转换base64字符串时出错

Question

I am trying to convert a base64 string to HTML code which i am getting from my e-mail through Gmail Api in WinRT. But the problem is that while converting the base64 string to byte array, an FormatException is thrown.Also when i decoded it on the base64decode website, it perfectly decodes the base64 string.

This is the base64 string which i am trying to decode:

PGltZyBzcmM9Imh0dHA6Ly9jY3Byb21haWwuY29tOjgxODEvYWRtaW1wLmpzcD9pZD1UaUUtbldrcGhESUJuaHNUS2t0NENpclQzTHVvY09ZOVV2czJDb1lFbGo4YjZJWjl0TWp6SzNvRUpERkVHZnBnIiBib3JkZXI9IjAiIGhlaWdodD0iMCIgd2lkdGg9IjAiPjxkaXYgc3R5bGU9IndpZHRoOjYwMHB4OyAgZm9udC1mYW1pbHk6QXJpYWwsIEhlbHZldGljYSwgc2Fucy1zZXJpZjsgZm9udC1zaXplOiAxOHB4OyBsaW5lLWhlaWdodDoyMnB4OyB0ZXh0LWFsaWduOiBjZW50ZXI7Ij4gIDxmb250IHN0eWxlPSJmb250LXdlaWdodDogYm9sZDsgZm9udC1zaXplOiAyMHB4OyBjb2xvcjogcmVkOyI-IEdST1cgWU9VUiBPV04gSEFJUiBGT1IgSlVTVCAxIFJTLjwvZm9udD4gIDxicj4gIDxicj4gIDxmb250IHN0eWxlPSJjb2xvcjpncmVlbjtmb250LXdlaWdodDogYm9sZDsiPkludGVyZXN0IEZyZWUgRU1JPGJyPjxicj4xIFJzLiBEb3duIFBheW1lbnQ8L2ZvbnQ-ICA8YnI-ICA8YnI-ICA8YSBocmVmPSJodHRwOi8vY2Nwcm9tYWlsLmNvbTo4MTgxL3RyYWNrLmpzcD9pZD1UaUUtbldrcGhESUJuaHNUS2t0NENpclQzTHVvY09ZOVV2czJDb1lFbGo4YjZJWjl0TWp6SzNvRUpERkVHZnBnIiBzdHlsZT0iY29sb3I6IGJsdWU7IGJhY2tncm91bmQtY29sb3I6IHllbGxvdzsiPkdldCBZb3VyIEFwcG9pbnRtZW50IE5vdzwvYT4gIDxicj4gIDxicj4gPC9kaXY-PGJyLz48ZGl2IHN0eWxlPSJmb250LXNpemU6OXB4O3dpZHRoOjYwMHB4 O21hcmdpbi1sZWZ0OiAxNTBweDsiPjxhIGhyZWY9Imh0dHA6Ly9jY3Byb21haWwuY29tOjgxODEvdHJhY2suanNwP2lkPVRpRS1uV2twaERJQm5oc1RLa3Q0Q2lyVDNMdW9jT1k5VXZzMkNvWUVsajhiNklaOXRNanpLM29FSkRGRUdmcGcmdT1hIj5DbGljayBIZXJlPC9hPiB0byB1bnN1YnNjcmliZSBmcm9tIHRoaXMgbmV3c2xldHRlci48YnIvPjwvZGl2Pg0K

EDIT: This is the code that i am using to decode the string:

string raw = "PGltZyBzcmM9Imh0dHA6Ly9jY3Byb21haWwuY29tOjgxODEvYWRtaW1wLmpzcD9pZD1UaUUtbldrcGhESUJuaHNUS2t0NENpclQzTHVvY09ZOVV2czJDb1lFbGo4YjZJWjl0TWp6SzNvRUpERkVHZnBnIiBib3JkZXI9IjAiIGhlaWdodD0iMCIgd2lkdGg9IjAiPjxkaXYgc3R5bGU9IndpZHRoOjYwMHB4OyAgZm9udC1mYW1pbHk6QXJpYWwsIEhlbHZldGljYSwgc2Fucy1zZXJpZjsgZm9udC1zaXplOiAxOHB4OyBsaW5lLWhlaWdodDoyMnB4OyB0ZXh0LWFsaWduOiBjZW50ZXI7Ij4gIDxmb250IHN0eWxlPSJmb250LXdlaWdodDogYm9sZDsgZm9udC1zaXplOiAyMHB4OyBjb2xvcjogcmVkOyI-IEdST1cgWU9VUiBPV04gSEFJUiBGT1IgSlVTVCAxIFJTLjwvZm9udD4gIDxicj4gIDxicj4gIDxmb250IHN0eWxlPSJjb2xvcjpncmVlbjtmb250LXdlaWdodDogYm9sZDsiPkludGVyZXN0IEZyZWUgRU1JPGJyPjxicj4xIFJzLiBEb3duIFBheW1lbnQ8L2ZvbnQ-ICA8YnI-ICA8YnI-ICA8YSBocmVmPSJodHRwOi8vY2Nwcm9tYWlsLmNvbTo4MTgxL3RyYWNrLmpzcD9pZD1UaUUtbldrcGhESUJuaHNUS2t0NENpclQzTHVvY09ZOVV2czJDb1lFbGo4YjZJWjl0TWp6SzNvRUpERkVHZnBnIiBzdHlsZT0iY29sb3I6IGJsdWU7IGJhY2tncm91bmQtY29sb3I6IHllbGxvdzsiPkdldCBZb3VyIEFwcG9pbnRtZW50IE5vdzwvYT4gIDxicj4gIDxicj4gPC9kaXY-PGJyLz48ZGl2IHN0eWxlPSJmb250LXNpemU6OXB4O3dpZHRoOjYwMHB4O21hcmdpbi1sZWZ0OiAxNTBweDsiPjxhIGhyZWY9Imh0dHA6Ly9jY3Byb21haWwuY29tOjgxODEvdHJhY2suanNwP2lkPVRpRS1uV2twaERJQm5oc1RLa3Q0Q2lyVDNMdW9jT1k5VXZzMkNvWUVsajhiNklaOXRNanpLM29FSkRGRUdmcGcmdT1hIj5DbGljayBIZXJlPC9hPiB0byB1bnN1YnNjcmliZSBmcm9tIHRoaXMgbmV3c2xldHRlci48YnIvPjwvZGl2Pg0K";
string codedbody = raw.Replace("-", "+");
codedbody = raw.Replace("_", "/");
byte[] data = Convert.FromBase64String(codedbody);
string body = Encoding.UTF8.GetString(data);

Answer 1

How about

string input = "PGltZyBzcmM9Imh0dHA6Ly9jY3Byb21haWwuY29tOjgxODEvYWRtaW1wLmpzcD9pZD1UaUUtbldrcGhESUJuaHNUS2t0NENpclQzTHVvY09ZOVV2czJDb1lFbGo4YjZJWjl0TWp6SzNvRUpERkVHZnBnIiBib3JkZXI9IjAiIGhlaWdodD0iMCIgd2lkdGg9IjAiPjxkaXYgc3R5bGU9IndpZHRoOjYwMHB4OyAgZm9udC1mYW1pbHk6QXJpYWwsIEhlbHZldGljYSwgc2Fucy1zZXJpZjsgZm9udC1zaXplOiAxOHB4OyBsaW5lLWhlaWdodDoyMnB4OyB0ZXh0LWFsaWduOiBjZW50ZXI7Ij4gIDxmb250IHN0eWxlPSJmb250LXdlaWdodDogYm9sZDsgZm9udC1zaXplOiAyMHB4OyBjb2xvcjogcmVkOyI-IEdST1cgWU9VUiBPV04gSEFJUiBGT1IgSlVTVCAxIFJTLjwvZm9udD4gIDxicj4gIDxicj4gIDxmb250IHN0eWxlPSJjb2xvcjpncmVlbjtmb250LXdlaWdodDogYm9sZDsiPkludGVyZXN0IEZyZWUgRU1JPGJyPjxicj4xIFJzLiBEb3duIFBheW1lbnQ8L2ZvbnQ-ICA8YnI-ICA8YnI-ICA8YSBocmVmPSJodHRwOi8vY2Nwcm9tYWlsLmNvbTo4MTgxL3RyYWNrLmpzcD9pZD1UaUUtbldrcGhESUJuaHNUS2t0NENpclQzTHVvY09ZOVV2czJDb1lFbGo4YjZJWjl0TWp6SzNvRUpERkVHZnBnIiBzdHlsZT0iY29sb3I6IGJsdWU7IGJhY2tncm91bmQtY29sb3I6IHllbGxvdzsiPkdldCBZb3VyIEFwcG9pbnRtZW50IE5vdzwvYT4gIDxicj4gIDxicj4gPC9kaXY-PGJyLz48ZGl2IHN0eWxlPSJmb250LXNpemU6OXB4O3dpZHRoOjYwMHB4O21hcmdpbi1sZWZ0OiAxNTBweDsiPjxhIGhyZWY9Imh0dHA6Ly9jY3Byb21haWwuY29tOjgxODEvdHJhY2suanNwP2lkPVRpRS1uV2twaERJQm5oc1RLa3Q0Q2lyVDNMdW9jT1k5VXZzMkNvWUVsajhiNklaOXRNanpLM29FSkRGRUdmcGcmdT1hIj5DbGljayBIZXJlPC9hPiB0byB1bnN1YnNjcmliZSBmcm9tIHRoaXMgbmV3c2xldHRlci48YnIvPjwvZGl2Pg0K";
string result = System.Text.Encoding.Default.GetString(System.Convert.FromBase64String(input.Replace("-", "+").Replace("_", "/")));

I kept your replacements because - and _ are invalid characters for BASE64

Answer 2

Your attempt to decode looks like you are trying to decode a base64 URL encoded string.

public byte[] Decode(string text) {
    var output = text;
    output = output.Replace('-', '+'); // 62nd char of encoding
    output = output.Replace('_', '/'); // 63rd char of encoding
    switch (output.Length % 4) { // Pad with trailing '='s
        case 0: break; // No pad chars in this case
        case 2: output += "=="; break; // Two pad chars
        case 3: output += "="; break;  // One pad char
        default: throw new FormatException("invalid format", text));
    }
    var converted = Convert.FromBase64String(output); // Standard base64 decoder
    return converted;
}

With the above code you can do

string raw = "PGltZyBzcmM9Imh0dHA6Ly9jY3Byb21haWwuY29tOjgxODEvYWRtaW1wLmpzcD9pZD1UaUUtbldrcGhESUJuaHNUS2t0NENpclQzTHVvY09ZOVV2czJDb1lFbGo4YjZJWjl0TWp6SzNvRUpERkVHZnBnIiBib3JkZXI9IjAiIGhlaWdodD0iMCIgd2lkdGg9IjAiPjxkaXYgc3R5bGU9IndpZHRoOjYwMHB4OyAgZm9udC1mYW1pbHk6QXJpYWwsIEhlbHZldGljYSwgc2Fucy1zZXJpZjsgZm9udC1zaXplOiAxOHB4OyBsaW5lLWhlaWdodDoyMnB4OyB0ZXh0LWFsaWduOiBjZW50ZXI7Ij4gIDxmb250IHN0eWxlPSJmb250LXdlaWdodDogYm9sZDsgZm9udC1zaXplOiAyMHB4OyBjb2xvcjogcmVkOyI-IEdST1cgWU9VUiBPV04gSEFJUiBGT1IgSlVTVCAxIFJTLjwvZm9udD4gIDxicj4gIDxicj4gIDxmb250IHN0eWxlPSJjb2xvcjpncmVlbjtmb250LXdlaWdodDogYm9sZDsiPkludGVyZXN0IEZyZWUgRU1JPGJyPjxicj4xIFJzLiBEb3duIFBheW1lbnQ8L2ZvbnQ-ICA8YnI-ICA8YnI-ICA8YSBocmVmPSJodHRwOi8vY2Nwcm9tYWlsLmNvbTo4MTgxL3RyYWNrLmpzcD9pZD1UaUUtbldrcGhESUJuaHNUS2t0NENpclQzTHVvY09ZOVV2czJDb1lFbGo4YjZJWjl0TWp6SzNvRUpERkVHZnBnIiBzdHlsZT0iY29sb3I6IGJsdWU7IGJhY2tncm91bmQtY29sb3I6IHllbGxvdzsiPkdldCBZb3VyIEFwcG9pbnRtZW50IE5vdzwvYT4gIDxicj4gIDxicj4gPC9kaXY-PGJyLz48ZGl2IHN0eWxlPSJmb250LXNpemU6OXB4O3dpZHRoOjYwMHB4O21hcmdpbi1sZWZ0OiAxNTBweDsiPjxhIGhyZWY9Imh0dHA6Ly9jY3Byb21haWwuY29tOjgxODEvdHJhY2suanNwP2lkPVRpRS1uV2twaERJQm5oc1RLa3Q0Q2lyVDNMdW9jT1k5VXZzMkNvWUVsajhiNklaOXRNanpLM29FSkRGRUdmcGcmdT1hIj5DbGljayBIZXJlPC9hPiB0byB1bnN1YnNjcmliZSBmcm9tIHRoaXMgbmV3c2xldHRlci48YnIvPjwvZGl2Pg0K";
byte[] data = Decode(raw);
string body = Encoding.UTF8.GetString(data);

which produces html output

<img src="http://ccpromail.com:8181/admimp.jsp?id=TiE-nWkphDIBnhsTKkt4CirT3LuocOY9Uvs2CoYElj8b6IZ9tMjzK3oEJDFEGfpg" border="0" height="0" width="0"><div style="width:600px;  font-family:Arial, Helvetica, sans-serif; font-size: 18px; line-height:22px; text-align: center;">  <font style="font-weight: bold; font-size: 20px; color: red;"> GROW YOUR OWN HAIR FOR JUST 1 RS.</font>  <br>  <br>  <font style="color:green;font-weight: bold;">Interest Free EMI<br><br>1 Rs. Down Payment</font>  <br>  <br>  <a href="http://ccpromail.com:8181/track.jsp?id=TiE-nWkphDIBnhsTKkt4CirT3LuocOY9Uvs2CoYElj8b6IZ9tMjzK3oEJDFEGfpg" style="color: blue; background-color: yellow;">Get Your Appointment Now</a>  <br>  <br> </div><br/><div style="font-size:9px;width:600px;margin-left: 150px;"><a href="http://ccpromail.com:8181/track.jsp?id=TiE-nWkphDIBnhsTKkt4CirT3LuocOY9Uvs2CoYElj8b6IZ9tMjzK3oEJDFEGfpg&u=a">Click Here</a> to unsubscribe from this newsletter.<br/></div>

URL applications

Base64 encoding can be helpful when fairly lengthy identifying information is used in an HTTP environment. For example, a database persistence framework for Java objects might use Base64 encoding to encode a relatively large unique id (generally 128-bit UUIDs) into a string for use as an HTTP parameter in HTTP forms or HTTP GET URLs. Also, many applications need to encode binary data in a way that is convenient for inclusion in URLs, including in hidden web form fields, and Base64 is a convenient encoding to render them in a compact way.

Using standard Base64 in URL requires encoding of '+', '/' and '=' characters into special percent-encoded hexadecimal sequences ('+' becomes '%2B', '/' becomes '%2F' and '=' becomes '%3D'), which makes the string unnecessarily longer.

For this reason, modified Base64 for URL variants exist, where the '+' and '/' characters of standard Base64 are respectively replaced by '-' and '_', so that using URL encoders/decoders is no longer necessary and have no impact on the length of the encoded value, leaving the same encoded form intact for use in relational databases, web forms, and object identifiers in general. Some variants allow or require omitting the padding '=' signs to avoid them being confused with field separators, or require that any such padding be percent-encoded. Some libraries (like org.bouncycastle.util.encoders.UrlBase64Encoder) will encode '=' to '.'.

Reference Base64 : From Wikipedia

Answer 3

You make mistake in your code: You again take to replace raw instead codebody

codedbody = raw.Replace("_", "/");

Should be:

codedbody = codedbody.Replace("_", "/");