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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;
import com.yahoo.security.KeyUtils;
import java.security.KeyPair;
import java.security.interfaces.XECPrivateKey;
import java.security.interfaces.XECPublicKey;
import java.util.function.Supplier;
import static com.yahoo.security.ArrayUtils.concat;
import static com.yahoo.security.hpke.Constants.EAE_PRK_LABEL;
import static com.yahoo.security.hpke.Constants.EMPTY_LABEL;
import static com.yahoo.security.hpke.Constants.SHARED_SECRET_LABEL;
import static com.yahoo.security.hpke.LabeledKdfUtils.labeledExpandForSuite;
import static com.yahoo.security.hpke.LabeledKdfUtils.labeledExtractForSuite;
/**
* KEM implementation using Diffie-Hellman over X25519 curves as the shared
* secret deriver and HKDF-SHA256 as its key derivation function.
*
* HPKE KEM spec: DHKEM(X25519, HKDF-SHA256)
*
* @author vekterli
*/
final class DHKemX25519HkdfSha256 implements Kem {
private static final HkdfSha256 HKDF = HkdfSha256.getInstance();
private final Supplier<KeyPair> keyPairGen;
DHKemX25519HkdfSha256(Supplier<KeyPair> keyPairGen) {
this.keyPairGen = keyPairGen;
}
// Section 4.1 DH-Based KEM (DHKEM):
// "The implicit suite_id value used within LabeledExtract and LabeledExpand
// is defined as follows, where kem_id is defined in Section 7.1:"
//
// suite_id = concat("KEM", I2OSP(kem_id, 2))"
//
// The ID of our KEM suite, DHKEM(X25519, HKDF-SHA256), is 0x0020, so just hard code this.
private static final byte[] DHKEM_SUITE_ID_LABEL = new byte[] { 'K','E','M', 0x00, 0x20 };
@Override public short nSecret() { return 32; }
@Override public short nEnc() { return 32; }
@Override public short nPk() { return 32; }
@Override public short nSk() { return 32; }
@Override public short kemId() { return 0x0020; }
private static byte[] serializePublicKey(XECPublicKey publicKey) {
return KeyUtils.toRawX25519PublicKeyBytes(publicKey);
}
private static XECPublicKey deserializePublicKey(byte[] enc) {
return KeyUtils.fromRawX25519PublicKey(enc);
}
/**
* Section 4.1 DH-Based KEM (DHKEM):
*
* <pre>
* def ExtractAndExpand(dh, kem_context):
* eae_prk = LabeledExtract("", "eae_prk", dh)
* shared_secret = LabeledExpand(eae_prk, "shared_secret",
* kem_context, Nsecret)
* return shared_secret
* </pre>
*/
private byte[] extractAndExpand(byte[] dh, byte[] kemContext) {
byte[] eaePrk = labeledExtractForSuite(HKDF, DHKEM_SUITE_ID_LABEL, EMPTY_LABEL, EAE_PRK_LABEL, dh);
return labeledExpandForSuite(HKDF, eaePrk, DHKEM_SUITE_ID_LABEL, SHARED_SECRET_LABEL, kemContext, nSecret());
}
/**
* Section 4.1 DH-Based KEM (DHKEM):
*
* <pre>
* def Encap(pkR):
* skE, pkE = GenerateKeyPair()
* dh = DH(skE, pkR)
* enc = SerializePublicKey(pkE)
*
* pkRm = SerializePublicKey(pkR)
* kem_context = concat(enc, pkRm)
*
* shared_secret = ExtractAndExpand(dh, kem_context)
* return shared_secret, enc
* </pre>
*/
@Override
public EncapResult encap(XECPublicKey pkR) {
var kpE = keyPairGen.get();
var skE = (XECPrivateKey)kpE.getPrivate();
var pkE = (XECPublicKey)kpE.getPublic();
byte[] dh = KeyUtils.ecdh(skE, pkR);
byte[] enc = serializePublicKey(pkE);
byte[] pkRm = serializePublicKey(pkR);
byte[] kemContext = concat(enc, pkRm);
byte[] sharedSecret = extractAndExpand(dh, kemContext);
return new EncapResult(sharedSecret, enc);
}
/**
* Section 4.1 DH-Based KEM (DHKEM):
*
* <pre>
* def Decap(enc, skR):
* pkE = DeserializePublicKey(enc)
* dh = DH(skR, pkE)
*
* pkRm = SerializePublicKey(pk(skR))
* kem_context = concat(enc, pkRm)
*
* shared_secret = ExtractAndExpand(dh, kem_context)
* return shared_secret
* </pre>
*
* Implementation note: we take in the key pair to avoid needing to compute the public key (TODO!)
*/
@Override
public byte[] decap(byte[] enc, KeyPair kpR) {
var pkE = deserializePublicKey(enc);
var skR = (XECPrivateKey)kpR.getPrivate();
var pkR = (XECPublicKey)kpR.getPublic();
byte[] dh = KeyUtils.ecdh(skR, pkE);
byte[] pkRm = serializePublicKey(pkR);
byte[] kemContext = concat(enc, pkRm);
return extractAndExpand(dh, kemContext);
}
}
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