[lifthrasiir]

Base64 doesn't have a concept of "stream". Conceptually base64-encoded string with padding is a concatenation of fragments that are always 4 bytes long but can encode one to three bytes. Concatenating two base64-encoded strings with padding therefore don't destroy fragment structures and can be decoded into a byte sequence that is a concatenation of two original input sequences. Without padding, fragments can be also 2 or 3 bytes and short fragments are not distinguishable from long fragments, so the concatenation will destroy fragment structures.

[Aardwolf]

Oh I see, so it's for concatenating multiple base64 fragments of the same single piece of data? But where is this used? Never seen that. Javascript's base64 decoder gives an error for ='s in the middle (but I just found out the Linux base64 -d command supports it!)

[lifthrasiir]

I actually don't know if it's an intention, but it is the only explanation that makes sense. It should be noted that the original PEM specification (RFC 989) did have a similar use case where alternating encrypted and unencrypted bytes can be intermixed by `*` characters, but you are still required to pad each portion to 4n bytes (e.g. `c2VjcmV0LCA=*cHVibGlj*IGFuZCBzZWNyZXQgYWdhaW4=`). It is still the closest to what I think padding characters are required for.