Scott Hanselman

Decoding an SSH Key from PEM to BASE64 to HEX to ASN.1 to prime decimal numbers

August 24, '18 Comments [6] Posted in Musings | Open Source
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I'm reading a new chapter of The Imposter's Handbook: Season 2 that Rob and I are working on. He's digging into the internals of what's exactly in your SSH Key.

Decoding a certificate

I generated a key with no password:

ssh-keygen -t rsa -C

Inside the generated file is this text, that we've all seen before but few have cracked open.


The private key is an ASN.1 (Abstract Syntax Notation One) encoded data structure. It's a funky format but it's basically a packed format with the ability for nested trees that can hold booleans, integers, etc.

However, ASN.1 is just the binary packed "payload." It's not the "container." For example, there are envelopes and there are letters inside them. The envelope is the PEM (Privacy Enhanced Mail) format. Such things start with ----- BEGIN SOMETHING ----- and end with ----- END SOMETHING ------. If you're familiar with BASE64, your spidey sense may tell you that this is a BASE64 encoded file. Not everything that's BASE64 turns into a friendly ASCII string. This turns into a bunch of bytes you can view in HEX.

We can first decode the PEM file into HEX. Yes, I know there's lots of ways to do this stuff at the command line, but I like showing and teaching using some of the many encoding/decoding websites and utilities there are out there. I also love using for these things as you can build a visual pipeline.


This ASN.1 JavaScript decoder can take the HEX and parse it for you. Or you can that ASN.1 packed format at the *nix command line and see that there's nine big integers inside (I trimmed them for this blog).

openssl asn1parse -in notreal
0:d=0 hl=4 l=1188 cons: SEQUENCE
4:d=1 hl=2 l= 1 prim: INTEGER :00
7:d=1 hl=4 l= 257 prim: INTEGER :B5DF00B3CE6C3948E3923575DAE8CC2199A1C9E81791C9866935A4DF5F6F401DFE7292215
268:d=1 hl=2 l= 3 prim: INTEGER :010001
273:d=1 hl=4 l= 257 prim: INTEGER :8195EB82324A9A667CFFE867991AD697FA4774FD920DA671C6F51A0CAE8B2E3C30D8A1967
534:d=1 hl=3 l= 129 prim: INTEGER :E22B40AC22007370E309E68A29C64EEF085B6937D8B808F74D2862EDEDCBC8CB18BEAAD48
666:d=1 hl=3 l= 129 prim: INTEGER :CDDC00D77FD6E9E7EE83AD95F1805076FAE3690D817C47ED4FA05B7872B5631C6174DEAA7
798:d=1 hl=3 l= 128 prim: INTEGER :747EECEFFD0F9866D43B3C497C5D0E3967679659D2C270B3D9456D37BADDD5CE6F2F7ED4B
929:d=1 hl=3 l= 128 prim: INTEGER :742961DC388A184144E8CEE9DE75AE72050E8FF80C0A0A38520745B648DC2BCA170030A97
1060:d=1 hl=3 l= 129 prim: INTEGER :997742BA4458617495BE121EFF68893C9BC0A1A38226E6F14E16B9FDF5EE072981790499E

Per the spec the format is this:

   An RSA private key shall have ASN.1 type RSAPrivateKey:

RSAPrivateKey ::= SEQUENCE {
version Version,
modulus INTEGER, -- n
publicExponent INTEGER, -- e
privateExponent INTEGER, -- d
prime1 INTEGER, -- p
prime2 INTEGER, -- q
exponent1 INTEGER, -- d mod (p-1)
exponent2 INTEGER, -- d mod (q-1)
coefficient INTEGER -- (inverse of q) mod p }

I found the description for how RSA works in this blog post very helpful as it uses small numbers as examples. The variable names here like p, q, and n are agreed upon and standard.

The fields of type RSAPrivateKey have the following meanings:
o version is the version number, for compatibility
with future revisions of this document. It shall
be 0 for this version of the document.
o modulus is the modulus n.
o publicExponent is the public exponent e.
o privateExponent is the private exponent d.
o prime1 is the prime factor p of n.
o prime2 is the prime factor q of n.
o exponent1 is d mod (p-1).
o exponent2 is d mod (q-1).
o coefficient is the Chinese Remainder Theorem
coefficient q-1 mod p.

Let's look at that first number q, the prime factor p of n. It's super long in Hexadecimal.


That hexadecimal number converted to decimal is this long ass number. It's 308 digits long!


It's hard work to prove this number is prime but there's a great Integer Factorization Calculator that actually uses WebAssembly and your own local CPU to check such things. Expect to way a long time, sometimes until the heat death of the universe. ;)

Rob and I are finding it really cool to dig just below the surface of common things we look at all the time. I have often opened a key file in a text file but never drawn a straight and complete line through decoding, unpacking, decoding, all the way to a math mathematical formula. I feel I'm filling up some major gaps in my knowledge!

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About Scott

Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.

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How do you even know this crap?

August 22, '18 Comments [24] Posted in Musings
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Imposter's HandbookThis post won't be well organized so lower your expectations first. When Rob Conery first wrote "The Imposter's Handbook" I was LOVING IT. It's a fantastic book written for imposters by an imposter. Remember, I'm the original phony.

Now he's working on The Imposter's Handbook: Season 2 and I'm helping. The book is currently in Presale and we're releasing PDFs every 2 to 3 weeks. Some of the ideas from the book will come from blog posts like or similar to this one. Since we are using Continuous Delivery and an Iterative Process to ship the book, some of the blog posts (like this one) won't be fully baked until they show up in the book (or not). See how I equivocated there? ;)

The next "Season" of The Imposter's Handbook is all about the flow of information. Information flowing through encoding, encryption, and transmission over a network. I'm also interested in the flow of information through one's brain as they move through the various phases of being a developer. Bear with me (and help me in the comments!).

I was recently on a call with two other developers, and it would be fair that we were of varied skill levels. We were doing some HTML and CSS work that I would say I'm competent at, but by no means an expert. Since our skill levels didn't fall on a single axis, we'd really we'd need some Dungeons & Dragon's Cards to express our competencies.

D&D Cards from Battle Grip

I might be HTML 8, CSS 6, Computer Science 9, Obscure Trivia 11, for example.

We were asked to make a little banner with some text that could be later closed with some iconography that would represent close/dismiss/go away.

  • One engineer suggested "Here's some text + ICON.PNG"
  • The next offered a more scalable option with "Here's some text + ICON.SVG"

Both are fine ideas that would work, while perhaps later having DPI or maintenance issues, but truly, perfectly cromulent ideas.

I have never been given this task, I am not a designer, and I am a mediocre front-end person. I asked what they wanted it to look like and they said "maybe a square with an X or a circle with an X or a circle with a line."

I offered, you know, there MUST be a Unicode Glyph for that. I mean, there's one for poop." Apparently I say poop in business meetings more than any other middle manager at the company, but that's fodder for another blog post.

We searched and lo and behold we found ☒ and ⊝ and simply added them to the end of the string. They scale visibly, require no downloads or extra dependencies, and can be colored and styled nicely because they are text.

One of the engineers said "how do you even know this crap?" I smiled and shrugged and we moved on to the doing.

To be clear, this post isn't self-congratulatory. Perhaps you had the same idea. This interaction was all of 10 minutes long. But I'm interested in the HOW did I know this? Note that I didn't actually KNOW that these glyphs existed. I knew only that they SHOULD exist. They MUST.

How many times have you been coding and said "You know, there really must be a function/site/tool that does x/y/z?" All the time, right? You don't know the answers but you know someone must have AND must have solved it in a specific way such that you could find it. A new developer doesn't have this intuition - this sense of technical smell - yet.

How is technical gut and intuition and smell developed? Certainly by doing, by osmosis, by time, by sleeping, and waking, and doing it again.

I think it's exposure. It's exposure to a diverse set of technical problems that all build on a solid base of fundamentals.

Rob and I are going to try to expand on how this technical confidence gets developed in The Imposter's Handbook: Season 2 as topics like Logic, Binary and Logical Circuits, Compression and Encoding, Encryption and Cryptanalysis, and Networking and Protocols are discussed. But I want to also understand how/if/when these topics and examples excite the reader...and most importantly do they provide the reader with that missing Tetris Piece of Knowledge that moves you from a journeyperson developer to someone who can more confidently wear the label Computer Science 9, Obscure Trivia 11.


What do you think? Sound off in the comments and help me and Rob understand!

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About Scott

Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.

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11 essential characteristics for being a good technical advocate or interviewer

August 8, '18 Comments [11] Posted in Musings
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14265784357_5b2773e123_oI was talking to my friend Rob Caron today. He produces Azure Friday with me - it's our weekly video podcast on Azure and the Cloud. We were talking about the magic for a successful episode, but then realized the ingredients that Rob came up with were generic enough that they were the essential for anyone who is teaching or advocating for a technology.

Personally I don't believe in "evangelism" in a technical context and I dislike the term "Technology Evangelism." Not only does it evoke unnecessary zealotry but it also implies that your religion technology is not only what's best for someone, but that it's the only solution. Java people shouldn't try to convert PHP people. That's all nonsense, of course. I like the word "advocate" because you're (hopefully) advocating for the right solution regardless of technology.

Here's the 11 herbs and spices that are needed for a great technical talk, a good episode of a podcast or show, or a decent career talking and teaching about tech.

  1. Empathy for the guest – When talking to another person, never let someone flounder and fail – compensate when necessary so they are successful.
  2. Empathy for the audience – Stay conscious that you're delivering an talk/episode/post that people want to watch/read.
  3. Improvisation – Learn how to think on your feet and keep the conversation going (“Yes, and…”) Consider ComedySportz or other mind exercises.
  4. Listening – Don't just wait to for your turn to speak, just to say something, and never interrupt to say it. Be present and conscious and respond to what you’re hearing
  5. Speaking experience – Do the work. Hundreds of talks. Hundreds of interviews. Hundreds of shows. This ain’t your first rodeo. Being good means hard work and putting in the hours, over years, whether it's 10 people in a lunch presentation or 2000 people in a keynote, you know what to articulate.
  6. Technical experience – You have to know the technology. Strive to have context and personal experiences to reference. If you've never built/shipped/deployed something real (multiple times) you're just talking.
  7. Be a customer – You use the product, every day, and more than just to demo stuff. Run real sites, ship real apps, multiple times. Maintain sites, have sites go down and wake up to fix them. Carry the proverbial pager.
  8. Physical mannerisms – Avoid having odd personal ticks and/or be conscious of your performance on video. I know what my ticks are and I'm always trying to correct them. It's not self-critical, it's self-aware.
  9. Personal brand – I'm not a fan of "personal branding" but here's how I think of it. Show up. (So important.) You’re a known quantity in the community. You're reliable and kind. This lends credibility to your projects. Lend your voice and amplify others. Be yourself consistently and advocate for others, always.
  10. Confidence – Don't be timid in what you have to say BUT be perfectly fine with saying something that the guest later corrects. You're NOT the smartest person in the room. It's OK just to be a person in the room.
  11. Production awareness – Know how to ensure everything is set to produce a good presentation/blog/talk/video/sample (font size, mic, physical blocking, etc.) Always do tech checks. Always.

These are just a few tips but they've always served me well. We've done 450 episodes of Azure Friday and I've done nearly 650 episodes of the Hanselminutes Tech Podcast. Please Subscribe!

Related Links

* pic from stevebustin used under CC.

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About Scott

Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.

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You got this! You know the fundamentals. You are a learner. Plus The Imposter's Handbook

January 26, '18 Comments [8] Posted in Musings
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Sometimes we all get overwhelmed. There's a million (no irony there) reasons to be overwhelmed today, to be sure. I got an email from a community member who was feeling like they hadn't kept up on the latest tech. Of course, anything you learn today will be obsolete tomorrow, right? I'm overwhelmed thinking of it!

I wrote a little thread about this on Twitter and I wanted to expand on it here.

Maybe you're a dev who's been keeping up and fresh on the latest since jump, or maybe you've been using the same reliable framework for your whole career.

It can be totally overwhelming when you "wake up" and look around and notice that you don't know NOUN.js or ASPNET 10 or the like. You feel like it's over, and you've missed the boat. I want to encourage you. You're a developer! You have a good base to build on!

You may not know today's JavaScript/Java/C# but you DO know JavaScript/Java/C#. Yes, the Internet moved your cheese while you were sleeping, but you DID grow. When talking to employers, emphasize the base of knowledge you bring. Frameworks come and go. Fundamentals remain.

I really recommend Rob Conery's "The Imposter's Handbook" as a great way to reinforce those fundamentals and core concepts.Rob has been programming for years but without a CS degree. This book is about all the things he learned and all the gaps that got filled in while he was overwhelmed.

Yes this is a squishy blog post, but sometimes that's what's needed. You are smart, you are capable. Look at the replies to the twitter thread and you'll see you are not alone. Your job as a programmer is to be the figure-outer.

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About Scott

Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.

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Scaling Mentorship

October 15, '17 Comments [3] Posted in Musings
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You may have had a mentor in the past. Often these are more senior/elder people who are further along in their career. The presumption usually is that if they are "ahead" of you that they likely have something profound to offer you in the areas of advice or strategy.

This is a classic mentor/mentee situation and while I think it has value, it has a few problems that are worth pointing out. Does it scale? Is a senior person the right mentor for you? Is just one mentor the right number? Does that person's time support mentoring you?

I've been blessed to have several mentors over the years and I've been fortunate to be a mentor myself. But there's only so much time in the day. Even if I could truly mentor 4 people a week, and meet with them a few times a month, that could fill up many days. Plus, I have to ask my self - am I giving them what they need? Personal advice? Career advice? Technical advice? Getting promoted advice? Life advice?

Create a Board of Directors for Your Life

I've been experimenting with a few other models for mentorship. Five years ago I set up a Board of Directors for my life. You can learn more at

Companies have mission statements and a Board of Directors. Your life is pretty important. Why not create a Life Board of Directors to help you through it? Pick 2 to 5 of your friends. Not necessarily your closest friends, but friends that are close enough where you can really confide but not so close that they can't see the big picture. Email them one a month, once a quarter or "once a crisis." Ask them for advice, lean on them, trust them and help them as well.

Assemble "Team You" and use your team to brainstorm directions and implementations of big decisions like moving to New York, or changing your business's direction, starting a new venture, or getting fit.

Use your personal Board of Directors as one of the compasses in your life. You've got family, friends, perhaps faith, hobbies, values, etc. Add your Team to this list of personal compasses.

It might sound like a silly mind game, but that's common with many hacks. Hacks feel insignificant but can have huge effects. The trick is to remember that it is a hack - you're hacking yourself. The idea of life's board of directors is a relationship hack meant to remind you in difficult times that you can agree on something fundamental and you have a team to support you in your endeavors. Set a direction and head in that direction with the confidence you've got a supportive group behind you.

Go assemble your Life's Board today.

Host Mentorship Meals

Over the last several months I've been quietly hosting "Dinner for people on the come up." These are dinners where everything is FrieNDA and we talk frankly about our jobs, our levels, our work situations, and most importantly - we find new mentors and people with whom to brainstorm. It's a mentorship multiplier. We encourage folks to pull from the pool of potential peer mentors.

Tonight we had one with almost 20 people. These were 20 mostly young people, many women and people of color who were all trying to find their way in tech. I have some life experiences to offer this group, but most of all what I can lend is my privilege. I can use my standing within the company and the industry to invite folks together and let them take over and mentor each other.

I host the mean, kick it off, sometimes invite guests to speak, and the attendees often break off into small groups, meet up separately and network. Peer mentorship is just as important as "elder/senior" mentorship.

It also helps mentor people in the the fullness of their personalities. Where I might help with speaking at conferences or technical issues, someone else can better speak to issues of harassment, or how to get a promotion, or how to be better seen and heard in meetings. I can also learn from younger people - and I do - every day.

The goal of mentorship isn't to lecture and preach, it's to guide and counsel, inspire and motivate. Most of all, to listen. Once you've truly heard your mentee, then you can help them think strategically and better plan their career, no matter what their challenges and strengths.

What do you recommend as positive ways to Scale Mentorship?

* Stock photo from The Jopwell Collection

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About Scott

Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.

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Disclaimer: The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.