PACE Protocol Explained: Step-by-Step with Logs

Monssif HAKKOURMonssif HAKKOUR
7 min read

In this article, we’ll dive deep into the PACE (Password Authenticated Connection Establishment) protocol with a particular focus on the Integrated Mapping. This protocol strengthens eMRTD (electronic Machine-Readable Travel Document) security by replacing the legacy BAC (Basic Access Control).

I’ll guide you through a full PACE transaction executed using a real smart card configured as follows:

  • PACE Protocol: ID_PACE_ECDH_IM_AES_CBC_CMAC_256

  • Elliptic Curve: BrainpoolP256r1

  • Password Type: CAN (Card Access Number)

During this walkthrough, I'll refer to the relevant paragraphs from the ICAO specification defined in [1].

Step 1: Read EF.CardAccess (EF 011C)

  • Purpose: Identify supported PACE protocols and retrieve mapping/crypto algorithms.

  • ICAO Reference: [1] §2.1.1 & 5.4

This file contains the PACEInfo object [1] §5.3.1 which tells the Terminal how to set up the secure communication. the PACEInfo object includes:

  • Protocol identifier “ProtocolOID“ (e.g. 04007F00070202040404 = id-PACE-ECDH-IM-AES-CBC-CMAC-256)

  • Version (must be 0×02)

  • ParameterId “Curve/algorithm information” (e.g. 0×0D = BrainpoolP256r1)

#-----------------------------------------------------------------------------------
# Step 1: Read EF.CardAccess (EF 011C)
#-----------------------------------------------------------------------------------
>> 00 B0 9C00 00
<< 31143012060A04007F0007020204040402010202010D

# Algorithm OID: 04007F00070202040404 (AES/CBC/NoPadding)
# Domain Parameter = 0x0D (13 = BrainpoolP256r1)

Step 2: MSE:SET AT — Select and Initialize PACE Protocol

  • Purpose: Tell the chip to activate PACE using the selected parameters

  • ICAO Reference: [1] §3.5.1

This command informs the chip of the selected PACEInfo. Here, we selected the OID id-PACE-ECDH-IM-AES-CBC-CMAC-256 with BrainpoolP256r1.

#-----------------------------------------------------------------------------------
# MSE SET AT - Select and initialize PACE protocol
#-----------------------------------------------------------------------------------
>> 00 22 C1A4 12 
    800A 04007F00070202040404 
    8301 02 
    8401 0D

This is where the secure session is about to begin!

Step 3: General Authenticate #1 (Request) — Get Encrypted Nonce

  • Purpose: Retrieve a nonce encrypted under a key derived from the password (CAN)

  • ICAO Reference: [1] §3.5.2 & 3.5.3

The Terminal sends the General Authenticate command requesting the chip's nonce.

#-----------------------------------------------------------------------------------
# General Authenticate #1 - Get encrypted nonce
#-----------------------------------------------------------------------------------
>> 10 86 0000 02 7C00 00

Step 4: General Authenticate #1 (Response)

  • Purpose: Receive encrypted nonce Z

  • ICAO Reference: [1] §3.5.2 & 3.5.3

You get the encrypted nonce Z, which the Terminal now needs to decrypt using the password-derived key K_pi.

#-----------------------------------------------------------------------------------
# General Authenticate #1 Response: <SW 9000>
#-----------------------------------------------------------------------------------
<< 7C22 8020 E0F1F5BFAA44F62BC55151E3FBFA21B87C2FE3995FAE5D287B7BBD7774450498

Step 5: Decrypt the Encrypted Nonce Z to get Nonce s

  • ICAO Reference: [1] §3.2, 3.4.2 & 5.8

Using the password (CAN), you derive K_pi and decrypt the encrypted nonce to get s. This nonce will later be mapped onto the curve.

K_pi derivation: The key is derived by applying the SHA-256 algorithm to the password combined with the static string "00000003". The size of K_pi in our case is 32 bytes.

# CAN: 300829
# K_PI: B653727C26263104EEE97E89BBF6312A911427C454E19C6DFF58EAB08D7FC6B9
# encrypted nonce: E0F1F5BFAA44F62BC55151E3FBFA21B87C2FE3995FAE5D287B7BBD7774450498
# decrypted nonce: 1171811C54032A1A86E2A18538C1296A5D9591CF130294794947536CA8DC97A8

Step 6: Nonce Mapping (General Authenticate #2)

  • Purpose: Map the nonce onto a curve point G_map

  • ICAO Reference: [1] §3.2, 3.4.2.2.1 & 3.5.3

For the integrated mapping, you send a randomly generated nonce t which is the same size as the nonce s.

#-----------------------------------------------------------------------------------
# General Authenticate #2 - Send mapped the nonce
#-----------------------------------------------------------------------------------
>> 10 86 0000 24 
    7C22 8120 B04E65450A54674ED7A48C62B00D58892BFC73D551721907636102622800687B 00

Step 7: General Authenticate #2 (Response) — Get Mapped Point G_map

  • Purpose: PICC confirms nonce mapping

  • ICAO Reference: [1] §3.2, 3.4.2 & 3.5.3

This confirms that both sides are now aligned on the mapped point G_map which is calculated using a pseudo random function [1] §3.4.2.2.3 and a point encoding function [1] §A.

#-----------------------------------------------------------------------------------
# General Authenticate #2 Response: <SW 9000>
#-----------------------------------------------------------------------------------
<< 7C02 8200

# New mapped G: 04895203FD3EF9355D3C54BC2785095C64F852E24C0EDD72D6A157478A81D846FF3B95F82A7950815B4FAEF2D3F55F0B2BF74546207DADEFFE8507BDE971325788

Step 8: General Authenticate #3 — Perform Key Agreement

  • Purpose: Perform ECDH key agreement using ephemeral key pairs

  • ICAO Reference: [1] §3.2, 3.5.2 & 3.5.3

Each side generates an ephemeral key pair (sk_PCD, pk_PCD) and (sk_PICC, pk_PICC) in the group field where the generator is the mapped point G_map. They then exchange their public keys.

# Ephemeral PCD Public Key: 0493E1C675765380F07FB7620A1DAA1202B1F92000FAB048CED1F51637954B168B69865421215B764CF42F2A21B3981FB888DAD45091F9EB952511B1C72C912DC8
# Ephemeral PCD Private Key: 23F6179CD2689C71BC4670A2CC495983C50F96F4FF67E255A1D3F6B0E4EB637F

#-----------------------------------------------------------------------------------
# General Authenticate #3 - Perform Key agreement
#-----------------------------------------------------------------------------------
>> 10 86 0000 45
    7C43 8341
    0493E1C675765380F07FB7620A1DAA1202B1F92000FAB048CED1F51637954B168B69865421215B764CF42F2A21B3981FB888DAD45091F9EB952511B1C72C912DC800

#-----------------------------------------------------------------------------------
# General Authenticate #3 Response: <SW 9000>
#-----------------------------------------------------------------------------------
<< 7C43 8441
    04705A5B3F5688F404A225F797695D5B0E63DA0F7BADB9F46CBB1127FDB0F55B94338439EAC12CD0737A60E0F8B7D49BEF6A29891EEFAA30BC5513EC3C148008EE

# Ephemeral PICC Public Key: 04705A5B3F5688F404A225F797695D5B0E63DA0F7BADB9F46CBB1127FDB0F55B94338439EAC12CD0737A60E0F8B7D49BEF6A29891EEFAA30BC5513EC3C148008EE

Step 9: Compute the Shared Secret

  • Purpose: The Terminal calculates the Shared Secret using sk_PCD and pk_PICC, while the card calculates the same value using sk_PICC and pk_PCD. The result will be the same due to Diffie-Hellman. For the ECDH scheme, the ECSVDP-DH key agreement algorithm is used to compute the Shared Secret.

  • ICAO Reference: [1] §3.2 & 5.6

This shared secret Z is used to derive the session keys.

# Shared Secret: 876E5413F3D3CB70212189D8B7489DA5C4637521ED2A64E5B563C753B32F4C35

Step 10: Derive Session Keys K_enc and K_mac

  • Purpose: Use the shared secret Z to derive K_enc and K_mac keys

  • ICAO Reference: [1] §3.2 & 5.8

The derivation uses a Key Derivation Function (KDF) defined in the spec. It uses SHA-1 or SHA-256 to calculate the keys as explained below. Protocols based on Integrated or Generic Mapping with 128-bit keys use SHA-1 while those with 192 or 256-bit keys use SHA-256. Protocols based on Chip Authentication Mapping use SHA-256:

  • K_enc (encryption): SHA-X(Shared Secret || 00000001)

  • K_mac (integrity): SHA-X(Shared Secret || 00000002)

# Kenc: CE4D305A33AF93D9791A4465B1574BD8D78C13B15B7F0D5092204CD593311FA9
# Kmac: 2019F04A24AE368AC47C2F4CD157FA7C47B218E912375B6D2D5EF663BA9A7D76

Step 11: Terminal Authentication Token — MAC Token Creation

  • Purpose: Prove terminal knowledge of shared secret

  • ICAO Reference: [1] §3.2 & 3.4.3

The Terminal computes a MAC over its public key using K_mac.

# Terminal Authentication Token: 17CEBFBD204F3105

Step 12: General Authenticate #4 — Mutual Authentication

  • Purpose: The Terminal sends the MAC token using K_mac. The card verifies it and responds with its own MAC token

  • ICAO Reference: [1] §3.5.3

#-----------------------------------------------------------------------------------
# General Authenticate #4 - Mutual Authentication
#-----------------------------------------------------------------------------------
>> 00 86 0000 0C 7C0A 8508 17CEBFBD204F3105 00
#-----------------------------------------------------------------------------------
# General Authenticate #4 Response: <SW 9000>
#-----------------------------------------------------------------------------------
<< 7C0A 8608 4C2521729B2CAD25

Step 13: Verify Chip Authentication Token

  • Purpose: The Terminal verifies the PICC's token

  • ICAO Reference: [1] §3.2 & 3.4.3

If the values match, Mutual Authentication is complete.

# Chip Authentication Token: 4C2521729B2CAD25
# Computed Chip Authentication Token: 4C2521729B2CAD25

# PACE successfully performed

🎉

PACE Successfully Performed!

At this point, the secure messaging context is established using K_enc and K_mac. Future APDUs can be wrapped and secured using these keys.

Complete APDU Log

At the end, I provide the full trace of APDUs exchanged between the Terminal and the Card during the PACE session:

#-----------------------------------------------------------------------------------
# Step 1: Read EF.CardAccess (EF 011C)
#-----------------------------------------------------------------------------------
>> 00 B0 9C00 00
<< 31143012060A04007F0007020204040402010202010D

# Algorithm OID: 04007F00070202040404 (AES/CBC/NoPadding)
# Domain Parameter = 0x0D (13 = BrainpoolP256r1)

#-----------------------------------------------------------------------------------
# MSE SET AT - Select and initialize PACE protocol
#-----------------------------------------------------------------------------------
>> 00 22 C1A4 12 
    800A 04007F00070202040404 
    8301 02 
    8401 0D

#-----------------------------------------------------------------------------------
# General Authenticate #1 - Get encrypted nonce
#-----------------------------------------------------------------------------------
>> 10 86 0000 02 7C00 00

#-----------------------------------------------------------------------------------
# General Authenticate #1 Response: <SW 9000>
#-----------------------------------------------------------------------------------
<< 7C22 8020 E0F1F5BFAA44F62BC55151E3FBFA21B87C2FE3995FAE5D287B7BBD7774450498

# CAN: 300829
# K_PI: B653727C26263104EEE97E89BBF6312A911427C454E19C6DFF58EAB08D7FC6B9
# encrypted nonce: E0F1F5BFAA44F62BC55151E3FBFA21B87C2FE3995FAE5D287B7BBD7774450498
# decrypted nonce: 1171811C54032A1A86E2A18538C1296A5D9591CF130294794947536CA8DC97A8

#-----------------------------------------------------------------------------------
# General Authenticate #2 - Send mapped the nonce
#-----------------------------------------------------------------------------------
>> 10 86 0000 24 
    7C22 8120 B04E65450A54674ED7A48C62B00D58892BFC73D551721907636102622800687B 00

#-----------------------------------------------------------------------------------
# General Authenticate #2 Response: <SW 9000>
#-----------------------------------------------------------------------------------
<< 7C02 8200

# New mapped G: 04895203FD3EF9355D3C54BC2785095C64F852E24C0EDD72D6A157478A81D846FF3B95F82A7950815B4FAEF2D3F55F0B2BF74546207DADEFFE8507BDE971325788

# Ephemeral PCD Public Key: 0493E1C675765380F07FB7620A1DAA1202B1F92000FAB048CED1F51637954B168B69865421215B764CF42F2A21B3981FB888DAD45091F9EB952511B1C72C912DC8
# Ephemeral PCD Private Key: 23F6179CD2689C71BC4670A2CC495983C50F96F4FF67E255A1D3F6B0E4EB637F

#-----------------------------------------------------------------------------------
# General Authenticate #3 - Perform Key agreement
#-----------------------------------------------------------------------------------
>> 10 86 0000 45
    7C43 8341
    0493E1C675765380F07FB7620A1DAA1202B1F92000FAB048CED1F51637954B168B69865421215B764CF42F2A21B3981FB888DAD45091F9EB952511B1C72C912DC800

#-----------------------------------------------------------------------------------
# General Authenticate #3 Response: <SW 9000>
#-----------------------------------------------------------------------------------
<< 7C43 8441
    04705A5B3F5688F404A225F797695D5B0E63DA0F7BADB9F46CBB1127FDB0F55B94338439EAC12CD0737A60E0F8B7D49BEF6A29891EEFAA30BC5513EC3C148008EE

# Ephemeral PICC Public Key: 04705A5B3F5688F404A225F797695D5B0E63DA0F7BADB9F46CBB1127FDB0F55B94338439EAC12CD0737A60E0F8B7D49BEF6A29891EEFAA30BC5513EC3C148008EE

# Shared Secret: 876E5413F3D3CB70212189D8B7489DA5C4637521ED2A64E5B563C753B32F4C35
# Kenc: CE4D305A33AF93D9791A4465B1574BD8D78C13B15B7F0D5092204CD593311FA9
# Kmac: 2019F04A24AE368AC47C2F4CD157FA7C47B218E912375B6D2D5EF663BA9A7D76

# Terminal Authentication Token: 17CEBFBD204F3105

#-----------------------------------------------------------------------------------
# General Authenticate #4 - Mutual Authentication
#-----------------------------------------------------------------------------------
>> 00 86 0000 0C 7C0A 8508 17CEBFBD204F3105 00
#-----------------------------------------------------------------------------------
# General Authenticate #4 Response: <SW 9000>
#-----------------------------------------------------------------------------------
<< 7C0A 8608 4C2521729B2CAD25

# Chip Authentication Token: 4C2521729B2CAD25
# Computed Chip Authentication Token: 4C2521729B2CAD25

# PACE successfully performed
📢
Do you need more specific information about PACE? Are you implementing it manually from scratch or using an existing crypto library? Are you migrating from BAC to PACE? Let’s discuss in the comments.

[1] : Supplemental Access Control for Machine Readable Travel Documents - Version 1.1:

https://www2023.icao.int/Security/FAL/TRIP/Documents/TR%20-%20Supplemental%20Access%20Control%20V1.1.pdf

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eMRTDPassword Authenticated Connection EstablishmentBasic Access Controlicao 9303icao smartcardSmart Card in GovernmentSecurityauthenticationdiffie hellmanelliptic curve cryptographypacebacsupplemental access controlSAC

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Monssif HAKKOUR
Monssif HAKKOUR