3 files changed, 48 insertions, 3 deletions
diff --git a/autotests/testolmsession.cpp b/autotests/testolmsession.cpp
index 2f7a82e9..fc151621 100644
--- a/autotests/testolmsession.cpp
+++ b/autotests/testolmsession.cpp
@@ -52,9 +52,9 @@ void TestOlmSession::olmEncryptDecrypt()
         QVERIFY(std::get<bool>(inboundSession->matchesInboundSession(m)));
     }
 
-    //const auto decrypted = inboundSession->decrypt(encrypted);
+    const auto decrypted = std::get<QString>(inboundSession->decrypt(encrypted));
 
-    //QCOMPARE(decrypted, "Hello world!");
+    QCOMPARE(decrypted, "Hello world!");
 #endif
 }
 
diff --git a/lib/olm/session.cpp b/lib/olm/session.cpp
index d0493fe8..a05e0786 100644
--- a/lib/olm/session.cpp
+++ b/lib/olm/session.cpp
@@ -6,6 +6,7 @@
 #include "olm/session.h"
 #include "olm/utils.h"
 #include "logging.h"
+#include <cstring>
 
 using namespace Quotient;
 
@@ -142,6 +143,46 @@ Message QOlmSession::encrypt(const QString &plaintext)
     return Message(messageBuf, messageType);
 }
 
+std::variant<QString, OlmError> QOlmSession::decrypt(const Message &message) const
+{
+    const auto messageType = message.type();
+    const auto ciphertext = message.toCiphertext();
+    const auto messageTypeValue = messageType == Message::Type::General
+        ? OLM_MESSAGE_TYPE_MESSAGE : OLM_MESSAGE_TYPE_PRE_KEY;
+
+    // We need to clone the message because
+    // olm_decrypt_max_plaintext_length destroys the input buffer
+    QByteArray messageBuf(ciphertext.length(), '0');
+    std::copy(message.begin(), message.end(), messageBuf.begin());
+
+    const auto plaintextMaxLen = olm_decrypt_max_plaintext_length(m_session, messageTypeValue,
+            reinterpret_cast<uint8_t *>(messageBuf.data()), messageBuf.length());
+
+    if (plaintextMaxLen == olm_error()) {
+        return lastError(m_session);
+    }
+
+    QByteArray plaintextBuf(plaintextMaxLen, '0');
+    QByteArray messageBuf2(ciphertext.length(), '0');
+    std::copy(message.begin(), message.end(), messageBuf2.begin());
+
+    const auto plaintextResultLen = olm_decrypt(m_session, messageTypeValue,
+            reinterpret_cast<uint8_t *>(messageBuf2.data()), messageBuf2.length(),
+            reinterpret_cast<uint8_t *>(plaintextBuf.data()), plaintextMaxLen);
+
+    if (plaintextResultLen == olm_error()) {
+        const auto lastErr = lastError(m_session);
+        if (lastErr == OlmError::OutputBufferTooSmall) {
+            throw lastErr;
+        }
+        return lastErr;
+    }
+    QByteArray output(plaintextResultLen, '0');
+    std::memcpy(output.data(), plaintextBuf.data(), plaintextResultLen);
+    plaintextBuf.clear();
+    return output;
+}
+
 Message::Type QOlmSession::encryptMessageType()
 {
     const auto messageTypeResult = olm_encrypt_message_type(m_session);
diff --git a/lib/olm/session.h b/lib/olm/session.h
index c45b6898..3f1622c7 100644
--- a/lib/olm/session.h
+++ b/lib/olm/session.h
@@ -32,7 +32,11 @@ public:
     static std::variant<std::unique_ptr<QOlmSession>, OlmError> unpickle(QByteArray &pickled, const PicklingMode &mode);
     //! Encrypts a plaintext message using the session.
     Message encrypt(const QString &plaintext);
-    // TODO: WiP
+
+    //! Decrypts a message using this session. Decoding is lossy, meaing if
+    //! the decrypted plaintext contains invalid UTF-8 symbols, they will
+    //! be returned as `U+FFFD` (�).
+    std::variant<QString, OlmError> decrypt(const Message &message) const;
 
     //! Get a base64-encoded identifier for this session.
     QByteArray sessionId() const;