# SP 800-56C HKDF Extract and Expand Walkthrough Show each HMAC step of HKDF Extract+Expand for protocol key derivation, including PRK, T(n), and OKM. > Canonical page: https://elysiatools.com/en/tools/sp800-56c-hkdf-extract-and-expand-walkthrough - **Category:** Security - **Keywords:** hkdf, sp 800-56c, rfc 5869, key derivation, hmac, ecdh, tls ## Overview A deterministic SP 800-56C/RFC 5869 learning walkthrough for ECDH shared-secret derivation. It is for development and test vectors, not a secret-management service. ## Inputs - **Input preset** (select) - **Hash** (select) - **Input encoding** (select) - **Input keying material (IKM)** (textarea): Hex, text, or Base64 according to input encoding - **Salt** (textarea): Optional; empty uses a zero-filled HashLen salt - **Info** (textarea): Optional context / protocol label - **Output length (bytes)** (number) ## When to use - Verify an RFC 5869 HKDF implementation against the published SHA-256 test case. - Study how SP 800-56C or HKDF Extract and Expand derives protocol key material from ECDH input. - Create development and test vectors with different hashes, encodings, salts, context info, and output lengths. ## How it works - Select a preset, or provide the input keying material using Hex, UTF-8 text, or Base64. - Choose SHA-256, SHA-384, or SHA-512, then optionally enter a salt and protocol context in the Info field. - Set the desired output length from 1 to 4096 bytes. - The walkthrough calculates and displays the Extract PRK, each Expand block T(n), and the final OKM. ## Use cases - Compare HKDF output with RFC 5869 test vectors during cryptographic implementation testing. - Trace ECDH shared-secret derivation for protocol design and debugging. - Generate reproducible examples for explaining HMAC-based key derivation. ## Frequently asked questions ### What does this HKDF walkthrough calculate? It shows the HMAC steps for HKDF Extract and Expand, including PRK, T(n), and OKM. ### Which hash algorithms are supported? You can select SHA-256, SHA-384, or SHA-512. ### Which input encodings can I use? IKM, salt, and info values can be entered as Hex, UTF-8 text, or Base64 according to the selected encoding. ### What happens if I omit the salt? An empty salt uses a zero-filled salt with a length equal to the selected hash output length. ### Is this a secret-management service? No. It is a deterministic learning walkthrough for development and test vectors. ## Related tools - [SSH Key Pair Generator](https://elysiatools.com/en/tools/ssh-key-generator): Generate an SSH key pair (Ed25519 recommended, RSA 4096, or ECDSA P-256) with an OpenSSH-format public key ready for ~/.ssh/authorized_keys, a SHA256 fingerprint, and a PEM private key. Optional passphrase encryption. 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