Comment 1:

Adam, summary from that URL:
In src/pkg/crypto/cipher/cfb.go:
func (x *cfb) XORKeyStream(dst, src []byte) {
....
        if !x.decrypt {
            copy(x.next[x.outUsed:], dst) // BUG? `dst` should be `src`
        }

Owner changed to @agl.

Comment 2:

I don't know whether the code is correct or not, but either way the tests should have
some test vectors.  The tests don't actually check the code implements CFB correctly,
only that the decode(encode(x)) == x.
http://golang.org/src/pkg/crypto/cipher/cfb_test.go

Comment 3:

Have replied on the StackOverflow page. https://golang.org/cl/136720043 coming
to add test vectors for CFB mode.

Status changed to Accepted.

Comment 4:

CL https://golang.org/cl/136720043 mentions this issue.

Comment 5:

This issue was closed by revision a6cd733.

Status changed to Fixed.