Support for SHA3 algorithm

[Wow, my response was completely wrong, confusing SHA-2 and SHA-3. See RandomBits’s answer below. Sorry about the mixup.]

Does swift have default support for SHA3-256 hashing algorithm for all devices iOS 12 and above?

Yes.

Just to be clear, SHA-3 is a family of algorithms based on the bit size (224, 256, 384, 512). Most folks leave out the 3, rendering SHA-3/256 as SHA-256.

With that in mind:

• Apple CryptoKit supports the 256-, 384-, and 512-bit variants. This requires iOS 13.

• Common Crypto (<CommonCrypto/CommonDigest.h>) supports all four variants all the way back to the dawn of iOS.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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According to the linked Apple CryptoKit documentation, only the SHA-2 algorithms are supported, e.g.:

An implementation of Secure Hashing Algorithm 2 (SHA-2) hashing with a 256-bit digest.

This is easily verified by comparing the result of using CryptoKit with any of the online digest tools. For example, the SHA-2/256 bit digest for the empty string is:

E3B0 C442 98FC 1C14 9AFB F4C8 996F B924 27AE 41E4 649B 934C A495 991B 7852 B855

The SHA-3 family of algorithms is supported by the CryptoSwift package available from GitHub. Note, this is not the Apple curated swift-crypto package (which is excellent, but mostly mirrors CryptoKit).

My understanding is that the SHA-2 family of algorithms are often referred to as simply SHA while the SHA-3 family of algorithms are explicitly referred to as SHA-3 since they are more recent (released 2015).