std/crypto: implement the HKDF construction

lib/std/crypto.zig

@@ -95,6 +95,11 @@ pub const stream = struct {
     pub const ChaCha20With64BitNonce = @import("crypto/chacha20.zig").ChaCha20With64BitNonce;
 };
 
+/// Key derivation functions.
+pub const kdf = struct {
+    pub const hkdf = @import("crypto/hkdf.zig");
+};
+
 const std = @import("std.zig");
 pub const randomBytes = std.os.getrandom;
 

lib/std/crypto/hkdf.zig

@@ -0,0 +1,68 @@
+const std = @import("../std.zig");
+const assert = std.debug.assert;
+const hmac = std.crypto.auth.hmac;
+const mem = std.mem;
+
+/// HKDF-SHA256
+pub const HkdfSha256 = Hkdf(hmac.sha2.HmacSha256);
+
+/// HKDF-SHA512
+pub const HkdfSha512 = Hkdf(hmac.sha2.HmacSha512);
+
+/// The Hkdf construction takes some source of initial keying material and
+/// derives one or more uniform keys from it.
+pub fn Hkdf(comptime Hmac: type) type {
+    return struct {
+        pub const prk_length = Hmac.mac_length;
+
+        /// Return a master key from a salt and initial keying material.
+        fn extract(salt: []const u8, ikm: []const u8) [Hmac.mac_length]u8 {
+            var prk: [Hmac.mac_length]u8 = undefined;
+            Hmac.create(&prk, ikm, salt);
+            return prk;
+        }
+
+        /// Derive a subkey from a master key `prk` and a subkey description `ctx`.
+        fn expand(out: []u8, ctx: []const u8, prk: [Hmac.mac_length]u8) void {
+            assert(out.len < Hmac.mac_length * 255); // output size is too large for the Hkdf construction
+            var i: usize = 0;
+            var counter = [1]u8{1};
+            while (i + Hmac.mac_length <= out.len) : (i += Hmac.mac_length) {
+                var st = Hmac.init(&prk);
+                if (i != 0) {
+                    st.update(out[i - Hmac.mac_length ..][0..Hmac.mac_length]);
+                }
+                st.update(ctx);
+                st.update(&counter);
+                st.final(out[i..][0..Hmac.mac_length]);
+                counter[0] += 1;
+            }
+            const left = out.len % Hmac.mac_length;
+            if (left > 0) {
+                var st = Hmac.init(&prk);
+                if (i != 0) {
+                    st.update(out[i - Hmac.mac_length ..][0..Hmac.mac_length]);
+                }
+                st.update(ctx);
+                st.update(&counter);
+                var tmp: [Hmac.mac_length]u8 = undefined;
+                st.final(tmp[0..Hmac.mac_length]);
+                mem.copy(u8, out[i..][0..left], tmp[0..left]);
+            }
+        }
+    };
+}
+
+const htest = @import("test.zig");
+
+test "Hkdf" {
+    const ikm = [_]u8{0x0b} ** 22;
+    const salt = [_]u8{ 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c };
+    const context = [_]u8{ 0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9 };
+    const kdf = HkdfSha256;
+    const prk = kdf.extract(&salt, &ikm);
+    htest.assertEqual("077709362c2e32df0ddc3f0dc47bba6390b6c73bb50f9c3122ec844ad7c2b3e5", &prk);
+    var out: [42]u8 = undefined;
+    kdf.expand(&out, &context, prk);
+    htest.assertEqual("3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865", &out);
+}