RLP Encoding (Recursive Length Prefix)

RLP Encoding (Recursive Length Prefix) is a deterministic, space-efficient serialization scheme designed for Ethereum to encode structured data like transactions, account states, and block headers into a byte array. Its core function is to provide a canonical representation of complex, nested data structures—such as lists of lists—ensuring that any valid structure has one and only one RLP encoding. This property is critical for generating consistent cryptographic hashes, which underpin the blockchain's integrity. Unlike formats like JSON or Protocol Buffers, RLP does not encode specific data types (e.g., strings, integers); it only distinguishes between a byte string (a sequence of bytes) and a list (an ordered collection of RLP-encoded items).

The encoding process applies two simple rules based on the input data. For a single byte string, if it is a single byte with a value in the [0x00, 0x7f] range, it is encoded as itself. Otherwise, a length prefix is added. For lists, the encodings of all its items are concatenated, and a prefix is added to indicate the total length of that concatenation. This prefix is the key to the 'recursive' nature: the length of a list is calculated from the recursively encoded lengths of its contents. The result is a flat, unambiguous byte sequence that can be perfectly reconstructed back into the original nested structure.

RLP's design is integral to Ethereum's Merkle Patricia Trie, where the hashed RLP encoding of nodes forms the cryptographic backbone of the state and storage. Its simplicity avoids complexity-induced bugs and ensures minimal overhead. However, a significant limitation is the lack of native support for common types; integers must be represented as big-endian byte strings without leading zeros, and decoding requires prior knowledge of the expected data schema. This is why higher-level specifications, like Ethereum's execution payloads, define the exact structure of the RLP-encoded lists.

RLP (Recursive Length Prefix) encoding is a deterministic, space-efficient serialization format designed to encode arbitrarily nested arrays of binary data. It is the foundational method for encoding all account states, transactions, and blocks in Ethereum. Unlike formats like JSON or Protocol Buffers, RLP does not encode data types (e.g., strings, integers) directly; it encodes byte arrays and lists, leaving the interpretation of the data to the higher-level protocol. This simplicity and determinism are critical for generating consistent cryptographic hashes across all network nodes.

The encoding process follows a simple set of rules based on the length of the input. For a single byte with a value in the range [0x00, 0x7f], it is encoded as itself. For other byte strings, a length prefix is added, which indicates the length of the following bytes. Lists, which are recursive structures containing other byte strings and lists, are encoded by concatenating the RLP encodings of their items and then prefixing the total length. This recursive nature allows for the construction of complex, nested data trees, which is essential for representing Merkle Patricia Tries.

A key property of RLP is its canonical encoding, meaning there is exactly one valid RLP encoding for any given data structure. This is non-negotiable for consensus; if two nodes serialize the same data differently, they will compute different hashes, leading to a chain fork. For example, the integer 1024 (0x0400 as bytes) is not encoded as a number but as a byte string, resulting in the encoding 0x820400 (where 0x82 is the prefix for a 2-byte string). This eliminates ambiguity from integer size or endianness.

RLP's design directly supports Ethereum's core data structures. The state, stored as a Merkle Patricia Trie, is entirely built from RLP-encoded nodes. Similarly, every field of a transaction—nonce, gas price, recipient, value, data, and signatures—is serialized into an RLP-encoded list before being signed and broadcast. The resulting RLP-encoded transaction is then hashed to produce its unique transaction ID (txid), ensuring data integrity throughout its lifecycle on the network.

While RLP is elegant for its purpose, developers typically interact with it through client libraries (like Web3.js or Ethers.js) rather than manually. Understanding RLP is crucial for debugging low-level data, implementing light clients, or writing protocol-level code. Its successor in Ethereum 2.0's beacon chain, Simple Serialize (SSZ), was designed for efficiency in proof systems but RLP remains the bedrock of Ethereum's execution layer serialization.

RLP (Recursive Length Prefix) is a space-efficient serialization method used primarily in Ethereum to encode arbitrarily nested arrays of binary data. It transforms complex data structures—like transactions, state, and blocks—into a single, deterministic byte sequence. The core principle is to prefix data with a length indicator, allowing the original structure to be perfectly reconstructed without ambiguity. This deterministic encoding is crucial for generating consistent cryptographic hashes, such as transaction IDs and block hashes.

The encoding process follows strict rules based on the data's type and size. For a single byte with a value between 0x00 and 0x7f, RLP encodes it as itself. For short strings (0-55 bytes), it adds a single-byte prefix of 0x80 plus the string's length. Longer strings and lists (nested arrays) receive a more complex prefix that indicates both the length of the length and the payload itself. This recursive nature allows a list to contain other lists, forming a tree-like structure.

Consider encoding the list [ "cat", "dog" ]. First, the string "cat" (bytes [0x63, 0x61, 0x74]) is encoded as [0x83, 0x63, 0x61, 0x74] where 0x83 is the prefix for a 3-byte string. Similarly, "dog" becomes [0x83, 0x64, 0x6f, 0x67]. The outer list contains two items with a total payload of 8 bytes. Since this is a short list (total payload ≤ 55 bytes), it receives the prefix 0xc0 + length (8), resulting in the final RLP output: [0xc8, 0x83, 0x63, 0x61, 0x74, 0x83, 0x64, 0x6f, 0x67].

This encoding is foundational for Ethereum's Merkle Patricia Trie, where all state data is stored as RLP-encoded nodes. Its properties—determinism, simplicity, and efficiency—make it ideal for consensus-critical applications. Unlike formats like JSON or Protocol Buffers, RLP does not define specific data types (integers, floats); it deals only with bytes and lists, pushing the interpretation of the data (e.g., big-endian integers) to the protocol layer, which keeps the serialization layer minimal and robust.