What exactly is the info_Hash in a torrent file

Question

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?

Answer 1

So I finally figured it out.

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

1. ITEM: length(size) and path (path with filename)
2. Name: The name to search for
3. Piece length: The length(size) of a single piece
4. Pieces: SHA1 Hash of EVERY piece of this torrent
5. 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.

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.

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.

Answer 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: