diff --git a/src/cmd/dist/buildtool.go b/src/cmd/dist/buildtool.go
index 37b3d45977a..e7bedfb84ee 100644
--- a/src/cmd/dist/buildtool.go
+++ b/src/cmd/dist/buildtool.go
@@ -53,6 +53,7 @@ var bootstrapDirs = []string{
 	"cmd/compile/internal/x86",
 	"cmd/compile/internal/wasm",
 	"cmd/internal/bio",
+	"cmd/internal/codesign",
 	"cmd/internal/gcprog",
 	"cmd/internal/dwarf",
 	"cmd/internal/edit",
diff --git a/src/cmd/internal/codesign/codesign.go b/src/cmd/internal/codesign/codesign.go
new file mode 100644
index 00000000000..0517a10640a
--- /dev/null
+++ b/src/cmd/internal/codesign/codesign.go
@@ -0,0 +1,268 @@
+// Copyright 2020 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 codesign provides basic functionalities for
+// ad-hoc code signing of Mach-O files.
+//
+// This is not a general tool for code-signing. It is made
+// specifically for the Go toolchain. It uses the same
+// ad-hoc signing algorithm as the Darwin linker.
+package codesign
+
+import (
+	"crypto/sha256"
+	"debug/macho"
+	"encoding/binary"
+	"io"
+)
+
+// Code signature layout.
+//
+// The code signature is a block of bytes that contains
+// a SuperBlob, which contains one or more Blobs. For ad-hoc
+// signing, a single CodeDirectory Blob suffices.
+//
+// A SuperBlob starts with its header (the binary representation
+// of the SuperBlob struct), followed by a list of (in our case,
+// one) Blobs (offset and size). A CodeDirectory Blob starts
+// with its head (the binary representation of CodeDirectory struct),
+// followed by the identifier (as a C string) and the hashes, at
+// the corresponding offsets.
+//
+// The signature data must be included in the __LINKEDIT segment.
+// In the Mach-O file header, an LC_CODE_SIGNATURE load command
+// points to the data.
+
+const (
+	pageSizeBits = 12
+	pageSize     = 1 << pageSizeBits
+)
+
+const LC_CODE_SIGNATURE = 0x1d
+
+// Constants and struct layouts are from
+// https://opensource.apple.com/source/xnu/xnu-4903.270.47/osfmk/kern/cs_blobs.h
+
+const (
+	CSMAGIC_REQUIREMENT        = 0xfade0c00 // single Requirement blob
+	CSMAGIC_REQUIREMENTS       = 0xfade0c01 // Requirements vector (internal requirements)
+	CSMAGIC_CODEDIRECTORY      = 0xfade0c02 // CodeDirectory blob
+	CSMAGIC_EMBEDDED_SIGNATURE = 0xfade0cc0 // embedded form of signature data
+	CSMAGIC_DETACHED_SIGNATURE = 0xfade0cc1 // multi-arch collection of embedded signatures
+
+	CSSLOT_CODEDIRECTORY = 0 // slot index for CodeDirectory
+)
+
+const (
+	CS_HASHTYPE_SHA1             = 1
+	CS_HASHTYPE_SHA256           = 2
+	CS_HASHTYPE_SHA256_TRUNCATED = 3
+	CS_HASHTYPE_SHA384           = 4
+)
+
+const (
+	CS_EXECSEG_MAIN_BINARY     = 0x1   // executable segment denotes main binary
+	CS_EXECSEG_ALLOW_UNSIGNED  = 0x10  // allow unsigned pages (for debugging)
+	CS_EXECSEG_DEBUGGER        = 0x20  // main binary is debugger
+	CS_EXECSEG_JIT             = 0x40  // JIT enabled
+	CS_EXECSEG_SKIP_LV         = 0x80  // skip library validation
+	CS_EXECSEG_CAN_LOAD_CDHASH = 0x100 // can bless cdhash for execution
+	CS_EXECSEG_CAN_EXEC_CDHASH = 0x200 // can execute blessed cdhash
+)
+
+type Blob struct {
+	typ    uint32 // type of entry
+	offset uint32 // offset of entry
+	// data follows
+}
+
+func (b *Blob) put(out []byte) []byte {
+	out = put32be(out, b.typ)
+	out = put32be(out, b.offset)
+	return out
+}
+
+const blobSize = 2 * 4
+
+type SuperBlob struct {
+	magic  uint32 // magic number
+	length uint32 // total length of SuperBlob
+	count  uint32 // number of index entries following
+	// blobs []Blob
+}
+
+func (s *SuperBlob) put(out []byte) []byte {
+	out = put32be(out, s.magic)
+	out = put32be(out, s.length)
+	out = put32be(out, s.count)
+	return out
+}
+
+const superBlobSize = 3 * 4
+
+type CodeDirectory struct {
+	magic         uint32 // magic number (CSMAGIC_CODEDIRECTORY)
+	length        uint32 // total length of CodeDirectory blob
+	version       uint32 // compatibility version
+	flags         uint32 // setup and mode flags
+	hashOffset    uint32 // offset of hash slot element at index zero
+	identOffset   uint32 // offset of identifier string
+	nSpecialSlots uint32 // number of special hash slots
+	nCodeSlots    uint32 // number of ordinary (code) hash slots
+	codeLimit     uint32 // limit to main image signature range
+	hashSize      uint8  // size of each hash in bytes
+	hashType      uint8  // type of hash (cdHashType* constants)
+	_pad1         uint8  // unused (must be zero)
+	pageSize      uint8  // log2(page size in bytes); 0 => infinite
+	_pad2         uint32 // unused (must be zero)
+	scatterOffset uint32
+	teamOffset    uint32
+	_pad3         uint32
+	codeLimit64   uint64
+	execSegBase   uint64
+	execSegLimit  uint64
+	execSegFlags  uint64
+	// data follows
+}
+
+func (c *CodeDirectory) put(out []byte) []byte {
+	out = put32be(out, c.magic)
+	out = put32be(out, c.length)
+	out = put32be(out, c.version)
+	out = put32be(out, c.flags)
+	out = put32be(out, c.hashOffset)
+	out = put32be(out, c.identOffset)
+	out = put32be(out, c.nSpecialSlots)
+	out = put32be(out, c.nCodeSlots)
+	out = put32be(out, c.codeLimit)
+	out = put8(out, c.hashSize)
+	out = put8(out, c.hashType)
+	out = put8(out, c._pad1)
+	out = put8(out, c.pageSize)
+	out = put32be(out, c._pad2)
+	out = put32be(out, c.scatterOffset)
+	out = put32be(out, c.teamOffset)
+	out = put32be(out, c._pad3)
+	out = put64be(out, c.codeLimit64)
+	out = put64be(out, c.execSegBase)
+	out = put64be(out, c.execSegLimit)
+	out = put64be(out, c.execSegFlags)
+	return out
+}
+
+const codeDirectorySize = 13*4 + 4 + 4*8
+
+// CodeSigCmd is Mach-O LC_CODE_SIGNATURE load command.
+type CodeSigCmd struct {
+	Cmd      uint32 // LC_CODE_SIGNATURE
+	Cmdsize  uint32 // sizeof this command (16)
+	Dataoff  uint32 // file offset of data in __LINKEDIT segment
+	Datasize uint32 // file size of data in __LINKEDIT segment
+}
+
+func FindCodeSigCmd(f *macho.File) (CodeSigCmd, bool) {
+	get32 := f.ByteOrder.Uint32
+	for _, l := range f.Loads {
+		data := l.Raw()
+		cmd := get32(data)
+		if cmd == LC_CODE_SIGNATURE {
+			return CodeSigCmd{
+				cmd,
+				get32(data[4:]),
+				get32(data[8:]),
+				get32(data[12:]),
+			}, true
+		}
+	}
+	return CodeSigCmd{}, false
+}
+
+func put32be(b []byte, x uint32) []byte { binary.BigEndian.PutUint32(b, x); return b[4:] }
+func put64be(b []byte, x uint64) []byte { binary.BigEndian.PutUint64(b, x); return b[8:] }
+func put8(b []byte, x uint8) []byte     { b[0] = x; return b[1:] }
+func puts(b, s []byte) []byte           { n := copy(b, s); return b[n:] }
+
+// Size computes the size of the code signature.
+// id is the identifier used for signing (a field in CodeDirectory blob, which
+// has no significance in ad-hoc signing).
+func Size(codeSize int64, id string) int64 {
+	nhashes := (codeSize + pageSize - 1) / pageSize
+	idOff := int64(codeDirectorySize)
+	hashOff := idOff + int64(len(id)+1)
+	cdirSz := hashOff + nhashes*sha256.Size
+	return int64(superBlobSize+blobSize) + cdirSz
+}
+
+// Sign generates an ad-hoc code signature and writes it to out.
+// out must have length at least Size(codeSize, id).
+// data is the file content without the signature, of size codeSize.
+// textOff and textSize is the file offset and size of the text segment.
+// isMain is true if this is a main executable.
+// id is the identifier used for signing (a field in CodeDirectory blob, which
+// has no significance in ad-hoc signing).
+func Sign(out []byte, data io.Reader, id string, codeSize, textOff, textSize int64, isMain bool) {
+	nhashes := (codeSize + pageSize - 1) / pageSize
+	idOff := int64(codeDirectorySize)
+	hashOff := idOff + int64(len(id)+1)
+	sz := Size(codeSize, id)
+
+	// emit blob headers
+	sb := SuperBlob{
+		magic:  CSMAGIC_EMBEDDED_SIGNATURE,
+		length: uint32(sz),
+		count:  1,
+	}
+	blob := Blob{
+		typ:    CSSLOT_CODEDIRECTORY,
+		offset: superBlobSize + blobSize,
+	}
+	cdir := CodeDirectory{
+		magic:        CSMAGIC_CODEDIRECTORY,
+		length:       uint32(sz) - (superBlobSize + blobSize),
+		version:      0x20400,
+		flags:        0x20002, // adhoc | linkerSigned
+		hashOffset:   uint32(hashOff),
+		identOffset:  uint32(idOff),
+		nCodeSlots:   uint32(nhashes),
+		codeLimit:    uint32(codeSize),
+		hashSize:     sha256.Size,
+		hashType:     CS_HASHTYPE_SHA256,
+		pageSize:     uint8(pageSizeBits),
+		execSegBase:  uint64(textOff),
+		execSegLimit: uint64(textSize),
+	}
+	if isMain {
+		cdir.execSegFlags = CS_EXECSEG_MAIN_BINARY
+	}
+
+	outp := out
+	outp = sb.put(outp)
+	outp = blob.put(outp)
+	outp = cdir.put(outp)
+
+	// emit the identifier
+	outp = puts(outp, []byte(id+"\000"))
+
+	// emit hashes
+	var buf [pageSize]byte
+	h := sha256.New()
+	p := 0
+	for p < int(codeSize) {
+		n, err := io.ReadFull(data, buf[:])
+		if err == io.EOF {
+			break
+		}
+		if err != nil && err != io.ErrUnexpectedEOF {
+			panic(err)
+		}
+		if p+n > int(codeSize) {
+			n = int(codeSize) - p
+		}
+		p += n
+		h.Reset()
+		h.Write(buf[:n])
+		b := h.Sum(nil)
+		outp = puts(outp, b[:])
+	}
+}