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// Note: Consider the API unstable until the code supports at least three different image formats or transports.
// NOTE: Keep this in sync with docs/atomic-signature.md and docs/atomic-signature-embedded.json!
package signature
import (
"encoding/json"
"errors"
"fmt"
"time"
"github.com/containers/image/v5/signature/internal"
"github.com/containers/image/v5/version"
digest "github.com/opencontainers/go-digest"
)
const (
signatureType = "atomic container signature"
)
// InvalidSignatureError is returned when parsing an invalid signature.
type InvalidSignatureError = internal.InvalidSignatureError
// Signature is a parsed content of a signature.
// The only way to get this structure from a blob should be as a return value from a successful call to verifyAndExtractSignature below.
type Signature struct {
DockerManifestDigest digest.Digest
DockerReference string // FIXME: more precise type?
}
// untrustedSignature is a parsed content of a signature.
type untrustedSignature 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
}
// UntrustedSignatureInformation is information available in an untrusted signature.
// This may be useful when debugging signature verification failures,
// or when managing a set of signatures on a single image.
//
// WARNING: Do not use the contents of this for ANY security decisions,
// and be VERY CAREFUL about showing this information to humans in any way which suggest that these values “are probably” reliable.
// There is NO REASON to expect the values to be correct, or not intentionally misleading
// (including things like “✅ Verified by $authority”)
type UntrustedSignatureInformation struct {
UntrustedDockerManifestDigest digest.Digest
UntrustedDockerReference string // FIXME: more precise type?
UntrustedCreatorID *string
UntrustedTimestamp *time.Time
UntrustedShortKeyIdentifier string
}
// newUntrustedSignature returns an untrustedSignature object with
// the specified primary contents and appropriate metadata.
func newUntrustedSignature(dockerManifestDigest digest.Digest, dockerReference string) untrustedSignature {
// 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 := "atomic " + version.Version
timestamp := time.Now().Unix()
return untrustedSignature{
untrustedDockerManifestDigest: dockerManifestDigest,
untrustedDockerReference: dockerReference,
untrustedCreatorID: &creatorID,
untrustedTimestamp: ×tamp,
}
}
// A compile-time check that untrustedSignature and *untrustedSignature implements json.Marshaler
var _ json.Marshaler = untrustedSignature{}
var _ json.Marshaler = (*untrustedSignature)(nil)
// MarshalJSON implements the json.Marshaler interface.
func (s untrustedSignature) MarshalJSON() ([]byte, error) {
if s.untrustedDockerManifestDigest == "" || s.untrustedDockerReference == "" {
return nil, errors.New("Unexpected empty signature content")
}
critical := map[string]any{
"type": signatureType,
"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 untrustedSignature implements json.Unmarshaler
var _ json.Unmarshaler = (*untrustedSignature)(nil)
// UnmarshalJSON implements the json.Unmarshaler interface
func (s *untrustedSignature) UnmarshalJSON(data []byte) error {
return internal.JSONFormatToInvalidSignatureError(s.strictUnmarshalJSON(data))
}
// strictUnmarshalJSON is UnmarshalJSON, except that it may return the internal.JSONFormatError error type.
// Splitting it into a separate function allows us to do the internal.JSONFormatError → InvalidSignatureError in a single place, the caller.
func (s *untrustedSignature) strictUnmarshalJSON(data []byte) error {
var critical, optional json.RawMessage
if err := internal.ParanoidUnmarshalJSONObjectExactFields(data, map[string]any{
"critical": &critical,
"optional": &optional,
}); err != nil {
return err
}
var creatorID string
var timestamp float64
var gotCreatorID, gotTimestamp = false, false
if err := internal.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 internal.NewInvalidSignatureError("Field optional.timestamp is not an integer")
}
s.untrustedTimestamp = &intTimestamp
}
var t string
var image, identity json.RawMessage
if err := internal.ParanoidUnmarshalJSONObjectExactFields(critical, map[string]any{
"type": &t,
"image": &image,
"identity": &identity,
}); err != nil {
return err
}
if t != signatureType {
return internal.NewInvalidSignatureError(fmt.Sprintf("Unrecognized signature type %s", t))
}
var digestString string
if err := internal.ParanoidUnmarshalJSONObjectExactFields(image, map[string]any{
"docker-manifest-digest": &digestString,
}); err != nil {
return err
}
digestValue, err := digest.Parse(digestString)
if err != nil {
return internal.NewInvalidSignatureError(fmt.Sprintf(`invalid docker-manifest-digest value %q: %v`, digestString, err))
}
s.untrustedDockerManifestDigest = digestValue
return internal.ParanoidUnmarshalJSONObjectExactFields(identity, map[string]any{
"docker-reference": &s.untrustedDockerReference,
})
}
// Sign formats the signature and returns a blob signed using mech and keyIdentity
// (If it seems surprising that this is a method on untrustedSignature, note that there
// isn’t a good reason to think that a key used by the user is trusted by any component
// of the system just because it is a private key — actually the presence of a private key
// on the system increases the likelihood of an a successful attack on that private key
// on that particular system.)
func (s untrustedSignature) sign(mech SigningMechanism, keyIdentity string, passphrase string) ([]byte, error) {
json, err := json.Marshal(s)
if err != nil {
return nil, err
}
if newMech, ok := mech.(signingMechanismWithPassphrase); ok {
return newMech.SignWithPassphrase(json, keyIdentity, passphrase)
}
if passphrase != "" {
return nil, errors.New("signing mechanism does not support passphrases")
}
return mech.Sign(json, keyIdentity)
}
// signatureAcceptanceRules specifies how to decide whether an untrusted signature is acceptable.
// We centralize the actual parsing and data extraction in verifyAndExtractSignature; 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 signatureAcceptanceRules struct {
validateKeyIdentity func(string) error
validateSignedDockerReference func(string) error
validateSignedDockerManifestDigest func(digest.Digest) error
}
// verifyAndExtractSignature verifies that unverifiedSignature has been signed, and that its principal components
// match expected values, both as specified by rules, and returns it
func verifyAndExtractSignature(mech SigningMechanism, unverifiedSignature []byte, rules signatureAcceptanceRules) (*Signature, error) {
signed, keyIdentity, err := mech.Verify(unverifiedSignature)
if err != nil {
return nil, err
}
if err := rules.validateKeyIdentity(keyIdentity); err != nil {
return nil, err
}
var unmatchedSignature untrustedSignature
if err := json.Unmarshal(signed, &unmatchedSignature); err != nil {
return nil, internal.NewInvalidSignatureError(err.Error())
}
if err := rules.validateSignedDockerManifestDigest(unmatchedSignature.untrustedDockerManifestDigest); err != nil {
return nil, err
}
if err := rules.validateSignedDockerReference(unmatchedSignature.untrustedDockerReference); err != nil {
return nil, err
}
// signatureAcceptanceRules have accepted this value.
return &Signature{
DockerManifestDigest: unmatchedSignature.untrustedDockerManifestDigest,
DockerReference: unmatchedSignature.untrustedDockerReference,
}, nil
}
// GetUntrustedSignatureInformationWithoutVerifying extracts information available in an untrusted signature,
// WITHOUT doing any cryptographic verification.
// This may be useful when debugging signature verification failures,
// or when managing a set of signatures on a single image.
//
// WARNING: Do not use the contents of this for ANY security decisions,
// and be VERY CAREFUL about showing this information to humans in any way which suggest that these values “are probably” reliable.
// There is NO REASON to expect the values to be correct, or not intentionally misleading
// (including things like “✅ Verified by $authority”)
func GetUntrustedSignatureInformationWithoutVerifying(untrustedSignatureBytes []byte) (*UntrustedSignatureInformation, error) {
// NOTE: This should eventually do format autodetection.
mech, _, err := NewEphemeralGPGSigningMechanism([]byte{})
if err != nil {
return nil, err
}
defer mech.Close()
untrustedContents, shortKeyIdentifier, err := mech.UntrustedSignatureContents(untrustedSignatureBytes)
if err != nil {
return nil, err
}
var untrustedDecodedContents untrustedSignature
if err := json.Unmarshal(untrustedContents, &untrustedDecodedContents); err != nil {
return nil, internal.NewInvalidSignatureError(err.Error())
}
var timestamp *time.Time // = nil
if untrustedDecodedContents.untrustedTimestamp != nil {
ts := time.Unix(*untrustedDecodedContents.untrustedTimestamp, 0)
timestamp = &ts
}
return &UntrustedSignatureInformation{
UntrustedDockerManifestDigest: untrustedDecodedContents.untrustedDockerManifestDigest,
UntrustedDockerReference: untrustedDecodedContents.untrustedDockerReference,
UntrustedCreatorID: untrustedDecodedContents.untrustedCreatorID,
UntrustedTimestamp: timestamp,
UntrustedShortKeyIdentifier: shortKeyIdentifier,
}, nil
}
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