What all encryption uses == in the last? - encryption

Question

I am aware that most of the Base64 encoding has == at the end. Is there any other which uses does the same?
For example, I found this:
nijdRcCHIUnketWzFbcxmvqQKKDnFW05LSE3ttTjoqyBna7JT87AwxeKdoOszXYODMRm6UfA8jK97qgV8A==
But it is not a Base64 kind. What else can it be?

Answer 1

The string you have posted is a valid Base64 string.
A Base64 string will end with == if and only if the number of bytes it encodes, mod 3, equals 1.
>>> for i in range(10):
... print(i, base64.b64encode(b"\x00"*i))
...
0 b''
1 b'AA=='
2 b'AAA='
3 b'AAAA'
4 b'AAAAAA=='
5 b'AAAAAAA='
6 b'AAAAAAAA'
7 b'AAAAAAAAAA=='
8 b'AAAAAAAAAAA='
9 b'AAAAAAAAAAAA'
Do you see the pattern?
It happens that 16-byte (128-bit) encryption keys are very commonly encoded in Base64, and since 16 mod 3 = 1, their encoding will end with ==. But your string, decoded, is 61 bytes (488 bits) long. That is too big to be most sorts of encryption key, and too small to be an RSA key.
This is your string, decoded, and then hexdumped:
00000000 9e 28 dd 45 c0 87 21 49 e4 7a d5 b3 15 b7 31 9a |.(.E..!I.z....1.|
00000010 fa 90 28 a0 e7 15 6d 39 2d 21 37 b6 d4 e3 a2 ac |..(...m9-!7.....|
00000020 81 9d ae c9 4f ce c0 c3 17 8a 76 83 ac cd 76 0e |....O.....v...v.|
00000030 0c c4 66 e9 47 c0 f2 32 bd ee a8 15 f0 |..f.G..2.....|
0000003d
I don't see anything in there to tell me what it actually is, and file(1) is also stumped. It could be random enough to be encrypted, but I can't tell for sure by eye. (And if it is random, that doesn't mean it's encrypted! It could just be the output of a random number generator.)
It is important to understand that Base64 is not encryption, because it has no key. I didn't need to know or guess any piece of secret information to reverse the Base64 encoding of your string. (The term 'encoding' can be confusing — it is more general. UTF-8, Base64, and DEFLATE are all encodings, and so is AES-CBC, but of all of them, only AES-CBC is encryption.)