1 files changed, 136 insertions, 0 deletions
diff --git a/wallet/src/androidMain/kotlin/net/taler/lib/crypto/RsaBlinding.kt b/wallet/src/androidMain/kotlin/net/taler/lib/crypto/RsaBlinding.kt
new file mode 100644
index 0000000..732ac10
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+++ b/wallet/src/androidMain/kotlin/net/taler/lib/crypto/RsaBlinding.kt
@@ -0,0 +1,136 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.lib.crypto
+
+import java.math.BigInteger
+import kotlin.math.abs
+import kotlin.math.ceil
+import kotlin.math.floor
+
+internal object RsaBlinding {
+
+    fun rsaBlind(hm: ByteArray, bks: ByteArray, rsaPubEnc: ByteArray): ByteArray {
+        val rsaPub = rsaPubDecode(rsaPubEnc)
+        val data = rsaFullDomainHash(hm, rsaPub)
+        val r = rsaBlindingKeyDerive(rsaPub, bks)
+        val rE = r.modPow(rsaPub.e, rsaPub.n)
+        val bm = rE.multiply(data).mod(rsaPub.n)
+        return bm.toByteArrayWithoutSign()
+    }
+
+    fun rsaUnblind(sig: ByteArray, rsaPubEnc: ByteArray, bks: ByteArray): ByteArray {
+        val rsaPub = rsaPubDecode(rsaPubEnc)
+        val blindedSig = BigInteger(1, sig)
+        val r = rsaBlindingKeyDerive(rsaPub, bks)
+        val rInv = r.modInverse(rsaPub.n)
+        val s = blindedSig.multiply(rInv).mod(rsaPub.n)
+        return s.toByteArrayWithoutSign()
+    }
+
+    fun rsaVerify(hm: ByteArray, rsaSig: ByteArray, rsaPubEnc: ByteArray): Boolean {
+        val rsaPub = rsaPubDecode(rsaPubEnc)
+        val d = rsaFullDomainHash(hm, rsaPub)
+        val sig = BigInteger(1, rsaSig)
+        val sigE = sig.modPow(rsaPub.e, rsaPub.n)
+        return sigE == d
+    }
+
+    private fun rsaBlindingKeyDerive(rsaPub: RsaPublicKey, bks: ByteArray): BigInteger {
+        val salt = "Blinding KDF extrator HMAC key".encodeToByteArray()
+        val info = "Blinding KDF".encodeToByteArray()
+        return kdfMod(rsaPub.n, bks, salt, info)
+    }
+
+    private fun rsaPubDecode(publicKey: ByteArray): RsaPublicKey {
+        val modulusLength = abs((publicKey[0].toInt() shl 8) or publicKey[1].toInt())
+        val exponentLength = abs((publicKey[2].toInt() shl 8) or publicKey[3].toInt())
+        if (4 + exponentLength + modulusLength != publicKey.size) {
+            throw Error("invalid RSA public key (format wrong)")
+        }
+        val modulus = publicKey.copyOfRange(4, 4 + modulusLength)
+        val exponent = publicKey.copyOfRange(
+            4 + modulusLength,
+            4 + modulusLength + exponentLength
+        )
+        return RsaPublicKey(BigInteger(1, modulus), BigInteger(1, exponent))
+    }
+
+    private fun rsaFullDomainHash(hm: ByteArray, rsaPublicKey: RsaPublicKey): BigInteger {
+        val info = "RSA-FDA FTpsW!".encodeToByteArray()
+        val salt = rsaPubEncode(rsaPublicKey)
+        val r = kdfMod(rsaPublicKey.n, hm, salt, info)
+        rsaGcdValidate(r, rsaPublicKey.n)
+        return r
+    }
+
+    private fun rsaPubEncode(rsaPublicKey: RsaPublicKey): ByteArray {
+        val mb = rsaPublicKey.n.toByteArrayWithoutSign()
+        val eb = rsaPublicKey.e.toByteArrayWithoutSign()
+        val out = ByteArray(4 + mb.size + eb.size)
+        out[0] = ((mb.size ushr 8) and 0xff).toByte()
+        out[1] = (mb.size and 0xff).toByte()
+        out[2] = ((eb.size ushr 8) and 0xff).toByte()
+        out[3] = (eb.size and 0xff).toByte()
+        mb.copyInto(out, destinationOffset = 4)
+        eb.copyInto(out, destinationOffset = 4 + mb.size)
+        return out
+    }
+
+    private fun kdfMod(n: BigInteger, ikm: ByteArray, salt: ByteArray, info: ByteArray): BigInteger {
+        val nBits = n.bitLength()
+        val bufLen = floor((nBits.toDouble() - 1) / 8 + 1).toInt()
+        val mask = (1 shl (8 - (bufLen * 8 - nBits))) - 1
+        var counter = 0
+        while (true) {
+            val ctx = ByteArray(info.size + 2)
+            info.copyInto(ctx)
+            ctx[ctx.size - 2] = ((counter ushr 8) and 0xff).toByte()
+            ctx[ctx.size - 1] = (counter and 0xff).toByte()
+            val buf = CryptoJvmImpl.kdf(bufLen, ikm, salt, ctx)
+            val arr = buf.copyOf()
+            arr[0] = (arr[0].toInt() and mask).toByte()
+            val r = BigInteger(1, arr)
+            if (r < n) return r
+            counter++
+        }
+    }
+
+    /**
+     * Test for malicious RSA key.
+     *
+     * Assuming n is an RSA modulous and r is generated using a call to
+     * GNUNET_CRYPTO_kdf_mod_mpi, if gcd(r,n) != 1 then n must be a
+     * malicious RSA key designed to deanomize the user.
+     *
+     * @param r KDF result
+     * @param n RSA modulus of the public key
+     */
+    private fun rsaGcdValidate(r: BigInteger, n: BigInteger) {
+        if (r.gcd(n) != BigInteger.ONE) throw Error("malicious RSA public key")
+    }
+
+    // TODO check that this strips *only* the sign correctly
+    private fun BigInteger.toByteArrayWithoutSign(): ByteArray = this.toByteArray().let {
+        val byteLength = ceil(this.bitLength().toDouble() / 8).toInt()
+        val signBitPosition = ceil((this.bitLength() + 1).toDouble() / 8).toInt()
+        val start = signBitPosition - byteLength
+        it.copyOfRange(start, it.size)  // stripping least significant byte (sign)
+    }
+
+}
+
+internal class RsaPublicKey(val n: BigInteger, val e: BigInteger)