, so that the required metadata can be added.
pub fn seal(io: std.Io, c: []u8, m: []const u8, public_key: [public_length]u8) (WeakPublicKeyError || IdentityElementError)!void {
debug.assert(c.len == m.len + seal_length);
var ekp = KeyPair.generate(io);
const nonce = createNonce(ekp.public_key, public_key);
c[0..public_length].* = ekp.public_key;
try Box.seal(c[Box.public_length..], m, nonce, public_key, ekp.secret_key);
crypto.secureZero(u8, ekp.secret_key[0..]);
}
/// Decrypt a message using a key pair.
/// `m` must be exactly `seal_length` bytes smaller than `c`, as `c` also includes metadata.
pub fn open(m: []u8, c: []const u8, keypair: KeyPair) (IdentityElementError || WeakPublicKeyError || AuthenticationError)!void {
if (c.len < seal_length) {
return error.AuthenticationFailed;
}
const epk = c[0..public_length];
const nonce = createNonce(epk.*, keypair.public_key);
return Box.open(m, c[public_length..], nonce, epk.*, keypair.secret_key);
}
}