flesh out some docstrings/comments
This commit is contained in:
lukechampine
parent
72d3c39081
commit
6ff1174970
154
blake3.go
154
blake3.go
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@ -1,7 +1,4 @@
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// Package blake3 implements the BLAKE3 cryptographic hash function.
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//
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// This is a direct port of the Rust reference implementation. It is not
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// optimized for performance.
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package blake3
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import (
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@ -34,6 +31,21 @@ var iv = [8]uint32{
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0x510E527F, 0x9B05688C, 0x1F83D9AB, 0x5BE0CD19,
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}
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// helper functions for converting between bytes and BLAKE3 "words"
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func bytesToWords(bytes []byte, words []uint32) {
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for i := range words {
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words[i] = binary.LittleEndian.Uint32(bytes[i*4:])
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}
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}
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func wordsToBytes(words []uint32, bytes []byte) {
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for i, w := range words {
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binary.LittleEndian.PutUint32(bytes[i*4:], w)
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}
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}
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// The g function, split into two parts so that the compiler will inline it.
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func gx(state *[16]uint32, a, b, c, d int, mx uint32) {
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state[a] += state[b] + mx
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state[d] = bits.RotateLeft32(state[d]^state[a], -16)
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@ -79,17 +91,29 @@ func permute(m *[16]uint32) {
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}
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}
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// Each chunk or parent node can produce either an 8-word chaining value or, by
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// setting flagRoot, any number of final output bytes. The node struct
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// captures the state just prior to choosing between those two possibilities.
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// A node represents a chunk or parent in the BLAKE3 Merkle tree. In BLAKE3
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// terminology, the elements of the bottom layer (aka "leaves") of the tree are
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// called chunk nodes, and the elements of upper layers (aka "interior nodes")
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// are called parent nodes.
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//
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// Computing a BLAKE3 hash involves splitting the input into chunk nodes, then
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// repeatedly merging these nodes into parent nodes, until only a single "root"
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// node remains. The root node can then be used to generate up to 2^64 - 1 bytes
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// of pseudorandom output.
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type node struct {
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// the chaining value from the previous state
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cv [8]uint32
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// the current state
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block [16]uint32
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counter uint64
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blockLen uint32
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flags uint32
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}
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// compress is the core hash function, generating 16 pseudorandom words from a
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// node. When nodes are being merged into parents, only the first 8 words are
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// used. When the root node is being used to generate output, the full 16 words
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// are used.
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func (n node) compress() [16]uint32 {
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state := [16]uint32{
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n.cv[0], n.cv[1], n.cv[2], n.cv[3],
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@ -119,25 +143,19 @@ func (n node) compress() [16]uint32 {
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return state
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}
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// chainingValue returns the first 8 words of the compressed node. This is used
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// in two places. First, when a chunk node is being constructed, its cv is
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// overwritten with this value after each block of input is processed. Second,
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// when two nodes are merged into a parent, each of their chaining values
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// supplies half of the new node's block. Second, when
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func (n node) chainingValue() (cv [8]uint32) {
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full := n.compress()
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copy(cv[:], full[:8])
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return
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}
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func bytesToWords(bytes []byte, words []uint32) {
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for i := range words {
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words[i] = binary.LittleEndian.Uint32(bytes[i*4:])
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}
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}
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func wordsToBytes(words []uint32, bytes []byte) {
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for i, w := range words {
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binary.LittleEndian.PutUint32(bytes[i*4:], w)
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}
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}
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// An OutputReader produces an seekable stream of 2^64 - 1 output bytes.
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// An OutputReader produces an seekable stream of 2^64 - 1 pseudorandom output
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// bytes.
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type OutputReader struct {
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n node
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block [blockSize]byte
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@ -201,6 +219,7 @@ func (or *OutputReader) Seek(offset int64, whence int) (int64, error) {
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return int64(or.off), nil
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}
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// chunkState manages the state involved in hashing a single chunk of input.
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type chunkState struct {
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n node
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block [blockSize]byte
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@ -208,24 +227,30 @@ type chunkState struct {
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bytesConsumed int
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}
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// chunkCounter is the index of this chunk, i.e. the number of chunks that have
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// been processed prior to this one.
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func (cs *chunkState) chunkCounter() uint64 {
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return cs.n.counter
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}
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// update incorporates input into the chunkState.
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func (cs *chunkState) update(input []byte) {
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for len(input) > 0 {
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// If the block buffer is full, compress it and clear it. More
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// input is coming, so this compression is not flagChunkEnd.
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if cs.blockLen == blockSize {
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// copy the chunk block (bytes) into the node block and chain it.
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bytesToWords(cs.block[:], cs.n.block[:])
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cs.n.cv = cs.n.chainingValue()
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// clear the start flag for all but the first block
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cs.n.flags &^= flagChunkStart
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// reset the chunk block. It must contain zeros, because BLAKE3
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// blocks are zero-padded.
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cs.block = [blockSize]byte{}
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cs.blockLen = 0
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// After the first chunk has been compressed, clear the start flag.
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cs.n.flags &^= flagChunkStart
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}
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// Copy input bytes into the block buffer.
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// Copy input bytes into the chunk block.
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n := copy(cs.block[cs.blockLen:], input)
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cs.blockLen += n
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cs.bytesConsumed += n
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@ -233,6 +258,8 @@ func (cs *chunkState) update(input []byte) {
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}
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}
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// node returns a node containing the chunkState's current state, with the
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// ChunkEnd flag set.
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func (cs *chunkState) node() node {
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n := cs.n
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bytesToWords(cs.block[:], n.block[:])
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@ -241,18 +268,20 @@ func (cs *chunkState) node() node {
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return n
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}
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func newChunkState(key [8]uint32, chunkCounter uint64, flags uint32) chunkState {
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func newChunkState(iv [8]uint32, chunkCounter uint64, flags uint32) chunkState {
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return chunkState{
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n: node{
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cv: key,
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cv: iv,
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counter: chunkCounter,
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blockLen: blockSize,
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// compress the first chunk with the start flag set
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// compress the first block with the start flag set
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flags: flags | flagChunkStart,
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},
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}
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}
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// parentNode returns a node that incorporates the chaining values of two child
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// nodes.
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func parentNode(left, right [8]uint32, key [8]uint32, flags uint32) node {
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var blockWords [16]uint32
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copy(blockWords[:8], left[:])
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@ -260,8 +289,8 @@ func parentNode(left, right [8]uint32, key [8]uint32, flags uint32) node {
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return node{
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cv: key,
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block: blockWords,
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counter: 0, // Always 0 for parent nodes.
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blockLen: blockSize, // Always blockSize (64) for parent nodes.
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counter: 0, // counter is reset for parents
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blockLen: blockSize, // block is full: 8 words from left, 8 from right
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flags: flags | flagParent,
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}
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}
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@ -271,7 +300,7 @@ type Hasher struct {
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cs chunkState
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key [8]uint32
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chainStack [54][8]uint32 // space for 54 subtrees (2^54 * chunkSize = 2^64)
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stackSize int // index within chainStack
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stackSize int // number of chainStack elements that are valid
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flags uint32
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size int // output size, for Sum
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}
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@ -296,14 +325,15 @@ func New(size int, key []byte) *Hasher {
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return newHasher(keyWords, flagKeyedHash, size)
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}
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// addChunkChainingValue appends a chunk to the right edge of the Merkle tree.
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func (h *Hasher) addChunkChainingValue(cv [8]uint32, totalChunks uint64) {
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// This chunk might complete some subtrees. For each completed subtree,
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// its left child will be the current top entry in the CV stack, and
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// its right child will be the current value of `cv`. Pop each left
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// child off the stack, merge it with `cv`, and overwrite `cv`
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// with the result. After all these merges, push the final value of
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// `cv` onto the stack. The number of completed subtrees is given
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// by the number of trailing 0-bits in the new total number of chunks.
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// This chunk might complete some subtrees. For each completed subtree, its
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// left child will be the current top entry in the CV stack, and its right
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// child will be the current value of cv. Pop each left child off the stack,
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// merge it with cv, and overwrite cv with the result. After all these
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// merges, push the final value of cv onto the stack. The number of
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// completed subtrees is given by the number of trailing 0-bits in the new
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// total number of chunks.
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for totalChunks&1 == 0 {
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// pop and merge
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h.stackSize--
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@ -314,7 +344,9 @@ func (h *Hasher) addChunkChainingValue(cv [8]uint32, totalChunks uint64) {
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h.stackSize++
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}
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func (h *Hasher) finalNode() node {
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// rootNode computes the root of the Merkle tree. It does not modify the
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// chainStack.
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func (h *Hasher) rootNode() node {
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// Starting with the node from the current chunk, compute all the
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// parent chaining values along the right edge of the tree, until we
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// have the root node.
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@ -342,8 +374,9 @@ func (h *Hasher) Size() int { return h.size }
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func (h *Hasher) Write(p []byte) (int, error) {
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lenp := len(p)
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for len(p) > 0 {
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// If the current chunk is complete, finalize it and reset the
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// chunk state. More input is coming, so this chunk is not flagRoot.
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// If the current chunk is complete, finalize it and add it to the tree,
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// then reset the chunk state (but keep incrementing the counter across
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// chunks).
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if h.cs.bytesConsumed == chunkSize {
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cv := h.cs.node().chainingValue()
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totalChunks := h.cs.chunkCounter() + 1
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@ -363,16 +396,24 @@ func (h *Hasher) Write(p []byte) (int, error) {
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}
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// Sum implements hash.Hash.
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func (h *Hasher) Sum(b []byte) []byte {
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ret, fill := sliceForAppend(b, h.Size())
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h.XOF().Read(fill)
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return ret
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func (h *Hasher) Sum(b []byte) (sum []byte) {
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// We need to append h.Size() bytes to b. Reuse b's capacity if possible;
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// otherwise, allocate a new slice.
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if total := len(b) + h.Size(); cap(b) >= total {
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sum = b[:total]
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} else {
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sum = make([]byte, total)
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copy(sum, b)
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}
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// Read into the appended portion of sum
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h.XOF().Read(sum[len(b):])
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return
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}
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// XOF returns an OutputReader initialized with the current hash state.
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func (h *Hasher) XOF() *OutputReader {
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return &OutputReader{
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n: h.finalNode(),
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n: h.rootNode(),
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}
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}
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@ -392,7 +433,17 @@ func Sum512(b []byte) (out [64]byte) {
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return
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}
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// DeriveKey derives a subkey from ctx and srcKey.
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// DeriveKey derives a subkey from ctx and srcKey. ctx should be hardcoded,
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// globally unique, and application-specific. A good format for ctx strings is:
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//
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// [application] [commit timestamp] [purpose]
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//
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// e.g.:
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//
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// example.com 2019-12-25 16:18:03 session tokens v1
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//
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// The purpose of these requirements is to ensure that an attacker cannot trick
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// two different applications into using the same context string.
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func DeriveKey(subKey []byte, ctx string, srcKey []byte) {
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// construct the derivation Hasher
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const derivationIVLen = 32
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@ -400,22 +451,11 @@ func DeriveKey(subKey []byte, ctx string, srcKey []byte) {
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h.Write([]byte(ctx))
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var derivationIV [8]uint32
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bytesToWords(h.Sum(make([]byte, 0, derivationIVLen)), derivationIV[:])
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h = newHasher(derivationIV, flagDeriveKeyMaterial, len(subKey))
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h = newHasher(derivationIV, flagDeriveKeyMaterial, 0)
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// derive the subKey
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h.Write(srcKey)
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h.Sum(subKey[:0])
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h.XOF().Read(subKey)
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}
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// ensure that Hasher implements hash.Hash
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var _ hash.Hash = (*Hasher)(nil)
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func sliceForAppend(in []byte, n int) (head, tail []byte) {
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if total := len(in) + n; cap(in) >= total {
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head = in[:total]
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} else {
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head = make([]byte, total)
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copy(head, in)
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}
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tail = head[len(in):]
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return
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}
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