1 files changed, 121 insertions, 0 deletions
diff --git a/challenge/c12.go b/challenge/c12.go
new file mode 100644
index 0000000..fbb9fbb
--- /dev/null
+++ b/challenge/c12.go
@@ -0,0 +1,121 @@
+// Copyright © 2020 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+	"ricketyspace.net/cryptopals/lib"
+)
+
+var sheep byte = 65
+
+func C12() {
+	if !isOracleUsingECB() {
+		panic("oracle not using ecb mode")
+	}
+
+	blocksize := findBlockSize()
+	nbl, us_sz := findUnknownStringCharacteristics(blocksize)
+	in := freshSheepBlock(blocksize)
+	ds := make([]byte, 0) // deciphered unknown string in bytes
+	for i := 0; i < nbl; i++ {
+		nby := blocksize
+		if i == nbl-1 {
+			nby = us_sz % blocksize
+		}
+		for j := 0; j < nby; j++ {
+			in, ds = decipherUnknownStringIter(blocksize, i+1, j+1,
+				in, ds)
+		}
+		s := 16 * i
+		e := s + 16
+		copy(in, ds[s:e])
+	}
+	fmt.Printf("Unknown String:\n%v", lib.BytesToStr(ds))
+}
+
+func freshSheepBlock(blocksize int) []byte {
+	in := make([]byte, blocksize)
+	for i := 0; i < blocksize; i++ {
+		in[i] = sheep
+	}
+	return in
+}
+
+func findBlockSize() int {
+	in := make([]byte, 0)
+
+	in = append(in, sheep)
+	is := len(lib.OracleAESEncryptECB(in)) // initial size
+	bs := 0                                // block size
+	for {
+		in = append(in, sheep)
+		bs = len(lib.OracleAESEncryptECB(in))
+		if bs != is {
+			return (bs - is)
+		}
+	}
+}
+
+func findUnknownStringCharacteristics(blocksize int) (int, int) {
+	in := make([]byte, 0)
+	c_sz := len(lib.OracleAESEncryptECB(in)) // Cipher size
+	nblocks := c_sz / 16                     // number of blocks
+
+	// Figure out ize of unknown string.
+	for {
+		in = append(in, sheep)
+		bs := len(lib.OracleAESEncryptECB(in))
+		if bs != c_sz {
+			return nblocks, (c_sz - len(in))
+		}
+	}
+
+}
+
+func isOracleUsingECB() bool {
+	in := lib.StrToBytes("OliverMkTukudzi OliverMkTukudzi OliverMkTukudzi")
+	oo := lib.OracleAESEncryptECB(in)
+	if lib.CipherUsesECB(oo) != nil {
+		return true
+	}
+	return false
+}
+
+// `blocksize` is the size of a block
+// `block` is the nth block that is being deciphered
+// `n` is the nth byte of the block `block` that is going to be deciphered.
+// `in` (n-1)th block that is known
+// `ds` deciphered unknown string
+func decipherUnknownStringIter(blocksize, block, n int, in, ds []byte) ([]byte, []byte) {
+	oo := lib.OracleAESEncryptECB(in[0:(blocksize - n)])
+
+	s := 16 * (block - 1)
+	e := s + 16
+	nbl := oo[s:e] // nth block of the cipher
+
+	// Shift `in` to the left by one place.
+	for i := 0; i < blocksize-1; i++ {
+		in[i] = in[i+1]
+	}
+
+	// Try all combinations.
+	for i := 0; i < 256; i++ {
+		in[15] = byte(i)
+		oo = lib.OracleAESEncryptECB(in)
+
+		if lib.BlocksEqual(nbl, oo[0:16]) {
+			ds = append(ds, in[15])
+			return in, ds
+		}
+	}
+	panic("not found!")
+}
+
+// Output:
+// Unknown String:
+// Rollin' in my 5.0
+// With my rag-top down so my hair can blow
+// The girlies on standby waving just to say hi
+// Did you stop? No, I just drove by