<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><span style="orphans: 2; text-align: -webkit-auto; widows: 2;">On Sep 20, 2014, at 11:18 AM, Adeola Bannis <<a href="mailto:abannis@ucla.edu">abannis@ucla.edu</a>> wrote:</span><br><div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr"><br><div class="gmail_extra"><div class="gmail_quote">On Fri, Sep 19, 2014 at 11:18 PM, Yingdi Yu <span dir="ltr"><<a href="mailto:yingdi@cs.ucla.edu" target="_blank">yingdi@cs.ucla.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex; position: static; z-index: auto;"><div style="word-wrap:break-word"><div><span style="border-collapse: separate; font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;"><span style="border-collapse: separate; font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;"><div style="word-wrap:break-word"><br></div></span></span></div><div><span class=""><div>@Adeola, you probably want to forbid KeyDigest in KeyLocator for this HMAC signature. Because if key size is longer than hash output, the key digest is used instead. If we allow KeyDigest in KeyLocator, then some careless programmers may leak the secret. </div></span></div></div></blockquote><div><br></div><div>Well, a careless programmer could put a passphrase, or something used to derive the key, into a KeyLocator KeyName as well. We can go ahead and make the restriction, but there are other ways a programmer could shoot herself in the foot here.</div></div></div></div></blockquote></div><br><div>I did not mean that "careless" as you describe. I mean those who do not know the details of HMAC.</div><div><br></div><div>The problem is that when key is shorter than hash output, it is safe to put key digest in the key locator. However, when key is longer than the hash output, putting key digest into key locator directly expose the secret to other, i.e., attackers can directly use the key digest to construct any legitimate HMAC. Therefore you either explicitly specify what should be used when KeyDigest is used as the KeyLocator, or you can disable the usage of KeyDigest in KeyLocator.</div><div><br></div><div>Yingdi </div></body></html>