Abstract signing hash data location

This makes the interface a little cleaner by not requiring the class
user to know how we laid out the data in the object.

llarp/crypto/crypto_libsodium.cpp

@@ -194,7 +194,7 @@ namespace llarp
       // PrivateKeys will come from a hash of the root key's s concatenated with
       // the derivation hash.
       crypto_hash_sha512_init(&hs);
-      crypto_hash_sha512_update(&hs, privkey.data() + 32, 32);
+      crypto_hash_sha512_update(&hs, privkey.signingHash(), 32);
       crypto_hash_sha512_update(&hs, buf.base, buf.sz);
       crypto_hash_sha512_final(&hs, nonce);
       crypto_core_ed25519_scalar_reduce(nonce, nonce);
@@ -362,9 +362,9 @@ namespace llarp
       // s' = H(h || s)
       std::array< byte_t, 64 > buf;
       std::copy(h.begin(), h.end(), buf.begin());
-      std::copy(a.begin() + 32, a.end(), buf.begin() + 32);
+      std::copy(a.signingHash(), a.signingHash() + 32, buf.begin() + 32);
       return -1
-          != crypto_generichash_blake2b(out_key.data() + 32, 32, buf.data(),
+          != crypto_generichash_blake2b(out_key.signingHash(), 32, buf.data(),
                                         buf.size(), nullptr, 0);
 
       return true;

llarp/crypto/types.hpp

@@ -156,6 +156,22 @@ namespace llarp
     {
     }
 
+    /// Returns a pointer to the beginning of the 32-byte hash which is used for
+    /// pseudorandomness when signing with this private key.
+    const byte_t *
+    signingHash() const
+    {
+      return data() + 32;
+    }
+
+    /// Returns a pointer to the beginning of the 32-byte hash which is used for
+    /// pseudorandomness when signing with this private key.
+    byte_t *
+    signingHash()
+    {
+      return data() + 32;
+    }
+
     std::ostream &
     print(std::ostream &stream, int level, int spaces) const
     {