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// Copyright 2019 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 note defines the notes signed by the Go module database server.
//
// This package is part of a DRAFT of what the Go module database server will look like.
// Do not assume the details here are final!
//
// A note is text signed by one or more server keys.
// The text should be ignored unless the note is signed by
// a trusted server key and the signature has been verified
// using the server's public key.
//
// A server's public key is identified by a name, typically the "host[/path]"
// giving the base URL of the server's transparency log.
// The syntactic restrictions on a name are that it be non-empty,
// well-formed UTF-8 containing neither Unicode spaces nor plus (U+002B).
//
// A Go module database server signs texts using public key cryptography.
// A given server may have multiple public keys, each
// identified by the first 32 bits of the SHA-256 hash of
// the concatenation of the server name, a newline, and
// the encoded public key.
//
// Verifying Notes
//
// A Verifier allows verification of signatures by one server public key.
// It can report the name of the server and the uint32 hash of the key,
// and it can verify a purported signature by that key.
//
// The standard implementation of a Verifier is constructed
// by NewVerifier starting from a verifier key, which is a
// plain text string of the form "<name>+<hash>+<keydata>".
//
// A Verifiers allows looking up a Verifier by the combination
// of server name and key hash.
//
// The standard implementation of a Verifiers is constructed
// by VerifierList from a list of known verifiers.
//
// A Note represents a text with one or more signatures.
// An implementation can reject a note with too many signatures
// (for example, more than 100 signatures).
//
// A Signature represents a signature on a note, verified or not.
//
// The Open function takes as input a signed message
// and a set of known verifiers. It decodes and verifies
// the message signatures and returns a Note structure
// containing the message text and (verified or unverified) signatures.
//
// Signing Notes
//
// A Signer allows signing a text with a given key.
// It can report the name of the server and the hash of the key
// and can sign a raw text using that key.
//
// The standard implementation of a Signer is constructed
// by NewSigner starting from an encoded signer key, which is a
// plain text string of the form "PRIVATE+KEY+<name>+<hash>+<keydata>".
// Anyone with an encoded signer key can sign messages using that key,
// so it must be kept secret. The encoding begins with the literal text
// "PRIVATE+KEY" to avoid confusion with the public server key.
//
// The Sign function takes as input a Note and a list of Signers
// and returns an encoded, signed message.
//
// Signed Note Format
//
// A signed note consists of a text ending in newline (U+000A),
// followed by a blank line (only a newline),
// followed by one or more signature lines of this form:
// em dash (U+2014), space (U+0020),
// server name, space, base64-encoded signature, newline.
//
// Signed notes must be valid UTF-8 and must not contain any
// ASCII control characters (those below U+0020) other than newline.
//
// A signature is a base64 encoding of 4+n bytes.
//
// The first four bytes in the signature are the uint32 key hash
// stored in big-endian order, which is to say they are the first
// four bytes of the truncated SHA-256 used to derive the key hash
// in the first place.
//
// The remaining n bytes are the result of using the specified key
// to sign the note text (including the final newline but not the
// separating blank line).
//
// Generating Keys
//
// There is only one key type, Ed25519 with algorithm identifier 1.
// New key types may be introduced in the future as needed,
// although doing so will require deploying the new algorithms to all clients
// before starting to depend on them for signatures.
//
// The GenerateKey function generates and returns a new signer
// and corresponding verifier.
//
// Example
//
// Here is a well-formed signed note:
//
// If you think cryptography is the answer to your problem,
// then you don't know what your problem is.
//
// — PeterNeumann x08go/ZJkuBS9UG/SffcvIAQxVBtiFupLLr8pAcElZInNIuGUgYN1FFYC2pZSNXgKvqfqdngotpRZb6KE6RyyBwJnAM=
//
// It can be constructed and displayed using:
//
// skey := "PRIVATE+KEY+PeterNeumann+c74f20a3+AYEKFALVFGyNhPJEMzD1QIDr+Y7hfZx09iUvxdXHKDFz"
// text := "If you think cryptography is the answer to your problem,\n" +
// "then you don't know what your problem is.\n"
//
// signer, err := note.NewSigner(skey)
// if err != nil {
// log.Fatal(err)
// }
//
// msg, err := note.Sign(&note.Note{Text: text}, signer)
// if err != nil {
// log.Fatal(err)
// }
// os.Stdout.Write(msg)
//
// The note's text is two lines, including the final newline,
// and the text is purportedly signed by a server named
// "PeterNeumann". (Although server names are canonically
// base URLs, the only syntactic requirement is that they
// not contain spaces or newlines).
//
// If Open is given access to a Verifiers including the
// Verifier for this key, then it will succeed at verifiying
// the encoded message and returning the parsed Note:
//
// vkey := "PeterNeumann+c74f20a3+ARpc2QcUPDhMQegwxbzhKqiBfsVkmqq/LDE4izWy10TW"
// msg := []byte("If you think cryptography is the answer to your problem,\n" +
// "then you don't know what your problem is.\n" +
// "\n" +
// "— PeterNeumann x08go/ZJkuBS9UG/SffcvIAQxVBtiFupLLr8pAcElZInNIuGUgYN1FFYC2pZSNXgKvqfqdngotpRZb6KE6RyyBwJnAM=\n")
//
// verifier, err := note.NewVerifier(vkey)
// if err != nil {
// log.Fatal(err)
// }
// verifiers := note.VerifierList(verifier)
//
// n, err := note.Open([]byte(msg), verifiers)
// if err != nil {
// log.Fatal(err)
// }
// fmt.Printf("%s (%08x):\n%s", n.Sigs[0].Name, n.Sigs[0].Hash, n.Text)
//
// You can add your own signature to this message by re-signing the note:
//
// skey, vkey, err := note.GenerateKey(rand.Reader, "EnochRoot")
// if err != nil {
// log.Fatal(err)
// }
// _ = vkey // give to verifiers
//
// me, err := note.NewSigner(skey)
// if err != nil {
// log.Fatal(err)
// }
//
// msg, err := note.Sign(n, me)
// if err != nil {
// log.Fatal(err)
// }
// os.Stdout.Write(msg)
//
// This will print a doubly-signed message, like:
//
// If you think cryptography is the answer to your problem,
// then you don't know what your problem is.
//
// — PeterNeumann x08go/ZJkuBS9UG/SffcvIAQxVBtiFupLLr8pAcElZInNIuGUgYN1FFYC2pZSNXgKvqfqdngotpRZb6KE6RyyBwJnAM=
// — EnochRoot rwz+eBzmZa0SO3NbfRGzPCpDckykFXSdeX+MNtCOXm2/5n2tiOHp+vAF1aGrQ5ovTG01oOTGwnWLox33WWd1RvMc+QQ=
//
package note
import (
"bytes"
"crypto/sha256"
"encoding/base64"
"encoding/binary"
"errors"
"fmt"
"io"
"strconv"
"strings"
"unicode"
"unicode/utf8"
"golang.org/x/crypto/ed25519"
)
// A Verifier verifies messages signed with a specific key.
type Verifier interface {
// Name returns the server name associated with the key.
Name() string
// KeyHash returns the key hash.
KeyHash() uint32
// Verify reports whether sig is a valid signature of msg.
Verify(msg, sig []byte) bool
}
// A Signer signs messages using a specific key.
type Signer interface {
// Name returns the server name associated with the key.
Name() string
// KeyHash returns the key hash.
KeyHash() uint32
// Sign returns a signature for the given message.
Sign(msg []byte) ([]byte, error)
}
// keyHash computes the key hash for the given server name and encoded public key.
func keyHash(name string, key []byte) uint32 {
h := sha256.New()
h.Write([]byte(name))
h.Write([]byte("\n"))
h.Write(key)
sum := h.Sum(nil)
return binary.BigEndian.Uint32(sum)
}
var (
errVerifierID = errors.New("malformed verifier id")
errVerifierAlg = errors.New("unknown verifier algorithm")
errVerifierHash = errors.New("invalid verifier hash")
)
const (
algEd25519 = 1
)
// isValidName reports whether name is valid.
// It must be non-empty and not have any Unicode spaces or pluses.
func isValidName(name string) bool {
return name != "" && utf8.ValidString(name) && strings.IndexFunc(name, unicode.IsSpace) < 0 && !strings.Contains(name, "+")
}
// NewVerifier construct a new Verifier from an encoded verifier key.
func NewVerifier(vkey string) (Verifier, error) {
name, vkey := chop(vkey, "+")
hash16, key64 := chop(vkey, "+")
hash, err1 := strconv.ParseUint(hash16, 16, 32)
key, err2 := base64.StdEncoding.DecodeString(key64)
if len(hash16) != 8 || err1 != nil || err2 != nil || !isValidName(name) || len(key) == 0 {
return nil, errVerifierID
}
if uint32(hash) != keyHash(name, key) {
return nil, errVerifierHash
}
v := &verifier{
name: name,
hash: uint32(hash),
}
alg, key := key[0], key[1:]
switch alg {
default:
return nil, errVerifierAlg
case algEd25519:
if len(key) != 32 {
return nil, errVerifierID
}
v.verify = func(msg, sig []byte) bool {
return ed25519.Verify(key, msg, sig)
}
}
return v, nil
}
// chop chops s at the first instance of sep, if any,
// and returns the text before and after sep.
// If sep is not present, chop returns before is s and after is empty.
func chop(s, sep string) (before, after string) {
i := strings.Index(s, sep)
if i < 0 {
return s, ""
}
return s[:i], s[i+len(sep):]
}
// verifier is a trivial Verifier implementation.
type verifier struct {
name string
hash uint32
verify func([]byte, []byte) bool
}
func (v *verifier) Name() string { return v.name }
func (v *verifier) KeyHash() uint32 { return v.hash }
func (v *verifier) Verify(msg, sig []byte) bool { return v.verify(msg, sig) }
// NewSigner constructs a new Signer from an encoded signer key.
func NewSigner(skey string) (Signer, error) {
priv1, skey := chop(skey, "+")
priv2, skey := chop(skey, "+")
name, skey := chop(skey, "+")
hash16, key64 := chop(skey, "+")
hash, err1 := strconv.ParseUint(hash16, 16, 32)
key, err2 := base64.StdEncoding.DecodeString(key64)
if priv1 != "PRIVATE" || priv2 != "KEY" || len(hash16) != 8 || err1 != nil || err2 != nil || !isValidName(name) || len(key) == 0 {
return nil, errSignerID
}
// Note: hash is the hash of the public key and we have the private key.
// Must verify hash after deriving public key.
s := &signer{
name: name,
hash: uint32(hash),
}
var pubkey []byte
alg, key := key[0], key[1:]
switch alg {
default:
return nil, errSignerAlg
case algEd25519:
if len(key) != 32 {
return nil, errSignerID
}
key = ed25519.NewKeyFromSeed(key)
pubkey = append([]byte{algEd25519}, key[32:]...)
s.sign = func(msg []byte) ([]byte, error) {
return ed25519.Sign(key, msg), nil
}
}
if uint32(hash) != keyHash(name, pubkey) {
return nil, errSignerHash
}
return s, nil
}
var (
errSignerID = errors.New("malformed verifier id")
errSignerAlg = errors.New("unknown verifier algorithm")
errSignerHash = errors.New("invalid verifier hash")
)
// signer is a trivial Signer implementation.
type signer struct {
name string
hash uint32
sign func([]byte) ([]byte, error)
}
func (s *signer) Name() string { return s.name }
func (s *signer) KeyHash() uint32 { return s.hash }
func (s *signer) Sign(msg []byte) ([]byte, error) { return s.sign(msg) }
// GenerateKey generates a signer and verifier key pair for a named server.
// The signer key skey is private and must be kept secret.
func GenerateKey(rand io.Reader, name string) (skey, vkey string, err error) {
pub, priv, err := ed25519.GenerateKey(rand)
if err != nil {
return "", "", err
}
pubkey := append([]byte{algEd25519}, pub...)
privkey := append([]byte{algEd25519}, priv.Seed()...)
h := keyHash(name, pubkey)
skey = fmt.Sprintf("PRIVATE+KEY+%s+%08x+%s", name, h, base64.StdEncoding.EncodeToString(privkey))
vkey = fmt.Sprintf("%s+%08x+%s", name, h, base64.StdEncoding.EncodeToString(pubkey))
return skey, vkey, nil
}
// NewEd25519VerifierKey returns an encoded verifier key using the given name
// and Ed25519 public key.
func NewEd25519VerifierKey(name string, key ed25519.PublicKey) (string, error) {
if len(key) != ed25519.PublicKeySize {
return "", fmt.Errorf("invalid public key size %d, expected %d", len(key), ed25519.PublicKeySize)
}
pubkey := append([]byte{algEd25519}, key...)
hash := keyHash(name, pubkey)
b64Key := base64.StdEncoding.EncodeToString(pubkey)
return fmt.Sprintf("%s+%08x+%s", name, hash, b64Key), nil
}
// A Verifiers is a collection of known verifier keys.
type Verifiers interface {
// Verifier returns the Verifier associated with the key
// identified by the name and hash.
// If the name, hash pair is unknown, Verifier should return
// an UnknownVerifierError.
Verifier(name string, hash uint32) (Verifier, error)
}
// An UnknownVerifierError indicates that the given key is not known.
// The Open function records signatures without associated verifiers as
// unverified signatures.
type UnknownVerifierError struct {
Name string
KeyHash uint32
}
func (e *UnknownVerifierError) Error() string {
return fmt.Sprintf("unknown key %s+%08x", e.Name, e.KeyHash)
}
// An ambiguousVerifierError indicates that the given name and hash
// match multiple keys passed to VerifierList.
// (If this happens, some malicious actor has taken control of the
// verifier list, at which point we may as well give up entirely,
// but we diagnose the problem instead.)
type ambiguousVerifierError struct {
name string
hash uint32
}
func (e *ambiguousVerifierError) Error() string {
return fmt.Sprintf("ambiguous key %s+%08x", e.name, e.hash)
}
// VerifierList returns a Verifiers implementation that uses the given list of verifiers.
func VerifierList(list ...Verifier) Verifiers {
m := make(verifierMap)
for _, v := range list {
k := nameHash{v.Name(), v.KeyHash()}
m[k] = append(m[k], v)
}
return m
}
type nameHash struct {
name string
hash uint32
}
type verifierMap map[nameHash][]Verifier
func (m verifierMap) Verifier(name string, hash uint32) (Verifier, error) {
v, ok := m[nameHash{name, hash}]
if !ok {
return nil, &UnknownVerifierError{name, hash}
}
if len(v) > 1 {
return nil, &ambiguousVerifierError{name, hash}
}
return v[0], nil
}
// A Note is a text and signatures.
type Note struct {
Text string // text of note
Sigs []Signature // verified signatures
UnverifiedSigs []Signature // unverified signatures
}
// A Signature is a single signature found in a note.
type Signature struct {
// Name and Hash give the name and key hash
// for the key that generated the signature.
Name string
Hash uint32
// Base64 records the base64-encoded signature bytes.
Base64 string
}
// An UnverifiedNoteError indicates that the note
// successfully parsed but had no verifiable signatures.
type UnverifiedNoteError struct {
Note *Note
}
func (e *UnverifiedNoteError) Error() string {
return "note has no verifiable signatures"
}
// An InvalidSignatureError indicates that the given key was known
// and the associated Verifier rejected the signature.
type InvalidSignatureError struct {
Name string
Hash uint32
}
func (e *InvalidSignatureError) Error() string {
return fmt.Sprintf("invalid signature for key %s+%08x", e.Name, e.Hash)
}
var (
errMalformedNote = errors.New("malformed note")
errInvalidSigner = errors.New("invalid signer")
sigSplit = []byte("\n\n")
sigPrefix = []byte("")
)
// Open opens and parses the message msg, checking signatures from the known verifiers.
//
// For each signature in the message, Open calls known.Verifier to find a verifier.
// If known.Verifier returns a verifier and the verifier accepts the signature,
// Open records the signature in the returned note's Sigs field.
// If known.Verifier returns a verifier but the verifier rejects the signature,
// Open returns an InvalidSignatureError.
// If known.Verifier returns an UnknownVerifierError,
// Open records the signature in the returned note's UnverifiedSigs field.
// If known.Verifier returns any other error, Open returns that error.
//
// If no known verifier has signed an otherwise valid note,
// Open returns an UnverifiedNoteError.
// In this case, the unverified note can be fetched from inside the error.
func Open(msg []byte, known Verifiers) (*Note, error) {
if known == nil {
// Treat nil Verifiers as empty list, to produce useful error instead of crash.
known = VerifierList()
}
// Must have valid UTF-8 with no non-newline ASCII control characters.
for i := 0; i < len(msg); {
r, size := utf8.DecodeRune(msg[i:])
if r < 0x20 && r != '\n' || r == utf8.RuneError && size == 1 {
return nil, errMalformedNote
}
i += size
}
// Must end with signature block preceded by blank line.
split := bytes.LastIndex(msg, sigSplit)
if split < 0 {
return nil, errMalformedNote
}
text, sigs := msg[:split+1], msg[split+2:]
if len(sigs) == 0 || sigs[len(sigs)-1] != '\n' {
return nil, errMalformedNote
}
n := &Note{
Text: string(text),
}
var buf bytes.Buffer
buf.Write(text)
// Parse and verify signatures.
// Ignore duplicate signatures.
seen := make(map[nameHash]bool)
seenUnverified := make(map[string]bool)
numSig := 0
for len(sigs) > 0 {
// Pull out next signature line.
// We know sigs[len(sigs)-1] == '\n', so IndexByte always finds one.
i := bytes.IndexByte(sigs, '\n')
line := sigs[:i]
sigs = sigs[i+1:]
if !bytes.HasPrefix(line, sigPrefix) {
return nil, errMalformedNote
}
line = line[len(sigPrefix):]
name, b64 := chop(string(line), " ")
sig, err := base64.StdEncoding.DecodeString(b64)
if err != nil || !isValidName(name) || b64 == "" || len(sig) < 5 {
return nil, errMalformedNote
}
hash := binary.BigEndian.Uint32(sig[0:4])
sig = sig[4:]
if numSig++; numSig > 100 {
// Avoid spending forever parsing a note with many signatures.
return nil, errMalformedNote
}
v, err := known.Verifier(name, hash)
if _, ok := err.(*UnknownVerifierError); ok {
// Drop repeated identical unverified signatures.
if seenUnverified[string(line)] {
continue
}
seenUnverified[string(line)] = true
n.UnverifiedSigs = append(n.UnverifiedSigs, Signature{Name: name, Hash: hash, Base64: b64})
continue
}
if err != nil {
return nil, err
}
// Drop repeated signatures by a single verifier.
if seen[nameHash{name, hash}] {
continue
}
seen[nameHash{name, hash}] = true
ok := v.Verify(text, sig)
if !ok {
return nil, &InvalidSignatureError{name, hash}
}
n.Sigs = append(n.Sigs, Signature{Name: name, Hash: hash, Base64: b64})
}
// Parsed and verified all the signatures.
if len(n.Sigs) == 0 {
return nil, &UnverifiedNoteError{n}
}
return n, nil
}
// Sign signs the note with the given signers and returns the encoded message.
// The new signatures from signers are listed in the encoded message after
// the existing signatures already present in n.Sigs.
// If any signer uses the same key as an existing signature,
// the existing signature is elided from the output.
func Sign(n *Note, signers ...Signer) ([]byte, error) {
var buf bytes.Buffer
if !strings.HasSuffix(n.Text, "\n") {
return nil, errMalformedNote
}
buf.WriteString(n.Text)
// Prepare signatures.
var sigs bytes.Buffer
have := make(map[nameHash]bool)
for _, s := range signers {
name := s.Name()
hash := s.KeyHash()
have[nameHash{name, hash}] = true
if !isValidName(name) {
return nil, errInvalidSigner
}
sig, err := s.Sign(buf.Bytes()) // buf holds n.Text
if err != nil {
return nil, err
}
var hbuf [4]byte
binary.BigEndian.PutUint32(hbuf[:], hash)
b64 := base64.StdEncoding.EncodeToString(append(hbuf[:], sig...))
sigs.WriteString("")
sigs.WriteString(name)
sigs.WriteString(" ")
sigs.WriteString(b64)
sigs.WriteString("\n")
}
buf.WriteString("\n")
// Emit existing signatures not replaced by new ones.
for _, list := range [][]Signature{n.Sigs, n.UnverifiedSigs} {
for _, sig := range list {
name, hash := sig.Name, sig.Hash
if !isValidName(name) {
return nil, errMalformedNote
}
if have[nameHash{name, hash}] {
continue
}
// Double-check hash against base64.
raw, err := base64.StdEncoding.DecodeString(sig.Base64)
if err != nil || len(raw) < 4 || binary.BigEndian.Uint32(raw) != hash {
return nil, errMalformedNote
}
buf.WriteString("")
buf.WriteString(sig.Name)
buf.WriteString(" ")
buf.WriteString(sig.Base64)
buf.WriteString("\n")
}
}
buf.Write(sigs.Bytes())
return buf.Bytes(), nil
}
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