Question

我正在使用Gmail API从Gmail帐户中读取电子邮件。

在我体内，我正在替换在论坛中阅读时需要的一些字符：

 String codedBody = body.Replace("-", "+");
 codedBody = codedBody.Replace("_", "/");

问题在于，当我尝试转换它时

byte[] data = Convert.FromBase64String(codedBody);

有一个例外，其中包含一些电子邮件：

System.FormatException：＆＃39;输入不是有效的Base-64字符串，因为它包含非基本64个字符，两个以上的填充字符或填充字符中的非法字符。＆＃39;

来自请求的字符串是：

＆＃34; 0J7QsdGP0LLQsDogSGVhbHRoY2FyZSBTZXJ2aWNlIFJlcHJlc2VudGF0aXZlIHdpdGggRHV0Y2gsIEdlcm1hbiANCiDQktCw0LbQvdC-ISDQnNC-0LvRjywg0L3QtSDQvtGC0LPQvtCy0LDRgNGP0LnRgtC1INC90LAg0YLQvtC30LggZW1haWwuICANCiAg0KLQvtC30LggZW1haWwg0LUg0LjQt9C_0YDQsNGC0LXQvSDQv9GA0LXQtyBqb2JzLmJnINC-0YIg0LjQvNC10YLQviDQvdCwINCa0YDQuNGB0YLQuNCw0L0g0JrRitC90LXQsiAg0JfQsCDQtNCwINGB0LUg0YHQstGK0YDQttC10YLQtSDRgSDQutCw0L3QtNC40LTQsNGC0LAg0YfRgNC10LcgZW1haWwg0LjQt9C_0L7Qu9C30LLQsNC50YLQtToga3Jpc3RpYW5fdG9uaUBhYnYuYmcgIA0KICDQodGK0L7QsdGJ0LXQvdC40LUg0L7RgiDQutCw0L3QtNC40LTQsNGC0LA6ICANCiAg0LHQu9Cw0LHQu9Cw0LHQu9Cw0LHQu9CwDQoNCg0KDQoNCg0KICA = PEhUTUw-PEJPRFk-DQrQntCx0Y_QstCwOiBIZWFsdGhjYXJlIFNlcnZpY2UgUmVwcmVzZW50YXRpdmUgd2l0aCBEdXRjaCwgR2VybWFuPGRpdj48YnI-PGRpdj7QktCw0LbQvdC-ISDQnNC-0LvRjywg0L3QtSDQvtGC0LPQvtCy0LDRgNGP0LnRgtC1INC90LAg0YLQvtC30LggZW1haWwuPC9kaXY-PGRpdj48YnI-PC9kaXY-PGRpdj7QotC-0LfQuCBlbWFpbCDQtSDQuNC30L_RgNCw0YLQtdC9INC_0YDQtdC3IGpvYnMuYmcg0L7RgiDQuNC80LXRgtC-INC90LAg0JrRgNC40YHRgtC40LDQvSDQmtGK0L3QtdCyPC9k AXY-PGRpdj7Ql9CwINC00LAg0YHQtSDRgdCy0YrRgNC20LXRgtC1INGBINC60LDQvdC00LjQtNCw0YLQsCDRh9GA0LXQtyBlbWFpbCDQuNC30L_QvtC70LfQstCw0LnRgtC1OiBrcmlzdGlhbl90b25pQGFidi5iZzwvZGl2PjxkaXY-PGJyPjwvZGl2PjxkaXY-0KHRitC-0LHRidC10L3QuNC1INC-0YIg0LrQsNC90LTQuNC00LDRgtCwOjwvZGl2PjxkaXY-PGJyPjwvZGl2PjxkaXY-0LHQu9Cw0LHQu9Cw0LHQu9Cw0LHQu9CwPGJyPjxicj48YnI-PGJyPjxicj48YnI-PC9kaXY-PC9kaXY-PC9CT0RZPjwvSFRNTD4NCg ==＆＃34;

是什么导致了这个问题？

Answer 1

Your source Base64 string is not valid. It contains a padding character = at position 604 in the middle of the string.

It appears as if you have two valid Base64 string that have been concatenated together. Go back to your source and ensure that you're collecting them correctly.

The source has to provide some detail on this as Base64 itself provides no means to determine if you have two values joined like this. If the first source byte array had a length which was a multiple of 3, there would be no padding character in the middle, and it would have decoded successfully and given garbage.

For what it's worth, replacing those characters appears to be correct as there is no de-facto standard for which two symbols characters are used in Base64. However, make sure you've gotten them right way around.

Update

Having investigated further (learning is fun) there is a defined Base64 standard, which defines two separate Base64 encodings.

The Base 64 Alphabet defines + and / for the two symbols, and = for the padding character.

The same RFC also specifies The "URL and Filename safe" Base 64 Alphabet which uses - and _ for the two symbols, and = (or %3D) for the padding character.

It appears your source data uses the "URL and Filename safe" format, while FromBase64String() only accepts the normal format. Therefore you are quite correct to replace - with + and _ with / to convert from one to the other.

从阅读电子邮件中编码字符串

1 个答案: