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// Copyright © 2021 siddharth ravikumar <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"ricketyspace.net/cryptopals/lib"
)
func C29() {
// Original message.
msg := lib.StrToBytes("comment1=cooking%20MCs;userdata=foo;" +
"comment2=%20like%20a%20pound%20of%20bacon")
// Random secret (unknown to attacker)
sec, err := lib.RandomBytes(int(lib.RandomInt(8, 100)))
if err != nil {
fmt.Printf("Error: unable generate secret\n")
}
// `m` is the original message.
// `hvs` is the hash values.
// `sl` is the secret key length (guess).
genForgedMsgMac := func(m []byte, hvs []uint32, sl int) ([]byte, []byte) {
mf := make([]byte, len(m))
copy(mf, m)
mf = append(mf, lib.MDPadding(sl+len(m))...)
mf = append(mf, lib.StrToBytes(";admin=true")...)
// Generate SHA1 MAC for forged message.
sha1 := lib.Sha1{}
sha1.Init(hvs)
sha1.Message(lib.StrToBytes(";admin=true"))
// Fudge message length to forged message length.
sha1.MsgLen = sl + len(mf)
return mf, sha1.Hash()
}
// Returns true if the message is forged.
isForged := func(msg, mac []byte) bool {
sha1 := lib.Sha1{}
sha1.Init([]uint32{})
if sha1.MacVerify(sec, msg, mac) {
return true
}
return false
}
// Generate SHA1 MAC of original message and get the hash
// values.
sha1 := lib.Sha1{}
sha1.Init([]uint32{})
msgHVs := lib.BytesToUint32s(sha1.Mac(sec, msg)) // Hash values.
// Try to forge message with different secret prefix lengths.
sl := 1
for {
mf, mac := genForgedMsgMac(msg, msgHVs, sl)
if isForged(mf, mac) {
fmt.Printf("SHA1-MAC successfully forged: %x\n", mac)
fmt.Printf("Forged Message: %s\n", mf)
fmt.Printf("Secret prefix length: %v\n", sl)
break
}
sl += 1
}
}
// Output:
// SHA1-MAC successfully forged: 0cb8bbbaa6090c86a8b7110788d6241b20df7c99
// Forged Message: comment1=cooking%20MCs;userdata=foo;comment2=%20like%20a%20pound%20of%20baconh;admin=true
// Secret prefix length: 96
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