ac:laboratoare:11

Lab 11: EMV crypto - Verifying a DDA Signature

Lab 11: EMV crypto - Verifying a DDA Signature

Install python tools to work with your smartcard

Do the following on Linux (this is for Ubuntu/Debian – you might need root access):

Install pcsclite-dev:

sudo apt-get install libpcsclite-dev

Then also install these packages:

sudo apt-get install swig python3-dev libudev-dev python3-pip

Get and install Pyscard using pip (install pip if needed)

pip3 install pyscard

Install Pyserial

pip3 install pyserial

If this doesn't work, then get pyserial from here

Install pcsc related libs:

sudo apt-get install libusb-dev libccid pcscd libpcsclite1

You might also want to install these additional card tools from here:

sudo apt-get install libpcsc-perl pcsc-tools

See details here.

For Windows drivers you can check 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:

pcsc_scan

This should show you applications on the card. If not, don't worry, we'll do it ourselves below.

Writing a terminal emulator to interact with your card

Start with files for accessing card data in this zip file. (for Python 2 you may use the code here, but is obsolete and no longer mantained).

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:

python3 sclink.py --scterminal terminal.txt gg

Select finantial app

We shall now first select the main financial application on the card via the general `1PAY.SYS.DDF01' file available on some EMV cards followed by selection of the Application ID. See EMV Book 1, sections 11.3 and 12 for details.

Newer EMV cards may not support the 1PAY.SYS.DDF01 selection method described below, but you may need to use the Application ID list method or some other variant, as explained in the EMV Book 1, chapter 12.

In summary, the main steps are these:

Send the first SELECT command with `1PAY.SYS.DDF01': 00A404000E315041592E5359532E4444463031
Decode the response using 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 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).

terminal.txt

00A404000E315041592E5359532E4444463031
00B2010C00
00A4040007A0000000041010
80A80000028300
0000000000

As mentioned above, now run this terminal emulator with the following code:

python3 sclink.py --scterminal terminal.txt gg

Reading data from card

Your next goal is to be able to read all the application files with READ RECORD commands (for each file).

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).

In response you should get the Application Interchange Profile (AIP) bytes (2 bytes, coded according to Book 3, Appendix C) followed by a list of Application File Locators (AFL, coded as explained in Book 3, Section 10.2).

After you decode this (TLV decodeer might help), you will find one or more groups of 4 bytes as follows:

1st byte: SFI « 3
2nd byte: first record_number
3rd byte: last record_number
4th byte: [you don't need it]

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.

SFI is like a directory with multiple records that can be read.

To read a file, you need to issue a READ RECORD command which looks like this: 00B2 <record_number> <SFI || 100>

The <record_number> is a byte (you need to write a READ RECORD command for each record_number). <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.

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:

00 B2 01 14 00

Read public key material

Using the READ RECORD commands mentioned earlier and the 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

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.

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).

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.

If your card doesn't work with the standard Payment application ID (the one in terminal.txt), try using one from here.

A short list might be this one:

// 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" };

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 here, here, here, or 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).

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:

'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%%

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:

  openssl asn1parse -genconf ca_pk.asn1 -out ca_pk.der -noout

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:

  cat issuer_pk.bytes | xxd -r -p > issuer_pk.bin

At this point you can verify/decrypt the issuer certificate using openssl as follows:

  openssl rsautl -verify -in issuer_pk_cert.bin -inkey ca_pk.der -pubin -keyform DER -raw

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 15 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.

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.

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 TLV decode)

> 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

> 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

> 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

> 00 B2 04 14 CA
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

> 00 B2 05 14 B4
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

> 00 88 00 00 04 30 85 C1 63
<  [] 61 87
135
> 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

For this card the public key modulus is this (1152 bit):

A6DA428387A502D7DDFB7A74D3F412BE762627197B25435B7A81716A700157DDD06F7CC99D6CA28C2470527E2C03616B9C59217357C2674F583B3BA5C7DCF2838692D023E3562420B4615C439CA97C44DC9A249CFCE7B3BFB22F68228C3AF13329AA4A613CF8DD853502373D62E49AB256D2BC17120E54AEDCED6D96A4287ACC5C04677D4A5A320DB8BEE2F775E5FEC5

And the exponent is 0x03.

In case of big big trouble, is the ASN1 file that you should obtain for the Issuer Public Key:

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
[rsa_alg]
algorithm=OID:rsaEncryption
parameter=NULL
 
# Actual public key: modulus and exponent
[rsapubkey]
n=INTEGER:0xcb3a3f60cceccabb300a57d0c7c7fc974a34d8fa4728b9bb4719fec80a41b0cd04eb2c9fdfdd9139f87de2b3cbee69ecdf2889a37888beadc7a5ed5cc51da52940b000ef806ab277d0276386493da941f390f8a1354a3040dc84a7611b0a6e46874efd463a0f0607459ea58eEF2FD9E83BD90C88A1A65884B35A636908403659831AFB41743C61E50FCE32491CD39A81
 
e=INTEGER:0x3

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