crypto: add FSChaCha20, a rekeying wrapper around ChaCha20

This adds the FSChaCha20 stream cipher as specified in BIP324, a
wrapper around the ChaCha20 stream cipher (specified in RFC8439
section 2.4) which automatically rekeys every N messages, and
manages the nonces used for encryption.

Co-authored-by: dhruv <856960+dhruv@users.noreply.github.com>

Author: sipa

src/crypto/chacha20.cpp

@@ -7,6 +7,7 @@
 
 #include <crypto/common.h>
 #include <crypto/chacha20.h>
+#include <support/cleanse.h>
 
 #include <algorithm>
 #include <string.h>
@@ -42,6 +43,11 @@ ChaCha20Aligned::ChaCha20Aligned()
     memset(input, 0, sizeof(input));
 }
 
+ChaCha20Aligned::~ChaCha20Aligned()
+{
+    memory_cleanse(input, sizeof(input));
+}
+
 ChaCha20Aligned::ChaCha20Aligned(const unsigned char* key32)
 {
     SetKey32(key32);
@@ -318,3 +324,38 @@ void ChaCha20::Crypt(const unsigned char* m, unsigned char* c, size_t bytes)
         m_bufleft = 64 - bytes;
     }
 }
+
+ChaCha20::~ChaCha20()
+{
+    memory_cleanse(m_buffer, sizeof(m_buffer));
+}
+
+FSChaCha20::FSChaCha20(Span<const std::byte> key, uint32_t rekey_interval) noexcept :
+    m_chacha20(UCharCast(key.data())), m_rekey_interval(rekey_interval)
+{
+    assert(key.size() == KEYLEN);
+}
+
+void FSChaCha20::Crypt(Span<const std::byte> input, Span<std::byte> output) noexcept
+{
+    assert(input.size() == output.size());
+
+    // Invoke internal stream cipher for actual encryption/decryption.
+    m_chacha20.Crypt(UCharCast(input.data()), UCharCast(output.data()), input.size());
+
+    // Rekey after m_rekey_interval encryptions/decryptions.
+    if (++m_chunk_counter == m_rekey_interval) {
+        // Get new key from the stream cipher.
+        std::byte new_key[KEYLEN];
+        m_chacha20.Keystream(UCharCast(new_key), sizeof(new_key));
+        // Update its key.
+        m_chacha20.SetKey32(UCharCast(new_key));
+        // Wipe the key (a copy remains inside m_chacha20, where it'll be wiped on the next rekey
+        // or on destruction).
+        memory_cleanse(new_key, sizeof(new_key));
+        // Set the nonce for the new section of output.
+        m_chacha20.Seek64({0, ++m_rekey_counter}, 0);
+        // Reset the chunk counter.
+        m_chunk_counter = 0;
+    }
+}

src/crypto/chacha20.h

@@ -5,6 +5,10 @@
 #ifndef BITCOIN_CRYPTO_CHACHA20_H
 #define BITCOIN_CRYPTO_CHACHA20_H
 
+#include <span.h>
+
+#include <array>
+#include <cstddef>
 #include <cstdlib>
 #include <stdint.h>
 #include <utility>
@@ -29,6 +33,9 @@ class ChaCha20Aligned
     /** Initialize a cipher with specified 32-byte key. */
     ChaCha20Aligned(const unsigned char* key32);
 
+    /** Destructor to clean up private memory. */
+    ~ChaCha20Aligned();
+
     /** set 32-byte key. */
     void SetKey32(const unsigned char* key32);
 
@@ -72,6 +79,9 @@ class ChaCha20
     /** Initialize a cipher with specified 32-byte key. */
     ChaCha20(const unsigned char* key32) : m_aligned(key32) {}
 
+    /** Destructor to clean up private memory. */
+    ~ChaCha20();
+
     /** set 32-byte key. */
     void SetKey32(const unsigned char* key32)
     {
@@ -98,4 +108,43 @@ class ChaCha20
     void Crypt(const unsigned char* input, unsigned char* output, size_t bytes);
 };
 
+/** Forward-secure ChaCha20
+ *
+ * This implements a stream cipher that automatically transitions to a new stream with a new key
+ * and new nonce after a predefined number of encryptions or decryptions.
+ *
+ * See BIP324 for details.
+ */
+class FSChaCha20
+{
+private:
+    /** Internal stream cipher. */
+    ChaCha20 m_chacha20;
+
+    /** The number of encryptions/decryptions before a rekey happens. */
+    const uint32_t m_rekey_interval;
+
+    /** The number of encryptions/decryptions since the last rekey. */
+    uint32_t m_chunk_counter{0};
+
+    /** The number of rekey operations that have happened. */
+    uint64_t m_rekey_counter{0};
+
+public:
+    /** Length of keys expected by the constructor. */
+    static constexpr unsigned KEYLEN = 32;
+
+    // No copy or move to protect the secret.
+    FSChaCha20(const FSChaCha20&) = delete;
+    FSChaCha20(FSChaCha20&&) = delete;
+    FSChaCha20& operator=(const FSChaCha20&) = delete;
+    FSChaCha20& operator=(FSChaCha20&&) = delete;
+
+    /** Construct an FSChaCha20 cipher that rekeys every rekey_interval Crypt() calls. */
+    FSChaCha20(Span<const std::byte> key, uint32_t rekey_interval) noexcept;
+
+    /** Encrypt or decrypt a chunk. */
+    void Crypt(Span<const std::byte> input, Span<std::byte> output) noexcept;
+};
+
 #endif // BITCOIN_CRYPTO_CHACHA20_H

src/test/crypto_tests.cpp

@@ -182,6 +182,46 @@ static void TestChaCha20(const std::string &hex_message, const std::string &hexk
     }
 }
 
+static void TestFSChaCha20(const std::string& hex_plaintext, const std::string& hexkey, uint32_t rekey_interval, const std::string& ciphertext_after_rotation)
+{
+    auto key = ParseHex<std::byte>(hexkey);
+    BOOST_CHECK_EQUAL(FSChaCha20::KEYLEN, key.size());
+
+    auto plaintext = ParseHex<std::byte>(hex_plaintext);
+
+    auto fsc20 = FSChaCha20{key, rekey_interval};
+    auto c20 = ChaCha20{UCharCast(key.data())};
+
+    std::vector<std::byte> fsc20_output;
+    fsc20_output.resize(plaintext.size());
+
+    std::vector<std::byte> c20_output;
+    c20_output.resize(plaintext.size());
+
+    for (size_t i = 0; i < rekey_interval; i++) {
+        fsc20.Crypt(plaintext, fsc20_output);
+        c20.Crypt(UCharCast(plaintext.data()), UCharCast(c20_output.data()), plaintext.size());
+        BOOST_CHECK(c20_output == fsc20_output);
+    }
+
+    // At the rotation interval, the outputs will no longer match
+    fsc20.Crypt(plaintext, fsc20_output);
+    auto c20_copy = c20;
+    c20.Crypt(UCharCast(plaintext.data()), UCharCast(c20_output.data()), plaintext.size());
+    BOOST_CHECK(c20_output != fsc20_output);
+
+    std::byte new_key[FSChaCha20::KEYLEN];
+    c20_copy.Keystream(UCharCast(new_key), sizeof(new_key));
+    c20.SetKey32(UCharCast(new_key));
+    c20.Seek64({0, 1}, 0);
+
+    // Outputs should match again after simulating key rotation
+    c20.Crypt(UCharCast(plaintext.data()), UCharCast(c20_output.data()), plaintext.size());
+    BOOST_CHECK(c20_output == fsc20_output);
+
+    BOOST_CHECK_EQUAL(HexStr(fsc20_output), ciphertext_after_rotation);
+}
+
 static void TestPoly1305(const std::string &hexmessage, const std::string &hexkey, const std::string& hextag)
 {
     auto key = ParseHex<std::byte>(hexkey);
@@ -696,6 +736,20 @@ BOOST_AUTO_TEST_CASE(chacha20_testvector)
                  "fd565dea5addbdb914208fde7950f23e0385f9a727143f6a6ac51d84b1c0fb3e"
                  "2e3b00b63d6841a1cc6d1538b1d3a74bef1eb2f54c7b7281e36e484dba89b351"
                  "c8f572617e61e342879f211b0e4c515df50ea9d0771518fad96cd0baee62deb6");
+
+    // Forward secure ChaCha20
+    TestFSChaCha20("000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f",
+                   "0000000000000000000000000000000000000000000000000000000000000000",
+                   256,
+                   "a93df4ef03011f3db95f60d996e1785df5de38fc39bfcb663a47bb5561928349");
+    TestFSChaCha20("01",
+                   "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f",
+                   5,
+                   "ea");
+    TestFSChaCha20("e93fdb5c762804b9a706816aca31e35b11d2aa3080108ef46a5b1f1508819c0a",
+                   "8ec4c3ccdaea336bdeb245636970be01266509b33f3d2642504eaf412206207a",
+                   4096,
+                   "8bfaa4eacff308fdb4a94a5ff25bd9d0c1f84b77f81239f67ff39d6e1ac280c9");
 }
 
 BOOST_AUTO_TEST_CASE(chacha20_midblock)

src/test/fuzz/crypto_chacha20.cpp

@@ -8,6 +8,8 @@
 #include <test/fuzz/util.h>
 #include <test/util/xoroshiro128plusplus.h>
 
+#include <array>
+#include <cstddef>
 #include <cstdint>
 #include <vector>
 
@@ -151,3 +153,21 @@ FUZZ_TARGET(chacha20_split_keystream)
     FuzzedDataProvider provider{buffer.data(), buffer.size()};
     ChaCha20SplitFuzz<false>(provider);
 }
+
+FUZZ_TARGET(crypto_fschacha20)
+{
+    FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
+
+    auto key = fuzzed_data_provider.ConsumeBytes<std::byte>(FSChaCha20::KEYLEN);
+    key.resize(FSChaCha20::KEYLEN);
+
+    auto fsc20 = FSChaCha20{key, fuzzed_data_provider.ConsumeIntegralInRange<uint32_t>(1, 1024)};
+
+    LIMITED_WHILE(fuzzed_data_provider.ConsumeBool(), 10000)
+    {
+        auto input = fuzzed_data_provider.ConsumeBytes<std::byte>(fuzzed_data_provider.ConsumeIntegralInRange(0, 4096));
+        std::vector<std::byte> output;
+        output.resize(input.size());
+        fsc20.Crypt(input, output);
+    }
+}