crypto: add the ChaCha20Poly1305 AEAD as specified in RFC8439

This adds an implementation of the ChaCha20Poly1305 AEAD exactly matching
the version specified in RFC8439 section 2.8, including tests and official
test vectors.

4 files changed, 231 insertions, 0 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index d8ea4bbd28..b6fe256335 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -548,6 +548,8 @@ crypto_libbitcoin_crypto_base_la_SOURCES = \
   crypto/aes.h \
   crypto/chacha20.h \
   crypto/chacha20.cpp \
+  crypto/chacha20poly1305.h \
+  crypto/chacha20poly1305.cpp \
   crypto/common.h \
   crypto/hkdf_sha256_32.cpp \
   crypto/hkdf_sha256_32.h \
diff --git a/src/crypto/chacha20poly1305.cpp b/src/crypto/chacha20poly1305.cpp
new file mode 100644
index 0000000000..5f27857213
--- /dev/null
+++ b/src/crypto/chacha20poly1305.cpp
@@ -0,0 +1,101 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <crypto/chacha20poly1305.h>
+
+#include <crypto/common.h>
+#include <crypto/chacha20.h>
+#include <crypto/poly1305.h>
+#include <span.h>
+
+#include <assert.h>
+#include <cstdint>
+#include <cstddef>
+#include <iterator>
+
+AEADChaCha20Poly1305::AEADChaCha20Poly1305(Span<const std::byte> key) noexcept : m_chacha20(UCharCast(key.data()))
+{
+    assert(key.size() == KEYLEN);
+}
+
+void AEADChaCha20Poly1305::SetKey(Span<const std::byte> key) noexcept
+{
+    assert(key.size() == KEYLEN);
+    m_chacha20.SetKey32(UCharCast(key.data()));
+}
+
+namespace {
+
+#ifndef HAVE_TIMINGSAFE_BCMP
+#define HAVE_TIMINGSAFE_BCMP
+
+int timingsafe_bcmp(const unsigned char* b1, const unsigned char* b2, size_t n) noexcept
+{
+    const unsigned char *p1 = b1, *p2 = b2;
+    int ret = 0;
+    for (; n > 0; n--)
+        ret |= *p1++ ^ *p2++;
+    return (ret != 0);
+}
+
+#endif
+
+/** Compute poly1305 tag. chacha20 must be set to the right nonce, block 0. Will be at block 1 after. */
+void ComputeTag(ChaCha20& chacha20, Span<const std::byte> aad, Span<const std::byte> cipher, Span<std::byte> tag) noexcept
+{
+    static const std::byte PADDING[16] = {{}};
+
+    // Get block of keystream (use a full 64 byte buffer to avoid the need for chacha20's own buffering).
+    std::byte first_block[64];
+    chacha20.Keystream(UCharCast(first_block), sizeof(first_block));
+
+    // Use the first 32 bytes of the first keystream block as poly1305 key.
+    Poly1305 poly1305{Span{first_block}.first(Poly1305::KEYLEN)};
+
+    // Compute tag:
+    // - Process the padded AAD with Poly1305.
+    const unsigned aad_padding_length = (16 - (aad.size() % 16)) % 16;
+    poly1305.Update(aad).Update(Span{PADDING}.first(aad_padding_length));
+    // - Process the padded ciphertext with Poly1305.
+    const unsigned cipher_padding_length = (16 - (cipher.size() % 16)) % 16;
+    poly1305.Update(cipher).Update(Span{PADDING}.first(cipher_padding_length));
+    // - Process the AAD and plaintext length with Poly1305.
+    std::byte length_desc[Poly1305::TAGLEN];
+    WriteLE64(UCharCast(length_desc), aad.size());
+    WriteLE64(UCharCast(length_desc + 8), cipher.size());
+    poly1305.Update(length_desc);
+
+    // Output tag.
+    poly1305.Finalize(tag);
+}
+
+} // namespace
+
+void AEADChaCha20Poly1305::Encrypt(Span<const std::byte> plain, Span<const std::byte> aad, Nonce96 nonce, Span<std::byte> cipher) noexcept
+{
+    assert(cipher.size() == plain.size() + EXPANSION);
+
+    // Encrypt using ChaCha20 (starting at block 1).
+    m_chacha20.Seek64(nonce, 1);
+    m_chacha20.Crypt(UCharCast(plain.data()), UCharCast(cipher.data()), plain.size());
+
+    // Seek to block 0, and compute tag using key drawn from there.
+    m_chacha20.Seek64(nonce, 0);
+    ComputeTag(m_chacha20, aad, cipher.first(plain.size()), cipher.last(EXPANSION));
+}
+
+bool AEADChaCha20Poly1305::Decrypt(Span<const std::byte> cipher, Span<const std::byte> aad, Nonce96 nonce, Span<std::byte> plain) noexcept
+{
+    assert(cipher.size() == plain.size() + EXPANSION);
+
+    // Verify tag (using key drawn from block 0).
+    m_chacha20.Seek64(nonce, 0);
+    std::byte expected_tag[EXPANSION];
+    ComputeTag(m_chacha20, aad, cipher.first(plain.size()), expected_tag);
+    if (timingsafe_bcmp(UCharCast(expected_tag), UCharCast(cipher.data() + plain.size()), EXPANSION)) return false;
+
+    // Decrypt (starting at block 1).
+    m_chacha20.Crypt(UCharCast(cipher.data()), UCharCast(plain.data()), plain.size());
+    return true;
+}
diff --git a/src/crypto/chacha20poly1305.h b/src/crypto/chacha20poly1305.h
new file mode 100644
index 0000000000..0007ee63a9
--- /dev/null
+++ b/src/crypto/chacha20poly1305.h
@@ -0,0 +1,51 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_CRYPTO_CHACHA20POLY1305_H
+#define BITCOIN_CRYPTO_CHACHA20POLY1305_H
+
+#include <cstddef>
+#include <cstdlib>
+#include <stdint.h>
+
+#include <crypto/chacha20.h>
+#include <crypto/poly1305.h>
+#include <span.h>
+
+/** The AEAD_CHACHA20_POLY1305 authenticated encryption algorithm from RFC8439 section 2.8. */
+class AEADChaCha20Poly1305
+{
+    /** Internal stream cipher. */
+    ChaCha20 m_chacha20;
+
+public:
+    /** Expected size of key argument in constructor. */
+    static constexpr unsigned KEYLEN = 32;
+
+    /** Expansion when encrypting. */
+    static constexpr unsigned EXPANSION = Poly1305::TAGLEN;
+
+    /** Initialize an AEAD instance with a specified 32-byte key. */
+    AEADChaCha20Poly1305(Span<const std::byte> key) noexcept;
+
+    /** Switch to another 32-byte key. */
+    void SetKey(Span<const std::byte> key) noexcept;
+
+    /** 96-bit nonce type. */
+    using Nonce96 = ChaCha20::Nonce96;
+
+    /** Encrypt a message with a specified 96-bit nonce and aad.
+     *
+     * Requires cipher.size() = plain.size() + EXPANSION.
+     */
+    void Encrypt(Span<const std::byte> plain, Span<const std::byte> aad, Nonce96 nonce, Span<std::byte> cipher) noexcept;
+
+    /** Decrypt a message with a specified 96-bit nonce and aad. Returns true if valid.
+     *
+     * 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;
+};
+
+#endif // BITCOIN_CRYPTO_CHACHA20POLY1305_H
diff --git a/src/test/crypto_tests.cpp b/src/test/crypto_tests.cpp
index 809b897998..f62072f5bb 100644
--- a/src/test/crypto_tests.cpp
+++ b/src/test/crypto_tests.cpp
@@ -4,6 +4,7 @@
 
 #include <crypto/aes.h>
 #include <crypto/chacha20.h>
+#include <crypto/chacha20poly1305.h>
 #include <crypto/hkdf_sha256_32.h>
 #include <crypto/hmac_sha256.h>
 #include <crypto/hmac_sha512.h>
@@ -207,6 +208,24 @@ static void TestPoly1305(const std::string &hexmessage, const std::string &hexke
     }
 }
 
+static void TestChaCha20Poly1305(const std::string& plain_hex, const std::string& aad_hex, const std::string& key_hex, ChaCha20::Nonce96 nonce, const std::string& cipher_hex)
+{
+    auto plain = ParseHex<std::byte>(plain_hex);
+    auto aad = ParseHex<std::byte>(aad_hex);
+    auto key = ParseHex<std::byte>(key_hex);
+    auto expected_cipher = ParseHex<std::byte>(cipher_hex);
+
+    std::vector<std::byte> cipher(plain.size() + AEADChaCha20Poly1305::EXPANSION);
+    AEADChaCha20Poly1305 aead{key};
+    aead.Encrypt(plain, aad, nonce, cipher);
+    BOOST_CHECK(cipher == expected_cipher);
+
+    std::vector<std::byte> decipher(cipher.size() - AEADChaCha20Poly1305::EXPANSION);
+    bool ret = aead.Decrypt(cipher, aad, nonce, decipher);
+    BOOST_CHECK(ret);
+    BOOST_CHECK(decipher == plain);
+}
+
 static void TestHKDF_SHA256_32(const std::string &ikm_hex, const std::string &salt_hex, const std::string &info_hex, const std::string &okm_check_hex) {
     std::vector<unsigned char> initial_key_material = ParseHex(ikm_hex);
     std::vector<unsigned char> salt = ParseHex(salt_hex);
@@ -818,6 +837,64 @@ BOOST_AUTO_TEST_CASE(poly1305_testvector)
                  "0e410fa9d7a40ac582e77546be9a72bb");
 }
 
+BOOST_AUTO_TEST_CASE(chacha20poly1305_testvectors)
+{
+    // Note that in our implementation, the authentication is suffixed to the ciphertext.
+    // The RFC test vectors specify them separately.
+
+    // RFC 8439 Example from section 2.8.2
+    TestChaCha20Poly1305("4c616469657320616e642047656e746c656d656e206f662074686520636c6173"
+                         "73206f66202739393a204966204920636f756c64206f6666657220796f75206f"
+                         "6e6c79206f6e652074697020666f7220746865206675747572652c2073756e73"
+                         "637265656e20776f756c642062652069742e",
+                         "50515253c0c1c2c3c4c5c6c7",
+                         "808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9f",
+                         {7, 0x4746454443424140},
+                         "d31a8d34648e60db7b86afbc53ef7ec2a4aded51296e08fea9e2b5a736ee62d6"
+                         "3dbea45e8ca9671282fafb69da92728b1a71de0a9e060b2905d6a5b67ecd3b36"
+                         "92ddbd7f2d778b8c9803aee328091b58fab324e4fad675945585808b4831d7bc"
+                         "3ff4def08e4b7a9de576d26586cec64b61161ae10b594f09e26a7e902ecbd060"
+                         "0691");
+
+    // RFC 8439 Test vector A.5
+    TestChaCha20Poly1305("496e7465726e65742d4472616674732061726520647261667420646f63756d65"
+                         "6e74732076616c696420666f722061206d6178696d756d206f6620736978206d"
+                         "6f6e74687320616e64206d617920626520757064617465642c207265706c6163"
+                         "65642c206f72206f62736f6c65746564206279206f7468657220646f63756d65"
+                         "6e747320617420616e792074696d652e20497420697320696e617070726f7072"
+                         "6961746520746f2075736520496e7465726e65742d4472616674732061732072"
+                         "65666572656e6365206d6174657269616c206f7220746f206369746520746865"
+                         "6d206f74686572207468616e206173202fe2809c776f726b20696e2070726f67"
+                         "726573732e2fe2809d",
+                         "f33388860000000000004e91",
+                         "1c9240a5eb55d38af333888604f6b5f0473917c1402b80099dca5cbc207075c0",
+                         {0, 0x0807060504030201},
+                         "64a0861575861af460f062c79be643bd5e805cfd345cf389f108670ac76c8cb2"
+                         "4c6cfc18755d43eea09ee94e382d26b0bdb7b73c321b0100d4f03b7f355894cf"
+                         "332f830e710b97ce98c8a84abd0b948114ad176e008d33bd60f982b1ff37c855"
+                         "9797a06ef4f0ef61c186324e2b3506383606907b6a7c02b0f9f6157b53c867e4"
+                         "b9166c767b804d46a59b5216cde7a4e99040c5a40433225ee282a1b0a06c523e"
+                         "af4534d7f83fa1155b0047718cbc546a0d072b04b3564eea1b422273f548271a"
+                         "0bb2316053fa76991955ebd63159434ecebb4e466dae5a1073a6727627097a10"
+                         "49e617d91d361094fa68f0ff77987130305beaba2eda04df997b714d6c6f2c29"
+                         "a6ad5cb4022b02709beead9d67890cbb22392336fea1851f38");
+
+    // Test vectors exercising aad and plaintext which are multiples of 16 bytes.
+    TestChaCha20Poly1305("8d2d6a8befd9716fab35819eaac83b33269afb9f1a00fddf66095a6c0cd91951"
+                         "a6b7ad3db580be0674c3f0b55f618e34",
+                         "",
+                         "72ddc73f07101282bbbcf853b9012a9f9695fc5d36b303a97fd0845d0314e0c3",
+                         {0x3432b75f, 0xb3585537eb7f4024},
+                         "f760b8224fb2a317b1b07875092606131232a5b86ae142df5df1c846a7f6341a"
+                         "f2564483dd77f836be45e6230808ffe402a6f0a3e8be074b3d1f4ea8a7b09451");
+    TestChaCha20Poly1305("",
+                         "36970d8a704c065de16250c18033de5a400520ac1b5842b24551e5823a3314f3"
+                         "946285171e04a81ebfbe3566e312e74ab80e94c7dd2ff4e10de0098a58d0f503",
+                         "77adda51d6730b9ad6c995658cbd49f581b2547e7c0c08fcc24ceec797461021",
+                         {0x1f90da88, 0x75dafa3ef84471a4},
+                         "aaae5bb81e8407c94b2ae86ae0c7efbe");
+}
+
 BOOST_AUTO_TEST_CASE(hkdf_hmac_sha256_l32_tests)
 {
     // Use rfc5869 test vectors but truncated to 32 bytes (our implementation only support length 32)