Reading ASN.1 DER-encoded RSA Public Key

Question

I am writing an application to better understand DKIM. The specification says that I am retrieving the ASN.1 DER-encoded public key from the domain TXT record. I can see the key on "s1024._domainkey.yahoo.com" = "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDrEee0Ri4Juz + QfiWYui / E9UGSXau / 2P8LjnTD8V4Unn32B4FEZEB3GEBFEZ

How can I use this key from .net? The examples I saw get the key from X509Certificate2 or an XML file containing RSAP parameters.

CORRECTION . I copy / paste the key from the DNS DNS tool network-tools.com, which was supposed to shorten it. 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 = 1024 bit key;"

So, Abelenki was on the right track with BASE64 ..

Answer 1

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

Here is the 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
            :         }
            :       }
            :   }

OBJECT IDENTIFIER indicates that the next BIT STRING contains the RSAPublicKey encoding. INTEGER are a module and a public indicator.

base64 Convert.FromBase64String, , .NET PublicKeyInfos, , BouncyCastle.

Answer 2

, , System.Security.Cryptography.X509Certificates.PublicKey, DER.

Answer 3

base-64. -64 BLOB, ASN.1.

Answer 4

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