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// Note: Consider the API unstable until the code supports at least three different image formats or transports.
package signature
import (
"bytes"
"errors"
"fmt"
"io/ioutil"
"strings"
"golang.org/x/crypto/openpgp"
)
// SigningMechanism abstracts a way to sign binary blobs and verify their signatures.
// Each mechanism should eventually be closed by calling Close().
// FIXME: Eventually expand on keyIdentity (namespace them between mechanisms to
// eliminate ambiguities, support CA signatures and perhaps other key properties)
type SigningMechanism interface {
// Close removes resources associated with the mechanism, if any.
Close() error
// SupportsSigning returns nil if the mechanism supports signing, or a SigningNotSupportedError.
SupportsSigning() error
// Sign creates a (non-detached) signature of input using keyIdentity.
// Fails with a SigningNotSupportedError if the mechanism does not support signing.
Sign(input []byte, keyIdentity string) ([]byte, error)
// Verify parses unverifiedSignature and returns the content and the signer's identity
Verify(unverifiedSignature []byte) (contents []byte, keyIdentity string, err error)
// UntrustedSignatureContents returns UNTRUSTED contents of the signature WITHOUT ANY VERIFICATION,
// along with a short identifier of the key used for signing.
// WARNING: The short key identifier (which correponds to "Key ID" for OpenPGP keys)
// is NOT the same as a "key identity" used in other calls ot this interface, and
// the values may have no recognizable relationship if the public key is not available.
UntrustedSignatureContents(untrustedSignature []byte) (untrustedContents []byte, shortKeyIdentifier string, err error)
}
// SigningNotSupportedError is returned when trying to sign using a mechanism which does not support that.
type SigningNotSupportedError string
func (err SigningNotSupportedError) Error() string {
return string(err)
}
// NewGPGSigningMechanism returns a new GPG/OpenPGP signing mechanism for the user’s default
// GPG configuration ($GNUPGHOME / ~/.gnupg)
// The caller must call .Close() on the returned SigningMechanism.
func NewGPGSigningMechanism() (SigningMechanism, error) {
return newGPGSigningMechanismInDirectory("")
}
// NewEphemeralGPGSigningMechanism returns a new GPG/OpenPGP signing mechanism which
// recognizes _only_ public keys from the supplied blob, and returns the identities
// of these keys.
// The caller must call .Close() on the returned SigningMechanism.
func NewEphemeralGPGSigningMechanism(blob []byte) (SigningMechanism, []string, error) {
return newEphemeralGPGSigningMechanism(blob)
}
// gpgUntrustedSignatureContents returns UNTRUSTED contents of the signature WITHOUT ANY VERIFICATION,
// along with a short identifier of the key used for signing.
// WARNING: The short key identifier (which correponds to "Key ID" for OpenPGP keys)
// is NOT the same as a "key identity" used in other calls ot this interface, and
// the values may have no recognizable relationship if the public key is not available.
func gpgUntrustedSignatureContents(untrustedSignature []byte) (untrustedContents []byte, shortKeyIdentifier string, err error) {
// This uses the Golang-native OpenPGP implementation instead of gpgme because we are not doing any cryptography.
md, err := openpgp.ReadMessage(bytes.NewReader(untrustedSignature), openpgp.EntityList{}, nil, nil)
if err != nil {
return nil, "", err
}
if !md.IsSigned {
return nil, "", errors.New("The input is not a signature")
}
content, err := ioutil.ReadAll(md.UnverifiedBody)
if err != nil {
// Coverage: An error during reading the body can happen only if
// 1) the message is encrypted, which is not our case (and we don’t give ReadMessage the key
// to decrypt the contents anyway), or
// 2) the message is signed AND we give ReadMessage a correspnding public key, which we don’t.
return nil, "", err
}
// Uppercase the key ID for minimal consistency with the gpgme-returned fingerprints
// (but note that key ID is a suffix of the fingerprint only for V4 keys, not V3)!
return content, strings.ToUpper(fmt.Sprintf("%016X", md.SignedByKeyId)), nil
}
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