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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 "s1024._domainkey.yahoo.com" = "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDrEee0Ri4Juz+QfiWYui/E9UGSXau/2P8LjnTD8V4Unn+2FAZVGE3kL23bzeoULYv4PeleB3gfm".

How can I use this key from .net? The examples I've seen get the key from a X509Certificate2, or an XML file containing the RSAParameters.

CORRECTION: I copy/pasted the key above from the network-tools.com DNS tool, which must've cut it short. nslookup gives me the full key:

s1024._domainkey.yahoo.com text = "k=rsa; t=y; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDrEee0Ri4Juz+QfiWYui/E9UGSXau2P8LjnTD8V4Unn+2FAZVGE3kL23bzeoULYv4PeleB3gfm" "JiDJOKU3Ns5L4KJAUUHjFwDebt0NP+sBK0VKeTATL2Yr/S3bTxhy+1xtj4RkdV7fVxTn56Lb4udUnwuxK4V5b5PdOKj+XcwIDAQAB; n=A 1024 bit key;"

So abelenky was on the right track with BASE64..

+1  A: 

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.

abelenky
I've tried converting it from BASE64 - no luck. This post suggests DER-encoded is something similar? http://stackoverflow.com/questions/657989/identifying-whether-a-certificate-is-der-encoded-or-base-64-encoded
russau
+3  A: 

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.

Rasmus Faber
the key was cut short.. i've updated the question with the full key. this does make a lot more sense now, thanks.
russau
A: 

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

iffi