Add OlmPkDecryption functions (#141)

* Add OlmPkDecryption functions

* Trim result to the valid size

crypto/olm/pk.go

@@ -109,3 +109,62 @@ func (p *PkSigning) SignJSON(obj interface{}) (string, error) {
 func (p *PkSigning) lastError() error {
 	return convertError(C.GoString(C.olm_pk_signing_last_error((*C.OlmPkSigning)(p.int))))
 }
+
+type PkDecryption struct {
+	int       *C.OlmPkDecryption
+	mem       []byte
+	PublicKey []byte
+}
+
+func pkDecryptionSize() uint {
+	return uint(C.olm_pk_decryption_size())
+}
+
+func pkDecryptionPublicKeySize() uint {
+	return uint(C.olm_pk_key_length())
+}
+
+func NewPkDecryption(privateKey []byte) (*PkDecryption, error) {
+	memory := make([]byte, pkDecryptionSize())
+	p := &PkDecryption{
+		int: C.olm_pk_decryption(unsafe.Pointer(&memory[0])),
+		mem: memory,
+	}
+	p.Clear()
+	pubKey := make([]byte, pkDecryptionPublicKeySize())
+
+	if C.olm_pk_key_from_private((*C.OlmPkDecryption)(p.int),
+		unsafe.Pointer(&pubKey[0]), C.size_t(len(pubKey)),
+		unsafe.Pointer(&privateKey[0]), C.size_t(len(privateKey))) == errorVal() {
+		return nil, p.lastError()
+	}
+	p.PublicKey = pubKey
+
+	return p, nil
+}
+
+func (p *PkDecryption) Decrypt(ephemeralKey []byte, mac []byte, ciphertext []byte) ([]byte, error) {
+	maxPlaintextLength := uint(C.olm_pk_max_plaintext_length((*C.OlmPkDecryption)(p.int), C.size_t(len(ciphertext))))
+	plaintext := make([]byte, maxPlaintextLength)
+
+	size := C.olm_pk_decrypt((*C.OlmPkDecryption)(p.int),
+		unsafe.Pointer(&ephemeralKey[0]), C.size_t(len(ephemeralKey)),
+		unsafe.Pointer(&mac[0]), C.size_t(len(mac)),
+		unsafe.Pointer(&ciphertext[0]), C.size_t(len(ciphertext)),
+		unsafe.Pointer(&plaintext[0]), C.size_t(len(plaintext)))
+	if size == errorVal() {
+		return nil, p.lastError()
+	}
+
+	return plaintext[:size], nil
+}
+
+// Clear clears the underlying memory of a PkDecryption object.
+func (p *PkDecryption) Clear() {
+	C.olm_clear_pk_decryption((*C.OlmPkDecryption)(p.int))
+}
+
+// lastError returns the last error that happened in relation to this PkDecryption object.
+func (p *PkDecryption) lastError() error {
+	return convertError(C.GoString(C.olm_pk_decryption_last_error((*C.OlmPkDecryption)(p.int))))
+}