| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| // |
| //////////////////////////////////////////////////////////////////////////////// |
| |
| package com.google.crypto.tink.hybrid.subtle; |
| |
| import com.google.crypto.tink.Aead; |
| import com.google.crypto.tink.HybridDecrypt; |
| import com.google.crypto.tink.aead.subtle.AeadFactory; |
| import com.google.crypto.tink.subtle.Hkdf; |
| import java.nio.ByteBuffer; |
| import java.security.GeneralSecurityException; |
| import java.security.interfaces.RSAPrivateKey; |
| import javax.crypto.Cipher; |
| |
| /** |
| * Hybrid encryption with RSA-KEM as defined in Shoup's ISO standard proposal as KEM, and AEAD as |
| * DEM and HKDF as KDF. |
| * |
| * <p>Shoup's ISO standard proposal is available at https://www.shoup.net/iso/std6.pdf. |
| */ |
| public final class RsaKemHybridDecrypt implements HybridDecrypt { |
| private final RSAPrivateKey recipientPrivateKey; |
| private final String hkdfHmacAlgo; |
| private final byte[] hkdfSalt; |
| private final AeadFactory aeadFactory; |
| |
| public RsaKemHybridDecrypt( |
| final RSAPrivateKey recipientPrivateKey, |
| String hkdfHmacAlgo, |
| final byte[] hkdfSalt, |
| AeadFactory aeadFactory) |
| throws GeneralSecurityException { |
| RsaKem.validateRsaModulus(recipientPrivateKey.getModulus()); |
| this.recipientPrivateKey = recipientPrivateKey; |
| this.hkdfSalt = hkdfSalt; |
| this.hkdfHmacAlgo = hkdfHmacAlgo; |
| this.aeadFactory = aeadFactory; |
| } |
| |
| @Override |
| public byte[] decrypt(final byte[] ciphertext, final byte[] contextInfo) |
| throws GeneralSecurityException { |
| int modSizeInBytes = RsaKem.bigIntSizeInBytes(recipientPrivateKey.getModulus()); |
| if (ciphertext.length < modSizeInBytes) { |
| throw new GeneralSecurityException( |
| String.format( |
| "Ciphertext must be of at least size %d bytes, but got %d", |
| modSizeInBytes, ciphertext.length)); |
| } |
| |
| // Decrypt the token to obtain the raw shared secret. |
| ByteBuffer cipherBuffer = ByteBuffer.wrap(ciphertext); |
| byte[] token = new byte[modSizeInBytes]; |
| cipherBuffer.get(token); |
| Cipher rsaCipher = Cipher.getInstance("RSA/ECB/NoPadding"); |
| rsaCipher.init(Cipher.DECRYPT_MODE, recipientPrivateKey); |
| byte[] sharedSecret = rsaCipher.doFinal(token); |
| |
| // KDF: derive a DEM key from the shared secret, salt, and contextInfo. |
| byte[] demKey = |
| Hkdf.computeHkdf( |
| hkdfHmacAlgo, sharedSecret, hkdfSalt, contextInfo, aeadFactory.getKeySizeInBytes()); |
| |
| // DEM: decrypt the payload. |
| Aead aead = aeadFactory.createAead(demKey); |
| byte[] demPayload = new byte[cipherBuffer.remaining()]; |
| cipherBuffer.get(demPayload); |
| return aead.decrypt(demPayload, RsaKem.EMPTY_AAD); |
| } |
| } |