What is ASN.1 DER tag value 0xA0?

Question

Here is an example of a certificate encoded in ASN.1 DER

30 82 01 8F 30 81 F9 **A0** 03 02 01 02 02 01 01 30
0D 06 09 2A 86 48 86 F7 0D 01 01 05 05 00 30 0D
31 0B 30 09 06 03 55 04 03 0C 02 43 41 30 20 17
0D 31 33 30 39 31 35 31 35 33 35 30 32 5A 18 0F
32 31 31 33 30 39 32 32 31 35 33 35 30 32 5A 30
0D 31 0B 30 09 06 03 55 04 03 0C 02 43 41 30 81
9F 30 0D 06 09 2A 86 48 86 F7 0D 01 01 01 05 00
03 81 8D 00 30 81 89 02 81 81 00 8D 80 B5 8E 80
8E 94 D1 04 03 6A 45 1A 54 5E 7E EE 6D 0C CB 0B
82 03 F1 7D C9 6F ED 52 02 B2 08 C3 48 D1 24 70
C3 50 C2 1C 40 BC B5 9D F8 E8 A8 41 16 7B 0B 34
1F 27 8D 32 2D 38 BA 18 A5 31 A9 E3 15 20 3D E4
0A DC D8 CD 42 B0 E3 66 53 85 21 7C 90 13 E9 F9
C9 26 5A F3 FF 8C A8 92 25 CD 23 08 69 F4 A2 F8
7B BF CD 45 E8 19 33 F1 AA E0 2B 92 31 22 34 60
27 2E D7 56 04 8B 1B 59 64 77 5F 02 03 01 00 01
30 0D 06 09 2A 86 48 86 F7 0D 01 01 05 05 00 03
81 81 00 0A 1C ED 77 F4 79 D5 EC 73 51 32 25 09
61 F7 00 C4 64 74 29 86 5B 67 F2 3D A9 39 34 6B
3C A9 92 B8 BF 07 13 0B A0 9B DF 41 E2 8A F6 D3
17 53 E1 BA 7F C0 D0 BC 10 B7 9B 63 4F 06 D0 7B
AC C6 FB CE 95 F7 8A 72 AA 10 EA B0 D1 6D 74 69
5E 20 68 5D 1A 66 28 C5 59 33 43 DB EE DA 00 80
99 5E DD 17 AC 43 36 1E D0 5B 06 0F 8C 6C 82 D3
BB 3E 2B A5 F1 94 FB 53 7B B0 54 22 6F F6 4C 18
1B 72 1C

What does the highlighted tag 0xA0 mean? What type it encodes?

The next value is 0x03 which is the length of the 0xA0 tag. But the next byte after the length is actually INTEGER type - 0x02.

Who can explain this?

Is there a typo? A value of 0xA0 is highlighted. You talk about 0x0A in your post. — Thomas Matthews, Mar 27 '17 at 19:39

score 7 · Accepted Answer · edited Jul 08 '21 at 16:04

7

0xA0 is a Context Specific tag. In your example it contains an integer as you correctly deduced. In ASN.1 you can have 4 encoding of class of the 1-byte tag (more in section 8.1.2.2 of ITU-T X.690):

Universal (00xxxxxx)
Application (01xxxxxx)
Context-specific (10xxxxxx)
Private (11xxxxxx)

Personally I've never seen anything other than Universal (i.e sequence, integer, octet string) and context-specific.

You can explore the structure of the certificate by using ASN.1 Editor. I've selected the context specific tag here:

edited Jul 08 '21 at 16:04

Sean Bright

118,630
17
138
146

answered Mar 27 '17 at 21:14

pepo

8,644
2
27
42

What does "Context specific" actually mean here? And what distinguishes `0xa0=0b10100000` from other values that fit `0b10xxxxxx`, which would also be context specific? – nog642 Mar 21 '22 at 20:49

What is ASN.1 DER tag value 0xA0?

1 Answers1