crypto/hkdf: remove useless call to Reset

HKDF is commonly used to read keys that are the the same length (or
smaller) than the size of the hash digest, which means the loop inside
Read only runs once.

In that case, calling Reset is unnecesssary overhead.

name                  old time/op    new time/op    delta
16ByteMD5Single-8       1.39µs ± 1%    1.22µs ± 0%  -11.95%  (p=0.000 n=10+9)
20ByteSHA1Single-8       826ns ± 0%     746ns ± 0%   -9.70%  (p=0.000 n=9+10)
32ByteSHA256Single-8     838ns ± 1%     744ns ± 0%  -11.29%  (p=0.000 n=10+10)
64ByteSHA512Single-8    5.12µs ± 0%    4.57µs ± 0%  -10.78%  (p=0.000 n=8+10)
8ByteMD5Stream-8         137ns ± 0%     138ns ± 0%   +0.27%  (p=0.009 n=9+6)
16ByteMD5Stream-8        264ns ± 0%     265ns ± 0%   +0.29%  (p=0.000 n=10+10)
8ByteSHA1Stream-8       64.1ns ± 0%    64.4ns ± 0%   +0.60%  (p=0.000 n=9+9)
20ByteSHA1Stream-8       145ns ± 0%     146ns ± 1%   +0.69%  (p=0.000 n=9+10)
8ByteSHA256Stream-8     42.9ns ± 1%    43.1ns ± 0%   +0.48%  (p=0.005 n=10+10)
32ByteSHA256Stream-8     151ns ± 0%     152ns ± 0%   +0.35%  (p=0.006 n=10+8)
8ByteSHA512Stream-8      139ns ± 0%     139ns ± 0%   +0.08%  (p=0.035 n=9+10)
64ByteSHA512Stream-8    1.07µs ± 0%    1.07µs ± 0%   +0.33%  (p=0.000 n=9+10)

name                  old speed      new speed      delta
16ByteMD5Single-8     11.6MB/s ± 0%  13.1MB/s ± 0%  +13.50%  (p=0.000 n=9+9)
20ByteSHA1Single-8    24.2MB/s ± 0%  26.8MB/s ± 0%  +10.75%  (p=0.000 n=9+10)
32ByteSHA256Single-8  38.2MB/s ± 1%  43.0MB/s ± 0%  +12.72%  (p=0.000 n=10+10)
64ByteSHA512Single-8  12.5MB/s ± 0%  14.0MB/s ± 0%  +12.06%  (p=0.000 n=8+10)
8ByteMD5Stream-8      58.2MB/s ± 0%  58.1MB/s ± 0%   -0.27%  (p=0.004 n=9+9)
16ByteMD5Stream-8     60.6MB/s ± 0%  60.5MB/s ± 0%   -0.27%  (p=0.000 n=9+10)
8ByteSHA1Stream-8      125MB/s ± 0%   124MB/s ± 0%   -0.59%  (p=0.000 n=9+9)
20ByteSHA1Stream-8     138MB/s ± 0%   137MB/s ± 1%   -0.69%  (p=0.000 n=9+10)
8ByteSHA256Stream-8    186MB/s ± 1%   185MB/s ± 0%   -0.47%  (p=0.005 n=10+10)
32ByteSHA256Stream-8   211MB/s ± 0%   211MB/s ± 0%   -0.37%  (p=0.003 n=10+8)
8ByteSHA512Stream-8   57.4MB/s ± 1%  57.4MB/s ± 0%     ~     (p=0.137 n=10+10)
64ByteSHA512Stream-8  59.9MB/s ± 0%  59.7MB/s ± 0%   -0.33%  (p=0.000 n=9+10)

name                  old alloc/op   new alloc/op   delta
16ByteMD5Single-8       1.17kB ± 0%    0.98kB ± 0%  -16.42%  (p=0.000 n=10+10)
20ByteSHA1Single-8      1.25kB ± 0%    1.06kB ± 0%  -15.37%  (p=0.000 n=10+10)
32ByteSHA256Single-8    1.36kB ± 0%    1.14kB ± 0%  -16.46%  (p=0.000 n=10+10)
64ByteSHA512Single-8    2.26kB ± 0%    1.84kB ± 0%  -18.43%  (p=0.000 n=10+10)
8ByteMD5Stream-8         2.00B ± 0%     2.00B ± 0%     ~     (all equal)
16ByteMD5Stream-8        5.00B ± 0%     5.00B ± 0%     ~     (all equal)
8ByteSHA1Stream-8        2.00B ± 0%     2.00B ± 0%     ~     (all equal)
20ByteSHA1Stream-8       5.00B ± 0%     5.00B ± 0%     ~     (all equal)
8ByteSHA256Stream-8      1.00B ± 0%     1.00B ± 0%     ~     (all equal)
32ByteSHA256Stream-8     6.00B ± 0%     6.00B ± 0%     ~     (all equal)
8ByteSHA512Stream-8      1.00B ± 0%     1.00B ± 0%     ~     (all equal)
64ByteSHA512Stream-8     9.00B ± 0%     9.00B ± 0%     ~     (all equal)

name                  old allocs/op  new allocs/op  delta
16ByteMD5Single-8         16.0 ± 0%      14.0 ± 0%  -12.50%  (p=0.000 n=10+10)
20ByteSHA1Single-8        16.0 ± 0%      14.0 ± 0%  -12.50%  (p=0.000 n=10+10)
32ByteSHA256Single-8      16.0 ± 0%      14.0 ± 0%  -12.50%  (p=0.000 n=10+10)
64ByteSHA512Single-8      16.0 ± 0%      14.0 ± 0%  -12.50%  (p=0.000 n=10+10)
8ByteMD5Stream-8          0.00           0.00          ~     (all equal)
16ByteMD5Stream-8         1.00 ± 0%      1.00 ± 0%     ~     (all equal)
8ByteSHA1Stream-8         0.00           0.00          ~     (all equal)
20ByteSHA1Stream-8        1.00 ± 0%      1.00 ± 0%     ~     (all equal)
8ByteSHA256Stream-8       0.00           0.00          ~     (all equal)
32ByteSHA256Stream-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
8ByteSHA512Stream-8       0.00           0.00          ~     (all equal)
64ByteSHA512Stream-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Change-Id: I08e5deb7045ce9aa63428e0ac7ae6140226414b4
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/423414
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>

hkdf/hkdf.go

@@ -56,7 +56,9 @@ func (f *hkdf) Read(p []byte) (int, error) {
 
 	// Fill the rest of the buffer
 	for len(p) > 0 {
-		f.expander.Reset()
+		if f.counter > 1 {
+			f.expander.Reset()
+		}
 		f.expander.Write(f.prev)
 		f.expander.Write(f.info)
 		f.expander.Write([]byte{f.counter})