net.i2p.data

Class Hash

java.lang.Object

net.i2p.data.SimpleDataStructure

net.i2p.data.Hash

All Implemented Interfaces:

DataStructure

Direct Known Subclasses:

LocalHash, Peer

public class Hash
extends SimpleDataStructure

SHA-256 hash implementation for I2P data identification and integrity.

Hash provides cryptographic hash functionality throughout I2P:

• Algorithm: SHA-256 cryptographic hash function
• Fixed Size: Always 32 bytes (256 bits)
• Uniqueness: Extremely low collision probability
• Standardized: Consistent hash across all I2P components

Key Uses:

• Identification: Unique identifiers for destinations and routers
• Integrity: Verify data hasn't been modified or corrupted
• Indexing: Fast lookup keys in network database (NetDb)
• Routing: Keys for distributed hash table (DHT) operations
• Caching: Efficient storage and retrieval of frequently used hashes

Performance Features:

• LRU Caching: Least-recently-used cache with size limits
• Factory Methods: Static creation methods for cache efficiency
• Efficient Comparison: Optimized equals() and hashCode() methods
• Base32 Conversion: Cached .b32.i2p address generation
• Memory Management: Automatic cache size adjustment

Security Properties:

• Cryptographic Strength: SHA-256 provides 128-bit security against collisions
• Preimage Resistance: Computationally infeasible to reverse
• Second Preimage: Computationally infeasible to find similar inputs
• Collision Resistance: Practically impossible to find colliding inputs

Common Operations:

• Data Hashing: Calculate hash of any byte array or data structure
• Stream Hashing: Incremental hashing of large data streams
• File Verification: Verify file integrity using hash comparison
• Address Generation: Convert to Base32 for .b32.i2p addresses

Constants:

• HASH_LENGTH - Standard 32-byte hash size
• FAKE_HASH - All-zero hash for testing/placeholder use

Thread Safety:

• Immutable Data: Hash data cannot be modified after creation
• Thread-Safe Cache: Static factory methods are thread-safe
• Safe Sharing: Instances can be safely shared between threads

Best Practices:

• Factory Methods: Use static create() methods for cache efficiency
• Hash Verification: Always verify critical data hashes
• Constant Time: Use hash comparison for data integrity checks
• Memory Efficiency: Reuse hash instances when possible

Field Summary

Fields

Modifier and Type	Field and Description
static Hash	FAKE_HASH
static int	HASH_LENGTH

Fields inherited from class net.i2p.data.SimpleDataStructure

_data

Constructor Summary

Constructors

Constructor and Description
Hash()
Hash(byte[] data)

Method Summary

Modifier and Type	Method and Description
static void	clearCache() Clear the hash cache.
static Hash	create(byte[] data) Pull from cache or return new WARNING - If the SDS is found in the cache, the passed-in byte array will be returned to the SimpleByteCache for reuse.
static Hash	create(byte[] data, int off) Pull from cache or return new
static Hash	create(InputStream in) Pull from cache or return new
int	hashCode() a Hash is a hash, so just use the first 4 bytes for speed
int	length() The legal length of the byte array in this data structure
void	readBytes(InputStream in) Sets the data.
void	setData(byte[] data) Sets the data.
String	toBase32() For convenience.
String	toBase64() render the structure into modified base 64 notation

Methods inherited from class net.i2p.data.SimpleDataStructure

calculateHash, equals, fromBase64, fromByteArray, getData, read, toByteArray, toString, writeBytes

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

FAKE_HASH

public static final Hash FAKE_HASH

HASH_LENGTH

public static final int HASH_LENGTH

See Also:

Constant Field Values

Constructor Detail

Hash

public Hash()

Hash

public Hash(byte[] data)

Throws:

IllegalArgumentException - if data is not 32 bytes (null is ok)

Method Detail

clearCache

public static void clearCache()

Clear the hash cache.

create

public static Hash create(byte[] data)

Pull from cache or return new WARNING - If the SDS is found in the cache, the passed-in byte array will be returned to the SimpleByteCache for reuse. Do NOT save a reference to the passed-in data, or use or modify it, after this call. Ignore this warning and you WILL corrupt the cache or other data structures.

Throws:

IllegalArgumentException - if data is not the correct number of bytes

create

public static Hash create(byte[] data,
                          int off)

Pull from cache or return new

Throws:

ArrayIndexOutOfBoundsException - if not enough bytes

create

public static Hash create(InputStream in)
                   throws IOException

Pull from cache or return new

Throws:

IOException

hashCode

public int hashCode()

a Hash is a hash, so just use the first 4 bytes for speed

Overrides:

hashCode in class SimpleDataStructure

length

public int length()

Description copied from class: SimpleDataStructure

The legal length of the byte array in this data structure

Specified by:

length in class SimpleDataStructure

readBytes

public void readBytes(InputStream in)
               throws DataFormatException,
                      IOException

Description copied from class: SimpleDataStructure

Sets the data.

Specified by:

readBytes in interface DataStructure

Overrides:

readBytes in class SimpleDataStructure

Parameters:

in - the stream to read

Throws:

DataFormatException - if the data is improperly formatted

IOException - if there was a problem reading the stream

setData

public void setData(byte[] data)

Description copied from class: SimpleDataStructure

Sets the data.

Overrides:

setData in class SimpleDataStructure

Parameters:

data - of correct length, or null

Throws:

IllegalArgumentException - if data is not 32 bytes (null is ok)

toBase32

public String toBase32()

For convenience.

Returns:

"{52 chars}.b32.i2p" or null if data not set.

toBase64

public String toBase64()

Description copied from interface: DataStructure

render the structure into modified base 64 notation

Specified by:

toBase64 in interface DataStructure

Overrides:

toBase64 in class SimpleDataStructure

Returns:

null on error