https://openjdk.java.net/ 
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package sun.security.ssl;

import java.security.NoSuchAlgorithmException;
import java.security.InvalidKeyException;
import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.ShortBufferException;
import javax.crypto.spec.SecretKeySpec;
import java.util.Objects;


An implementation of the HKDF key derivation algorithm outlined in RFC 5869,
specific to the needs of TLS 1.3 key derivation in JSSE.  This is not a
general purpose HKDF implementation and is suited only to single-key output
derivations.
HKDF objects are created by specifying a message digest algorithm.  That
digest algorithm will be used by the HMAC function as part of the HKDF
derivation process.

/**
 * An implementation of the HKDF key derivation algorithm outlined in RFC 5869,
 * specific to the needs of TLS 1.3 key derivation in JSSE.  This is not a
 * general purpose HKDF implementation and is suited only to single-key output
 * derivations.
 *
 * HKDF objects are created by specifying a message digest algorithm.  That
 * digest algorithm will be used by the HMAC function as part of the HKDF
 * derivation process.
 */
final class HKDF {
    private final String hmacAlg;
    private final Mac hmacObj;
    private final int hmacLen;

    
Create an HDKF object, specifying the underlying message digest
algorithm.

Params:
  • hashAlg – a standard name corresponding to a supported message
digest algorithm.
Throws:
  • NoSuchAlgorithmException – if that message digest algorithm does
not have an HMAC variant supported on any available provider.
/**
     * Create an HDKF object, specifying the underlying message digest
     * algorithm.
     *
     * @param hashAlg a standard name corresponding to a supported message
     * digest algorithm.
     *
     * @throws NoSuchAlgorithmException if that message digest algorithm does
     * not have an HMAC variant supported on any available provider.
     */
    HKDF(String hashAlg) throws NoSuchAlgorithmException {
        Objects.requireNonNull(hashAlg,
                "Must provide underlying HKDF Digest algorithm.");
        hmacAlg = "Hmac" + hashAlg.replace("-", "");
        hmacObj = JsseJce.getMac(hmacAlg);
        hmacLen = hmacObj.getMacLength();
    }

    
Perform the HMAC-Extract derivation.

Params:
  • salt – a salt value, implemented as a SecretKey. A null value is allowed, which will internally use an array of zero bytes the same size as the underlying hash output length.
  • inputKey – the input keying material provided as a SecretKey.
  • keyAlg – the algorithm name assigned to the resulting SecretKey object.
Throws:
  • InvalidKeyException – if the salt parameter cannot be used to initialize the underlying HMAC.
Returns:a SecretKey that is the result of the HKDF extract operation.
/**
     * Perform the HMAC-Extract derivation.
     *
     * @param salt a salt value, implemented as a {@code SecretKey}.  A
     * {@code null} value is allowed, which will internally use an array of
     * zero bytes the same size as the underlying hash output length.
     * @param inputKey the input keying material provided as a
     * {@code SecretKey}.
     * @param keyAlg the algorithm name assigned to the resulting
     * {@code SecretKey} object.
     *
     * @return a {@code SecretKey} that is the result of the HKDF extract
     * operation.
     *
     * @throws InvalidKeyException if the {@code salt} parameter cannot be
     * used to initialize the underlying HMAC.
     */
    SecretKey extract(SecretKey salt, SecretKey inputKey, String keyAlg)
            throws InvalidKeyException {
        if (salt == null) {
            salt = new SecretKeySpec(new byte[hmacLen], "HKDF-Salt");
        }
        hmacObj.init(salt);

        return new SecretKeySpec(hmacObj.doFinal(inputKey.getEncoded()),
                keyAlg);
    }

    
Perform the HMAC-Extract derivation.

Params:
  • salt – a salt value as cleartext bytes. A null value is allowed, which will internally use an array of zero bytes the same size as the underlying hash output length.
  • inputKey – the input keying material provided as a SecretKey.
  • keyAlg – the algorithm name assigned to the resulting SecretKey object.
Throws:
  • InvalidKeyException – if the salt parameter cannot be used to initialize the underlying HMAC.
Returns:a SecretKey that is the result of the HKDF extract operation.
/**
     * Perform the HMAC-Extract derivation.
     *
     * @param salt a salt value as cleartext bytes.  A {@code null} value is
     * allowed, which will internally use an array of zero bytes the same
     * size as the underlying hash output length.
     * @param inputKey the input keying material provided as a
     * {@code SecretKey}.
     * @param keyAlg the algorithm name assigned to the resulting
     * {@code SecretKey} object.
     *
     * @return a {@code SecretKey} that is the result of the HKDF extract
     * operation.
     *
     * @throws InvalidKeyException if the {@code salt} parameter cannot be
     * used to initialize the underlying HMAC.
     */
    SecretKey extract(byte[] salt, SecretKey inputKey, String keyAlg)
            throws InvalidKeyException {
        if (salt == null) {
            salt = new byte[hmacLen];
        }
        return extract(new SecretKeySpec(salt, "HKDF-Salt"), inputKey, keyAlg);
    }

    
Perform the HKDF-Expand derivation for a single-key output.

Params:
  • pseudoRandKey – the pseudo random key (PRK).
  • info – optional context-specific info. A null value is allowed in which case a zero-length byte array will be used.
  • outLen – the length of the resulting SecretKey
  • keyAlg – the algorithm name applied to the resulting SecretKey
Throws:
  • InvalidKeyException – if the underlying HMAC operation cannot be initialized using the provided pseudoRandKey object.
Returns:the resulting key derivation as a SecretKey object
/**
     * Perform the HKDF-Expand derivation for a single-key output.
     *
     * @param pseudoRandKey the pseudo random key (PRK).
     * @param info optional context-specific info.  A {@code null} value is
     * allowed in which case a zero-length byte array will be used.
     * @param outLen the length of the resulting {@code SecretKey}
     * @param keyAlg the algorithm name applied to the resulting
     * {@code SecretKey}
     *
     * @return the resulting key derivation as a {@code SecretKey} object
     *
     * @throws InvalidKeyException if the underlying HMAC operation cannot
     * be initialized using the provided {@code pseudoRandKey} object.
     */
    SecretKey expand(SecretKey pseudoRandKey, byte[] info, int outLen,
            String keyAlg) throws InvalidKeyException {
        byte[] kdfOutput;

        // Calculate the number of rounds of HMAC that are needed to
        // meet the requested data.  Then set up the buffers we will need.
        Objects.requireNonNull(pseudoRandKey, "A null PRK is not allowed.");

        // Output from the expand operation must be <= 255 * hmac length
        if (outLen > 255 * hmacLen) {
            throw new IllegalArgumentException("Requested output length " +
                    "exceeds maximum length allowed for HKDF expansion");
        }
        hmacObj.init(pseudoRandKey);
        if (info == null) {
            info = new byte[0];
        }
        int rounds = (outLen + hmacLen - 1) / hmacLen;
        kdfOutput = new byte[rounds * hmacLen];
        int offset = 0;
        int tLength = 0;

        for (int i = 0; i < rounds ; i++) {

            // Calculate this round
            try {
                 // Add T(i).  This will be an empty string on the first
                 // iteration since tLength starts at zero.  After the first
                 // iteration, tLength is changed to the HMAC length for the
                 // rest of the loop.
                hmacObj.update(kdfOutput,
                        Math.max(0, offset - hmacLen), tLength);
                hmacObj.update(info);                       // Add info
                hmacObj.update((byte)(i + 1));              // Add round number
                hmacObj.doFinal(kdfOutput, offset);

                tLength = hmacLen;
                offset += hmacLen;                       // For next iteration
            } catch (ShortBufferException sbe) {
                // This really shouldn't happen given that we've
                // sized the buffers to their largest possible size up-front,
                // but just in case...
                throw new RuntimeException(sbe);
            }
        }

        return new SecretKeySpec(kdfOutput, 0, outLen, keyAlg);
    }
}