[dev.boringcrypto] cmd/internal/notsha256: add new package

Package notsha256 implements the NOTSHA256 hash,
defined as bitwise NOT of SHA-256.

It will be used from the Go compiler toolchain where an
arbitrary hash is needed and the code currently reaches
for MD5, SHA1, or SHA256. The problem with all of those
is that when we add GOEXPERIMENT=boringcrypto, the
bootstrap process will not converge if the compiler itself
depends on the boringcrypto cgo code.
Using notsha256 avoids boringcrypto.

It is possible that I don't fully understand the convergence
problem and that there is a way to make the compiler converge
when using cgo, but keeping cgo out of the compiler seems safest.
It also makes clear that (except for the hack in codesign)
the code using this package doesn't care which hash is used.

For #51940.

Change-Id: Ie7c661183eacf8413a9d2074c96cbb9361e125ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/402594
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 141 insertions, 0 deletions

diff --git a/src/cmd/internal/notsha256/sha256.go b/src/cmd/internal/notsha256/sha256.go
new file mode 100644
index 0000000000..080b344979
--- /dev/null
+++ b/src/cmd/internal/notsha256/sha256.go
@@ -0,0 +1,141 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package notsha256 implements the NOTSHA256 algorithm,
+// a hash defined as bitwise NOT of SHA256.
+// It is used in situations where exact fidelity to SHA256 is unnecessary.
+// In particular, it is used in the compiler toolchain,
+// which cannot depend directly on cgo when GOEXPERIMENT=boringcrypto
+// (and in that mode the real sha256 uses cgo).
+package notsha256
+
+import (
+	"encoding/binary"
+	"hash"
+)
+
+// The size of a checksum in bytes.
+const Size = 32
+
+// The blocksize in bytes.
+const BlockSize = 64
+
+const (
+	chunk = 64
+	init0 = 0x6A09E667
+	init1 = 0xBB67AE85
+	init2 = 0x3C6EF372
+	init3 = 0xA54FF53A
+	init4 = 0x510E527F
+	init5 = 0x9B05688C
+	init6 = 0x1F83D9AB
+	init7 = 0x5BE0CD19
+)
+
+// digest represents the partial evaluation of a checksum.
+type digest struct {
+	h   [8]uint32
+	x   [chunk]byte
+	nx  int
+	len uint64
+}
+
+func (d *digest) Reset() {
+	d.h[0] = init0
+	d.h[1] = init1
+	d.h[2] = init2
+	d.h[3] = init3
+	d.h[4] = init4
+	d.h[5] = init5
+	d.h[6] = init6
+	d.h[7] = init7
+	d.nx = 0
+	d.len = 0
+}
+
+// New returns a new hash.Hash computing the NOTSHA256 checksum.
+// state of the hash.
+func New() hash.Hash {
+	d := new(digest)
+	d.Reset()
+	return d
+}
+
+func (d *digest) Size() int {
+	return Size
+}
+
+func (d *digest) BlockSize() int { return BlockSize }
+
+func (d *digest) Write(p []byte) (nn int, err error) {
+	nn = len(p)
+	d.len += uint64(nn)
+	if d.nx > 0 {
+		n := copy(d.x[d.nx:], p)
+		d.nx += n
+		if d.nx == chunk {
+			block(d, d.x[:])
+			d.nx = 0
+		}
+		p = p[n:]
+	}
+	if len(p) >= chunk {
+		n := len(p) &^ (chunk - 1)
+		block(d, p[:n])
+		p = p[n:]
+	}
+	if len(p) > 0 {
+		d.nx = copy(d.x[:], p)
+	}
+	return
+}
+
+func (d *digest) Sum(in []byte) []byte {
+	// Make a copy of d so that caller can keep writing and summing.
+	d0 := *d
+	hash := d0.checkSum()
+	return append(in, hash[:]...)
+}
+
+func (d *digest) checkSum() [Size]byte {
+	len := d.len
+	// Padding. Add a 1 bit and 0 bits until 56 bytes mod 64.
+	var tmp [64]byte
+	tmp[0] = 0x80
+	if len%64 < 56 {
+		d.Write(tmp[0 : 56-len%64])
+	} else {
+		d.Write(tmp[0 : 64+56-len%64])
+	}
+
+	// Length in bits.
+	len <<= 3
+	binary.BigEndian.PutUint64(tmp[:], len)
+	d.Write(tmp[0:8])
+
+	if d.nx != 0 {
+		panic("d.nx != 0")
+	}
+
+	var digest [Size]byte
+
+	binary.BigEndian.PutUint32(digest[0:], d.h[0]^0xFFFFFFFF)
+	binary.BigEndian.PutUint32(digest[4:], d.h[1]^0xFFFFFFFF)
+	binary.BigEndian.PutUint32(digest[8:], d.h[2]^0xFFFFFFFF)
+	binary.BigEndian.PutUint32(digest[12:], d.h[3]^0xFFFFFFFF)
+	binary.BigEndian.PutUint32(digest[16:], d.h[4]^0xFFFFFFFF)
+	binary.BigEndian.PutUint32(digest[20:], d.h[5]^0xFFFFFFFF)
+	binary.BigEndian.PutUint32(digest[24:], d.h[6]^0xFFFFFFFF)
+	binary.BigEndian.PutUint32(digest[28:], d.h[7]^0xFFFFFFFF)
+
+	return digest
+}
+
+// Sum256 returns the SHA256 checksum of the data.
+func Sum256(data []byte) [Size]byte {
+	var d digest
+	d.Reset()
+	d.Write(data)
+	return d.checkSum()
+}