github.com/letsencrypt/trillian@v1.1.2-0.20180615153820-ae375a99d36a/merkle/map_verifier.go (about)

     1  // Copyright 2016 Google Inc. All Rights Reserved.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package merkle
    16  
    17  import (
    18  	"bytes"
    19  	"fmt"
    20  
    21  	"github.com/google/trillian/merkle/hashers"
    22  	"github.com/google/trillian/storage"
    23  )
    24  
    25  // VerifyMapInclusionProof verifies that the passed in expectedRoot can be
    26  // reconstructed correctly given the other parameters.
    27  //
    28  // The process is essentially the same as the inclusion proof checking for
    29  // append-only logs, but adds support for nil/"default" proof nodes.
    30  //
    31  // Returns nil on a successful verification, and an error otherwise.
    32  func VerifyMapInclusionProof(treeID int64, index, leaf, expectedRoot []byte, proof [][]byte, h hashers.MapHasher) error {
    33  	if got, want := len(index)*8, h.BitLen(); got != want {
    34  		return fmt.Errorf("index len: %d, want %d", got, want)
    35  	}
    36  	if got, want := len(proof), h.BitLen(); got != want {
    37  		return fmt.Errorf("proof len: %d, want %d", got, want)
    38  	}
    39  	for i, element := range proof {
    40  		if got, wanta, wantb := len(element), 0, h.Size(); got != wanta && got != wantb {
    41  			return fmt.Errorf("proof[%d] len: %d, want %d or %d", i, got, wanta, wantb)
    42  		}
    43  	}
    44  
    45  	var runningHash []byte
    46  	if len(leaf) != 0 {
    47  		leafHash, err := h.HashLeaf(treeID, index, leaf)
    48  		if err != nil {
    49  			return fmt.Errorf("HashLeaf(): %v", err)
    50  		}
    51  		runningHash = leafHash
    52  	}
    53  
    54  	nID := storage.NewNodeIDFromHash(index)
    55  	for height, sib := range nID.Siblings() {
    56  		pElement := proof[height]
    57  
    58  		// Since empty values are tied to a location and a level,
    59  		// HashEmpty(leve1) != HashChildren(E0, E0).
    60  		// Therefore we need to maintain an empty marker along the
    61  		// proof path until the first non-empty element so we can call
    62  		// HashEmpty once at the top of the empty branch.
    63  		if len(runningHash) == 0 && len(pElement) == 0 {
    64  			continue
    65  		}
    66  		// When we reach a level that has a neighbor, we compute the empty value
    67  		// for the branch that we are on before combining it with the neighbor.
    68  		if len(runningHash) == 0 && len(pElement) != 0 {
    69  			depth := nID.PrefixLenBits - height
    70  			emptyBranch := nID.Copy().MaskLeft(depth)
    71  			runningHash = h.HashEmpty(treeID, emptyBranch.Path, height)
    72  		}
    73  
    74  		if len(runningHash) != 0 && len(pElement) == 0 {
    75  			pElement = h.HashEmpty(treeID, sib.Path, height)
    76  		}
    77  		proofIsRightHandElement := nID.Bit(height) == 0
    78  		if proofIsRightHandElement {
    79  			runningHash = h.HashChildren(runningHash, pElement)
    80  		} else {
    81  			runningHash = h.HashChildren(pElement, runningHash)
    82  		}
    83  	}
    84  	if len(runningHash) == 0 {
    85  		depth := 0
    86  		emptyBranch := nID.Copy().MaskLeft(depth)
    87  		runningHash = h.HashEmpty(treeID, emptyBranch.Path, h.BitLen())
    88  	}
    89  
    90  	if got, want := runningHash, expectedRoot; !bytes.Equal(got, want) {
    91  		return fmt.Errorf("calculated root: %x, want \n%x", got, want)
    92  	}
    93  	return nil
    94  }