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OCTET STRING

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The ASN.1 OCTET STRING data type is encoded into a TLV triplet that begins with a Tag byte of 0x04. The OCTET STRING and BIT STRING data types are very similar. Thus, the two types are encoded in a similar manner except that, because the trailing byte of an OCTET STRING cannot have unused bits, no leading bytes must be added to the content. The following example, adapted from the CMC Encoded ASN.1 topic, shows how the name of a certificate template is encoded as a byte array.

30 17                                 ; SEQUENCE (17 Bytes)
|  06 09                              ; OBJECT_ID (9 Bytes)
|  |  2b 06 01 04 01 82 37 14  02     ;   1.3.6.1.4.1.311.20.2
|  04 0a                              ; OCTET_STRING (a Bytes)
|     1e 08 00 55 00 73 00 65  00 72  ;   ...U.s.e.r

If the byte array contains fewer than 128 bytes, the Length field of the TLV triplet requires only one byte to specify the content length. If it is more than 127 bytes, bit 7 of the Length field is set to 1 and bits 6 through 0 specify the number of additional bytes used to identify the content length. This is shown in the following example where the high order bit of the second byte on the first line is set to 1 and the byte indicates that there is a trailing Length byte. The third byte therefore specifies that the content is 0x80 bytes long.

04 81 80                       ; OCTET_STRING (80 Bytes)
   38 10 60 e2 70 69 91 4a     ;   8.`.pi.J
   8b b5 22 57 2a 62 ef de     ;   .."W*b..
   15 7d 59 d6 4e 20 9a 45     ;   .}Y.N .E
   2b e3 fd fc 68 ba af bf     ;   +...h...
   9c 17 b0 8e 6d c4 29 1e     ;   ....m.).
   e3 21 ac bb 5a 8a c9 67     ;   .!..Z..g
   0a d4 45 93 10 c0 26 eb     ;   ..E...&.
   0a 83 c2 b1 40 87 36 f7     ;   ....@.6.
   a0 26 da b9 bb 46 73 88     ;   .&...Fs.
   7a 67 b9 e6 b3 6f ea 59     ;   zg...o.Y
   28 8a d3 92 72 f6 7b 89     ;   (...r.{.
   a0 d8 2d 9e 40 eb 1e bb     ;   ..-.@...
   6e ae f0 5a ed 16 c9 e3     ;   n..Z....
   27 59 37 8f f3 4a 98 60     ;   'Y7..J.`
   f8 fb a7 0a ee 1b 6e 91     ;   ......n.
   95 96 cf 0d 56 ac ab 35     ;   ....V..5

ASN.1 Type System

DER Encoding of ASN.1 Types