DumpASN1 for Windows

A minimal test ASN.1 file minasn.dat.

> dumpasn1 minasn.dat
  0   4: SEQUENCE {
  2   2:   SEQUENCE {
  4   0:     NULL
       :     }
       :   }

0 warnings, 0 errors.

A minimal X.509 certificate smallca.cer

> dumpasn1 smallca.cer
  0 224: SEQUENCE {
  3 154:   SEQUENCE {
  6   1:     INTEGER 1
  9  13:     SEQUENCE {
 11   9:       OBJECT IDENTIFIER sha1WithRSAEncryption (1 2 840 113549 1 1 5)
 22   0:       NULL
       :       }
 24  12:     SEQUENCE {
 26  10:       SET {
 28   8:         SEQUENCE {
 30   3:           OBJECT IDENTIFIER commonName (2 5 4 3)
 35   1:           PrintableString 'A'
       :           }
       :         }
       :       }
 38  30:     SEQUENCE {
 40  13:       UTCTime 19/09/1999 01:08:47 GMT
 55  13:       UTCTime 31/12/2039 23:59:59 GMT
       :       }
 70  12:     SEQUENCE {
 72  10:       SET {
 74   8:         SEQUENCE {
 76   3:           OBJECT IDENTIFIER commonName (2 5 4 3)
 81   1:           PrintableString 'A'
       :           }
       :         }
       :       }
 84  74:     SEQUENCE {
 86  13:       SEQUENCE {
 88   9:         OBJECT IDENTIFIER rsaEncryption (1 2 840 113549 1 1 1)
 99   0:         NULL
       :         }
101  57:       BIT STRING, encapsulates {
104  54:         SEQUENCE {
106  49:           INTEGER
       :             02 F9 09 6A 7D 83 55 C2 71 AE F1 6A CB 45 41 BA
       :             B3 22 A2 83 B5 AD DE 70 E3 37 19 A7 C9 BB EE 76
       :             4B E2 FC B8 5C C7 9B E2 3F 27 1F 6F B7 B5 36 80
       :             19
157   1:           INTEGER 3
       :           }
       :         }
       :       }
       :     }
160  13:   SEQUENCE {
162   9:     OBJECT IDENTIFIER sha1WithRSAEncryption (1 2 840 113549 1 1 5)
173   0:     NULL
       :     }
175  50:   BIT STRING
       :     01 9A 9B B2 EC B9 CD FD 66 C6 94 5B 2E D6 96 DC
       :     32 87 68 DA 5E 6F 2E 5D 5A 7F E6 09 2E 60 8F 8C
       :     45 A5 18 7E 06 1C E9 81 AA EA D6 F2 E3 14 7D 25
       :     91
       :   }

Passing a base64-encoded value using the base64 utility.

> echo MAQwAgUA | base64 -d | dumpasn1 -
  0   4: SEQUENCE {
  2   2:   SEQUENCE {
  4   0:     NULL
       :     }
       :   }

A longer example using the base64-encoded value given in RFC7030 section 4.5.2

> echo MEEGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBYGCSqGSIb3DQEJDjEJBgcrBgEBAQEWBggqhkjOPQQDAw== | base64 -d | dumpasn1 -
  0  65: SEQUENCE {
  2   9:   OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7)
 13  18:   SEQUENCE {
 15   7:     OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
 24   7:     SET {
 26   5:       OBJECT IDENTIFIER secp384r1 (1 3 132 0 34)
       :       }
       :     }
 33  22:   SEQUENCE {
 35   9:     OBJECT IDENTIFIER extensionRequest (1 2 840 113549 1 9 14)
 46   9:     SET {
 48   7:       OBJECT IDENTIFIER '1 3 6 1 1 1 1 22'
       :       }
       :     }
 57   8:   OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3)
       :   }

The best explanation of ASN.1 we've found is in A Layman's Guide to a Subset of ASN.1, BER, and DER written in 1993 by Burt Kalinsky of RSA Laboratories ("RSA Data Security, Inc. Public-Key Cryptography Standards (PKCS)").

OSI's method of specifying abstract objects is called ASN.1 (Abstract Syntax Notation One, defined in X.208), and one set of rules for representing such objects as strings of ones and zeros is called the BER (Basic Encoding Rules, defined in X.209). ASN.1 is a flexible notation that allows one to define a variety data types, from simple types such as integers and bit strings to structured types such as sets and sequences, as well as complex types defined in terms of others. BER describes how to represent or encode values of each ASN.1 type as a string of eight-bit octets. There is generally more than one way to BER-encode a given value. Another set of rules, called the Distinguished Encoding Rules (DER), which is a subset of BER, gives a unique encoding to each ASN.1 value.