June 9th, 2021
Every so often, I get asked how people can enter
into information security, what they should
study or what certifications they should pursue. I
don't have a good answer to this question. Or rather,
I don't have an easy answer to this question.
I don't find much value in any specific
certification program, and I know that everybody's
path is different, so I'm hesitant to give any more
specific advice but a broad recommendation to work
with your organization's security team on practical
projects to gain experience and an understanding of
how the team functions.
However, at the same time, there are a number
of things that I regard as, well, a common body of
knowledge in the field, a set of core competencies.
They are, in no particular order, and with no claim
- How to read a CVE
announcement and assess the impact based on its CWE
score and description. Understand that CVSS scores are
relative and impact in your environment may
- How to read a hyped, name- and
logo branded, corporation backed vulnerability
announcement -- as well as the relevant RFCs and
actual research paper, if any, instead of just the
breathless webpage -- and distill the actual,
realistic threat to your environment.
- The difference between a
vulnerability, a threat, risk, an exploit, and attack
surface; the likelihood of an exploit based on an
adversary's motivation, goals, and capabilities; being
able to ballpark the cost of defense within a given
- The difference between authentication
(authN) and authorization
(authZ); between secrecy and authenticity;
between authenticity and integrity. Be able to
translate these concepts across different
- The difference between symmetric
or private and asymmetric
or public key cryptography; between
encryption and hashing; between a key
derivation function and an HMAC
used for message authentication.
- You don't need to be
cryptographer, but you should know roughly what the cryptographic
right answers for your developers' questions are
-- and why.
- The difference between encryption
in transit and encryption at rest.
- Know to use TLS >= 1.2, but you
don't need to know the details of all the ciphers and
algorithms. But you should understand mTLS and client
cert validation vs. server cert validation
- Understand the x509 PKI
CA bundles, cert chains, and common client
- Know how to
use 'openssl s_client' and 'openssl
x509' to troubleshoot TLS connections, including
- HTTP basics: be able to make
manual HTTP requests via telnet(1) /
openssl s_client; know enough HTTP
headers and the general concepts of CSP;
be able to use in-browser developer tools to
troubleshoot, debug, and replay requests.
and HTML DOM to understand the different XSS
attack types conceptually.
- How to use curl(1) to
post data to and pull information from an API; use
jq(1) to manipulate
- Be able
to use tcpdump(1) to at least get the
gist of what's going on on the wire. I.e., protocol,
type, port, TCP S/R/P/F/., sequence numbers,
- Be able to use e.g., Wireshark to
drill down into specific flows, filter and pick
outliers out of the noise, debug TLS with logged
- Be aware of the different ICMP
types beyond echo request/reply: time
exceeded, fragmentation required, destination
- Be able to spot certain CIDRs.
E.g., for IPv4: 127/8, 169.254/16, 224/4, 240/4,
RFC1918; for IPv6: ::1, fc00::/7, fe80::/10,
- Understand that RFC1918 does not
imply the host cannot be reached from the internet;
internalize that NAT
is not a security control.
- How to send arbitrary packets
between two hosts, e.g., via nc(1) or
- The difference between
"Connection refused", "Connection
timed out", and "Name or service not
known/host not found".
- Be able to use
dig(1), nslookup(1) and
host(1) and understand why they don't care
about what you put into /etc/hosts.
- Understand basic DNS resolution.
I.e., lookups from client->stub
- Understand domain registration,
NS records, TTLs, and zone delegation. Grasp the difference in threat model
between plain DNS, DoH, and DNSSEC.
- Understand cache poisoning as a
concept and as applied to different protocols, such as
ARP, DNS, HTTP Proxy, ...
- Have a general awareness of the
internet, peering, ASNs,
BGP hijacking, and how (and when) governments can (and
do) censor or intercept/inspect (parts of) the
internet for their jurisdiction.
to use ktrace(1) / strace(1) / dtrace(1)
to figure out just what
files or sockets a program is accessing.
- Be able to
use nmap(1) to identify
a host's open ports and fingerprint them.
- Be able to use
SSH port forwarding, SSH pubkey options
(from=, command= / ForceCommand),
ProxyCommand, and use of SSH
Unix skills, pipes and common tools:
grep(1), sed(1), awk(1),
sort(1), uniq(1), diff(1),
comm(1), tr(1), ...
- Know how to use screen(1)
to keep long sessions uninterrupted, to resume if
needed, to juggle multiple remote terminals in a
- Really understand the Unix permissions
model: owner, group, others; permissions on
directories (e.g., 1777, 0711), Unix
groups; su(1) and sudo(1); how
- Understand TOCTOU attacks,
mktemp(3), and umask(2).
- Be able to convert timestamps between
formats and know to look for UTC offset when
correlating log entries.
- Enough shell
scripting to automate the execution of repeated
commands, including flow control using loops,
functions, and variable expansion.
- Enough Python, Perl, PHP, C, Go,
code you come across and at least make some sense of
- Enough C to understand how a buffer
overflow works and how to spot one. (90% of the
time a sprintf(3) of a user-generated string
into a fixed-size buffer.)
- Enough SQL
to be able to explain Little Bobby Tables
and to pull records from multiple tables.
- Enough input- and shell
meta-characters escaping to detect, abuse, and fix
unsanitized system(3) / popen(3) command-injections
(in the various languages).
- Be able to efficiently use your
preferred packaged manager to identify file/package
ownership, dependencies, package integrity. Be able
to create a package in your commonly used package
manager format (.deb, .rpm, ...) for
a non-trivial piece of software to understand
packaging, install scripts, signatures, validation
AWS* to be able to spin up an instance when needed.
Grok the difference between NACLs and Security Groups.
Be able to manage simple IAM resources and to inspect
down an S3 bucket.
whatever, although, honestly, AWS remains the 600 lb
- Enough Kerberos
to understand the concept
of client authentication versus service
authorization and replay
PGP to be able to send/receive, encrypt/decrypt,
and sign/verify encrypted and signed
- Understand SMTP basics,
email headers, etc. Know enough about SPF/DKIM/DMARC
to identify those headers and understand what they are
- Shamir's 3 Laws of Cryptography:
- Absolutely secure systems do not exist
- To halve your vulnerability, you have to double your expenditure
- Cryptography is typically bypassed, not penetrated
- Schneier's Law (Any person
can invent a security system so clever that she or he
can't think of how to break it.) and not to
attempt to invent your own cryptographic
- Be able to explain core concepts like
Zero Trust, Defense in Depth, Least Privilege, Failing
Closed, and Kerckhoff's Principle vs. Security by
- How containers are different from
virtual machines, and what their respective trust- and
control boundaries are.
- How to file an
actionable, useful bug report. Know how to manage
your own ticket
- When to seek input from others:
there's not just domain specific, but plenty of
general security expertise outside of the information
- Nothing, absolutely nothing about
cryptocurrencies. "Crypto" means "cryptography".
That's all you need to know.
Now granted, the above list is shaped by my own
personal background and experience, and you may do
well without many of them, making up for
gaps with experience and knowledge in areas
that I lack. That's quite ok.
You may also notice that a lot of this overlaps with a
general understanding of... well, computering on the
internet, with operations and system
administration concepts. This is no
coincidence. Good ops is good security.
A fair bit of what I regard as essential in this
field is covered in my video
lectures; if you are interested, please do check
them out and don't hesitate to hit me up
with any follow-up questions you might have.
Oh, and if you note the absence of all the "soft
skills" here: well, those are harder.
Perhaps another time...
June 9th, 2021