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--- ac:laboratoare:11 [2024/01/18 14:11]
marios.choudary
+++ ac:laboratoare:11 [2024/12/12 02:17] (current)
dimitrie.valu
@@ Line 1: / Line 1: @@
-===== Lab 11: EMV crypto - Verifying a DDA Signature =====
+===== Lab ε - TOFU-based Authenticated Key Exchange =====
-==== Install python tools to work with your smartcard ====
+In this lab, we will implement Trust On First Use in a manner similar to that of SSH. We will base our implementation around ''%%PyCryptodome%%'' - you can find the relevant documentation [[https://www.pycryptodome.org/src/api |here]].
-Do the following on Linux (this is for Ubuntu/Debian -- you might need root access):
+==== 0. Init ====
-  * Install pcsclite-dev:
-<code>
-sudo apt-get install libpcsclite-dev
-</code>
-  * Then also install these packages:
-<code>
-sudo apt-get install swig python3-dev libudev-dev python3-pip
-</code>
-  * Get and install Pyscard using pip (install pip if needed)
-<code>
-pip3 install pyscard
-</code>
-  * Install Pyserial
-<code>
-pip3 install pyserial
-</code>
-If this doesn't work, then get pyserial from [[https://pypi.python.org/pypi/pyserial#downloads|here]]
-  * Install pcsc related libs:
-<code>
-sudo apt-get install libusb-dev libccid pcscd libpcsclite1
-</code>
-  * You might also want to install these additional card tools from here:
-<code>
-sudo apt-get install libpcsc-perl pcsc-tools
-</code>
-See details [[http://support.gemalto.com/fileadmin/user_upload/IAM/FAQ/How_to_install_the_PC-Link_reader_on_Linux.pdf|here]].
-For Windows drivers you can check [[https://supportportal.gemalto.com/csm/?id=kb_article_view&sys_kb_id=0adc96844f350700873b69d18110c76a&sysparm_article=KB0016522|here]]. However, we recommend using Linux, as the instructions below apply for the linux installation.
-==== Get information about your card ====
-Try this with your card in the smartcard reader:
+Use these commands to generate a private key and a Diffie-Hellman parameter file:
 <code>
-pcsc_scan
+openssl genrsa -out private.pem 2048
+openssl dhparam -out dhparam.pem 2048
 </code>
-This should show you applications on the card. If not, don't worry, we'll do it ourselves below.
+The ''%%dhparam%%'' file will be used as a hardcoded ''%%.pem%%'' that contains the necessary Diffie-Hellman parameters to generate our DH keys. These values are generally decided upon by convention and are hardcoded - check [[https://datatracker.ietf.org/doc/html/rfc7919#page-19 |RFC 7919]]. An example which can be found there is ''%%ffdhe2048%%''.
-==== Writing a terminal emulator to interact with your card ====
+To better understand its structure, you can use the following commands:
-Start with files for accessing card data in {{:ac:laboratoare:lab_emv_py3.zip|this}} zip file.
+<code bash>
-(for Python 2 you may use the code [[https://ocw.cs.pub.ro/courses/_media/ac/laboratoare/lab_emv.zip|here]], but is obsolete
+openssl dhparam -in dhparam.pem -text -noout
-and no longer mantained).
+openssl asn1parse < dhparam.pem
-Create a file named terminal.txt that will be populated as mentioned below.
-This file should end with a line containing the string '0000000000'.
-After updating this file (see below), we can run the terminal in this manner:
-<code>
-python3 sclink.py --scterminal terminal.txt gg
 </code>
-==== Select finantial app ====
+Create a Python environment or use an existing one, then install the required packages:
-We shall now first select the main financial application on the card via the general `1PAY.SYS.DDF01' file
+<code bash>
-available on some EMV cards followed by selection of the Application ID. See EMV Book 1, sections 11.3 and 12 for details.
+python3 -m venv create env
+source ./env/bin/activate
-<note>
+pip install --upgrade pip
-Newer EMV cards may not support the 1PAY.SYS.DDF01 selection method described below, but you may need to use
+pip install pycryptodome pyasn1 pyasn1-modules
-the Application ID list method or some other variant, as explained in the EMV Book 1, chapter 12.
-</note>
-In summary, the main steps are these:
-  - Send the first SELECT command with `1PAY.SYS.DDF01': 00A404000E315041592E5359532E4444463031
-  - Decode the response using [[http://www.emvlab.org|emvlab]]. Use the SFI response (e.g. 01, concatenated with the record number encoded in the last 3 bits):  (SFI << 3) | REC_NUM. E.g. If SFI=01 and REC_NUM=1, we get the Reference Control parameter (P2) 0x0C for the READ RECORD command, leading to the READ RECORD command 00B2010C00.
-  - Check the available apps by sending READ RECORD commands of the form  00B2010C00, 00B2020C00, etc. Check the responses by decoding them with [[http://www.emvlab.org|emvlab]]
-  - Eventually select one of them using SELECT, e.g.
-  * select particular app: 00A4040007XXXXXXXXXXXXXX (replace the X values based on the Application ID response to the 00B2XXX command above).
-    E.g. to get something like 00A4040007A0000000041010. (If the application has 7 bytes -- 14 hex characters for the Application ID).
-  * 00A4040007A0000000041010 (this must be updated for your card, based on the response to the 00B2XXX command above).
-  * start transaction with GET PROCESSING OPTS: 80A80000028300
-Now your terminal.txt file should look something like this
-(but again, replace the Application ID with the correct one and also use the correct READ RECORD commands -- from your trials).
-<code - terminal.txt>
-A404000E315041592E5359532E4444463031
-B2010C00
-A4040007A0000000041010
-A80000028300
-0000000000
 </code>
-As mentioned above, now run this terminal emulator with the following code:
+We will use the ''%%pyasn1%%'' library to parse the ''%%.pem%%'' file's binarized DER format more conveniently.
-<code>
-python3 sclink.py --scterminal terminal.txt gg
-</code>
-==== Reading data from card ====
+==== 1. Implement DH + RSA signature ====
-Your next goal is to be able to read all the application files with READ RECORD commands (for each file).
+Starting from {{:ac:laboratoare:lab_tofu.zip |these files}}, solve the TODO 1 series in ''%%dhe_server.py%%'' and ''%%dhe_client.py%%''.
-In order to find out the present files (which differ from card to card), you need to issue the GET PROCESSING OPTS command above (80A80000028300).
+On the server side, you should:
-In response you should get the Application Interchange Profile (AIP) bytes (2 bytes, coded according to Book 3, Appendix C)
+    * Send the RSA public key to the client.
-followed by a list of Application File Locators (AFL, coded as explained in Book 3, Section 10.2).
+    * Generate a DH key pair (use the data hardcoded in ''%%dhparam.pem%%'') and send the public key to the client.
+    * Generate a signature over the RSA and DH public keys (concatenate and hash them, then sign using the RSA private key) and send it to the client. Use [[https://pycryptodome.readthedocs.io/en/latest/src/signature/pkcs1_v1_5.html |this]] as your guide.
+    * Receive the client's DH public key and generate the shared DH secret.
+    * Derive a symmetric key from the shared secret. Use [[https://pycryptodome.readthedocs.io/en/latest/src/protocol/kdf.html#hkdf |HKDF]] for this.
-After you decode this ([[http://www.emvlab.org/tlvutils/|TLV decodeer]] might help), you will find one or more groups of 4 bytes as follows:
+On the client side, you should:
-  * 1st byte: SFI << 3
-  * 2nd byte: first record_number
-  * 3rd byte: last record_number
-  * 4th byte: [you don't need it]
-<note>
+    * Receive the RSA public key from the server.
-The response of the Get Processing Opts command can vary. Either it is a BER-TLV encoded value and you will see easily the AIP and AFL values, or it is a non TLV result (starting with tag 88) where the AIP (2 bytes) and the list of AFLs (each of 4 bytes) are just concatenated, i.e. you have something like 88 <LEN> <AIP> <AFL1> <AFL2> ... <AFLn>, where n>=1. Each AFL is encoded as mentioned above.
+    * Receive the server's DH public key.
-</note>
+    * Receive the server's signature over the RSA and DH public keys and [[https://pycryptodome.readthedocs.io/en/latest/src/signature/pkcs1_v1_5.html |verify]] it using the server's public key.
+    * Generate a DH key pair and send the public key to the server.
+    * Compute the shared DH secret.
+    * Derive a symmetric key from the shared secret.
-SFI is like a directory with multiple records that can be read.
+Generating a signature over the RSA and DH public keys is a way to authenticate the remote host. If the client successfully verifies this signature using the server's public key, then the server is authenticated unless the public key itself has been replaced by the attacker in a man in the middle attack. We will look at a way to (mostly) solve this issue in the next task.
-To read a file, you need to issue a READ RECORD command which looks like this:
+The symmetric key derived from the shared secret will be used to encrypt the communication between the client and server. Although we're stopping the tasks here, this key would be the one that you would use to encrypt and decrypt the communication between the client and server. If you want, you can check the [[https://pycryptodome.readthedocs.io/en/latest/src/cipher/aes.html |PyCryptodome documentation]] for more information on how to use AES. To see what ciphers SSH uses, run the following command:
-B2 <record_number> <SFI || 100>
-The <record_number> is a byte (you need to write a READ RECORD command for each record_number).
+<code bash>
-<SFI || 100> is a byte which contains the SFI number in the first 5 bits and 100 in the last 3 bits. This is the same as SFI << 3 + 0x04.
+ssh -Q cipher
-For example, if your AFL shows like "10 01 05 01", then you might want to read records between 01 and 05 using SFI 01 + 0x04, i.e. issuing READ RECORD commands like this:
-<code>
-B2 01 14 00
 </code>
-==== Read public key material ====
+==== 2. Do you like TOFU? ====
-Using the READ RECORD commands mentioned earlier and the [[https://emvlab.org/tlvutils/|TLV decoder]], find the public keys in
-your card, in particular:
-  * Issuer public key certificate
-  * Issuer public key exponent
-  * Issuer public key reminder
-  * ICC public key certificate
-  * ICC public key exponent
-  * ICC public key reminder
-<note>
-Depending on the application selected, you might have (or NOT) public keys available. If you don't find ones, then just select a different app at the beginning.
-</note>
-==== Get Dynamic signature from card ====
-After you get all the public key data, use an INTERNAL AUTHENTICATE command similar to this: 00880000043085C163.
-See the file trace_emv.txt for an example of trace as model for the set of commands you might have to issue (i.e. to add to your terminal.txt file) [[https://ocw.cs.pub.ro/courses/_media/ac/laboratoare/lab_emv.zip|here]].
-As discussed in class (see also the EMV book 2, section 6), modern EMV cards generally support dynamic signature generation (DDA).
-This works as follows:
-  * The terminal issues the INTERNAL AUTHENTICATE command with some random data (typically 4 bytes)
-  * The ICC makes a signature over some internal ICC data and the random bytes from the terminal
-  * The ICC sends the signature (signed dynamic data) to the terminal in response to the INTERNAL AUTHENTICATE command
-  * The terminal verifies the signature using a chain of certificates
-An example of an INTERNAL AUTHENTICATE command similar is the following: 00880000043085C163.
-You can look at the file trace_emv.txt for an example of trace [[https://ocw.cs.pub.ro/courses/_media/ac/laboratoare/lab_emv.zip|here]]
-<note>
-If your card doesn't work with the standard Payment application ID (the one in terminal.txt), try using one from
-[[https://www.eftlab.com/knowledge-base/211-emv-aid-rid-pix/|here]].
-A short list might be this one:
-<code>
-// EMV.AIDLIST:
-EMV.AIDLIST = new Array();
-EMV.AIDLIST[0] = { aid : "A00000002501", partial : true, name : "AMEX" };
-EMV.AIDLIST[1] = { aid : "A0000000031010", partial : false, name : "VISA" };
-EMV.AIDLIST[2] = { aid : "A0000000041010", partial : false, name : "MC" };
-</code>
-Check that you obtained a correct DDA signature and a successful "9000" response.
-To verify the DDA signature obtained earlier, the terminal must have access to the root CA public keys.
-You may find some of these available
-[[https://developer.elavon.com/na/docs/viaconex/1.0.0/emv-integration-guide/10_references/emv_production_public_keys.md|here]],
-[[https://technologypartner.visa.com/download.aspx?id=34|here]],
-[[https://www.mastercard.us/content/dam/public/mastercardcom/na/us/en/documents/mchip-payment-system-public-keys-12042018.pdf|here]],
-or
-[[https://www.eftlab.com/knowledge-base/243-ca-public-keys/|here]].
-You will need to know the card type (AMEX, VISA, Mastercard, etc.) and CA public key index, which is given by the ICC (see tag 8F).
-</note>
-The process to verify a DDA signature is as follows:
-  * The terminal verifies (RSA decrypts) the signed Issuer public key data (read from the ICC) using the CA public key, obtaining the Issuer public key
-  * The terminal verifies (RSA decrypts) the signed ICC public key data (read from the ICC) using the Issuer public key, obtaining the ICC public key
-  * The terminal verifies (RSA decrypts) the signed DDA data using the ICC public key (read from the ICC via the INTERNAL AUTHENTICATE command)
-At each step, the verification step includes decryption of the data and checking that the hash over the fields mentioned in Book 2 of EMV matches the hash in the decrypted data.
-=== Verify the signature returned by your card ===
-In short, you need to do the following (see EMV Book 2, sections 6, 6.1, 6.2, 6.3, 6.4 and 6.5)
-  * Decrypt Issuer public key from Issuer Certificate Public key using the root CA public key of your card scheme (section 6.3)
-  * Decrypt ICC public key from ICC Certificate Public Key using the Issuer public key (section 6.4)
-  * Decrypt DDA signature returned by the card using the ICC public key (section 6.5)
-  * Verify the DDA signature (section 6.5)
-To recover the data from the issuer public key certificate (same applies to the other signatures), you may also find useful the following notes (which are based on the EMV specs mentioned above, please refer to those).
-  * First, generate a template ASN1 file as follows:
-<file asn1 'template.asn1'>
-# Start with a SEQUENCE
-asn1=SEQUENCE:pubkeyinfo
-# pubkeyinfo contains an algorithm identifier and the public key wrapped
-# in a BIT STRING
-[pubkeyinfo]
-algorithm=SEQUENCE:rsa_alg
-pubkey=BITWRAP,SEQUENCE:rsapubkey
-# algorithm ID for RSA is just an OID and a NULL
-[rsa_alg]
-algorithm=OID:rsaEncryption
-parameter=NULL
-# Actual public key: modulus and exponent
-[rsapubkey]
-n=INTEGER:0x%%MODULUS%%
-e=INTEGER:0x%%EXPONENT%%
-</file>
-  * Then use this template for all the keys you need to generate. For example, for the CA root key, use the template and replace the %%MODULUS%% and %%EXPONENT%% part by the modulus and exponent bytes given in the list of public CA root public keys for your card. Say the resulting file is named ca_pk.asn1.
-  * Then use openssl asn1 parser to obtain a public key in DER format as follows:
-  <code>
-  openssl asn1parse -genconf ca_pk.asn1 -out ca_pk.der -noout
-  </code>
-  * Now copy the Issuer Certificate Public Key bytes obtained from the card into a file, say issuer_pk.bytes and then convert this to a binary file like this:
-  <code>
-  cat issuer_pk.bytes | xxd -r -p > issuer_pk.bin
-  </code>
-  * At this point you can verify/decrypt the issuer certificate using openssl as follows:
-  <code>
-  openssl rsautl -verify -in issuer_pk_cert.bin -inkey ca_pk.der -pubin -keyform DER -raw
-  </code>
-  Although it might be more convenient to see the output in hexa, using something like this:
-  <code>
-  openssl rsautl -verify -in issuer_pk_cert.bin -inkey ca_pk.der -pubin -keyform DER -raw | xxd -p
-  </code>
-To understand the meaning of the decrypted bytes, please refer to the respective EMV documentation (in particular sections 6.2-6.5
-in book 2). For example, for the Issuer public key certificate, to obtain the actual issuer public key you need to ignore the first
-bytes (metadata) as well as the last 21 bytes (hash result and trailer value "BC"). The reminder bytes are the first part of the
-Issuer Public key. For the second part of the Issuer Public key (which you need to concatenate to the first part to get the full
-public key), please see the card response with tag 92 (Issuer Public Key reminder).
-Apply the same/similar process to get the ICC public key and finally to decrypt/verify the DDA signature.
-<note>
-Check the decrypted DDA response format in the EMV specs (book 2, section 6.5).
-The response should follow this format (but please check it to confirm):
-  * response length (1 byte) -- say N
-  * signature (N - 21 bytes)
-  * a SHA-1 hash over 20 bytes (20*8 = 160 bits)
-  * a trailer byte with value "BC"
-The hash contained in the DDA reponse is computed over the signature bytes (N - 21 bytes) concatenated with the data sent for the DDA signature (typically the 4 random/unpredictable bytes). Hence, if you recompute the hash over the N-21 signature bytes concatenated with the 4 random bytes and this matches the 20 bytes of the hash in the DDA response this should confirm that the 4 random bytes were correctly input into the signature generation.
-</note>
-=== Responses from a card ===
-=== In case of trouble use existing signature ===
-If you don't manage to get a signature from your card, use these responses from a card (decode them with TLV decode):
-Start from the following responses of a card (decode them with [[http://www.emvlab.org/tlvutils/|TLV decode]])
-<code>
-> 00 B2 01 14 8A
-< 70 81 87 5F 25 03 08 02 01 5F 24 03 12 02 29 5A 08 54 00 49 51 48 65 15 96 5F 34 01 00 9F 07 02 FF 00 8E 14 00 00 00 00 00 00 00 00 42 01 44 03 41 03 42 03 5E 03 1F 03 8C 21 9F 02 06 9F 03 06 9F 1A 02 95 05 5F 2A 02 9A 03 9C 01 9F 37 04 9F 35 01 9F 45 02 9F 4C 08 9F 34 03 8D 0C 91 0A 8A 02 95 05 9F 37 04 9F 4C 08 9F 0D 05 F8 50 AC 08 00 9F 0E 05 00 00 00 00 00 9F 0F 05 F8 70 AC 98 00 5F 28 02 06 42 9F 4A 01 82
-</code>
-<code>
-> 00 B2 02 14 47
-< 70 45 9F 08 02 00 02 57 13 54 00 49 51 48 65 15 96 D1 20 22 01 00 00 08 28 00 00 0F 5F 20 17 43 48 4F 55 44 41 52 59 20 2F 4F 4D 41 52 20 53 41 4C 49 4D 20 44 4C 5F 30 02 02 01 9F 42 02 09 78 9F 44 01 02 8F 01 04
-</code>
-<code>
-> 00 B2 03 14 96
-< 70 81 93 90 81 90 0B 69 37 0D CF E1 E7 B0 9C 00 6F CC 12 91 38 C0 7A 69 80 87 3C 1E 0A 60 04 E6 8E 23 F5 BF B7 51 08 28 00 8B 37 F4 C3 D3 30 6A 0D AE 70 92 51 2F FB B1 E8 1E AE 26 23 1A 0D BF C8 30 B3 1C F1 F6 81 9C F3 12 37 FE 74 B3 5C 5B 57 62 0A 4D C1 96 ED 06 CC 94 45 AC 0A 5B 00 BB 8E BA 7F B4 1D 97 4C A1 F9 DD A4 45 1E B3 2E FC 55 5A 16 9D 60 09 47 4E 97 09 2B 33 21 AD D5 9D 1C 35 30 11 CC C1 C1 D6 19 65 B9 12 0E 07 FC 8F B3 72 4A C0 3A 15
-</code>
-<code>
-> 00 B2 04 14 CA
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
-</code>
-<code>
-> 00 B2 05 14 B4
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
-</code>
-<code>
-> 00 88 00 00 04 30 85 C1 63
-<  [] 61 87
-> 00 C0 00 00 87
-< 77 81 84 9F 4B 81 80 99 0B 9E 27 E7 51 B7 45 5F F8 64 82 3C 82 35 94 CD E0 26 AB 9A D9 1D 02 7F 6E B8 5E DE 27 23 B7 81 40 0D BD 85 FD 20 21 07 08 A3 6C B3 09 51 33 B2 D7 30 BF 12 8B 23 C9 4D 87 3F 28 56 63 1C 19 F5 21 BB BC E2 2F 45 B2 C9 0A 7E B2 F5 C7 02 03 3C B3 AB 1C 06 F5 5A CB 44 3A E0 93 84 42 33 FF 16 D0 CE BB 75 6C E1 39 09 A6 39 5A 89 48 D1 9A BF D8 5E 29 43 0F A0 CC 16 90 7A 5C 92 CA 74 3F
-</code>
-For this card the public key modulus is this (1152 bit):
-<code>
-A6DA428387A502D7DDFB7A74D3F412BE762627197B25435B7A81716A700157DDD06F7CC99D6CA28C2470527E2C03616B9C59217357C2674F583B3BA5C7DCF2838692D023E3562420B4615C439CA97C44DC9A249CFCE7B3BFB22F68228C3AF13329AA4A613CF8DD853502373D62E49AB256D2BC17120E54AEDCED6D96A4287ACC5C04677D4A5A320DB8BEE2F775E5FEC5
-</code>
-And the exponent is 0x03.
-In case of big big trouble, is the ASN1 file that you should obtain for the Issuer Public Key:
-<code asn1 issuer_pk.asn1>
-# Start with a SEQUENCE
-asn1=SEQUENCE:pubkeyinfo
-# pubkeyinfo contains an algorithm identifier and the public key wrapped
-# in a BIT STRING
-[pubkeyinfo]
-algorithm=SEQUENCE:rsa_alg
-pubkey=BITWRAP,SEQUENCE:rsapubkey
-# algorithm ID for RSA is just an OID and a NULL
+Now start solving the TODO 2 series by implementing Trust On First Use in ''%%dhe_client.py%%''. Do it as follows:
-[rsa_alg]
-algorithm=OID:rsaEncryption
-parameter=NULL
-# Actual public key: modulus and exponent
+    * Store the public key of the server in a file named ''%%known_hosts%%'' in the following format (the same way SSH does it):
-[rsapubkey]
-n=INTEGER:0xcb3a3f60cceccabb300a57d0c7c7fc974a34d8fa4728b9bb4719fec80a41b0cd04eb2c9fdfdd9139f87de2b3cbee69ecdf2889a37888beadc7a5ed5cc51da52940b000ef806ab277d0276386493da941f390f8a1354a3040dc84a7611b0a6e46874efd463a0f0607459ea58eEF2FD9E83BD90C88A1A65884B35A636908403659831AFB41743C61E50FCE32491CD39A81
-e=INTEGER:0x3
+<code text>
+hostname1 public_key1
+hostname2 public_key2
+...
 </code>
+    * If the client already has a public key for the given IP of the server, check that the public key of the server matches the one that is stored (if it's a first connection - i.e., **First Use** - just store the public key and **Trust** the host).
+    * If it matches, print a "connection established" message and proceed to use that key for verification of the signature over the DH share of the server.
+    * If it doesn't match, print a suggestive error message and exit.
+This is very similar to what SSH does when connecting to a server using a pair of public/private keys and is known as Trust On First Use (TOFU) authentication.

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