Add function to return the size of a key agreement output

Very useful when using "Raw" DH/ECDH via the FFI API.

10 files changed, 39 insertions, 7 deletions

diff --git a/src/lib/ffi/ffi.h b/src/lib/ffi/ffi.h
index fe847e94a..96cdf1de4 100644
--- a/src/lib/ffi/ffi.h
+++ b/src/lib/ffi/ffi.h
@@ -1203,6 +1203,8 @@ BOTAN_PUBLIC_API(2,0) int botan_pk_op_key_agreement_destroy(botan_pk_op_ka_t op)
 BOTAN_PUBLIC_API(2,0) int botan_pk_op_key_agreement_export_public(botan_privkey_t key,
                                                       uint8_t out[], size_t* out_len);
 
+BOTAN_PUBLIC_API(2,8) int botan_pk_op_key_agreement_size(botan_pk_op_ka_t op, size_t* out_len);
+
 BOTAN_PUBLIC_API(2,0)
 int botan_pk_op_key_agreement(botan_pk_op_ka_t op,
                               uint8_t out[], size_t* out_len,
diff --git a/src/lib/ffi/ffi_pk_op.cpp b/src/lib/ffi/ffi_pk_op.cpp
index 5f2437a10..4e548fff8 100644
--- a/src/lib/ffi/ffi_pk_op.cpp
+++ b/src/lib/ffi/ffi_pk_op.cpp
@@ -221,6 +221,13 @@ int botan_pk_op_key_agreement_export_public(botan_privkey_t key,
       });
    }
 
+int botan_pk_op_key_agreement_size(botan_pk_op_ka_t op, size_t* out_len)
+   {
+   return BOTAN_FFI_DO(Botan::PK_Key_Agreement, op, o, {
+      *out_len = o.agreed_value_size();
+      });
+   }
+
 int botan_pk_op_key_agreement(botan_pk_op_ka_t op,
                               uint8_t out[], size_t* out_len,
                               const uint8_t other_key[], size_t other_key_len,
diff --git a/src/lib/prov/pkcs11/p11_ecdh.cpp b/src/lib/prov/pkcs11/p11_ecdh.cpp
index d7b0fae5c..0cd671bcb 100644
--- a/src/lib/prov/pkcs11/p11_ecdh.cpp
+++ b/src/lib/prov/pkcs11/p11_ecdh.cpp
@@ -45,6 +45,7 @@ class PKCS11_ECDH_KA_Operation final : public PK_Ops::Key_Agreement
          : PK_Ops::Key_Agreement(), m_key(key), m_mechanism(MechanismWrapper::create_ecdh_mechanism(params))
          {}
 
+      size_t agreed_value_size() const override { return m_key.domain().get_p_bytes(); }
 
       /// The encoding in V2.20 was not specified and resulted in different implementations choosing different encodings.
       /// Applications relying only on a V2.20 encoding (e.g. the DER variant) other than the one specified now (raw) may not work with all V2.30 compliant tokens.
diff --git a/src/lib/pubkey/curve25519/curve25519.cpp b/src/lib/pubkey/curve25519/curve25519.cpp
index c43f526a8..68b4f5a4f 100644
--- a/src/lib/pubkey/curve25519/curve25519.cpp
+++ b/src/lib/pubkey/curve25519/curve25519.cpp
@@ -118,6 +118,8 @@ class Curve25519_KA_Operation final : public PK_Ops::Key_Agreement_with_KDF
          PK_Ops::Key_Agreement_with_KDF(kdf),
          m_key(key) {}
 
+      size_t agreed_value_size() const { return 32; }
+
       secure_vector<uint8_t> raw_agree(const uint8_t w[], size_t w_len) override
          {
          return m_key.agree(w, w_len);
diff --git a/src/lib/pubkey/dh/dh.cpp b/src/lib/pubkey/dh/dh.cpp
index ae6e9c589..75b0db2f6 100644
--- a/src/lib/pubkey/dh/dh.cpp
+++ b/src/lib/pubkey/dh/dh.cpp
@@ -93,6 +93,8 @@ class DH_KA_Operation final : public PK_Ops::Key_Agreement_with_KDF
                    [this](const BigInt& k) { return m_powermod_x_p(inverse_mod(k, m_p)); })
          {}
 
+      size_t agreed_value_size() const override { return m_p.bytes(); }
+
       secure_vector<uint8_t> raw_agree(const uint8_t w[], size_t w_len) override;
    private:
       const BigInt& m_p;
diff --git a/src/lib/pubkey/ecdh/ecdh.cpp b/src/lib/pubkey/ecdh/ecdh.cpp
index 59f245a00..e7e49a74f 100644
--- a/src/lib/pubkey/ecdh/ecdh.cpp
+++ b/src/lib/pubkey/ecdh/ecdh.cpp
@@ -34,6 +34,8 @@ class ECDH_KA_Operation final : public PK_Ops::Key_Agreement_with_KDF
          m_l_times_priv = m_group.inverse_mod_order(m_group.get_cofactor()) * key.private_value();
          }
 
+      size_t agreed_value_size() const override { return m_group.get_p_bytes(); }
+
       secure_vector<uint8_t> raw_agree(const uint8_t w[], size_t w_len) override
          {
          PointGFp input_point = m_group.get_cofactor() * m_group.OS2ECP(w, w_len);
diff --git a/src/lib/pubkey/ecies/ecies.cpp b/src/lib/pubkey/ecies/ecies.cpp
index 8f5c445ed..ebfd9121e 100644
--- a/src/lib/pubkey/ecies/ecies.cpp
+++ b/src/lib/pubkey/ecies/ecies.cpp
@@ -64,6 +64,8 @@ class ECIES_ECDH_KA_Operation final : public PK_Ops::Key_Agreement_with_KDF
          {
          }
 
+      size_t agreed_value_size() const { return m_key.domain().get_p_bytes(); }
+
       secure_vector<uint8_t> raw_agree(const uint8_t w[], size_t w_len) override
          {
          const EC_Group& group = m_key.domain();
diff --git a/src/lib/pubkey/pk_ops.h b/src/lib/pubkey/pk_ops.h
index 3f794f00d..63ef9fa9b 100644
--- a/src/lib/pubkey/pk_ops.h
+++ b/src/lib/pubkey/pk_ops.h
@@ -118,8 +118,10 @@ class BOTAN_PUBLIC_API(2,0) Key_Agreement
    {
    public:
       virtual secure_vector<uint8_t> agree(size_t key_len,
-                                        const uint8_t other_key[], size_t other_key_len,
-                                        const uint8_t salt[], size_t salt_len) = 0;
+                                           const uint8_t other_key[], size_t other_key_len,
+                                           const uint8_t salt[], size_t salt_len) = 0;
+
+      virtual size_t agreed_value_size() const = 0;
 
       virtual ~Key_Agreement() = default;
    };
diff --git a/src/lib/pubkey/pubkey.cpp b/src/lib/pubkey/pubkey.cpp
index fef81a620..bb0170548 100644
--- a/src/lib/pubkey/pubkey.cpp
+++ b/src/lib/pubkey/pubkey.cpp
@@ -210,6 +210,11 @@ PK_Key_Agreement::PK_Key_Agreement(PK_Key_Agreement&& other) :
    m_op(std::move(other.m_op))
    {}
 
+size_t PK_Key_Agreement::agreed_value_size() const
+   {
+   return m_op->agreed_value_size();
+   }
+
 SymmetricKey PK_Key_Agreement::derive_key(size_t key_len,
                                           const uint8_t in[], size_t in_len,
                                           const uint8_t salt[],
diff --git a/src/lib/pubkey/pubkey.h b/src/lib/pubkey/pubkey.h
index a1404135d..43e1f987a 100644
--- a/src/lib/pubkey/pubkey.h
+++ b/src/lib/pubkey/pubkey.h
@@ -412,7 +412,7 @@ class BOTAN_PUBLIC_API(2,0) PK_Verifier final
    };
 
 /**
-* Key used for key agreement
+* Object used for key agreement
 */
 class BOTAN_PUBLIC_API(2,0) PK_Key_Agreement final
    {
@@ -454,7 +454,7 @@ class BOTAN_PUBLIC_API(2,0) PK_Key_Agreement final
       PK_Key_Agreement& operator=(const PK_Key_Agreement&) = delete;
       PK_Key_Agreement(const PK_Key_Agreement&) = delete;
 
-      /*
+      /**
       * Perform Key Agreement Operation
       * @param key_len the desired key output size
       * @param in the other parties key
@@ -468,7 +468,7 @@ class BOTAN_PUBLIC_API(2,0) PK_Key_Agreement final
                               const uint8_t params[],
                               size_t params_len) const;
 
-      /*
+      /**
       * Perform Key Agreement Operation
       * @param key_len the desired key output size
       * @param in the other parties key
@@ -485,7 +485,7 @@ class BOTAN_PUBLIC_API(2,0) PK_Key_Agreement final
                            params, params_len);
          }
 
-      /*
+      /**
       * Perform Key Agreement Operation
       * @param key_len the desired key output size
       * @param in the other parties key
@@ -501,7 +501,7 @@ class BOTAN_PUBLIC_API(2,0) PK_Key_Agreement final
                            params.length());
          }
 
-      /*
+      /**
       * Perform Key Agreement Operation
       * @param key_len the desired key output size
       * @param in the other parties key
@@ -516,6 +516,13 @@ class BOTAN_PUBLIC_API(2,0) PK_Key_Agreement final
                            params.length());
          }
 
+      /**
+      * Return the underlying size of the value that is agreed.
+      * If derive_key is called with a length of 0 with a "Raw"
+      * KDF, it will return a value of this size.
+      */
+      size_t agreed_value_size() const;
+
    private:
       std::unique_ptr<PK_Ops::Key_Agreement> m_op;
    };