Integer values are encoded into a TLV triplet that begins with a Tag value of 0x02. The Value field of the TLV triplet contains the encoded integer if it is positive, or its two's complement if it is negative. If the integer is positive but the high order bit is set to 1, a leading 0x00 is added to the content to indicate that the number is not negative. For example, the high order byte of 0x8F (10001111) is 1. Therefore a leading zero byte is added to the content as shown in the following illustration.

DER encoding of BOOLEAN data type

If the integer contains fewer than 128 bytes, the Length field requires only one byte to specify the content length. If the integer 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. For more information, see Encoded Length and Value Bytes.

The following example, from PKCS #10 Encoded ASN.1, shows the encoding for a 128 byte public key. The first byte contains the Tag value for the INTEGER data type, 0x02. The second and third bytes contain the Length value. Bit 7 of the second byte is set to 1 because there are more than 127 bytes of content. Bits 0 through 6 of the second byte specify the number of trailing bytes needed, in this case one, to accurately specify the content length. The third byte specifies the number of content bytes, 0x81. The fourth byte, 0x00, is added to the content to indicate that the integer is indeed a positive value even though the sign bit of the leading content byte (0x8F) is set to 1.

02 81 81          ; INTEGER (81 Bytes)
|  00
|  8f e2 41 2a 08 e8 51 a8  8c b3 e8 53 e7 d5 49 50
|  b3 27 8a 2b cb ea b5 42  73 ea 02 57 cc 65 33 ee
|  88 20 61 a1 17 56 c1 24  18 e3 a8 08 d3 be d9 31
|  f3 37 0b 94 b8 cc 43 08  0b 70 24 f7 9c b1 8d 5d
|  d6 6d 82 d0 54 09 84 f8  9f 97 01 75 05 9c 89 d4
|  d5 c9 1e c9 13 d7 2a 6b  30 91 19 d6 d4 42 e0 c4
|  9d 7c 92 71 e1 b2 2f 5c  8d ee f0 f1 17 1e d2 5f
|  31 5b b1 9c bc 20 55 bf  3a 37 42 45 75 dc 90 65

The following example shows how the integer value 0x03 is encoded. The Tag byte contains 0x02, and the Length byte specifies that there is one byte of content.

02 01             ; INTEGER (1 Bytes)
|  03

Related topics

ASN.1 Type System
DER Encoding of ASN.1 Types