From ff82895eacd215ab8d9c6a46f22b840df53186e9 Mon Sep 17 00:00:00 2001 From: siddharth ravikumar Date: Sat, 8 Apr 2023 10:38:03 -0400 Subject: challenge: do challenge 41 --- challenge/c41.go | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ cryptopals.go | 2 ++ 2 files changed, 75 insertions(+) create mode 100644 challenge/c41.go diff --git a/challenge/c41.go b/challenge/c41.go new file mode 100644 index 0000000..4ef983d --- /dev/null +++ b/challenge/c41.go @@ -0,0 +1,73 @@ +// Copyright © 2023 siddharth ravikumar +// 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()) +} diff --git a/cryptopals.go b/cryptopals.go index 0452433..5d02e56 100644 --- a/cryptopals.go +++ b/cryptopals.go @@ -100,5 +100,7 @@ func main() { challenge.C39() case 40: challenge.C40() + case 41: + challenge.C41() } } -- cgit v1.2.3