crypto: Add MuHash3072 implementation

Co-authored-by: Pieter Wuille <pieter.wuille@gmail.com>

1 files changed, 68 insertions, 0 deletions

diff --git a/src/crypto/muhash.h b/src/crypto/muhash.h
index e5f6f0464f..0c710007c4 100644
--- a/src/crypto/muhash.h
+++ b/src/crypto/muhash.h
@@ -59,4 +59,72 @@ public:
     }
 };
 
+/** A class representing MuHash sets
+ *
+ * MuHash is a hashing algorithm that supports adding set elements in any
+ * order but also deleting in any order. As a result, it can maintain a
+ * running sum for a set of data as a whole, and add/remove when data
+ * is added to or removed from it. A downside of MuHash is that computing
+ * an inverse is relatively expensive. This is solved by representing
+ * the running value as a fraction, and multiplying added elements into
+ * the numerator and removed elements into the denominator. Only when the
+ * final hash is desired, a single modular inverse and multiplication is
+ * needed to combine the two. The combination is also run on serialization
+ * to allow for space-efficient storage on disk.
+ *
+ * As the update operations are also associative, H(a)+H(b)+H(c)+H(d) can
+ * in fact be computed as (H(a)+H(b)) + (H(c)+H(d)). This implies that
+ * all of this is perfectly parallellizable: each thread can process an
+ * arbitrary subset of the update operations, allowing them to be
+ * efficiently combined later.
+ *
+ * Muhash does not support checking if an element is already part of the
+ * set. That is why this class does not enforce the use of a set as the
+ * data it represents because there is no efficient way to do so.
+ * It is possible to add elements more than once and also to remove
+ * elements that have not been added before. However, this implementation
+ * is intended to represent a set of elements.
+ *
+ * See also https://cseweb.ucsd.edu/~mihir/papers/inchash.pdf and
+ * https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-May/014337.html.
+ */
+class MuHash3072
+{
+private:
+    static constexpr size_t BYTE_SIZE = 384;
+
+    Num3072 m_numerator;
+    Num3072 m_denominator;
+
+    Num3072 ToNum3072(Span<const unsigned char> in);
+
+public:
+    /* The empty set. */
+    MuHash3072() noexcept {};
+
+    /* A singleton with variable sized data in it. */
+    explicit MuHash3072(Span<const unsigned char> in) noexcept;
+
+    /* Insert a single piece of data into the set. */
+    MuHash3072& Insert(Span<const unsigned char> in) noexcept;
+
+    /* Remove a single piece of data from the set. */
+    MuHash3072& Remove(Span<const unsigned char> in) noexcept;
+
+    /* Multiply (resulting in a hash for the union of the sets) */
+    MuHash3072& operator*=(const MuHash3072& mul) noexcept;
+
+    /* Divide (resulting in a hash for the difference of the sets) */
+    MuHash3072& operator/=(const MuHash3072& div) noexcept;
+
+    /* Finalize into a 32-byte hash. Does not change this object's value. */
+    void Finalize(uint256& out) noexcept;
+
+    SERIALIZE_METHODS(MuHash3072, obj)
+    {
+        READWRITE(obj.m_numerator);
+        READWRITE(obj.m_denominator);
+    }
+};
+
 #endif // BITCOIN_CRYPTO_MUHASH_H