How to Perform URL-Safe Base64 Encoding and Decoding in ASP.NET?

URL Safe Base64 Encoding/Decoding in ASP.NET Framework

Base64 encoding is commonly used to encode binary data into a text format for transmission over a network. However, the standard Base64 encoding contains characters (especially " " and "/") that may interfere with URI templates and URLs.

To solve this problem, a modified version of Base64 encoding exists specifically for URLs. In this variant, " " and "/" are replaced by "-" and "_" respectively, and the padding character "=" is omitted.

Implementing a modified version of Base64 for URLs in the ASP.NET framework can use the following method:

Decoding:

1. Replace "-" with " " and "_" with "/" in Base64 encoded text.
2. Determine the number of missing "=" characters based on the length of the Base64 encoded text.
3. Append the required "=" characters to the Base64 encoded text.
4. Performs regular Base64 decoding on modified Base64 encoded text.

Encoding:

1. Perform regular Base64 encoding on the raw data.
2. Replace the "=" characters with the appropriate number of "0", "1" or "2" depending on the length of the Base64 string.
3. Replace " " with "-" and "/" with "_".

Alternative:

.NET Framework provides the HttpServerUtility class, which contains the UrlTokenEncode and UrlTokenDecode methods that handle URL-safe Base64 encoding and decoding.

Code:

// 编码
string base64UrlEncodedText = HttpServerUtility.UrlTokenEncode(Encoding.UTF8.GetBytes(plaintext));

// 解码
string plaintext = Encoding.UTF8.GetString(HttpServerUtility.UrlTokenDecode(base64UrlEncodedText));

Note: HttpServerUtility method returns a non-standard base64url implementation in which the "=" padding character is replaced with "0", "1" or "2". This is different from the RFC4648 standard, which uses the "=" character for padding.

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