<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Mar 29, 2021, at 16:53, Phillip Hallam-Baker <<a href="mailto:phill@hallambaker.com" class="">phill@hallambaker.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div dir="ltr" class=""><div dir="ltr" class=""><div dir="ltr" class=""><div class="gmail_default" style="font-size:small"><br class=""></div></div><div class="gmail_quote"><div class=""><br class=""></div><div class=""><div class="gmail_default" style="font-size:small">Same problem as with a stream cipher.</div><div class="gmail_default" style="font-size:small"><br class=""></div><div class="gmail_default" style="font-size:small">The problem here is to use a single key to permute 2^n identifiers so that nothing is given away from the sequences. </div><div class="gmail_default" style="font-size:small"><br class=""></div><div class="gmail_default" style="font-size:small">If we use a stream cipher (or CTR mode, CFB, GCM, etc) we are effectively XORing every value with the same mask.</div><div class="gmail_default" style="font-size:small"><br class=""></div><div class="gmail_default" style="font-size:small">Permute (0) = 0 XOR k</div>Permute (<span class="gmail_default" style="font-size:small">1</span>) = <span class="gmail_default" style="font-size:small">1</span><span class="gmail_default" style="font-size:small"></span><span class="gmail_default" style="font-size:small"></span><span class="gmail_default" style="font-size:small"></span> XOR <span class="gmail_default" style="font-size:small">k</span><br class=""></div><div class="">Permute (<span class="gmail_default" style="font-size:small">2</span>) = <span class="gmail_default" style="font-size:small">2</span> XOR <span class="gmail_default" style="font-size:small">k</span></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><div class="gmail_default" style="font-size:small">What I want is for Permute (x) to be an unpredictable bijection and to be able to do it in 8, 16, 24, 32, 40, etc chunks.</div></div></div></div></div></div></div></blockquote><br class=""></div><div>Similarly to what I said to Peter, it sounds like CFB works for you.</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>Jon</div><div><br class=""></div><br class=""></body></html>