I am wondering why conversion from base64 strings returns equal byte arrays for different strings?
const string s1 = "dg==";
const string s2 = "dq==";
byte[] a1 = Convert.FromBase64String(s1);
byte[] a2 = Convert.FromBase64String(s2);
Console.WriteLine(a1.SequenceEqual(a2)); // prints "True".
Because of the encoding rules. When the last four-character group contains two padding characters (as is the case here) it decodes to a single byte. This means that decoding will take into account all 6 bits encoded into the first character, plus 2 of the bits encoded into the second.
In the base64 alphabet g corresponds to decimal 32 and q to decimal 42. When converted to 6-bit binary both of these values have their 2 most significant bits set to 10:
Base64 Decimal Binary
g 32 100000
h 33 100001
....
q 42 101010
^^
Since only these two bits go into the decoded output, it follows that the output will also be identical (by the same token, any decimal in the range [32, 47] => any base64 digit in the range [d,v] will produce the same result when substituted in the second position). It all works like this:
Printable d g = =
Bits 011101 10xxxx - -
^^^^^^ ^^
| \-------------------------------\
Interpretation \-----------------------------------\ |
| |
v v
Result (binary) 01110110
Result (ASCII) the letter "v"
True