HKDFExpand()

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Second step of two-phase key derivation: expands an intermediate key (from HKDFExtract) into one or more output keys.

Use different info strings to derive separate keys for different purposes (e.g. encryption vs authentication).

Requires Extension: Crypto Extension

HKDFExpand( algorithm=string, prk=any, info=any, outputLength=numeric );

Returns: binary

Argument Description
algorithm
string, required 		edit

Hash algorithm: SHA256, SHA384, or SHA512
prk
any, required 		edit

Pseudo-random key from HKDFExtract
info
any, required 		edit

Context/application info (can be empty string or null)
outputLength
numeric, required 		edit

Number of bytes to derive

Examples

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// HKDFExpand is the second phase of HKDF: it expands a pseudorandom key (PRK)
// into one or more output keys. Use different "info" strings to derive separate keys.

// First, extract a PRK from your secret
prk = HKDFExtract( "SHA256", "salt", "master-secret" );

// Then expand into multiple keys for different purposes
encKey = HKDFExpand( "SHA256", prk, "encryption key", 32 );   // 32 bytes for AES-256
authKey = HKDFExpand( "SHA256", prk, "authentication key", 32 ); // 32 bytes for HMAC
ivBytes = HKDFExpand( "SHA256", prk, "iv", 16 );              // 16 bytes for AES IV

// Each key is different because the "info" string is different
// But they're all deterministically derived from the same master secret

See also