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hkdf: add prk_length and extractInit()

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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.
This commit is contained in:
Frank Denis 2022-12-29 11:29:50 +01:00 committed by Andrew Kelley
parent 9b665a59f2
commit 0d83487dd0
1 changed files with 34 additions and 11 deletions
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45
lib/std/crypto/hkdf.zig
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@ -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);
}