This document discusses hash and MAC algorithms. It provides details on hash functions, the Secure Hash Algorithm (SHA), and HMAC.
Hash functions take a message and produce a fixed-size hash value. SHA is a secure hash algorithm developed by NIST that produces 160-bit or longer hash values. It involves padding the message, initializing a buffer, processing the message in blocks through a compression function, and outputting the final hash.
HMAC is a MAC algorithm that incorporates a secret key into an existing hash function like MD5 or SHA. It pads and XORs the key, hashes the result with the message, then hashes again with a padded key to produce the MAC value.
If the four 32-bit constants of SHA-1 can be modified, then exploitable collisions can be constructed. No need to panic, this doesn’t affect the original SHA-1. However, vendors and customers of products with custom cryptography will be interested.
for more info, check http://malicioussha1.github.io/
If the four 32-bit constants of SHA-1 can be modified, then exploitable collisions can be constructed. No need to panic, this doesn’t affect the original SHA-1. However, vendors and customers of products with custom cryptography will be interested.
for more info, check http://malicioussha1.github.io/
Security Hash Algorithm (SHA) was developed in 1993 by the National Institute of Standards and Technology (NIST) and National Security Agency (NSA).
It was designed as the algorithm to be used for secure hashing in the US Digital Signature Standard.
• Hashing function is one of the most commonly used encryption methods. A hash is a special mathematical function that performs one-way encryption.
• SHA-l is a revised version of SHA designed by NIST and was published as a Federal Information Processing Standard (FIPS).
• Like MD5, SHA-l processes input data in 512-bit blocks.
• SHA-l generates a 160-bit message digest. Whereas MD5 generated message digest of 128 bits.
• The procedure is used to send a non secret but signed message from sender to receiver. In such a case following steps are followed:
1. Sender feeds a plaintext message into SHA-l algorithm and obtains a 160-bit SHA-l hash.
2. Sender then signs the hash with his RSA private key and sends both the plaintext message and the signed hash to the receiver.
3. After receiving the message, the receiver computes the SHA-l hash himself and also applies the sender's public key to the signed hash to obtain the original hash H.
An introduction to the SHA Hashing Algorithm. The origins of SHA are explained, along with the family taxonomy of SHA message digest functions. We also cover their uses in cryptography. http://boblandstrom.com
Security Hash Algorithm (SHA) was developed in 1993 by the National Institute of Standards and Technology (NIST) and National Security Agency (NSA).
It was designed as the algorithm to be used for secure hashing in the US Digital Signature Standard.
• Hashing function is one of the most commonly used encryption methods. A hash is a special mathematical function that performs one-way encryption.
• SHA-l is a revised version of SHA designed by NIST and was published as a Federal Information Processing Standard (FIPS).
• Like MD5, SHA-l processes input data in 512-bit blocks.
• SHA-l generates a 160-bit message digest. Whereas MD5 generated message digest of 128 bits.
• The procedure is used to send a non secret but signed message from sender to receiver. In such a case following steps are followed:
1. Sender feeds a plaintext message into SHA-l algorithm and obtains a 160-bit SHA-l hash.
2. Sender then signs the hash with his RSA private key and sends both the plaintext message and the signed hash to the receiver.
3. After receiving the message, the receiver computes the SHA-l hash himself and also applies the sender's public key to the signed hash to obtain the original hash H.
An introduction to the SHA Hashing Algorithm. The origins of SHA are explained, along with the family taxonomy of SHA message digest functions. We also cover their uses in cryptography. http://boblandstrom.com
Each month, join us as we highlight and discuss hot topics ranging from the future of higher education to wearable technology, best productivity hacks and secrets to hiring top talent. Upload your SlideShares, and share your expertise with the world!
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This presentation has information about what do you mean by an algorithm, what is hashing and various hashing algorithms and their applications. Approximate counting Algorithm and their applications, Counting Distinct Elements Algorithm and their applications and Frequency estimation algorithm and their applications . Also, the research papers we referenced.
This material covers Authentication requirement, Authentication function, MAC, Hash function, Security of hash function and MAC, SHA, Digital signature and authentication protocols, DSS, Authentication protocols like Kerberos and X.509, entity authentication
This material covers Authentication requirement, Authentication function, MAC, Hash function, Security of hash function and MAC, SHA, Digital signature and authentication protocols, DSS, Authentication protocols like Kerberos and X.509, entity authentication
GDG Cloud Southlake #33: Boule & Rebala: Effective AppSec in SDLC using Deplo...James Anderson
Effective Application Security in Software Delivery lifecycle using Deployment Firewall and DBOM
The modern software delivery process (or the CI/CD process) includes many tools, distributed teams, open-source code, and cloud platforms. Constant focus on speed to release software to market, along with the traditional slow and manual security checks has caused gaps in continuous security as an important piece in the software supply chain. Today organizations feel more susceptible to external and internal cyber threats due to the vast attack surface in their applications supply chain and the lack of end-to-end governance and risk management.
The software team must secure its software delivery process to avoid vulnerability and security breaches. This needs to be achieved with existing tool chains and without extensive rework of the delivery processes. This talk will present strategies and techniques for providing visibility into the true risk of the existing vulnerabilities, preventing the introduction of security issues in the software, resolving vulnerabilities in production environments quickly, and capturing the deployment bill of materials (DBOM).
Speakers:
Bob Boule
Robert Boule is a technology enthusiast with PASSION for technology and making things work along with a knack for helping others understand how things work. He comes with around 20 years of solution engineering experience in application security, software continuous delivery, and SaaS platforms. He is known for his dynamic presentations in CI/CD and application security integrated in software delivery lifecycle.
Gopinath Rebala
Gopinath Rebala is the CTO of OpsMx, where he has overall responsibility for the machine learning and data processing architectures for Secure Software Delivery. Gopi also has a strong connection with our customers, leading design and architecture for strategic implementations. Gopi is a frequent speaker and well-known leader in continuous delivery and integrating security into software delivery.
GraphRAG is All You need? LLM & Knowledge GraphGuy Korland
Guy Korland, CEO and Co-founder of FalkorDB, will review two articles on the integration of language models with knowledge graphs.
1. Unifying Large Language Models and Knowledge Graphs: A Roadmap.
https://arxiv.org/abs/2306.08302
2. Microsoft Research's GraphRAG paper and a review paper on various uses of knowledge graphs:
https://www.microsoft.com/en-us/research/blog/graphrag-unlocking-llm-discovery-on-narrative-private-data/
"Impact of front-end architecture on development cost", Viktor TurskyiFwdays
I have heard many times that architecture is not important for the front-end. Also, many times I have seen how developers implement features on the front-end just following the standard rules for a framework and think that this is enough to successfully launch the project, and then the project fails. How to prevent this and what approach to choose? I have launched dozens of complex projects and during the talk we will analyze which approaches have worked for me and which have not.
Neuro-symbolic is not enough, we need neuro-*semantic*Frank van Harmelen
Neuro-symbolic (NeSy) AI is on the rise. However, simply machine learning on just any symbolic structure is not sufficient to really harvest the gains of NeSy. These will only be gained when the symbolic structures have an actual semantics. I give an operational definition of semantics as “predictable inference”.
All of this illustrated with link prediction over knowledge graphs, but the argument is general.
PHP Frameworks: I want to break free (IPC Berlin 2024)Ralf Eggert
In this presentation, we examine the challenges and limitations of relying too heavily on PHP frameworks in web development. We discuss the history of PHP and its frameworks to understand how this dependence has evolved. The focus will be on providing concrete tips and strategies to reduce reliance on these frameworks, based on real-world examples and practical considerations. The goal is to equip developers with the skills and knowledge to create more flexible and future-proof web applications. We'll explore the importance of maintaining autonomy in a rapidly changing tech landscape and how to make informed decisions in PHP development.
This talk is aimed at encouraging a more independent approach to using PHP frameworks, moving towards a more flexible and future-proof approach to PHP development.
Essentials of Automations: Optimizing FME Workflows with ParametersSafe Software
Are you looking to streamline your workflows and boost your projects’ efficiency? Do you find yourself searching for ways to add flexibility and control over your FME workflows? If so, you’re in the right place.
Join us for an insightful dive into the world of FME parameters, a critical element in optimizing workflow efficiency. This webinar marks the beginning of our three-part “Essentials of Automation” series. This first webinar is designed to equip you with the knowledge and skills to utilize parameters effectively: enhancing the flexibility, maintainability, and user control of your FME projects.
Here’s what you’ll gain:
- Essentials of FME Parameters: Understand the pivotal role of parameters, including Reader/Writer, Transformer, User, and FME Flow categories. Discover how they are the key to unlocking automation and optimization within your workflows.
- Practical Applications in FME Form: Delve into key user parameter types including choice, connections, and file URLs. Allow users to control how a workflow runs, making your workflows more reusable. Learn to import values and deliver the best user experience for your workflows while enhancing accuracy.
- Optimization Strategies in FME Flow: Explore the creation and strategic deployment of parameters in FME Flow, including the use of deployment and geometry parameters, to maximize workflow efficiency.
- Pro Tips for Success: Gain insights on parameterizing connections and leveraging new features like Conditional Visibility for clarity and simplicity.
We’ll wrap up with a glimpse into future webinars, followed by a Q&A session to address your specific questions surrounding this topic.
Don’t miss this opportunity to elevate your FME expertise and drive your projects to new heights of efficiency.
LF Energy Webinar: Electrical Grid Modelling and Simulation Through PowSyBl -...DanBrown980551
Do you want to learn how to model and simulate an electrical network from scratch in under an hour?
Then welcome to this PowSyBl workshop, hosted by Rte, the French Transmission System Operator (TSO)!
During the webinar, you will discover the PowSyBl ecosystem as well as handle and study an electrical network through an interactive Python notebook.
PowSyBl is an open source project hosted by LF Energy, which offers a comprehensive set of features for electrical grid modelling and simulation. Among other advanced features, PowSyBl provides:
- A fully editable and extendable library for grid component modelling;
- Visualization tools to display your network;
- Grid simulation tools, such as power flows, security analyses (with or without remedial actions) and sensitivity analyses;
The framework is mostly written in Java, with a Python binding so that Python developers can access PowSyBl functionalities as well.
What you will learn during the webinar:
- For beginners: discover PowSyBl's functionalities through a quick general presentation and the notebook, without needing any expert coding skills;
- For advanced developers: master the skills to efficiently apply PowSyBl functionalities to your real-world scenarios.
Accelerate your Kubernetes clusters with Varnish CachingThijs Feryn
A presentation about the usage and availability of Varnish on Kubernetes. This talk explores the capabilities of Varnish caching and shows how to use the Varnish Helm chart to deploy it to Kubernetes.
This presentation was delivered at K8SUG Singapore. See https://feryn.eu/presentations/accelerate-your-kubernetes-clusters-with-varnish-caching-k8sug-singapore-28-2024 for more details.
JMeter webinar - integration with InfluxDB and GrafanaRTTS
Watch this recorded webinar about real-time monitoring of application performance. See how to integrate Apache JMeter, the open-source leader in performance testing, with InfluxDB, the open-source time-series database, and Grafana, the open-source analytics and visualization application.
In this webinar, we will review the benefits of leveraging InfluxDB and Grafana when executing load tests and demonstrate how these tools are used to visualize performance metrics.
Length: 30 minutes
Session Overview
-------------------------------------------
During this webinar, we will cover the following topics while demonstrating the integrations of JMeter, InfluxDB and Grafana:
- What out-of-the-box solutions are available for real-time monitoring JMeter tests?
- What are the benefits of integrating InfluxDB and Grafana into the load testing stack?
- Which features are provided by Grafana?
- Demonstration of InfluxDB and Grafana using a practice web application
To view the webinar recording, go to:
https://www.rttsweb.com/jmeter-integration-webinar
Builder.ai Founder Sachin Dev Duggal's Strategic Approach to Create an Innova...Ramesh Iyer
In today's fast-changing business world, Companies that adapt and embrace new ideas often need help to keep up with the competition. However, fostering a culture of innovation takes much work. It takes vision, leadership and willingness to take risks in the right proportion. Sachin Dev Duggal, co-founder of Builder.ai, has perfected the art of this balance, creating a company culture where creativity and growth are nurtured at each stage.
Dev Dives: Train smarter, not harder – active learning and UiPath LLMs for do...UiPathCommunity
💥 Speed, accuracy, and scaling – discover the superpowers of GenAI in action with UiPath Document Understanding and Communications Mining™:
See how to accelerate model training and optimize model performance with active learning
Learn about the latest enhancements to out-of-the-box document processing – with little to no training required
Get an exclusive demo of the new family of UiPath LLMs – GenAI models specialized for processing different types of documents and messages
This is a hands-on session specifically designed for automation developers and AI enthusiasts seeking to enhance their knowledge in leveraging the latest intelligent document processing capabilities offered by UiPath.
Speakers:
👨🏫 Andras Palfi, Senior Product Manager, UiPath
👩🏫 Lenka Dulovicova, Product Program Manager, UiPath
Epistemic Interaction - tuning interfaces to provide information for AI supportAlan Dix
Paper presented at SYNERGY workshop at AVI 2024, Genoa, Italy. 3rd June 2024
https://alandix.com/academic/papers/synergy2024-epistemic/
As machine learning integrates deeper into human-computer interactions, the concept of epistemic interaction emerges, aiming to refine these interactions to enhance system adaptability. This approach encourages minor, intentional adjustments in user behaviour to enrich the data available for system learning. This paper introduces epistemic interaction within the context of human-system communication, illustrating how deliberate interaction design can improve system understanding and adaptation. Through concrete examples, we demonstrate the potential of epistemic interaction to significantly advance human-computer interaction by leveraging intuitive human communication strategies to inform system design and functionality, offering a novel pathway for enriching user-system engagements.
3. Hash Functions
Hash functions
Takes an input message M
Produces an output hash value, H(M), for the message M.
M
Hash function
H(M)
4. Hash Functions
Hash functions
partitions it into L fixed-size blocks of b bits each
M
b bits b bits b bits b bits → L blocks × b bits
H(M)
Hash function
5. If necessary, the final block is padded to b bits
Modify the length of M to L blocks × b bits
→ L blocks × b bits
→ M + padding
M
b bits b bits b bits b bits
H(M)
M Padding
Hash function
Hash Functions
6. Compression function, f
Hash algorithm involves repeated use of compression
function, f
takes an n-bit input from previous step and a b-bit input from
message.
produces an n- bit output.
Hash Functions
7. …
Hash functions
IV or CV0 Initial value for 1st compression
CVi Output of the ith compression
CVL The final hash value, H(M)
n Length of hash code
Yi ith input block from message M
b Length of input block
Hash Functions
8. Secure Hash Algorithm
SHA (Secure Hash Algorithm)
developed by NIST and published as FIPS 180 in 1993
NIST, National Institute of Standards and Technology
FIPS, a federal information processing standard
revised version FIPS 180-1 was issued in 1995
referred to as SHA-1 that produces 160 bit hash value.
FIPS 180-2 in 2002 defined 3 versions of SHA
SHA-256, SHA-384 and SHA-512 for 256, 384 and 512 bits hash.
9. Secure Hash Algorithm
SHA-1 is based on the hash function MD4.
SHA-256, SHA-384, SHA-512
have the same underlying structure as SHA-1
also use the same types of modular arithmetic and logical binary
operation as SHA-1
Comparison of 4 version of SHA
10. Secure Hash Algorithm
SHA-512 Logic
Input : a maximum length of less than < 2128
bits
Output : a 512-bit message digest
12. Secure Hash Algorithm
Step 1: Append padding bits
The message is padded so that its length is congruent to 896 mod
1024, [ length ≡ 896 mod 1024 ]
Padding is always added, even if the length of message is satisfied.
If the length of message is 896 bits, padding is 1024 bits,
because 1920(=896 + 1024) mod 1024 = 448.
thus, 1 ≤ padding bits ≤ 1024
The padding consists of a single 1-bit followed by the necessary
number of 0-bits, (100…0)
1. append padding
2. append length
3. Initialize hash buffer
4. Process message
5. Output
1024 × (N-1) + 896 bits
13. Secure Hash Algorithm
Step 2: Append length
A block of 128 bit is appended to the message
contains the length of the original message (before padding)
After 2 steps, the length of message is a multiple of 1024
The expanded message is a sequence of 1024 bit block M1, …, MN
1. append padding
2. append length
3. Initialize hash buffer
4. Process message
5. Output
14. Secure Hash Algorithm
Step 3 : Initialize hash buffer
Secure hash algorithm use a 512-bit buffer.
holding the intermediate and final result of the hash function.
Eight 64-bit registers (a, b, c, d, e, f, g, h) are used.
IV( Initial vector ) of eight 64-bit registers in hexadecimal value.
These words were obtained by taking the first 64bits of the fractional
parts of the square roots of the first 80 prime numbers.
1. append padding
2. append length
3. Initialize hash buffer
4. Process message
5. Output
a = 6A09 E667 F3BC
C908
e = 510E 527F ADE6
82D1
b = BB67 AE85 84CA
A73B
f = 9B05 688C 2B3E
6C1F
c = 3C6E F372 FE94
F82B
g = 1F83 D9AB FB41
BD6B
15. Secure Hash Algorithm
Step 4 : Process message in 1024-bit (128-word) blocks.
The main function of the algorithm is module F in the below picture.
The module F is the compression function.
Mi is the ith input block of expanded message.
Hi is the intermediate hash result and HN is the final result.
The operation ( + ) is word-by word addition mod 264.
1. append padding
2. append length
3. Initialize MD buffer
4. Process message
5. Output
16. Secure Hash Algorithm
The module F consists of 80
rounds for 1 block, Mi
Let t-th round call round t
where 0 ≤ t ≤ 79
Round t takes as an input
the contents of 512bit buffer,
abcdefg
a 64-bit value, Wt
an additive constant, Kt
Round t updates
the contents of buffer for the t +1
round
17. Secure Hash Algorithm
Wt, a 64-bit value
A part of message block Mi is used at round t.
These values derived from the current 1024-bit block, Mi
Detail explain how to generate will be later.
Kt , an additive constant
An integer number is added at round t.
These words represent the first 64-bits of fractional parts of the cube
roots of the first 80 prime number.
Kt provides a “randomized” set of 64-bit patterns
which eliminate any regularities in the input data.
18. Secure Hash Algorithm
After 80th rounds, the contents
of the buffer is added to the
input to the first round (Hi-1) to
produce (Hi).
The addition is done independently
for each 8 words with each of the
corresponding words in Hi-1
using addition modulo 264
19. Secure Hash Algorithm
Step 5 : Output
After all N 1024 bits blocks have been processed, the output
form the Nth stage is the 512-bit message digest.
Summary of SHA-512
H0 = IV
Hi = SUM64(Hi-1, abcdefghi)
MD = HN
N = number of blocks in the expanded message
SUM64 = Addition modulo 264
performed separately on each word of
the pair of inputs
1. append padding
2. append length
3. Initialize MD buffer
4. Process message
5. Output
20. Secure Hash Algorithm
SHA-512 round function
Detail at the logic in each of the 80 steps of the processing
of on 512-bit block.
Each round is defined by the following set of equation :
T1 and T2 will be shown in the next slide.
a = T1+T2 e = d + T1
b = a f = e
c = b g = f
d = c h = g
22. Secure Hash Algorithm
a = T1+T2 e = d + T1
b = a f = e
c = b g = f
d = c h = g
),,(
)(
)(
),,(
512
02
512
1
1
cbaMaj
aT
KW
e
gfeCh
hT
tt
+
=
++
+
+
=
∑
∑
T2
T1
23. Secure Hash Algorithm
Wt , a 64-bit value
Wt are derived from the 1024-bit message.
The first 16 values of Wtare taken directly from the 16
words of the current block.
24. Secure Hash Algorithm
The remaining values are defined as follows.
hton the rigby zeroes
nxx
xxxx
xxxx
WWWWW
n
ttttt
paddingwith
bitsbyargumentbit-64theofshiftleft)(SHR
)(SHR)(ROTR)(ROTR)(
)(SHR)(ROTR)(ROTR)(
where
)()(
66119512
1
781512
0
2
512
1715
512
016
=
⊕⊕=
⊕⊕=
+++= −−−−
σ
σ
σσ
26. HMAC
MAC (A message authentication code)
defined FIPS SUB 113
The most common approach to construct a MAC
Recently, there has been increased interest in developing a
MAC.
The motivation
1. cryptographic hash function, MD5 and SHA-1, generally execute
faster in software than symmetric block cipher such as DES.
2. Library code for cryptographic hash functions is widely available.
27. HMAC
A hash function such as SHA
not designed for use as a MAC
cannot be used directly for that purpose because it does not rely on
the secret key.
There have been a number of proposals
for the incorporation of a secret key into an existing hash algorithm
HMAC[BELL96a] is most supported.
issued RFC 2104 and as a NIST(FIPS 198).
as the mandatory-to-implement MAC for IP security
used in other Internet protocol such as SSL.
28. HMAC
HMAC Design Objectives on RFC 2104 list
To use, without modification, available hash functions. In particular,
hash functions that perform well in software and code is freely and
widely available.
To allow for easy replaceability of the embedded hash function in
case faster or more secure hash function are found or required.
To preserve the original performance of the hash function without
incurring a significant degradation.
To use and handle key in a simple way.
To have a well understood cryptographic analysis of the strength of
the authentication mechanism based on reasonable assumption about
the embedded hash function.
29. HMAC
HMAC structure
IV = initial value input to hash function
M = message input to HMAC
K = secret key recommended length is
≥ n;
if key length is greater than b; the key is input
to the hash function to produce an n-bit key.
ipad = 00110110 repeated b/8 times
opad = 01011100 repeated b/8 times
30. HMAC
HMAC structure
Hash = embedded hash function
( MD5, SHA-1, RIPEMD-160)
Yi= ith block of M, 0 ≤ i ≤ (L-1)
K+
= K padded with 0 on left so that the
result is b bits in length
L = number of blocks in M
b = number of bits in a block
n = length of hash code produced by H
31. HMAC
HMAC Algorithm
1. Append zero to the left end of K to
create a b-bit string K+
if K is of length 160 bits and b = 512, K
will be appended with 44 zero bytes 0×00.
K+
= K padded with 0 on left so that the
result is b bits in length
2. XOR K+
with ipad to produce the b-bit
block Si
ipad = 00110110
3. Append M to Si
32. HMAC
4. Apply H to the stream generated in
step 3.
5. XOR K+
with opad to produce the b-bit
block So.
opad = 01011100
4. Append the hash result from step 4 to
So.
5. Apply H to the stream generated in
step 6 and output result.
33. HMAC
HMAC should execute in
approximately the same time as
the embedded hash function
for a long message.
HMAC adds 3 executions of the hash
compression function.
A more efficient implement is
possible by precomputing
))opad(,IV(f
))ipad(,IV(f
⊕
⊕
+
+
K
K
34. HMAC
These quantities only need to computed initially and every
time the key exchange.
The precomputed quantities substitute for the initial value.
Only one additional instance of the compression function is
added to the processing.
))opad(,IV(f
))ipad(,IV(f
⊕
⊕
+
+
K
K
35. HMAC
Security of HMAC
The security of any MAC function based on an embedded
hash function depends in some way on the cryptographic
strength of the underlying hash function.
The appeal of HMAC is that its designers have been able to
prove an exact relationship between the strength of the
embedded hash function and the strength of HMAC
36. HMAC
The security of HMAC is expressed in terms of the
probability of successful forgery with
a given amount of time spent by the forger
a given number of message-MAC pairs created with the same key.
For a given level of effort (time, message-MAC pairs) on
messages generated by a legitimate user and seen by the
attacker, the probability successful attack on HMAC is
equivalent to one of following attacks.
37. HMAC
The probability successful attack on HMAC
1. The attacker is able to compute an output of the
compression function even with an IV that is random,
secret, and unknown to the attacker.
2. The attacker finds collisions in the hash function even
when IV is random and secret.
38. HMAC
In the 1st attack, compression function as equivalent to the
hash function.
For this attack, the IV of the hash function is replaced by a secret,
random value of n bits.
An attack requires either
A brute-force attack on the key, a level of effort on the order of 2n
A birthday attack, a special case of 2nd attack.
39. HMAC
In the 2nd attack, the attack is looking for 2 messages M
and M’ that produce H(M)=H(M’)
A birthday attack requires a level of effort of 2n/2
for a hash length of n
MD5, 264
, looks feasible in today, so MD5 is unsuitable for HMAC?
The answer is no.
To attack MD5, attackers know the hash algorithm and IV, so they can
generate the hash code for any message
In HMAC, attackers don’t know K, so they can’t generate the hash code.
So, to attack HMAC, attackers must observe a sequence of messages.
For a hash code of 128 bits, this requires 264
observed blocks with using
the same key.
On a 1-Gbps, it takes 150,000 years to get a satisfied stream.
Thus, if speed is concern, MD5 is fully acceptable to use rather than
SHA-1 as the embedded hash function for HMAC.