Refactor api (#6)

* Refactor library, remove duplicates

* Rebuild structure to use Messages and Signature models

* Use PGPSplitMessage

* Remove signature model

* Various fixes

* Add helpers with tests

* Fixes, add some docs, add tests

* Add attachment helpers

* Add helpers Symmetric encryption

* Edit docs + examples

* Rename kr to keyRing

* Various fixes for documentation

* Edit JSON handling functions, add decrypt keyring via token

* Add proposal changes doc

* Fix CI

* Drop *Message functions, join CleartextMessage and BinaryMessage

* Change canonicalization and trimming only to text signatures

* Add cleartextsignature, detach signature from message model, move helpers

* Documentation, remove optional parameters

* Move verification to separate model

* Don't return message in VerifyDetached

* Update table of contents in readme

* Appease golint

* Run go fmt

* Rename Encrypt/DecryptMessageWithPassword to ..WithToken

These functions shouldn't be used with user-provided passwords,
as they don't do any key-stretching.

* Change key generation usernames

helper/cleartext.go

@@ -0,0 +1,82 @@
+package helper
+
+import (
+	"errors"
+	"strings"
+
+	"github.com/ProtonMail/gopenpgp/armor"
+	"github.com/ProtonMail/gopenpgp/crypto"
+	"github.com/ProtonMail/gopenpgp/internal"
+)
+
+// SignCleartextMessageArmored signs text given a private key and its passphrase, canonicalizes and trims the newlines,
+// and returns the PGP-compliant special armoring
+func SignCleartextMessageArmored(privateKey, passphrase, text string) (string, error) {
+	signingKeyRing, err := pgp.BuildKeyRingArmored(privateKey)
+	if err != nil {
+		return "", err
+	}
+
+	err = signingKeyRing.UnlockWithPassphrase(passphrase)
+	if err != nil {
+		return "", err
+	}
+
+	return SignCleartextMessage(signingKeyRing, text)
+}
+
+// VerifyCleartextMessageArmored verifies PGP-compliant armored signed plain text given the public key
+// and returns the text or err if the verification fails
+func VerifyCleartextMessageArmored(publicKey, armored string, verifyTime int64) (string, error) {
+	verifyKeyRing, err := pgp.BuildKeyRingArmored(publicKey)
+	if err != nil {
+		return "", err
+	}
+
+	return VerifyCleartextMessage(verifyKeyRing, armored, verifyTime)
+}
+
+// SignCleartextMessage signs text given a private keyring, canonicalizes and trims the newlines,
+// and returns the PGP-compliant special armoring
+func SignCleartextMessage(keyRing *crypto.KeyRing, text string) (string, error) {
+	text = canonicalizeAndTrim(text)
+	message := crypto.NewPlainMessageFromString(text)
+
+	signature, err := keyRing.SignDetached(message)
+	if err != nil {
+		return "", err
+	}
+
+	return armor.ArmorClearSignedMessage(message.GetBinary(), signature.GetBinary())
+}
+
+// VerifyCleartextMessage verifies PGP-compliant armored signed plain text given the public keyring
+// and returns the text or err if the verification fails
+func VerifyCleartextMessage(keyRing *crypto.KeyRing, armored string, verifyTime int64) (string, error) {
+	text, signatureData, err := armor.ReadClearSignedMessage(armored)
+	if err != nil {
+		return "", err
+	}
+
+	message := crypto.NewPlainMessageFromString(text)
+	signature := crypto.NewPGPSignature(signatureData)
+	ver, err := keyRing.VerifyDetached(message, signature, verifyTime)
+	if err != nil {
+		return "", err
+	}
+
+	if !ver.IsValid() {
+		return "", errors.New("gopenpgp: unable to verify attachment")
+	}
+
+	return message.GetString(), nil
+}
+
+// ----- INTERNAL FUNCTIONS -----
+
+// canonicalizeAndTrim alters a string canonicalizing and trimming the newlines
+func canonicalizeAndTrim(text string) string {
+	text = internal.TrimNewlines(text)
+	text = strings.Replace(strings.Replace(text, "\r\n", "\n", -1), "\n", "\r\n", -1)
+	return text
+}