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--- ac:laboratoare:11 [2025/11/06 09:54]
marios.choudary
+++ ac:laboratoare:11 [2025/11/06 10:36] (current)
marios.choudary
@@ Line 80: / Line 80: @@
 ==== 3. PQC ready? (4p) ====
-Now try to replace the key exchange used above with post-quantum cryptography. For this you can use different libraries.
+Now let's update the key exchange to use post-quantum cryptography.
-One option is using [[https://github.com/open-quantum-safe/liboqs |liboqs]]. To install it on Ubuntu, use the command below:
+For this you can either modify the work you did above to use a post-quantum key exchange method such as ML-KEM in Python.
+To do this in Python you can use a library such as [[https://pypi.org/project/mlkem/|ml-kem]].
+You can install this as follows:
+<code>
+pip install ml-kem
+</code>
+Below is an example of how to use this library in a simple client-server scenario. Note: this example is provided by Gemini AI, so use it with caution and double-check it:
+<code>
+# Install the library first: pip install ml-kem
+from mlkem.ml_kem import ML_KEM
+from mlkem.parameter_set import ML_KEM_768 # Recommended security level
+import secrets
+# --- Server (Alice) Side ---
+def server_keygen():
+    """Alice generates her ML-KEM key pair."""
+    # Initialize ML-KEM with the desired security level
+    ml_kem = ML_KEM(parameters=ML_KEM_768, randomness=secrets.token_bytes)
+    # Generate the encapsulation key (ek, public) and decapsulation key (dk, private)
+    ek, dk = ml_kem.key_gen()
+    print("Alice: Generated Public Key (ek) and Private Key (dk).")
+    return ek, dk, ml_kem
+def server_decapsulate(dk, c, ml_kem):
+    """Alice decapsulates the ciphertext to get the shared secret."""
+    try:
+        K_prime = ml_kem.decaps(dk, c)
+        print("Alice: Successfully decapsulated the Shared Secret (K').")
+        return K_prime
+    except ValueError as e:
+        print(f"Alice: Decapsulation failed! {e}")
+        return None
+# --- Client (Bob) Side ---
+def client_encapsulate(ek):
+    """Bob encapsulates a shared secret using Alice's public key."""
+    ml_kem = ML_KEM(parameters=ML_KEM_768, randomness=secrets.token_bytes)
+    # Encapsulate to get the shared secret (K) and the ciphertext (c)
+    K, c = ml_kem.encaps(ek)
+    print("Bob: Encapsulated a Shared Secret (K) and created Ciphertext (c).")
+    return K, c
+# --- Communication Flow Simulation ---
+# 1. Server (Alice) Key Generation
+ek_server, dk_server, ml_kem_instance = server_keygen()
+# 2. Public Key Transmission (ek_server is sent to the client)
+print("\n--- Network Transmission: ek sent to Bob ---")
+# 3. Client (Bob) Encapsulation
+K_client, c_client = client_encapsulate(ek_server)
+# 4. Ciphertext Transmission (c_client is sent back to the server)
+print("\n--- Network Transmission: c sent to Alice ---")
+# 5. Server (Alice) Decapsulation
+K_server = server_decapsulate(dk_server, c_client, ml_kem_instance)
+# 6. Verification
+print("\n--- Verification ---")
+if K_client is not None and K_server is not None:
+    if K_client == K_server:
+        print("Success! Alice's and Bob's shared secrets match.")
+        # The shared secret can now be used as an AES key, e.g., K_client
+        # The shared secret is bytes:
+        # print(f"Shared Secret: {K_client.hex()}")
+    else:
+        print("Failure! Shared secrets do not match.")
+else:
+    print("Failure in key exchange process.")
+</code>
+Otherwise, you can start from the Diffie-Hellman key exchange lab we did in OpenSSL/C. You may start from {{:ac:laboratoare:lab_dhe_solved.zip|this}} code, that provides a working solution for the Diffie-Hellman lab in OpenSSL.
+If you go for the C implementation in OpenSSL, one option to include post-quantum encryption is using [[https://github.com/open-quantum-safe/liboqs |liboqs]]. To install it on Ubuntu, use the command below:
 <code>
@@ Line 92: / Line 176: @@
 Alternatively, you can also use OpenSSL directly (starting with version 3.5). You can find some documentation for ML-KEM [[https://docs.openssl.org/master/man7/EVP_KEM-ML-KEM/ |here]] and [[https://docs.openssl.org/3.5/man7/EVP_PKEY-ML-KEM/ |here]].
-Below is an example code obtained through Gemini AI (not fully tested, but perhaps a good starting point -- you may do something similar to get a starting point for the server):
+Below is an example code obtained through Gemini AI (not fully tested, but perhaps a good starting point):
 <code>
 #include <stdio.h>
@@ Line 217: / Line 301: @@
 </code>
+Some instructions to use this code (also from Gemini AI):
+  * Save the code as mlkem_client.c.
+  * Generate a real ML-KEM-768 public key (if you didn't do this in the previous step) by running the following commands:
+<code>
+openssl genpkey -algorithm ML-KEM-768 -out server_private.pem
+openssl pkey -in server_private.pem -pubout -out server_public.pem
+</code>
+  * Extract the raw public key bytes to replace the placeholder in the C code. A common way to get the raw bytes is to convert the PEM file to a DER format and extract the key component. For ML-KEM, the raw key is often found within the ASN.1 structure.
+<note>
+See the DH lab for information about this process. But see also below more details for this process, as provided by Gemini AI.
+</note>
+  * Compile the code (adjusting paths to your OpenSSL 3.5 installation), using a command like this (check again also the DH lab for more details):
+<code>
+gcc -o mlkem_client mlkem_client.c -I/path/to/openssl/include -L/path/to/openssl/lib -lssl -lcrypto
+</code>
+The following GeminiAI-generated scripts (might have some bugs, please double-check, they are only given as reference/helper tools) generate an ML-KEM-768 key pair and, importantly, forces OpenSSL to save the private key in the FIPS 203 'dk' format (the one the C code expects for decapsulation) rather than the default 'seed' format, which is not what the raw C code imports:
+<code>
+#!/bin/bash
+KEY_ALG="ML-KEM-768"
+PRIV_FILE="server_private_dk.pem"
+PUB_FILE="server_public_ek.pem"
+RAW_PRIV_FILE="server_private_raw.bin"
+RAW_PUB_FILE="server_public_raw.bin"
+echo "--- 1. Generating $KEY_ALG Key Pair (forcing dk-only private key) ---"
+# Generate the private key, using '-provparam ml-kem.retain_seed=no'
+# to ensure it saves the full FIPS 203 'dk' decapsulation key,
+# NOT just the seed.
+openssl genpkey -algorithm "$KEY_ALG" \
+    -provparam ml-kem.retain_seed=no \
+    -out "$PRIV_FILE"
+# Extract the public key from the private key file
+openssl pkey -in "$PRIV_FILE" -pubout -out "$PUB_FILE"
+echo "Keys generated: $PRIV_FILE and $PUB_FILE"
+echo "----------------------------------------------------------------"
+</code>
+The ML-KEM public key is in the SubjectPublicKeyInfo structure in the PEM file. We use openssl pkey to output the key in DER format and then remove the ASN.1 header/wrapper to get the raw bytes:
+<code>
+echo "--- 2. Extracting RAW Public Key (608 bytes) ---"
+# The ML-KEM public key is 608 bytes long.
+# The 'pubout' option converts to DER, and 'tail' removes the ASN.1 wrapper.
+# NOTE: The ASN.1 wrapper size may vary by OpenSSL version/config. We must
+# determine the exact offset to trim off the SubjectPublicKeyInfo header.
+# For standard ML-KEM-768 PKCS#8 output, the header is typically 24 bytes.
+HEADER_BYTES=24 # Common offset for ML-KEM-768 PKCS#8 SubjectPublicKeyInfo header
+openssl pkey -in "$PUB_FILE" -pubin -outform DER -out "$RAW_PUB_FILE"
+# Trim the ASN.1 header to get the raw 608-byte key
+# (Skip first $HEADER_BYTES, save the next 608 bytes)
+dd if="$RAW_PUB_FILE" of="client_public_key_raw_final.bin" bs=1 skip=$HEADER_BYTES count=608 status=none
+echo "Raw public key saved to client_public_key_raw_final.bin"
+# Print the hex array for C code
+echo "Public Key Hex Array:"
+xxd -p "client_public_key_raw_final.bin" | tr -d '\n' | sed 's/../&, 0x/g' | sed 's/^, 0x/0x/' | sed 's/, $//'
+echo
+echo "----------------------------------------------------------------"
+</code>
+Similarly, the private key (the FIPS 203 'dk' component) is inside the PKCS#8 private key structure. We again convert to DER and trim the ASN.1 wrappers:
+<code>
+echo "--- 3. Extracting RAW Private Key (1184 bytes) ---"
+# The ML-KEM-768 private key (dk) is 1184 bytes long.
+# We convert to DER and again skip the ASN.1 header bytes.
+# For PKCS#8 'dk' private key, the header is typically 50 bytes.
+HEADER_BYTES=50 # Common offset for ML-KEM-768 PKCS#8 PrivateKeyInfo header
+openssl pkey -in "$PRIV_FILE" -outform DER -out "$RAW_PRIV_FILE"
+# Trim the ASN.1 header to get the raw 1184-byte key
+# (Skip first $HEADER_BYTES, save the next 1184 bytes)
+dd if="$RAW_PRIV_FILE" of="server_private_key_raw_final.bin" bs=1 skip=$HEADER_BYTES count=1184 status=none
+echo "Raw private key saved to server_private_key_raw_final.bin"
+# Print the hex array for C code
+echo "Private Key Hex Array:"
+xxd -p "server_private_key_raw_final.bin" | tr -d '\n' | sed 's/../&, 0x/g' | sed 's/^, 0x/0x/' | sed 's/, $//'
+echo
+echo "----------------------------------------------------------------"
+</code>
+You can now use the hex output from these scripts to reliably replace the placeholder arrays in your C client and server code.
+See also [[https://www.youtube.com/watch?v=FzLzH5W-mi8|this]] video about Post-Quantum Cryptography support in OpenSSL.
+Below is also an example code from Gemini AI as starting point for the server, to pair with the code above for the client.
+Note that these examples use hardcoded values for keys and do not imply any communication. Therefore, make sure you use them for a proper key exchange that uses communication between parties to exchange the public keys and ciphertexts, in a similar manner to what you did for the DH lab.
+<code>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <openssl/evp.h>
+#include <openssl/err.h>
+#include <openssl/provider.h>
+// --- HARDCODED ML-KEM-768 Private Key (Replace with a real key) ---
+// A real ML-KEM-768 private key is 1184 bytes long.
+unsigned char server_private_key_bytes[] = {
+x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
+    // ... 1168 more bytes of a real ML-KEM-768 private key would go here ...
+x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
+};
+size_t private_key_len = 1184; // ML-KEM-768 Private Key size
+// --- HARDCODED Ciphertext (Generated by the Client - Replace with real data) ---
+// A real ML-KEM-768 ciphertext is 1088 bytes long.
+unsigned char client_ciphertext_bytes[] = {
+xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf,
+    // ... 1072 more bytes of a real ML-KEM-768 ciphertext would go here ...
+xb0, 0xb1, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd, 0xbe, 0xbf,
+};
+size_t ciphertext_len = 1088; // ML-KEM-768 Ciphertext size
+// --- Hardcoded Shared Secret Size (32 bytes for ML-KEM-768) ---
+size_t shared_secret_len = 32;
+// Error handling macro
+#define HANDLE_ERROR(msg) \
+    { \
+        fprintf(stderr, "%s failed.\n", msg); \
+        ERR_print_errors_fp(stderr); \
+        goto cleanup; \
+    }
+int main(void) {
+    EVP_PKEY_CTX *kctx = NULL;
+    EVP_PKEY *server_priv_key = NULL;
+    OSSL_PARAM params[2];
+    unsigned char *recovered_secret = NULL;
+    int ret = 0;
+    printf("Starting ML-KEM Server Key Decapsulation (ML-KEM-768).\n");
+    // 1. Load the OpenSSL default provider
+    if (!OSSL_PROVIDER_load(NULL, "default")) {
+        HANDLE_ERROR("Failed to load default provider");
+    }
+    // 2. Import the raw private key bytes into an EVP_PKEY structure
+    printf("2. Importing ML-KEM-768 Private Key...\n");
+    // a. Create parameter array to describe the key components
+    // The ML-KEM private key is referred to as "priv" (or 'dk' in FIPS 203 terms)
+    params[0] = OSSL_PARAM_construct_octet_string("priv", server_private_key_bytes, private_key_len);
+    params[1] = OSSL_PARAM_construct_end();
+    // b. Import the key.
+    server_priv_key = EVP_PKEY_new_from_params(NULL, params, "ML-KEM-768");
+    if (!server_priv_key) {
+        HANDLE_ERROR("EVP_PKEY_new_from_params failed (Private Key Import)");
+    }
+    // 3. Initialize the Key Decapsulation Context
+    kctx = EVP_PKEY_CTX_new_from_pkey(NULL, server_priv_key, NULL);
+    if (!kctx) {
+        HANDLE_ERROR("EVP_PKEY_CTX_new_from_pkey failed");
+    }
+    if (EVP_PKEY_decapsulate_init(kctx) <= 0) {
+        HANDLE_ERROR("EVP_PKEY_decapsulate_init failed");
+    }
+    // 4. Allocate buffer for the shared secret
+    recovered_secret = OPENSSL_malloc(shared_secret_len);
+    if (!recovered_secret) {
+        HANDLE_ERROR("Memory allocation failed");
+    }
+    // 5. Perform the Decapsulation (Server's Key Exchange Step)
+    printf("5. Performing Key Decapsulation...\n");
+    // The ciphertext is passed as the input buffer (client_ciphertext_bytes).
+    // The recovered shared secret is written to the output buffer (recovered_secret).
+    if (EVP_PKEY_decapsulate(kctx, recovered_secret, &shared_secret_len, client_ciphertext_bytes, ciphertext_len) <= 0) {
+        HANDLE_ERROR("EVP_PKEY_decapsulate failed");
+    }
+    printf("✅ Decapsulation successful!\n");
+    printf("   The server has recovered the **Shared Secret**.\n");
+    printf("   - Recovered Shared Secret (First 16 bytes): ");
+    for (size_t i = 0; i < (shared_secret_len > 16 ? 16 : shared_secret_len); i++) {
+        printf("%02x", recovered_secret[i]);
+    }
+    printf("...\n");
+    printf("\n**NOTE: For a successful key exchange, the hash of this secret must match the hash of the client's secret.**\n");
+    ret = 0; // Success
+cleanup:
+    // Free all allocated resources
+    EVP_PKEY_CTX_free(kctx);
+    EVP_PKEY_free(server_priv_key);
+    OPENSSL_free(recovered_secret);
+    OSSL_PROVIDER_unload(OSSL_PROVIDER_load(NULL, "default")); // Unload provider
+    if (ret != 0) {
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}
+</code>

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