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Build SHA-256 from Scratch

Implement SHA-256 from raw bit operations. You'll build ROTR/Σ/σ/Ch/Maj, the message schedule, the 64-round compression function, padding, and the full Merkle-Damgård construction. Then add HMAC for message authentication. By the end you'll deeply understand the hash function used in Bitcoin, Git, TLS, and JWT.

intermediateFree3 hours12 lessons
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Curriculum

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Going further

Read

  • Serious Cryptography (Aumasson) chapter 6 — covers SHA-2 family in depth.
  • FIPS 180-4 (the spec itself, ~30 pages).
  • The official test vectors — verify your implementation against these before declaring victory.

Build next

  • HMAC-SHA-256: ~10 lines of outer(inner(message)) on top of what you have. Foundation of JWT, TLS, almost every authenticated message format.
  • PBKDF2 + HKDF: key derivation built on HMAC. PBKDF2 is the password hasher; HKDF is what TLS 1.3 uses to derive session keys.
  • SHA-3 / Keccak: a totally different design (sponge construction). Worth implementing once because the structure is so different from SHA-2.
  • BLAKE3: modern, faster, parallelizable. The reference implementation is small + readable.

Continue with our courses

  • Build JWT — uses HMAC-SHA-256 directly.
  • Build TLS 1.3 — uses HMAC, HKDF, and SHA-256 throughout.
  • Build a Blockchain — uses SHA-256 for proof of work and Merkle roots.
Build SHA-256 from Scratch