# double SHA-256

> cryptographic hash function created by composing SHA-256 with itself

**Wikidata**: [Q124624061](https://www.wikidata.org/wiki/Q124624061)  
**Source**: https://4ort.xyz/entity/double-sha-256

## Summary
Double SHA-256 is a cryptographic hash function created by applying the SHA-256 algorithm twice in succession (composing SHA-256 with itself). This enhanced version provides greater security and collision resistance compared to the standard SHA-256 algorithm.

## Key Facts
- Double SHA-256 is an instance of cryptographic hash function and iterated function
- It is based on SHA-256 (Q110651361)
- Double SHA-256 is used in Bitcoin mining
- Aliases include double SHA256, SHA-256², SHA256², SHA-256^2, SHA256^2, SHA-256d, SHA256d
- It is created by composing SHA-256 with itself

## FAQs
### Q: What is the difference between SHA-256 and double SHA-256?
A: Double SHA-256 is created by applying the SHA-256 algorithm twice in succession, whereas standard SHA-256 applies the algorithm only once. This double application provides enhanced security properties.

### Q: Why is double SHA-256 used in Bitcoin mining?
A: Double SHA-256 is used in Bitcoin mining because it provides greater collision resistance and security, making it more computationally expensive to find valid hashes and thus increasing the security of the Bitcoin network.

### Q: Is double SHA-256 more secure than SHA-256?
A: Yes, double SHA-256 offers increased security compared to standard SHA-256 due to the additional layer of hashing, though in practice, SHA-256 alone is already considered cryptographically secure for most purposes.

## Why It Matters
Double SHA-256 represents an important evolution in cryptographic hash functions, particularly for blockchain applications like Bitcoin. By applying SHA-256 twice, it creates a more computationally intensive hash that helps secure networks against potential attacks that might compromise single-hash systems. This additional security layer has become essential for cryptocurrencies that rely on proof-of-work mechanisms to validate transactions and secure the network against malicious actors. The use of double SHA-256 demonstrates how cryptographic algorithms can be adapted and strengthened as computational power increases and new vulnerabilities are discovered.

## Notable For
- Being the specific hash function used in Bitcoin mining operations
- Creating a hash that is twice as computationally expensive to produce as standard SHA-256
- Enhancing collision resistance while maintaining the security properties of SHA-256
- Providing a higher security threshold for blockchain consensus mechanisms

## Body
### Definition
Double SHA-256 is a cryptographic hash function created by composing SHA-256 with itself. This means that the input data is first hashed using SHA-256, and then the resulting hash is hashed again using SHA-256 to produce the final output.

### Technical Properties
- Double SHA-256 is classified as both a cryptographic hash function and an iterated function
- It is based on the standard SHA-256 algorithm
- The function notation can be expressed as SHA-256(SHA-256(input))
- This composition provides enhanced security properties compared to standard SHA-256 alone

### Applications
- Primarily used in Bitcoin mining operations
- Serves as the hash function for Bitcoin's proof-of-work mechanism
- Provides additional security for blockchain systems by increasing the computational difficulty of finding valid hashes

### Naming Conventions
- Also known as double SHA256
- Common mathematical notations include SHA-256², SHA256², SHA-256^2, SHA256^2, SHA-256d, SHA256d

### Relationship to SHA-256
- Inherits all security properties of SHA-256
- Provides an additional layer of hashing that makes certain types of attacks more computationally expensive
- Represents a simple yet effective method of increasing hash security through function composition