Commit `0d83487dd0`

Frank Denis <github@pureftpd.org>

2022-12-29 11:29:50

hkdf: add prk_length and extractInit()

The HKDF extract function uses HMAC under the hood, but requiring applications to directly use HMAC functions reduces clarity and feels like the wrong abstraction. So, in order to get the PRK length, add a `prk_length` constant that applications can use directly. Also add an `extractInit()` function for cases where the keying material has to be provided as multiple chunks.

master

1 parent 9b665a5

Changed files (1)

lib

std

crypto

hkdf.zig

@@ -13,39 +13,56 @@ pub const HkdfSha512 = Hkdf(hmac.sha2.HmacSha512);
 /// derives one or more uniform keys from it.
 pub fn Hkdf(comptime Hmac: type) type {
     return struct {
+        /// Length of a master key, in bytes.
+        pub const prk_length = Hmac.mac_length;
+
         /// Return a master key from a salt and initial keying material.
-        pub fn extract(salt: []const u8, ikm: []const u8) [Hmac.mac_length]u8 {
-            var prk: [Hmac.mac_length]u8 = undefined;
+        pub fn extract(salt: []const u8, ikm: []const u8) [prk_length]u8 {
+            var prk: [prk_length]u8 = undefined;
             Hmac.create(&prk, ikm, salt);
             return prk;
         }
 
+        /// Initialize the creation of a master key from a salt
+        /// and keying material that can be added later, possibly in chunks.
+        /// Example:
+        /// ```
+        /// var prk: [hkdf.prk_length]u8 = undefined;
+        /// var hkdf = HkdfSha256.extractInit(salt);
+        /// hkdf.update(ikm1);
+        /// hkdf.update(ikm2);
+        /// hkdf.final(&prk);
+        /// ```
+        pub fn extractInit(salt: []const u8) Hmac {
+            return Hmac.init(salt);
+        }
+
         /// Derive a subkey from a master key `prk` and a subkey description `ctx`.
-        pub 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
+        pub fn expand(out: []u8, ctx: []const u8, prk: [prk_length]u8) void {
+            assert(out.len <= prk_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) {
+            while (i + prk_length <= out.len) : (i += prk_length) {
                 var st = Hmac.init(&prk);
                 if (i != 0) {
-                    st.update(out[i - Hmac.mac_length ..][0..Hmac.mac_length]);
+                    st.update(out[i - prk_length ..][0..prk_length]);
                 }
                 st.update(ctx);
                 st.update(&counter);
-                st.final(out[i..][0..Hmac.mac_length]);
+                st.final(out[i..][0..prk_length]);
                 counter[0] +%= 1;
                 assert(counter[0] != 1);
             }
-            const left = out.len % Hmac.mac_length;
+            const left = out.len % prk_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(out[i - prk_length ..][0..prk_length]);
                 }
                 st.update(ctx);
                 st.update(&counter);
-                var tmp: [Hmac.mac_length]u8 = undefined;
-                st.final(tmp[0..Hmac.mac_length]);
+                var tmp: [prk_length]u8 = undefined;
+                st.final(tmp[0..prk_length]);
                 mem.copy(u8, out[i..][0..left], tmp[0..left]);
             }
         }
@@ -64,4 +81,10 @@ test "Hkdf" {
     var out: [42]u8 = undefined;
     kdf.expand(&out, &context, prk);
     try htest.assertEqual("3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865", &out);
+
+    var hkdf = kdf.extractInit(&salt);
+    hkdf.update(&ikm);
+    var prk2: [kdf.prk_length]u8 = undefined;
+    hkdf.final(&prk2);
+    try htest.assertEqual("077709362c2e32df0ddc3f0dc47bba6390b6c73bb50f9c3122ec844ad7c2b3e5", &prk2);
 }

Commit 0d83487dd0

Commit `0d83487dd0`