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// Copyright © 2021 rsiddharth <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"ricketyspace.net/cryptopals/lib"
)
// Cryptopals #18 - Implement CTR, the stream cipher mode
func C18() {
// Generates counter function for AES CTR mode.
genCTRFunc := func(nonce uint64) func() []byte {
ctr := uint64(0) // Counter
ff := uint64(0xFF)
cf := func() []byte {
cb := make([]byte, 16) // counter block
var i, j uint
// Put nonce in the first 8 bytes in cb in little endian format
for i = 0; i < 8; i++ {
n := nonce & (ff << (i * 8)) // Reset all except the i^th byte of the nonce
cb[i] = byte(n >> (i * 8)) // Retrieve i^th byte of the nonce
}
// Put counter in the next 8 bytes in cb in little endian format
for i, j = 8, 0; i < 16; i, j = i+1, j+1 {
n := ctr & (ff << (j * 8)) // Reset all except the j^th byte of the counter
cb[i] = byte(n >> (j * 8)) // Retrieve j^th byte of the counter
}
ctr += 1 // Increment counter by 1
return cb
}
return cf
}
cipher := lib.Base64ToBytes("L77na/nrFsKvynd6HzOoG7GHTLXsTVu9qvY/2syLXzhPweyyMTJULu/6/kXX0KSvoOLSFQ==")
key := lib.StrToBytes("YELLOW SUBMARINE")
ctrFunc := genCTRFunc(0)
plain, err := lib.AESDecryptCTR(cipher, key, ctrFunc)
if err != nil {
fmt.Printf("decryption failed: %v", err)
}
fmt.Printf("%v\n", lib.BytesToStr(plain))
}
// Output:
// Yo, VIP Let's kick it Ice, Ice, baby Ice, Ice, baby
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