Back to Basics: Secure Hash Algorithms
September 04, 2020 by Maxim IntegratedThe main idea behind a Secure Hash Algorithm (SHA) function is to take data of variable size and condense it into a fixedsize bit string output. Here, learn the basics of an SHA and its variants.
In this application note, we will discuss the Secure Hash Algorithms (SHA) that are widely used in symmetric key cryptography. The basic idea behind a SHA function is to take data of variable size and condense it into a fixedsize bit string output. This concept is called hashing. The SHA functions are a family of hashing algorithms that have been developed over time through oversight by the National Institute of Standards and Technology (NIST). The latest of these is the SHA3 function. Maxim has a family of secure authenticator products that provide both SHA2 and SHA3 functions.
Figure 1 shows the basic concept of secure hash generation.
Figure 1. Secure hash generation, basic concept.
SHA Characteristics
The SHA functions have the following characteristics:
 They have variable input length and fixed output length.
 They are oneway functions. Figure 1 shows that it is infeasible to use the resultant hash value to regenerate the input text other than trying each possible input text. This becomes computationally impossible for sufficiently large inputs.
 If the same input message is fed to the SHA function, it will always generate the same resultant hash.
 It is not possible to generate the same hash value using two different input values. This is called “collision resistance.”
 A small change in the input value, even a single bit, completely changes the resultant hash value. This is called the “avalanche effect.”
If a hash function satisfies all of the above, it is considered a strong hash function.
Types of SHA Functions
Some of the SHA functions currently in use are:
 SHA1
 SHA2
 SHA3
Because SHA1 is being phased out and is not recommended for any new designs, this application note only discusses SHA2 and SHA3.
SHA2
The SHA2 function has four main types based on output bit lengths as follows:
 SHA224 – hash is 224 bits long.
 SHA256 – hash is 256 bits long.
 SHA384 – hash is 384 bits long.
 SHA512 – hash is 512 bits long.
As an example, Figure 2 shows a block diagram of a SHA256 engine.
Figure 2. SHA256 – hash generation flow.
The input message is first padded to make sure that it will completely fit in “n” number of 512bit blocks.
SHA3
The SHA3 function has no predefined output length. The input and output lengths have no maximums either. For comparison purposes with SHA2, we can define four main types based on output bit lengths:
 SHA3224 – hash is 224 bits long.
 SHA3256 – hash is 256 bits long.
 SHA3384 – hash is 384 bits long.
 SHA3512 – hash is 512 bits long.
All SHA3 types use a Keccak sponge function. Just like a sponge, the first step is to soak in or absorb the input message. In the next phase, the output hash is squeezed out. Figure 3 illustrates these phases using the block diagram of a SHA3256 function.
Figure 3. SHA3256 – Keccak sponge hash generation flow.
The iteration function in the Figure 3 diagram takes in the 1600 bits of data, puts it through 24 rounds of permutation using a specific algorithm, and then passes it to the next stage as a 1600bit block. This continues until the absorbing phase is complete.
Once the absorbing phase is complete, the last 1600bit block is passed to the squeezing phase. In this case, as the SHA3256 output hash length is less than 1088 bits, the squeezing phase does not need any iteration functions. The first 256 bits from the last stage is the output hash.
If the required hash length was 2500 bits, for example, we would have needed three more instances of the iteration function to get the desired length hash.
Differences between a Secure Hash and an HMAC
Before exploring message authentication, it is important to understand the differences between a secure hash and a hashed message authentication code (HMAC), which are illustrated in Figure 4.
Figure 4. Secure hash and HMAC.
Essentially, the secure hash uses a hashing algorithm such as SHA3 to produce a fixedlength hash of the message regardless of the message length. HMAC is similar but uses a key as an additional input to the hashing engine. It also produces a fixedlength hash regardless of the input message length.
Message Authentication Using SHA3
Figure 5 illustrates an example of how a message can be authenticated using SHA3 showing all the concepts already discussed.
Figure 5. Message authentication using SHA3 HMAC.
In Figure 5, Alice calculates the HMAC of a message by feeding it to a SHA3 engine along with a specific key. Alice has securely shared this key previously with Bob.
Alice sends the resultant HMAC along with the message to Bob. Bob then generates his own HMAC of the message using the same key Alice shared with him earlier. Bob compares the HMAC he generated with the one he received from Alice. If they match, the message has not been tampered with and is authentic. In this scenario, someone could intercept the HMAC and the message and then alter the message and generate a new HMAC and send it to Bob. However, that will not work, because the interceptor will not have the secret key and the received HMAC will not match the computed HMAC. Thus, Bob will realize that the message was not authentic.
The most important aspect of this sequence is for Alice and Bob to keep their shared key a secret from everyone else.
Maxim’s secure authenticator products, such as the DS28E50, has builtin SHA engines and a multitude of secure features like ChipDNATM that help secure any key, per the user’s requirements. For more information, see Maxim Application Note 6767, How ChipDNA Physically Unclonable Function Technology Protects Embedded Systems.
Secure Authenticators with Builtin SHA Engines
Table 1 outlines the various secure authenticators that are available from Maxim with the other main features and target applications.
Table 1. Maxim Devices with Builtin SHA Engines for Cryptographic Applications (E: 1Wire, C: I2C)
Part  SHA Engine  Other Features  Typical Applications 
DS28E50  SHA3  2Kb memory, ChipDNA 

DS28E16  SHA3  256 bits of secure memory, low cost 

DS28E15 DS28EL15 
SHA256  512bit memory, the EL device is a lowvoltage (1.671V to 1.89V) device 

DS28E22 DS28EL22 
SHA256  2048bit memory, the EL device is a lowvoltage (1.671V to 1.89V) device 

DS28E25 DS28EL25 
SHA256  4096bit memory, bidirectional authentication, the EL device is a lowvoltage (1.671V to 1.89V) device 

DS1964S  SHA256  512bit memory, iButton 

DS28E36 DS28C36 
SHA256  8Kb memory, bidirectional ECDSA or SHA256 authentication, two GPIOs 

DS28E83  SHA256  10Kb of OTP memory, highradiation resistance 

DS28E84  SHA256  15Kb of FRAM, 10Kb of OTP memory, highradiation resistance 

Summary
Using SHA to secure/authenticate a physical item or a piece of intellectual property is a very straightforward process, provided the correct supporting tools are used. Maxim’s secure authenticators are ideal for these purposes. They have built in SHA engines with many features that make implementing security for any application a relatively simple process. Each device has comprehensive support systems like evaluation kits and free software, including Cbased demonstration codes, to assist a developer to quickly deploy their solution.