Update reference to CBC vulnerability in cipher.go

The original referenced paper is no longer available. I contacted the author, and this is the reference he supplied.

Change-Id: Ib5ad88932f2bfc12ff62639f7fb7c3ee568610d6
GitHub-Last-Rev: 2cf7d15c63a114952c90a018ea5afe085965202e
GitHub-Pull-Request: golang/crypto#132
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/228638
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/ssh/cipher.go b/ssh/cipher.go
index b0204ee..8bd6b3d 100644
--- a/ssh/cipher.go
+++ b/ssh/cipher.go
@@ -119,7 +119,7 @@
 	chacha20Poly1305ID: {64, 0, newChaCha20Cipher},
 	// CBC mode is insecure and so is not included in the default config.
-	// (See http://www.isg.rhul.ac.uk/~kp/SandPfinal.pdf). If absolutely
+	// (See https://www.ieee-security.org/TC/SP2013/papers/4977a526.pdf). If absolutely
 	// needed, it's possible to specify a custom Config to enable it.
 	// You should expect that an active attacker can recover plaintext if
 	// you do.