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One of the common questions during L3 certification - a test case requires SDA/CDA/DDA offline authentication and it's not working. Why?

You probably don't have your keys loaded. Let's get into this.

Note: This article's focus is on individuals who are certifying with the brands, but you can also loosely apply this to other use cases that require CAPKs to be loaded.

Edit:  : For the users of the C++ SDK, if you are getting errors with the length of the contactless CAPK modulus, flip the length bytes (i.e., use big-endian format, not little-endian). See the discussion further below.


Saving a CAPK with the Universal Demo:

Look for the 'Save CAPK' command, enter in the CAPK RID as well as the index.

Instructions on how to format the Data can be found further below.

You can grab the Universal Demo (most recent version) from here.


What do I need before I try to save a CAPK?

If you happen to be performing brand testing with a specific card - each test case lists which card is being used.

Often times, the cards are re-used among tests.

To pass these tests, some of them require SDA/CDA/DDA, which references a Public Key / Private Key pair.

The test will be using the private key on the card - you will need to load the respective public key onto the device that is being validated.

The below values in the 'Breaking the Data down' should be provided by the issuer/brand that requires the test.

Here's an example of how the data may be presented (heavily redacted) -


Save CAPK command Data Breakdown (Sample):

Name: 

a000009999e1

Data:

a000009999e10101f8707b9bedf031e58a9f843631b90c90d80ed69500000003700099c5b70aa61b4f4c51

b6f90b0e3bfb7a3ee0e7db41bc466888b3ec8e9977c762407ef1d79e0afb2823100a020c3e8020593db50e

90dbeac18b78d13f96bb2f57eeddc30f256592417cdf739ca6804a10a29d2806e774bfa751f22cf3b65b38f3

7f91b4daf8aec9b803f7610e06ac9e6b


Breaking the Data down (example AID):

AID RID (5 bytes)a000009999
CA Index (1 byte)e1
Hash Algorithm (1 byte)

01

Encryption Algorithm (1 byte)01
Hash Value (20 bytes)

f8707b9bedf031e58a9f843631b90c90d80ed695

Public Key Exponent (4 bytes)00 00 00 03

2 bytes Modulus Length (4 bytes)

NOTE: The bytes need to be FLIPPED

IE 00 70 in this case, evaluating to 112 length

70 00

Variable bytes Modulus

(variable bytes length indicated prior)

99c5b70aa61b4f4c51b6f90b0e3bfb7a3ee0e7db41bc466888b3ec8e9977c762407ef1d79e0afb2823100a020c3e802059

3db50e90dbeac18b78d13f96bb2f57eeddc30f256592417cdf739ca6804a10a29d2806e774bfa751f22cf3b65b38f37f91b4

daf8aec9b803f7610e06ac9e6b


Example 1:

We were given this from AMEX to put in our device - 


INDEX C8
Header ‘20’ (not used)
Service Identifier ‘00 00 00 00’ (not used)
Length of CAPK Modulus ‘90’ (Modulus Length, interpret as 90 00)
CAPK Algorithm Indicator ‘01’ (Hash Algorithm)
Length of CAPK Exponent ‘01’ 
RID ‘A0 00 00 00 25’ (AID Name)
CAPK Index C8 (CA Index)
CAPK Modulus ( Variable Bytes Modulus) BF0CFCED708FB6B048E3014336EA24AA007D7967B8AA4E613D26D015C4FE7805D9DB131CED0D2A8ED504C3B5CCD48C33199E5A5BF644DA043B54DBF60276F05B1750FAB39098C7511D04BABC649482DDCF7CC42C8C435BAB8DD0EB1A620C31111D1AAAF9AF6571EEBD4CF5A08496D57E7ABDBB5180E0A42DA869AB95FB620EFF2641C3702AF3BE0B0C138EAEF202E21D
CAPK Exponent ‘03’ (Public Key exponent, would be viewed as 00 00 00 03)
Hash Value 33BD7A059FAB094939B90A8F35845C9DC779BD50



Let's piece this together:


Step 1: AID Name would be RID + CA index.

A0 00 00 00 25 + C8 = a000000025c8


Step 2: Rest of the body is formed by AID Name + CA Index (1 byte) + Hash Algorithm (1 byte) + Encryption Algorithm (1 byte) + Hash Value (20 bytes) + Public Key Exponent (4 bytes) + 2 bytes Modulus Length (4 bytes) + Variable bytes Modulus



a000000025 + c8 + 01 + 01 + 33BD7A059FAB094939B90A8F35845C9DC779BD50 + 00 00 00 03+ 9000 + BF0CFCED708FB6B048E3014336EA24AA007D7967B8AA4E613D26D015C4FE7805D9DB131CED0D2A8ED504C3B5CCD48C33199E5A5BF644DA043B54DBF60276F05B1750FAB39098C7511D04BABC649482DDCF7CC42C8C435BAB8DD0EB1A620C31111D1AAAF9AF6571EEBD4CF5A08496D57E7ABDBB5180E0A42DA869AB95FB620EFF2641C3702AF3BE0B0C138EAEF202E21D

=

a000000025c8010133BD7A059FAB094939B90A8F35845C9DC779BD50000000039000BF0CFCED708FB6B048E3014336EA24AA007D7967B8AA4E613D26D015C4FE7805D9DB131CED0D2A8ED504C3B5CCD48C33199E5A5BF644DA043B54DBF60276F05B1750FAB39098C7511D04BABC649482DDCF7CC42C8C435BAB8DD0EB1A620C31111D1AAAF9AF6571EEBD4CF5A08496D57E7ABDBB5180E0A42DA869AB95FB620EFF2641C3702AF3BE0B0C138EAEF202E21D


Notes:

  • Encryption Algorithm / Hash Algorithm are usually value 01
  • If Hash value is not provided, you can calculate it as well (your acquirer should have provided this value). It is a SHA-1 hash of... see below. 



How do I form the Hash Value? Concatinate the 4 data elements listed below.


AID RID + CA Index + Modulus + Public Key Exponent (hash only relevant bytes) In this example, I was able to re-calculated the SHA-1 hash successfully by removing the first three 00 bytes of the Public Key Exponent.

Input for the hash :

A0 00 00 00 25 C8 BF0CFCED708FB6B048E3014336EA24AA007D7967B8AA4E613D26D015C4FE7805D9DB131CED0D2A8ED504C3B5CCD48C33199E5A5BF644DA043B54DBF60276F05B1750FAB39098C7511D04BABC649482DDCF7CC42C8C435BAB8DD0EB1A620C31111D1AAAF9AF6571EEBD4CF5A08496D57E7ABDBB5180E0A42DA869AB95FB620EFF2641C3702AF3BE0B0C138EAEF202E21D 03

Final hash:

33BD7A059FAB094939B90A8F35845C9DC779BD50

Use the tool at http://www.idtechproducts.com/hosted-files/tools/encryptiondecryptiontool.html to calculate SHA-1. (The tool will automatically remove spaces and newlines from the data.) 

Another Example:

a000009999 + 

e1+

99c5b70aa61b4f4c51b6f90b0e3bfb7a3ee0e7db41bc466888b3ec8e9977c762407ef1d79e0afb2823100a020c3e8020593db50e90dbeac18b78d13f96bb2f57eeddc30f256592417cdf739ca6804a10a29d2806e774bfa751f22cf3b65b38f37f91b4daf8aec9b803f7610e06ac9e6b +

03 (not 00 00 00 03)


a000009999e199c5b70aa61b4f4c51b6f90b0e3bfb7a3ee0e7db41bc466888b3ec8e9977c762407ef1d79e0afb2823100a020c3e8020593db50e90dbeac18b78d13f96bb2f57eeddc30f256592417cdf739ca6804a10a29d2806e774bfa751f22cf3b65b38f37f91b4daf8aec9b803f7610e06ac9e6b03


SHA-1 Hash on that value to obtain:

F8707B9BEDF031E58A9F843631B90C90D80ED695



CONTACTLESS CAPKS IN THE C++ / JAVA SDK

Known Issue: We have a minor edge case that has cropped up in recent days. This is being addressed.  


You can save this one in C++:
// Sample, note the bolded part
a000009999e50101ada2349afd118d55af782d37b64651af1ca61ee500000003 *0080* d4fdae94dedbecc6d20d38b01e91826dc6954338379917b2bb8a6b36b5d3b0c5eda60b337448baffebcc3abdba869e8dadec6c870110c42f5aab90a18f4f867f72e3386ffc7e67e7ff94eba079e531b3cf329517e81c5dd9b3dc65db5f9043190be0be897e5fe48adf5d3bfa0585e076e554f26ec69814797f15669f4a255c13 (edited)


In C# demo / C# SDK, the above one won't actually save. But this will save in C#:

a000009999e50101ada2349afd118d55af782d37b64651af1ca61ee500000003 *8000* d4fdae94dedbecc6d20d38b01e91826dc6954338379917b2bb8a6b36b5d3b0c5eda60b337448baffebcc3abdba869e8dadec6c870110c42f5aab90a18f4f867f72e3386ffc7e67e7ff94eba079e531b3cf329517e81c5dd9b3dc65db5f9043190be0be897e5fe48adf5d3bfa0585e076e554f26ec69814797f15669f4a255c13



In short, for your existing CAPK files you actually already loaded in, if we want to make them work with the original format

Swap the length bytes (i.e., use big-endian format in your code) and they should save.


*Original created AID from C# demo*
a000000004fa01017f5acbb96b589f74cb959ed1c35bdb965c3f410600010001 *00f8* a4203e0c7beb27097b63c103c19fdcda671aea7f813065756f3b9b81810cbd4bc4dec548fbf1f3cdae51f847235cbf2c8badd8aca7c93bea3d44e80ed6a7b70e29622619db420accce07e1dd4e6c354f359fbdc9c5b70813926f77d827e52b19daf09bfae5274438bb8f61d17753c9ec0a8efa3b7e46f02692160d2653cdbcc71b7d48bd37968316eb444f6504b9421b7dd3035a2c117d8b1f76a8975440da9563618102397b881cef8ada7689edface32482a2dffed656e7f951db841da78368c6293bfc1053a86a845bfa6578e4b69f100b42b558fde1aecec6d250741bc783aa8a68a4261e7bb9246b10587a498d68dd955ce8b2b2433

*Edited, the one you should use for C++ and moving on forwards*
a000000004fa01017f5acbb96b589f74cb959ed1c35bdb965c3f410600010001 *f800* a4203e0c7beb27097b63c103c19fdcda671aea7f813065756f3b9b81810cbd4bc4dec548fbf1f3cdae51f847235cbf2c8badd8aca7c93bea3d44e80ed6a7b70e29622619db420accce07e1dd4e6c354f359fbdc9c5b70813926f77d827e52b19daf09bfae5274438bb8f61d17753c9ec0a8efa3b7e46f02692160d2653cdbcc71b7d48bd37968316eb444f6504b9421b7dd3035a2c117d8b1f76a8975440da9563618102397b881cef8ada7689edface32482a2dffed656e7f951db841da78368c6293bfc1053a86a845bfa6578e4b69f100b42b558fde1aecec6d250741bc783aa8a68a4261e7bb9246b10587a498d68dd955ce8b2b2433



Questions, comments, concerns, want more examples? Leave a comment, and / or email me at jasonc@idtechproducts.com