crypto: add Poly1305 class with std::byte Span interface

src/crypto/poly1305.h

@@ -5,6 +5,9 @@
 #ifndef BITCOIN_CRYPTO_POLY1305_H
 #define BITCOIN_CRYPTO_POLY1305_H
 
+#include <span.h>
+
+#include <cassert>
 #include <cstdlib>
 #include <stdint.h>
 
@@ -32,6 +35,40 @@ void poly1305_finish(poly1305_context *st, unsigned char mac[16]) noexcept;
 
 }  // namespace poly1305_donna
 
+/** C++ wrapper with std::byte Span interface around poly1305_donna code. */
+class Poly1305
+{
+    poly1305_donna::poly1305_context m_ctx;
+
+public:
+    /** Length of the output produced by Finalize(). */
+    static constexpr unsigned TAGLEN = POLY1305_TAGLEN;
+
+    /** Length of the keys expected by the constructor. */
+    static constexpr unsigned KEYLEN = POLY1305_KEYLEN;
+
+    /** Construct a Poly1305 object with a given 32-byte key. */
+    Poly1305(Span<const std::byte> key) noexcept
+    {
+        assert(key.size() == KEYLEN);
+        poly1305_donna::poly1305_init(&m_ctx, UCharCast(key.data()));
+    }
+
+    /** Process message bytes. */
+    Poly1305& Update(Span<const std::byte> msg) noexcept
+    {
+        poly1305_donna::poly1305_update(&m_ctx, UCharCast(msg.data()), msg.size());
+        return *this;
+    }
+
+    /** Write authentication tag to 16-byte out. */
+    void Finalize(Span<std::byte> out) noexcept
+    {
+        assert(out.size() == TAGLEN);
+        poly1305_donna::poly1305_finish(&m_ctx, UCharCast(out.data()));
+    }
+};
+
 void poly1305_auth(unsigned char out[POLY1305_TAGLEN], const unsigned char *m, size_t inlen,
     const unsigned char key[POLY1305_KEYLEN]);