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// Copyright © 2023 siddharth ravikumar <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"math/big"
"ricketyspace.net/cryptopals/lib"
)
func C41() {
// Generate RSA key pair and setup paraphernalia.
kpair, err := lib.RSAGenKey()
if err != nil {
fmt.Printf("rsa gen key: %v\n", err)
return
}
serverDecrypt := func(cipher []byte) []byte {
return kpair.Private.Decrypt(cipher)
}
pub := kpair.Public
// N
N := pub.N()
// E
E := pub.E()
// Message.
msg := big.NewInt(0).SetBytes([]byte(`{time: 1356304276, social: '555-55-5555'}`))
// Encrypt.
c := big.NewInt(0).SetBytes(pub.Encrypt(msg.Bytes()))
// Decrypt
if msg.Cmp(big.NewInt(0).SetBytes(serverDecrypt(c.Bytes()))) != 0 {
fmt.Printf("Decryption failed: %v\n", serverDecrypt(c.Bytes()))
return
}
// S
rb, err := lib.RandomBytes(1024) // Random bytes.
if err != nil {
fmt.Printf("random bytes: %v\n", err)
return
}
S := big.NewInt(0).SetBytes(rb)
S = S.Mod(S, N)
// C' = ((S**E mod N) C) mod N
cp := big.NewInt(0).Exp(S, E, N) // (S**E mod N)
cp = big.NewInt(0).Mul(cp, c) // ((S**E mod N) C)
cp = cp.Mod(cp, N) // ((S**E mod N) C) mod N => C'
// P'
pp := big.NewInt(0).SetBytes(serverDecrypt(cp.Bytes()))
// Get P from P' => (P'/S mod N)
sp, err := lib.InvMod(S, N)
if err != nil {
fmt.Printf("modinv(S, N) failed")
return
}
plainh := big.NewInt(0).Mul(pp, sp)
plainh = plainh.Mod(plainh, N)
if !lib.BytesEqual(plainh.Bytes(), msg.Bytes()) {
fmt.Printf("Unable to deduce message: %s\n", plainh.Bytes())
return
}
fmt.Printf("Deduced message from ciphertext: %s\n", plainh.Bytes())
}
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