diff --git a/submissions/challenge1/jan_opalski/README.md b/submissions/challenge1/jan_opalski/README.md
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+# Language
+The code was written in Go. These instructions are based on assumption that user trying to run this solution has Go on their machine.
+# Running the solution
+- enter the directory
+- `go run .` (alternatively: `go build` and then `./challenge1`)
+## Flags
+By default all three solutions will be launched. If you want to see results of execution of only chosen tasks, you can achieve that by passing a subset of flags `-easy`, `-medium`, `-hard` respectively (passing all 3 is effectively the same as passing none of these).
+
+Solution of medium task (implementation of Mersenne Twister RNG) uses a default seed of 19650218 (taken from [English Wikipedia](https://en.wikipedia.org/wiki/Mersenne_Twister)). It can be changed to arbitrary value (assuming it can be converted to unsigned 32-bit integer) using flag `-seed <value>`.
+
+Example command with flags: `./challenge1 -medium -seed 42`. This will launch only the Mersenne Twister implementation with a starting seed of 42.
+
+# Done tasks
+- Easy: [PKCS#7 padding validation](https://cryptopals.com/sets/2/challenges/15)
+- Medium: [Implement the MT19937 Mersenne Twister RNG](https://cryptopals.com/sets/3/challenges/21)
+- Hard: [Recover the key from CBC with IV=Key](https://cryptopals.com/sets/4/challenges/27)
\ No newline at end of file
diff --git a/submissions/challenge1/jan_opalski/go.mod b/submissions/challenge1/jan_opalski/go.mod
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+++ b/submissions/challenge1/jan_opalski/go.mod
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+module challenge1
+
+go 1.24.4
diff --git a/submissions/challenge1/jan_opalski/main.go b/submissions/challenge1/jan_opalski/main.go
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+++ b/submissions/challenge1/jan_opalski/main.go
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+package main
+
+import (
+	"challenge1/solutions"
+	"flag"
+)
+
+func main() {
+	easyFlag := flag.Bool("easy", false, "launch solution of easy task")
+	mediumFlag := flag.Bool("medium", false, "launch solution of medium task")
+	hardFlag := flag.Bool("hard", false, "launch solution of hard task")
+	mediumSeedFlag := flag.Int("seed", 19650218, "seed for medium task")
+	flag.Parse()
+	all := false
+	if !(*easyFlag || *mediumFlag || *hardFlag) {
+		flag.Usage()
+		all = true
+	}
+	if *easyFlag || all {
+		solutions.Easy()
+	}
+	if *mediumFlag || all {
+		solutions.Medium(uint32(*mediumSeedFlag))
+	}
+	if *hardFlag || all {
+		solutions.Hard()
+	}
+}
diff --git a/submissions/challenge1/jan_opalski/solutions/easy.go b/submissions/challenge1/jan_opalski/solutions/easy.go
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+++ b/submissions/challenge1/jan_opalski/solutions/easy.go
@@ -0,0 +1,47 @@
+package solutions
+
+import (
+	"errors"
+	"fmt"
+)
+
+// https://cryptopals.com/sets/2/challenges/15
+func Easy() {
+	fmt.Println("=============================\n\t  EASY (15)\n=============================")
+	str1 := "ICE ICE BABY\x04\x04\x04\x04"
+	str2 := "ICE ICE BABY\x05\x05\x05\x05"
+	str3 := "ICE ICE BABY\x01\x02\x03\x04"
+	test_stripping(str1)
+	test_stripping(str2)
+	test_stripping(str3)
+	str4 := "\x02\x02"
+	test_stripping(str4)
+}
+
+func strip_padding(msg string) (string, error) {
+	if len(msg) == 0 {
+		return msg, nil
+	}
+	padding_len := msg[len(msg)-1]
+	first_pad := len(msg) - int(padding_len)
+
+	if first_pad < 0 {
+		return "", errors.New("padding invalid")
+	}
+	for i := first_pad; i < len(msg); i++ {
+		if msg[i] != padding_len {
+			return "", errors.New("padding invalid")
+		}
+	}
+	return msg[:first_pad], nil
+}
+
+func test_stripping(msg string) {
+	fmt.Printf("Testing stripping for %q\n", msg)
+	unpadded, err := strip_padding(msg)
+	if err != nil {
+		fmt.Println("Got error:", err)
+	} else {
+		fmt.Println("Correctly unpadded message", unpadded)
+	}
+}
diff --git a/submissions/challenge1/jan_opalski/solutions/hard.go b/submissions/challenge1/jan_opalski/solutions/hard.go
new file mode 100644
index 0000000..442c1ef
--- /dev/null
+++ b/submissions/challenge1/jan_opalski/solutions/hard.go
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+package solutions
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"slices"
+)
+
+const blocklen = 16
+const blockcount = 3
+
+// https://cryptopals.com/sets/4/challenges/27
+func Hard() {
+	fmt.Println("=============================\n\t  HARD (27)\n=============================")
+	// create a 128-bit key, assume sender and receiver know it while attacker doesn't
+	key := sender_receiver_setKey()
+	// SENDER
+	message := sender_setupMessage()
+	encrypted := sender_encryptMessage(message, key)
+
+	// ATTACKER
+	modified := attacker_modifyMessage(encrypted)
+
+	// RECEIVER
+	err := receiver_decryptMessage(modified, key)
+	if err == nil {
+		fmt.Println("Receiver didn't raise error <- attack failed")
+		return
+	}
+
+	// ATTACKER
+	decrypted, err := hex.DecodeString(err.Error())
+	if err != nil || len(decrypted) == 0 {
+		fmt.Println("Didn't get decrypted \"plaintext\" of positive length <- attack failed")
+		return
+	}
+	att_key := attacker_guessKey(decrypted)
+
+	// verification
+	if !slices.Equal(key, att_key) {
+		fmt.Println("Attacker got the key wrong")
+	} else {
+		fmt.Println("Attacker got the key right")
+	}
+}
+
+func sender_receiver_setKey() []byte {
+	key := make([]byte, blocklen)
+	_, _ = io.ReadFull(rand.Reader, key)
+	fmt.Println("Key:\t\t\t", hex.EncodeToString(key))
+	return key
+}
+
+func sender_setupMessage() []byte {
+	message := []byte("This is a very important and very secret message")
+	if len(message) != blockcount*blocklen {
+		panic("Message not padded to be exactly 3-blocks long")
+	}
+	return message
+}
+
+// (P1, P2, P3) -> (C1, C2, C3)
+func sender_encryptMessage(msg []byte, key []byte) []byte {
+	block, _ := aes.NewCipher(key)
+	// create CBC cipher which has unsafe behaviour of reusing key as iv
+	encrypter := cipher.NewCBCEncrypter(block, key)
+	encrypted := make([]byte, len(msg))
+	encrypter.CryptBlocks(encrypted, msg)
+	fmt.Println("Encrypted by sender:\t", hex.EncodeToString(encrypted))
+	return encrypted
+}
+
+// (C1, C2, C3) -> (C1, 0, C1)
+func attacker_modifyMessage(msg []byte) []byte {
+	modified := make([]byte, len(msg))
+	copy(modified[:blocklen], msg[:blocklen])
+	copy(modified[2*blocklen:3*blocklen], msg[:blocklen])
+	fmt.Println("Modified by attacker:\t", hex.EncodeToString(modified))
+	return modified
+}
+
+// The assumption in case of processing error (decrypted plaintext containing invalid ASCII value)
+// receiver returns some kind of error message which contains the decrypted plaintext.
+// Here for the sake of simplicity receiver_ans error message is strictly either
+// - hex string of decrypted "plaintext" if there was an invalid ASCII value (almost surely there'll be)
+// - nothing if there was no invalid ASCII value
+func receiver_decryptMessage(encrypted []byte, key []byte) error {
+	block, _ := aes.NewCipher(key)
+	decrypter := cipher.NewCBCDecrypter(block, key)
+	decrypted := make([]byte, len(encrypted))
+	decrypter.CryptBlocks(decrypted, encrypted)
+	for _, char := range decrypted {
+		if char > 0x7F {
+			// Invalid ASCII, return error answer
+			return errors.New(hex.EncodeToString(decrypted))
+		}
+	}
+	return nil // Kind of "OK" response
+}
+
+// decrypted "plaintext" block1 ^ block3 should be equal to iv aka key (it's the same here)
+// will work always as long as we get "plaintext" from receiver
+func attacker_guessKey(receiver_ans []byte) []byte {
+	fmt.Println("Receiver thorws error:\t", hex.EncodeToString(receiver_ans))
+	key := receiver_ans[:blocklen]
+	for i := range blocklen {
+		key[i] ^= receiver_ans[2*blocklen+i]
+	}
+	fmt.Println("Attacker guesses key:\t", hex.EncodeToString(key))
+	return key
+}
diff --git a/submissions/challenge1/jan_opalski/solutions/medium.go b/submissions/challenge1/jan_opalski/solutions/medium.go
new file mode 100644
index 0000000..7571fe1
--- /dev/null
+++ b/submissions/challenge1/jan_opalski/solutions/medium.go
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+package solutions
+
+import (
+	"fmt"
+)
+
+// https://cryptopals.com/sets/3/challenges/21
+func Medium(seed uint32) {
+	fmt.Println("=============================\n\t MEDIUM (21)\n=============================")
+	rng := newMt(seed)
+	fmt.Println("First 5 integers generated for seed", seed)
+	for range 5 {
+		fmt.Println(rng.extractNumber())
+	}
+}
+
+type my_mtrng struct {
+	mt    [624]uint32
+	index int
+}
+
+// used pseudocode from https://pl.wikipedia.org/wiki/Mersenne_Twister
+func newMt(seed uint32) my_mtrng {
+	ans := my_mtrng{}
+	ans.mt[0] = seed
+	for i := uint32(1); i < 624; i++ {
+		ans.mt[i] = 0x6c078965*(ans.mt[i-1]^(ans.mt[i-1]>>30)) + i
+	}
+	return ans
+}
+
+func (rng *my_mtrng) extractNumber() uint32 {
+	if rng.index == 0 {
+		rng.generateNumbers()
+	}
+	y := rng.mt[rng.index]
+	y ^= (y >> 11)
+	y ^= ((y << 7) & 0x9d2c5680)
+	y ^= ((y << 15) & 0xefc60000)
+	y ^= (y >> 18)
+	rng.index = (rng.index + 1) % 624
+	return y
+}
+
+func (rng *my_mtrng) generateNumbers() {
+	for i := range 624 {
+		y := rng.mt[i]&(1<<31) | rng.mt[(i+1)%624]&(^uint32(1<<31))
+		rng.mt[i] = rng.mt[(i+397)%624] ^ (y >> 1)
+		if y%2 == 1 {
+			rng.mt[i] ^= 0x9908b0df
+		}
+	}
+}