# The Hash Functions Found in BSV

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#### Merkle–Damgård Construction

A fundamental challenge in building a hash function is that it must handle **inputs of arbitrary length** while producing a **fixed-length output**. This creates two issues:

* It’s hard to achieve **second preimage resistance** when input lengths vary.
* A **collision-resistant compression function** requires fixed-length inputs longer than its output.

The solution, discovered independently by **Ralph Merkle** and **Ivan Damgård** in 1979, is to use a **preprocessing step** that:

* **Pads the input** to a fixed length (e.g., 512 or 1024 bits).
* Appends the **length of the input itself** (*Merkle–Damgård strengthening*).

The padded input is then passed through a **compression function** that operates in rounds, linked with **chaining variables**. Finally, the output is concatenated and presented — usually in **hexadecimal format**.

👉 **Key takeaway:** The **Merkle–Damgård method** ensures that if the compression function is collision resistant, the entire hash function is too.

### MD4 and MD5

The first widely adopted Merkle–Damgård hash functions were **MD4** and **MD5**, created by **Ron Rivest** in the early 1990s:

* **MD4** was published in 1992 but quickly considered insecure.
* **MD5** followed shortly after and gained broad adoption, despite early warnings of weakness.
* In 1996, **Hans Dobbertin** demonstrated a collision in MD5’s compression function.
* By 2004, **Xiaoyun Wang and her team** developed an attack that could find collisions in about an hour, marking the end of MD5’s security.
* In 2008, the **CMU Software Engineering Institute** declared MD5 **“cryptographically broken and unsuitable for further use.”**

Even so, **MD4 and MD5 laid the foundation** for newer, stronger hash functions, including those used in Bitcoin.

#### BSV's Hash Functions

In the BSV blockchain, two main hash functions are used:

* **SHA-256**
* **RIPEMD-160**

They are often applied in **double-hash forms**:

* **HASH-256 = SHA-256(SHA-256)** → Used for blocks and transactions.
* **HASH-160 = RIPEMD-160(SHA-256)** → Used for address creation.

| **Hash Function**     | **Output Length** | **Description**                                                               | **Example Application in BSV**                 |
| --------------------- | ----------------- | ----------------------------------------------------------------------------- | ---------------------------------------------- |
| SHA-256               | 32 Bytes          | Generates unique 256-bit value from input string                              | 1. Proof-of-work algorithm 2. Address Creation |
| RIPEMD-160            | 20 Bytes          | Generates unique 160-bit value from an input string                           | Address creation                               |
| HASH-256              | 32 Bytes          | SHA256 hash of a SHA256 hash                                                  | 1. Blocks 2. Transactions                      |
| HASH-160              | 20 Bytes          | RIPEMD160 hash of a SHA256 hash                                               | Address creation                               |
| SHA512                | 64 Bytes          | Generates unique 512-bit value from input string                              | Wallet encryption (AES)                        |
| SHA256HMAC/SHA512HMAC | 32 Bytes          | HMAC Prevents length extension attacks and can be used with any hash function | Address Creation                               |

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