What exactly is the info_Hash in a torrent file

hash torrent info-hash

59,433

Solution 1

So I finally figured it out.

The “infohash” is the SHA1 Hash over the part of a torrent file that includes:

ITEM: length(size) and path (path with filename)
Name: The name to search for
Piece length: The length(size) of a single piece
Pieces: SHA1 Hash of EVERY piece of this torrent
Private: flag for restricted access

To show this a little more I took a random torrent file and used the “BEncode Editor” from Ultima to make it more clearly to me.

BEncode Editor

As you can see the the red box marked the information part of the torrent file. The torrent file includes not the Hash of the items, but the hashes of every piece.

For item1 with: 1069496548
and item2 with: 223
It is together: 1069496771
With a piece size of: 524288
There are 2040 pieces. (1069496771/524288=2039.9032 approximately)
The pieces section includes 40800 byte of data what are 81600 + 2 chars in the file.
the +2 because 0x marks that this is hexadecimal.
A SHA1 hash has 40 0x chars or 20 Byte of data what are 2040 SHA1 hashes.

enter image description here

I am sorry that this information is about a torrent that leads to a illegal movie, but i wanted to use a torrent that realy exists.

Solution 2

I wanted to add another example, slightly more concrete.

We start with one of the smallest .torrent files i have:

64 34 3A 69 6E 66 6F 64 35 3A 66 69 6C 65 73 6C 64 36 3A 6C 65 6E 67 74 
68 69 36 31 35 65 34 3A 70 61 74 68 6C 32 36 3A 66 72 65 65 20 61 75 64 
69 6F 62 6F 6F 6B 20 76 65 72 73 69 6F 6E 2E 74 78 74 65 65 64 36 3A 6C 
65 6E 67 74 68 69 33 39 33 34 31 37 65 34 3A 70 61 74 68 6C 36 31 3A 57 
61 72 63 72 61 66 74 5F 20 4F 66 66 69 63 69 61 6C 20 4D 6F 76 69 65 20 
4E 6F 76 65 6C 69 7A 61 74 69 6F 6E 20 62 79 20 43 68 72 69 73 74 69 65 
20 47 6F 6C 64 65 6E 2E 65 70 75 62 65 65 65 34 3A 6E 61 6D 65 36 31 3A 
57 61 72 63 72 61 66 74 5F 20 4F 66 66 69 63 69 61 6C 20 4D 6F 76 69 65 
20 4E 6F 76 65 6C 69 7A 61 74 69 6F 6E 20 62 79 20 43 68 72 69 73 74 69 
65 20 47 6F 6C 64 65 6E 20 45 50 55 42 31 32 3A 70 69 65 63 65 20 6C 65 
6E 67 74 68 69 31 30 34 38 35 37 36 65 36 3A 70 69 65 63 65 73 32 30 3A 
43 92 4C 22 BB 42 9E EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9 65 65

If we decode the BEncoding:

64                                             ; DICTIONARY (d)
|  34 3A 69 6E 66 6F                           ;  - 4:info
|  64                                          ;  - DICTIONARY (d)
|  |  35 3A 66 69 6C 65 73                     ;     - 5:files
|  |  6C                                       ;     - LIST (l)
|  |  |  64                                    ;        - DICTIONARY (d)
|  |  |  |  36 3A 6C 65 6E 67 74 68            ;           - 6:length
|  |  |  |  69 36 31 35 65                     ;           - i615e
|  |  |  |  34 3A 70 61 74 68                  ;           - 4:path
|  |  |  |  6C                                 ;           - LIST (l)
|  |  |  |  |  32 36 3A 66 72 65 65 20 61 75   ;              - 26:free audiobook verison.txt
|  |  |  |  |  64 69 6F 62 6F 6F 6B 20 76 65   ;                  
|  |  |  |  |  72 73 69 6F 6E 2E 74 78 74      ;                  
|  |  |  |  65                                 ;              - END (e)
|  |  |  65                                    ;           - END (e)
|  |  |  64                                    ;           - DICTIONARY (d)
|  |  |  |  36 3A 6C 65 6E 67 74 68            ;              - 6:length
|  |  |  |  69 33 39 33 34 31 37 65            ;              - i393417e
|  |  |  |  34 3A 70 61 74 68                  ;              - 4:path
|  |  |  |  6C                                 ;              - LIST (l)
|  |  |  |  |  36 31 3A 57 61 72 63 72 61 66   ;                 - 61:Warcraft_ Official Movie Novelization by Christie Golden.epub
|  |  |  |  |  74 5F 20 4F 66 66 69 63 69 61   ;                  
|  |  |  |  |  6C 20 4D 6F 76 69 65 20 4E 6F   ;                  
|  |  |  |  |  76 65 6C 69 7A 61 74 69 6F 6E   ;                  
|  |  |  |  |  20 62 79 20 43 68 72 69 73 74   ;                  
|  |  |  |  |  69 65 20 47 6F 6C 64 65 6E 2E   ;                  
|  |  |  |  |  65 70 75 62                     ;                  
|  |  |  |  65                                 ;              - END (e)
|  |  |  65                                    ;           - END (e)
|  |  65                                       ;        - END (e)
|  |  34 3A 6E 61 6D 65                        ;        - 4:name
|  |  36 31 3A 57 61 72 63 72 61 66            ;        - 61:Warcraft_ Official Movie Novelization by Christie Golden EPUB 
|  |  74 5F 20 4F 66 66 69 63 69 61            ;         
|  |  6C 20 4D 6F 76 69 65 20 4E 6F            ;         
|  |  76 65 6C 69 7A 61 74 69 6F 6E            ;         
|  |  20 62 79 20 43 68 72 69 73 74            ;         
|  |  69 65 20 47 6F 6C 64 65 6E 20            ;         
|  |  45 50 55 42                              ;          
|  |  31 32 3A 70 69 65 63 65 20 6C            ;        - 12:piece length
|  |  65 6E 67 74 68                           ;         
|  |  69 31 30 34 38 35 37 36 65               ;        - i10485765e
|  |  36 3A 70 69 65 63 65 73                  ;        - 6:pieces
|  |  32 30 3A 43 92 4C 22 BB 42 9E            ;        - 20:43 92 4C 22 BB 42 9E EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9 65 65
|  |  EA BD FF 66 C6 79 4C 29 E4 F9            ;
|  |  D0 F3 B9                                 ;
|  65                                          ;     - END (e)
65                                             ;  - END (e)

Or, in pseudo-json:

{
   info: {
      files: [
         { length: 615,    path: ["free audiobook verison.txt"] },
         { length: 393417, path: ["Warcraft_ Official Movie Novelization by Christie Golden.epub"] }
      ],
      name:           "Warcraft_ Official Movie Novelization by Christie Golden EPUB",
      "piece length": 10485765,
      pieces:         43 92 4C 22 BB 42 9E EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9 65 65
   }
}

InfoHash is hash of the info

The InfoHash is the SHA-1 hash of the info dictionary contents.

We want to take the SHA-1 hash of value of the info dictionary key:

64                                          ;  - DICTIONARY (d)
|  35 3A 66 69 6C 65 73                     ;     - 5:files
|  6C                                       ;     - LIST (l)
|  |  64                                    ;        - DICTIONARY (d)
|  |  |  36 3A 6C 65 6E 67 74 68            ;           - 6:length
|  |  |  69 36 31 35 65                     ;           - i615e
|  |  |  34 3A 70 61 74 68                  ;           - 4:path
|  |  |  6C                                 ;           - LIST (l)
|  |  |  |  32 36 3A 66 72 65 65 20 61 75   ;              - 26:free audiobook verison.txt
|  |  |  |  64 69 6F 62 6F 6F 6B 20 76 65   ;                  
|  |  |  |  72 73 69 6F 6E 2E 74 78 74      ;                  
|  |  |  65                                 ;              - END (e)
|  |  65                                    ;           - END (e)
|  |  64                                    ;           - DICTIONARY (d)
|  |  |  36 3A 6C 65 6E 67 74 68            ;              - 6:length
|  |  |  69 33 39 33 34 31 37 65            ;              - i393417e
|  |  |  34 3A 70 61 74 68                  ;              - 4:path
|  |  |  6C                                 ;              - LIST (l)
|  |  |  |  36 31 3A 57 61 72 63 72 61 66   ;                 - 61:Warcraft_ Official Movie Novelization by Christie Golden.epub
|  |  |  |  74 5F 20 4F 66 66 69 63 69 61   ;                  
|  |  |  |  6C 20 4D 6F 76 69 65 20 4E 6F   ;                  
|  |  |  |  76 65 6C 69 7A 61 74 69 6F 6E   ;                  
|  |  |  |  20 62 79 20 43 68 72 69 73 74   ;                  
|  |  |  |  69 65 20 47 6F 6C 64 65 6E 2E   ;                  
|  |  |  |  65 70 75 62                     ;                  
|  |  |  65                                 ;              - END (e)
|  |  65                                    ;           - END (e)
|  65                                       ;        - END (e)
|  34 3A 6E 61 6D 65                        ;        - 4:name
|  36 31 3A 57 61 72 63 72 61 66            ;        - 61:Warcraft_ Official Movie Novelization by Christie Golden EPUB 
|  74 5F 20 4F 66 66 69 63 69 61            ;         
|  6C 20 4D 6F 76 69 65 20 4E 6F            ;         
|  76 65 6C 69 7A 61 74 69 6F 6E            ;         
|  20 62 79 20 43 68 72 69 73 74            ;         
|  69 65 20 47 6F 6C 64 65 6E 20            ;         
|  45 50 55 42                              ;          
|  31 32 3A 70 69 65 63 65 20 6C            ;        - 12:piece length
|  65 6E 67 74 68                           ;         
|  69 31 30 34 38 35 37 36 65               ;        - i10485765e
|  36 3A 70 69 65 63 65 73                  ;        - 6:pieces
|  32 30 3A 43 92 4C 22 BB 42 9E            ;        - 20:43 92 4C 22 BB 42 9E EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9 65 65
|  EA BD FF 66 C6 79 4C 29 E4 F9            ;
|  D0 F3 B9                                 ;
65                                          ;     - END (e)

We run all these bytes together:

64 35 3A 66 69 6C 65 73 6C 64 36 3A 6C 65 6E 67 74 68 69 36 31 35 65 34 
3A 70 61 74 68 6C 32 36 3A 66 72 65 65 20 61 75 64 69 6F 62 6F 6F 6B 20 
76 65 72 73 69 6F 6E 2E 74 78 74 65 65 64 36 3A 6C 65 6E 67 74 68 69 33 
39 33 34 31 37 65 34 3A 70 61 74 68 6C 36 31 3A 57 61 72 63 72 61 66 74 
5F 20 4F 66 66 69 63 69 61 6C 20 4D 6F 76 69 65 20 4E 6F 76 65 6C 69 7A 
61 74 69 6F 6E 20 62 79 20 43 68 72 69 73 74 69 65 20 47 6F 6C 64 65 6E 
2E 65 70 75 62 65 65 65 34 3A 6E 61 6D 65 36 31 3A 57 61 72 63 72 61 66 
74 5F 20 4F 66 66 69 63 69 61 6C 20 4D 6F 76 69 65 20 4E 6F 76 65 6C 69 
7A 61 74 69 6F 6E 20 62 79 20 43 68 72 69 73 74 69 65 20 47 6F 6C 64 65 
6E 20 45 50 55 42 31 32 3A 70 69 65 63 65 20 6C 65 6E 67 74 68 69 31 30 
34 38 35 37 36 65 36 3A 70 69 65 63 65 73 32 30 3A 43 92 4C 22 BB 42 9E 
EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9 65

And then take the SHA-1 hash to generate a 160-bit (20-byte) digest:

7EDA978ED7628595BB91C48B947F025BAE78CB77

Which is the right answer:

59,433

Author by

Emetiel

Updated on May 20, 2020

Comments

Emetiel about 4 years
I am reading lately a lot about hash from torrents, and magnetic links, etc. But there is a question I don't understand.

I have:
- hash of a file
- and the infohash of a torrent
Is the infohash = hash of the file ?

If yes what if the torrent describes 6 Files to download?

If no what does it stand for?
Tyzoid almost 8 years

If you need examples in the future, most linux distributions provide legal torrent downloads of their releases.
Mangat Rai Modi almost 8 years

or try mininova
EmptyData over 7 years

@Emetiel How to get 20 byte info_hash string?
lud about 2 years

Shouldn't the big hex at "We run all these bytes together" finish by F9 D0 F3 B9 65 65 65 (that is, 3* 65 at the end instead of one. I find this hash: 61FF4229183898FC2A184CEC914A933903DAFEEA. Why does 20:43 92 4C 22 BB 42 9E EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9 65 65 translate to 32 30 3A 43 92 4C 22 BB 42 9E EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9 ?
mistertodd about 2 years

@lud No. The info dictionary ends with ... E4 F9 D0 F3 B9 65. And when you hash the info dictionary using sha-1 you get 7eda978ed7628595bb91c48b947f025bae78cb77. That is the info dictionary hash, also known as the "infohash".
lud almost 2 years

Yeah sorry. I think there is a copy paste error, 20:43 92 4C 22 BB 42 9E EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9 65 65 should be 20:43 92 4C 22 BB 42 9E EA BD FF 66 C6 79 4C 29 E4 F9 D0 F3 B9. But I find the good hash now. Thank you.