# Hash Functions: Creating Cryptographic Fingerprints

Hash functions are **one-way mathematical operations** that take input data of any size and produce a fixed-size output called a **hash** or **digest**. Even changing a single bit in the input produces a completely different hash output, making hash functions ideal for verifying data integrity.

Bitcoin Script includes **five primary hash functions**:

**Established Hash Functions:**

* **OP\_RIPEMD160** — 20-byte output (160 bits)
* **OP\_SHA1** — 20-byte output (160 bits)

#### Single Hash Operations

Each hash opcode **consumes the top stack item** and **replaces it with the hash** of that data.

| Opcode        | Hex  | Input | Output | Output Size | Description                             |
| ------------- | ---- | ----- | ------ | ----------- | --------------------------------------- |
| OP\_RIPEMD160 | 0xa6 | data  | hash   | 20 bytes    | Hashes input using RIPEMD-160 algorithm |
| OP\_SHA1      | 0xa7 | data  | hash   | 20 bytes    | Hashes input using SHA-1 algorithm      |

**Security Note:** While OP\_SHA1 remains available for backward compatibility, **it should not be used in new applications** due to known cryptographic vulnerabilities.


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