Fix compilation for gomobile iOS (#17)

* Move signature verification to errors

* Move cleartext messages to ClearTextMessage struct

* Fix documentation

crypto/signature.go

@@ -0,0 +1,141 @@
+package crypto
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"math"
+	"time"
+
+	"golang.org/x/crypto/openpgp"
+	"golang.org/x/crypto/openpgp/packet"
+	pgpErrors "golang.org/x/crypto/openpgp/errors"
+
+	"github.com/ProtonMail/gopenpgp/constants"
+	"github.com/ProtonMail/gopenpgp/internal"
+)
+
+// SignatureVerificationError is returned from Decrypt and VerifyDetached functions when signature verification fails
+type SignatureVerificationError struct {
+	Status int
+	Message string
+}
+
+// Error is the base method for all errors
+func (e SignatureVerificationError) Error() string {
+	return fmt.Sprintf("Signature Verification Error: %v", e.Message)
+}
+
+// ------------------
+// Internal functions
+// ------------------
+
+// newSignatureFailed creates a new SignatureVerificationError, type SIGNATURE_FAILED
+func newSignatureFailed() SignatureVerificationError {
+	return SignatureVerificationError {
+		constants.SIGNATURE_FAILED,
+		"Invalid signature",
+	}
+}
+
+// newSignatureNotSigned creates a new SignatureVerificationError, type SIGNATURE_NOT_SIGNED
+func newSignatureNotSigned() SignatureVerificationError {
+	return SignatureVerificationError {
+		constants.SIGNATURE_NOT_SIGNED,
+		"Missing signature",
+	}
+}
+
+// newSignatureNoVerifier creates a new SignatureVerificationError, type SIGNATURE_NO_VERIFIER
+func newSignatureNoVerifier() SignatureVerificationError {
+	return SignatureVerificationError {
+		constants.SIGNATURE_NO_VERIFIER,
+		"No matching signature",
+	}
+}
+
+// processSignatureExpiration handles signature time verification manually, so we can add a margin to the
+// creationTime check.
+func processSignatureExpiration(md *openpgp.MessageDetails, verifyTime int64) {
+	if md.SignatureError == pgpErrors.ErrSignatureExpired {
+		if verifyTime > 0 {
+			created := md.Signature.CreationTime.Unix()
+			expires := int64(math.MaxInt64)
+			if md.Signature.SigLifetimeSecs != nil {
+				expires = int64(*md.Signature.SigLifetimeSecs) + created
+			}
+			if created-internal.CreationTimeOffset <= verifyTime && verifyTime <= expires {
+				md.SignatureError = nil
+			}
+		} else {
+			// verifyTime = 0: time check disabled, everything is okay
+			md.SignatureError = nil
+		}
+	}
+}
+
+// verifyDetailsSignature verifies signature from message details
+func verifyDetailsSignature(md *openpgp.MessageDetails, verifierKey *KeyRing) error {
+	if md.IsSigned {
+		if md.SignedBy != nil {
+			if len(verifierKey.entities) > 0 {
+				matches := verifierKey.entities.KeysById(md.SignedByKeyId)
+				if len(matches) > 0 {
+					if md.SignatureError == nil {
+						return nil
+					}
+					return newSignatureFailed()
+				}
+			} else {
+				return newSignatureNoVerifier()
+			}
+		} else {
+			return newSignatureNoVerifier()
+		}
+	}
+
+	return newSignatureNoVerifier()
+}
+
+// verifySignature verifies if a signature is valid with the entity list
+func verifySignature(pubKeyEntries openpgp.EntityList, origText io.Reader, signature []byte, verifyTime int64) error {
+	config := &packet.Config{}
+	if verifyTime == 0 {
+		config.Time = func() time.Time {
+			return time.Unix(0, 0)
+		}
+	} else {
+		config.Time = func() time.Time {
+			return time.Unix(verifyTime+internal.CreationTimeOffset, 0)
+		}
+	}
+	signatureReader := bytes.NewReader(signature)
+
+	signer, err := openpgp.CheckDetachedSignature(pubKeyEntries, origText, signatureReader, config)
+
+	if err == pgpErrors.ErrSignatureExpired && signer != nil {
+		if verifyTime > 0 { // if verifyTime = 0: time check disabled, everything is okay
+			// Maybe the creation time offset pushed it over the edge
+			// Retry with the actual verification time
+			config.Time = func() time.Time {
+				return time.Unix(verifyTime, 0)
+			}
+
+			_, err = signatureReader.Seek(0, io.SeekStart)
+			if err != nil {
+				return newSignatureFailed()
+			}
+
+			signer, err = openpgp.CheckDetachedSignature(pubKeyEntries, origText, signatureReader, config)
+			if err != nil {
+				return newSignatureFailed()
+			}
+		}
+	}
+
+	if signer == nil {
+		return newSignatureFailed()
+	}
+
+	return nil
+}