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sigstore_payload.go
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/
sigstore_payload.go
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package internal
import (
"bytes"
"crypto"
"encoding/base64"
"encoding/json"
"errors"
"fmt"
"time"
"github.com/containers/image/v5/version"
digest "github.com/opencontainers/go-digest"
sigstoreSignature "github.com/sigstore/sigstore/pkg/signature"
)
const (
sigstoreSignatureType = "cosign container image signature"
sigstoreHarcodedHashAlgorithm = crypto.SHA256
)
// UntrustedSigstorePayload is a parsed content of a sigstore signature payload (not the full signature)
type UntrustedSigstorePayload struct {
untrustedDockerManifestDigest digest.Digest
untrustedDockerReference string // FIXME: more precise type?
untrustedCreatorID *string
// This is intentionally an int64; the native JSON float64 type would allow to represent _some_ sub-second precision,
// but not nearly enough (with current timestamp values, a single unit in the last place is on the order of hundreds of nanoseconds).
// So, this is explicitly an int64, and we reject fractional values. If we did need more precise timestamps eventually,
// we would add another field, UntrustedTimestampNS int64.
untrustedTimestamp *int64
}
// NewUntrustedSigstorePayload returns an UntrustedSigstorePayload object with
// the specified primary contents and appropriate metadata.
func NewUntrustedSigstorePayload(dockerManifestDigest digest.Digest, dockerReference string) UntrustedSigstorePayload {
// Use intermediate variables for these values so that we can take their addresses.
// Golang guarantees that they will have a new address on every execution.
creatorID := "containers/image " + version.Version
timestamp := time.Now().Unix()
return UntrustedSigstorePayload{
untrustedDockerManifestDigest: dockerManifestDigest,
untrustedDockerReference: dockerReference,
untrustedCreatorID: &creatorID,
untrustedTimestamp: ×tamp,
}
}
// A compile-time check that UntrustedSigstorePayload and *UntrustedSigstorePayload implements json.Marshaler
var _ json.Marshaler = UntrustedSigstorePayload{}
var _ json.Marshaler = (*UntrustedSigstorePayload)(nil)
// MarshalJSON implements the json.Marshaler interface.
func (s UntrustedSigstorePayload) MarshalJSON() ([]byte, error) {
if s.untrustedDockerManifestDigest == "" || s.untrustedDockerReference == "" {
return nil, errors.New("Unexpected empty signature content")
}
critical := map[string]any{
"type": sigstoreSignatureType,
"image": map[string]string{"docker-manifest-digest": s.untrustedDockerManifestDigest.String()},
"identity": map[string]string{"docker-reference": s.untrustedDockerReference},
}
optional := map[string]any{}
if s.untrustedCreatorID != nil {
optional["creator"] = *s.untrustedCreatorID
}
if s.untrustedTimestamp != nil {
optional["timestamp"] = *s.untrustedTimestamp
}
signature := map[string]any{
"critical": critical,
"optional": optional,
}
return json.Marshal(signature)
}
// Compile-time check that UntrustedSigstorePayload implements json.Unmarshaler
var _ json.Unmarshaler = (*UntrustedSigstorePayload)(nil)
// UnmarshalJSON implements the json.Unmarshaler interface
func (s *UntrustedSigstorePayload) UnmarshalJSON(data []byte) error {
err := s.strictUnmarshalJSON(data)
if err != nil {
if formatErr, ok := err.(JSONFormatError); ok {
err = NewInvalidSignatureError(formatErr.Error())
}
}
return err
}
// strictUnmarshalJSON is UnmarshalJSON, except that it may return the internal JSONFormatError error type.
// Splitting it into a separate function allows us to do the JSONFormatError → InvalidSignatureError in a single place, the caller.
func (s *UntrustedSigstorePayload) strictUnmarshalJSON(data []byte) error {
var critical, optional json.RawMessage
if err := ParanoidUnmarshalJSONObjectExactFields(data, map[string]any{
"critical": &critical,
"optional": &optional,
}); err != nil {
return err
}
var creatorID string
var timestamp float64
var gotCreatorID, gotTimestamp = false, false
// /usr/bin/cosign generates "optional": null if there are no user-specified annotations.
if !bytes.Equal(optional, []byte("null")) {
if err := ParanoidUnmarshalJSONObject(optional, func(key string) any {
switch key {
case "creator":
gotCreatorID = true
return &creatorID
case "timestamp":
gotTimestamp = true
return ×tamp
default:
var ignore any
return &ignore
}
}); err != nil {
return err
}
}
if gotCreatorID {
s.untrustedCreatorID = &creatorID
}
if gotTimestamp {
intTimestamp := int64(timestamp)
if float64(intTimestamp) != timestamp {
return NewInvalidSignatureError("Field optional.timestamp is not is not an integer")
}
s.untrustedTimestamp = &intTimestamp
}
var t string
var image, identity json.RawMessage
if err := ParanoidUnmarshalJSONObjectExactFields(critical, map[string]any{
"type": &t,
"image": &image,
"identity": &identity,
}); err != nil {
return err
}
if t != sigstoreSignatureType {
return NewInvalidSignatureError(fmt.Sprintf("Unrecognized signature type %s", t))
}
var digestString string
if err := ParanoidUnmarshalJSONObjectExactFields(image, map[string]any{
"docker-manifest-digest": &digestString,
}); err != nil {
return err
}
s.untrustedDockerManifestDigest = digest.Digest(digestString)
return ParanoidUnmarshalJSONObjectExactFields(identity, map[string]any{
"docker-reference": &s.untrustedDockerReference,
})
}
// SigstorePayloadAcceptanceRules specifies how to decide whether an untrusted payload is acceptable.
// We centralize the actual parsing and data extraction in VerifySigstorePayload; this supplies
// the policy. We use an object instead of supplying func parameters to verifyAndExtractSignature
// because the functions have the same or similar types, so there is a risk of exchanging the functions;
// named members of this struct are more explicit.
type SigstorePayloadAcceptanceRules struct {
ValidateSignedDockerReference func(string) error
ValidateSignedDockerManifestDigest func(digest.Digest) error
}
// VerifySigstorePayload verifies unverifiedBase64Signature of unverifiedPayload was correctly created by publicKey, and that its principal components
// match expected values, both as specified by rules, and returns it.
// We return an *UntrustedSigstorePayload, although nothing actually uses it,
// just to double-check against stupid typos.
func VerifySigstorePayload(publicKey crypto.PublicKey, unverifiedPayload []byte, unverifiedBase64Signature string, rules SigstorePayloadAcceptanceRules) (*UntrustedSigstorePayload, error) {
verifier, err := sigstoreSignature.LoadVerifier(publicKey, sigstoreHarcodedHashAlgorithm)
if err != nil {
return nil, fmt.Errorf("creating verifier: %w", err)
}
unverifiedSignature, err := base64.StdEncoding.DecodeString(unverifiedBase64Signature)
if err != nil {
return nil, NewInvalidSignatureError(fmt.Sprintf("base64 decoding: %v", err))
}
// github.com/sigstore/cosign/pkg/cosign.verifyOCISignature uses signatureoptions.WithContext(),
// which seems to be not used by anything. So we don’t bother.
if err := verifier.VerifySignature(bytes.NewReader(unverifiedSignature), bytes.NewReader(unverifiedPayload)); err != nil {
return nil, NewInvalidSignatureError(fmt.Sprintf("cryptographic signature verification failed: %v", err))
}
var unmatchedPayload UntrustedSigstorePayload
if err := json.Unmarshal(unverifiedPayload, &unmatchedPayload); err != nil {
return nil, NewInvalidSignatureError(err.Error())
}
if err := rules.ValidateSignedDockerManifestDigest(unmatchedPayload.untrustedDockerManifestDigest); err != nil {
return nil, err
}
if err := rules.ValidateSignedDockerReference(unmatchedPayload.untrustedDockerReference); err != nil {
return nil, err
}
// SigstorePayloadAcceptanceRules have accepted this value.
return &unmatchedPayload, nil
}