Reading a ASN.1 DER-encoded RSA Public key

Question

I\'m writing an app to get a better understanding of DKIM. The spec says I retrieve a \"ASN.1 DER-encoded\" public key from the domain TXT record. I can seen the key on \"

Answer 1

That string looks like its some sort of base-64 encoding. If you convert that string from base-64 to a BLOB, it should then be in valid ASN.1 format.

Answer 2

This is the base64-encoding of the DER-encoding of an ASN.1 PublicKeyInfo containing an RSA public key.

Here is a translation:

   0 30  159: SEQUENCE {
   3 30   13:   SEQUENCE {
   5 06    9:     OBJECT IDENTIFIER '1 2 840 113549 1 1 1'
  16 05    0:     NULL
            :     }
  18 03  141:   BIT STRING 0 unused bits, encapsulates {
  22 30  137:       SEQUENCE {
  25 02  129:         INTEGER
            :           00 EB 11 E7 B4 46 2E 09 BB 3F 90 7E 25 98 BA 2F
            :           C4 F5 41 92 5D AB BF D8 FF 0B 8E 74 C3 F1 5E 14
            :           9E 7F B6 14 06 55 18 4D E4 2F 6D DB CD EA 14 2D
            :           8B F8 3D E9 5E 07 78 1F 98 98 83 24 E2 94 DC DB
            :           39 2F 82 89 01 45 07 8C 5C 03 79 BB 74 34 FF AC
            :           04 AD 15 29 E4 C0 4C BD 98 AF F4 B7 6D 3F F1 87
            :           2F B5 C6 D8 F8 46 47 55 ED F5 71 4E 7E 7A 2D BE
            :           2E 75 49 F0 BB 12 B8 57 96 F9 3D D3 8A 8F FF 97
            :           73
 157 02    3:         INTEGER 65537
            :         }
            :       }
            :   }

The OBJECT IDENTIFIER indicates that the following BIT STRING contains the encoding of an RSAPublicKey. The INTEGERs are the modulus and the public exponent.

You can decode the base64 with Convert.FromBase64String, but I don't think .NET has built-in functionality for parsing PublicKeyInfos, so you need to use a 3rd party tool like BouncyCastle.

Answer 3

Try the bouncycastle library, it provides great functionality for such cases.

Answer 4

For anyone interested in this matter I would suggest the System.Security.Cryptography.X509Certificates.PublicKey which can be used to read a DER encoded public key.