cpp: Allow setting the curve to use when generating ECC keys

lang/cpp/src/gpggencardkeyinteractor.h (class GpgGenCardKeyInteractor):
Add enum Curve. Add member function setCurve.
lang/cpp/src/gpggencardkeyinteractor.cpp
(class GpgGenCardKeyInteractor::Private): Initialize simple members
in-class. Add member curve.
(GpgGenCardKeyInteractor::~GpgGenCardKeyInteractor): Use default d'tor.
(GpgGenCardKeyInteractor::setCurve): New.
(GpgGenCardKeyInteractor::action): Return curve defaulting to
Curve25519.
--

This enables users of this interactor to request the generation of
ECC keys with a specific curve as smart card keys. It's up to the user
to specify a curve that is actually supported by the smart card.

GnuPG-bug-id: 4429

3 files changed, 43 insertions, 7 deletions

diff --git a/NEWS b/NEWS
index f9cd8651..fc434647 100644
--- a/NEWS
+++ b/NEWS
@@ -10,6 +10,15 @@ Noteworthy changes in version 1.18.1 (unreleased)
  * qt: Fix problem with expiration dates after 2038-01-19 on 32-bit systems
    when adding an existing subkey to another key.  [T6137]
 
+ * cpp: Allow setting the curve to use when generating ECC keys
+   for smart cards.  [T4429]
+
+ * Interface changes relative to the 1.18.0 release:
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ cpp: GpgGenCardKeyInteractor::Curve        NEW.
+ cpp: GpgGenCardKeyInteractor::setCurve     NEW.
+
+
 Noteworthy changes in version 1.18.0 (2022-08-10)
 -------------------------------------------------
 
diff --git a/lang/cpp/src/gpggencardkeyinteractor.cpp b/lang/cpp/src/gpggencardkeyinteractor.cpp
index 4d90aa0b..a28169ec 100644
--- a/lang/cpp/src/gpggencardkeyinteractor.cpp
+++ b/lang/cpp/src/gpggencardkeyinteractor.cpp
@@ -36,16 +36,17 @@ using namespace GpgME;
 class GpgGenCardKeyInteractor::Private
 {
 public:
-    Private() : keysize("2048"), backup(false), algo(RSA)
+    Private() : keysize("2048")
     {
-
     }
+
     std::string name, email, backupFileName, expiry, serial, keysize;
-    bool backup;
-    Algo algo;
+    bool backup = false;
+    Algo algo = RSA;
+    std::string curve;
 };
 
-GpgGenCardKeyInteractor::~GpgGenCardKeyInteractor() {}
+GpgGenCardKeyInteractor::~GpgGenCardKeyInteractor() = default;
 
 GpgGenCardKeyInteractor::GpgGenCardKeyInteractor(const std::string &serial):
     d(new Private)
@@ -88,6 +89,15 @@ void GpgGenCardKeyInteractor::setAlgo(Algo algo)
     d->algo = algo;
 }
 
+void GpgGenCardKeyInteractor::setCurve(Curve curve)
+{
+    if (curve == DefaultCurve) {
+        d->curve.clear();
+    } else if (curve >= 1 && curve <= LastCurve) {
+        d->curve = std::to_string(static_cast<int>(curve));
+    }
+}
+
 namespace GpgGenCardKeyInteractor_Private
 {
 enum {
@@ -141,7 +151,7 @@ const char *GpgGenCardKeyInteractor::action(Error &err) const
     case KEY_CURVE1:
     case KEY_CURVE2:
     case KEY_CURVE3:
-        return "1"; // Only cv25519 supported.
+        return d->curve.empty() ? "1" : d->curve.c_str(); // default is Curve25519
     case NAME:
         return d->name.c_str();
     case EMAIL:
diff --git a/lang/cpp/src/gpggencardkeyinteractor.h b/lang/cpp/src/gpggencardkeyinteractor.h
index 3d9c7136..09a73b6a 100644
--- a/lang/cpp/src/gpggencardkeyinteractor.h
+++ b/lang/cpp/src/gpggencardkeyinteractor.h
@@ -58,10 +58,27 @@ public:
 
     enum Algo {
         RSA = 1,
-        ECC = 2
+        ECC = 2,
     };
     void setAlgo(Algo algo);
 
+    // the enum values minus 1 have to match the indexes of the curves used by
+    // ask_curve() in gnupg's g10/keygen.c
+    enum Curve {
+        DefaultCurve = 0, // currently Curve25519
+        Curve25519 = 1,
+        Curve448,
+        NISTP256,
+        NISTP384,
+        NISTP521,
+        BrainpoolP256,
+        BrainpoolP384,
+        BrainpoolP512,
+        Secp256k1,
+        LastCurve = Secp256k1,
+    };
+    void setCurve(Curve curve);
+
     std::string backupFileName() const;
 
 private: