crypto: add FSChaCha20Poly1305, rekeying wrapper around ChaCha20Poly1305

This adds the FSChaCha20Poly1305 AEAD as specified in BIP324, a wrapper
around the ChaCha20Poly1305 AEAD (as specified in RFC8439 section 2.8) which
automatically rekeys every N messages, and automatically increments the nonce
every message.

src/crypto/chacha20poly1305.cpp

@@ -8,6 +8,7 @@
 #include <crypto/chacha20.h>
 #include <crypto/poly1305.h>
 #include <span.h>
+#include <support/cleanse.h>
 
 #include <assert.h>
 #include <cstdint>
@@ -99,3 +100,41 @@ bool AEADChaCha20Poly1305::Decrypt(Span<const std::byte> cipher, Span<const std:
     m_chacha20.Crypt(UCharCast(cipher.data()), UCharCast(plain.data()), plain.size());
     return true;
 }
+
+void AEADChaCha20Poly1305::Keystream(Nonce96 nonce, Span<std::byte> keystream) noexcept
+{
+    // Skip the first output block, as it's used for generating the poly1305 key.
+    m_chacha20.Seek64(nonce, 1);
+    m_chacha20.Keystream(UCharCast(keystream.data()), keystream.size());
+}
+
+void FSChaCha20Poly1305::NextPacket() noexcept
+{
+    if (++m_packet_counter == m_rekey_interval) {
+        // Generate a full block of keystream, to avoid needing the ChaCha20 buffer, even though
+        // we only need KEYLEN (32) bytes.
+        std::byte one_block[64];
+        m_aead.Keystream({0xFFFFFFFF, m_rekey_counter}, one_block);
+        // Switch keys.
+        m_aead.SetKey(Span{one_block}.first(KEYLEN));
+        // Wipe the generated keystream (a copy remains inside m_aead, which will be cleaned up
+        // once it cycles again, or is destroyed).
+        memory_cleanse(one_block, sizeof(one_block));
+        // Update counters.
+        m_packet_counter = 0;
+        ++m_rekey_counter;
+    }
+}
+
+void FSChaCha20Poly1305::Encrypt(Span<const std::byte> plain, Span<const std::byte> aad, Span<std::byte> cipher) noexcept
+{
+    m_aead.Encrypt(plain, aad, {m_packet_counter, m_rekey_counter}, cipher);
+    NextPacket();
+}
+
+bool FSChaCha20Poly1305::Decrypt(Span<const std::byte> cipher, Span<const std::byte> aad, Span<std::byte> plain) noexcept
+{
+    bool ret = m_aead.Decrypt(cipher, aad, {m_packet_counter, m_rekey_counter}, plain);
+    NextPacket();
+    return ret;
+}

src/crypto/chacha20poly1305.h

@@ -46,6 +46,68 @@ public:
      * Requires cipher.size() = plain.size() + EXPANSION.
      */
     bool Decrypt(Span<const std::byte> cipher, Span<const std::byte> aad, Nonce96 nonce, Span<std::byte> plain) noexcept;
+
+    /** Get a number of keystream bytes from the underlying stream cipher.
+     *
+     * This is equivalent to Encrypt() with plain set to that many zero bytes, and dropping the
+     * last EXPANSION bytes off the result.
+     */
+    void Keystream(Nonce96 nonce, Span<std::byte> keystream) noexcept;
+};
+
+/** Forward-secure wrapper around AEADChaCha20Poly1305.
+ *
+ * This implements an AEAD which automatically increments the nonce on every encryption or
+ * decryption, and cycles keys after a predetermined number of encryptions or decryptions.
+ *
+ * See BIP324 for details.
+ */
+class FSChaCha20Poly1305
+{
+private:
+    /** Internal AEAD. */
+    AEADChaCha20Poly1305 m_aead;
+
+    /** Every how many iterations this cipher rekeys. */
+    const uint32_t m_rekey_interval;
+
+    /** The number of encryptions/decryptions since the last rekey. */
+    uint32_t m_packet_counter{0};
+
+    /** The number of rekeys performed so far. */
+    uint64_t m_rekey_counter{0};
+
+    /** Update counters (and if necessary, key) to transition to the next message. */
+    void NextPacket() noexcept;
+
+public:
+    /** Length of keys expected by the constructor. */
+    static constexpr auto KEYLEN = AEADChaCha20Poly1305::KEYLEN;
+
+    /** Expansion when encrypting. */
+    static constexpr auto EXPANSION = AEADChaCha20Poly1305::EXPANSION;
+
+    // No copy or move to protect the secret.
+    FSChaCha20Poly1305(const FSChaCha20Poly1305&) = delete;
+    FSChaCha20Poly1305(FSChaCha20Poly1305&&) = delete;
+    FSChaCha20Poly1305& operator=(const FSChaCha20Poly1305&) = delete;
+    FSChaCha20Poly1305& operator=(FSChaCha20Poly1305&&) = delete;
+
+    /** Construct an FSChaCha20Poly1305 cipher that rekeys every rekey_interval operations. */
+    FSChaCha20Poly1305(Span<const std::byte> key, uint32_t rekey_interval) noexcept :
+        m_aead(key), m_rekey_interval(rekey_interval) {}
+
+    /** Encrypt a message with a specified aad.
+     *
+     * Requires cipher.size() = plain.size() + EXPANSION.
+     */
+    void Encrypt(Span<const std::byte> plain, Span<const std::byte> aad, Span<std::byte> cipher) noexcept;
+
+    /** Decrypt a message with a specified aad. Returns true if valid.
+     *
+     * Requires cipher.size() = plain.size() + EXPANSION.
+     */
+    bool Decrypt(Span<const std::byte> cipher, Span<const std::byte> aad, Span<std::byte> plain) noexcept;
 };
 
 #endif // BITCOIN_CRYPTO_CHACHA20POLY1305_H