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diff --git a/security-utils/src/main/java/com/yahoo/security/hpke/Kdf.java b/security-utils/src/main/java/com/yahoo/security/hpke/Kdf.java
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+++ b/security-utils/src/main/java/com/yahoo/security/hpke/Kdf.java
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+// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.security.hpke;
+
+/**
+ * Key derivation function (KDF)
+ *
+ * @author vekterli
+ */
+public interface Kdf {
+
+    /**
+     * Extract a pseudorandom key of fixed length {@link #nH()} bytes from input keying material
+     * <code>ikm</code> and an optional byte string <code>salt</code>.
+     *
+     * @param salt non-secret salt used as input to KDF
+     * @param labeledIkm secret input keying material
+     * @return nH bytes of PRK data
+     */
+    byte[] extract(byte[] salt, byte[] labeledIkm);
+
+    /**
+     * Expand a pseudorandom key <code>prk</code> using optional string <code>info</code> into
+     * <code>nBytesToExpand</code> bytes of output keying material.
+     *
+     * @param prk pseudo random key previously gotten from a call to extract.
+     * @param info contextual info for expansion; useful for key domain separation
+     * @param nBytesToExpand number of bytes to return
+     *
+     * @return <code>nBytesToExpand</code> bytes of output keying material.
+     */
+    byte[] expand(byte[] prk, byte[] info, int nBytesToExpand);
+
+    /** Output size of the extract() function in bytes */
+    short nH();
+
+    /** Predefined KDF ID, as given in RFC 9180 section 7.2 */
+    short kdfId();
+
+    static Kdf hkdfSha256() {
+        return HkdfSha256.getInstance();
+    }
+
+}